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Students can Download Chapter 14 Environmental Chemistry Notes, Plus One Chemistry Notes helps you to revise the complete Kerala State Syllabus and score more marks in your examinations.

Kerala Plus One Chemistry Notes Chapter 14 Environmental Chemistry

Introduction
Environmental chemistry deals with various chemical phenomena occurring in the environment. Environmental chemistry deals with the study of the origin, transport, reactions, effects and fates of chemical species in the environment.

Environmental Pollution
The undesirable changes which have harmful effect on plants, animals and human beings is called environmental pollution. A substance, which causes pollution, is known as pollutant.

Atmospheric Pollution
The atmosphere that surrounds the earth is not of ‘ the same thickness at all heights. There are concentric layers of air or regions and each layer has different density. The lowest region of atmosphere in which the human beings along with other organisms live is called troposphere. Above the troposphere, lies stratosphere. Troposphere is a turbulent, dusty zone containing air, much water vapour and clouds. This is the region of strong air movement and cloud formation. The stratosphere, on the other hand, contains dinitrogen, dioxygen, ozone and little water vapour. Atmospheric pollution is generally studied as tropospheric and stratospheric pollution. The presence of ozone in the stratosphere prevents about 99.5 per cent of the sun’s harmful ultraviolet (UV) radiations from reaching the earth’s surface and thereby protecting humans and other animals from its effect.

Tropospheric Pollution
Tropospheric pollution occurs due to the presence of undesirable solid or gaseous particles in the air. The following are the major gaseous and particulate pollutants present in the troposphere:
1) Gaseous air pollutants:
These are oxides of sulphur, nitrogen and carbon, hydrogen sulphide, hydrocarbons, ozone and other oxidants.
2) Particulate pollutants:
These are dust, mist, fumes, smoke, smog, etc.

1. Gaseous air pollutants
a) Oxides of Sulphur:
Oxides of sulphur are produced when sulphur containing fossil fuel is burnt. It has been reported that even a low concentration of sulphur dioxide causes respiratory diseases e.g., asthma, bronchitis, emphysema in human beings. Sulphur dioxide causes irritation to the eyes, resulting in tears and redness. High concentration of SO₂ leads to stiffness of flower buds which eventually fall off from plants.
2SO₂ (g) + O₂ (g) → 2SO₃ (g)
The reaction can also be promoted by ozone and hydrogen peroxide.
SO₂ (g) + O₃ (g) → SO₃ (g) + O₂ (g)
SO₂(g) + H₂O₂(l) → H₂SO₄(aq)

b) Oxides of Nitrogen :

NO reacts instantly with oxygen to give NO₂
2NO(g) + O₂ (g) → 2NO₂ (g)

Rate of production of N02 is faster when nitric oxide reacts with ozone in the stratosphere.
NO(g) + O₃ (g) → NO₂ (g) + O₂ (g)

The irritant red haze in the traffic and congested places is due to oxides of nitrogen. Higher concentrations of NO₂ damage the leaves of plants and retard the rate of photosynthesis. Nitrogen dioxide is a lung irritant that can lead to an acute respiratory disease in children. It is toxic to living tissues also. Nitrogen dioxide is also harmful to various textile fibres and metals.

c) Hydrocarbons:
Hydrocarbons are composed of hydrogen and carbon only and are formed by incomplete combustion of fuel used in automobiles. Hydrocarbons are carcinogenic, i.e., they cause cancer. They harm plants by causing ageing, breakdown of tissues and shedding of leaves, flowers and twigs.

d) Oxides of Carbon :
i) Carbon monoxide:
Carbon monoxide is mainly released into the air by automobile exhaust. Other sources, which produce CO, involve incomplete combustion of coal, firewood, petrol, etc. Many vehicles are poorly maintained and several have inadequate pollution control equipment resulting in the release of greater amount of carbon monoxide and other polluting gases. CO binds to haemoglobin to form carboxyhaemoglobin, which is about 300 times more stable than the oxygen-haemoglobin complex. In blood, when the concentration of carboxyhaemoglobin reaches about 3-4 per cent, the oxygen carrying capacity of blood is greatly reduced. This oxygen deficiency results into headache, weak eyesight, nervousness and cardiovascular disorder. This is the reason why people are advised not to smoke. In pregnant women who have the habit of smoking the increased CO level in blood may induce premature birth, spontaneous abortions and deformed babies.

ii) Carbon dioxide:
Carbon dioxide (CO₂) is released into the atmosphere by respiration, burning of fossil fuels for energy, and by decomposition of limestone during the manufacture of cement. It is also emitted during volcanic eruptions. Carbon dioxide gas is confined to troposphere only. Normally it forms about 0.03 per cent by volume of the atmosphere. With the increased use of fossil fuels, a large amount of carbon dioxide gets released into the atmosphere. Excess of CO₂ in the air is removed by green plants and this maintains an appropriate level of CO₂ in the atmosphere. Green plants require CO₂ for photosynthesis and they, in turn, emit oxygen, thus maintaining the delicate balance. As you know, deforestation and burning of fossil fuel increases the CO₂ level and disturb the balance in the atmosphere. The increased amount of CO₂ in the air is mainly responsible for global warming.

Global Warming and Greenhouse Effect
Greenhouse effect:
Though carbon dioxide is not toxic, the excess concentration of it can lead to changes in climatic conditions, especially by raising the global tempreature. Greenhouse ef- feet is the phenomenon in which earth’s atmosphere traps the heat from the sun and prevents it from escaping into outer space resulting in the rise of atmospheric temperature.

The earth’s atmosphere allows most of the sunlight that falls on it to pass through and heats the surface of the earth. But the heat radiated by the heated surface in the form of infrared radiation is absorbed by greenhouse gases such as CO₂, CH₄, O₃, chlorofluoro carbon compound, water vapour etc. Thus these gases prevent the heat radiation of the earth to go out in space.

As more and more infrared, radiations are trapped, the atmosphere becomes hotter and the global temperature rises up. This is known as global warming. There has been a marked increase in the levels of carbon dioxide in the atmosphere due to severe deforestation and burning of fossil
fuels. An increase in average global temperature is likely to increase infectious diseases such as yellow fever, dengue fever etc. Global warming leads to heating up of water which in turn results in an increase of water level in oceans, lakes, etc.

Acid Rain
Rain water has a pH of 5.6 due to the presence of H⁺ ions formed by the reaction of rainwater with carbon dioxide present in the atmosphere.
H₂P(l) + CO₂(g) \(\rightleftharpoons \) H₂CO₃(aq)
H₂CO₃(aq) \(\rightleftharpoons \) H⁺(aq) + HCO₃^–(aq)

When the pH of the rain water drops below 5.6, it is called acid rain. Acid rain is a byproduct of a variety of human activities that emit the oxides of sulphur and nitrogen in the atmosphere. As mentioned earlier, burning of fossil fuels (which contain sulphur and nitrogenous matter) such as coal and oil in power stations and furnaces or petrol and diesel in motor engines produce sulphur dioxide and nitrogen oxides. SO₂ and NO₂ after oxidation and reaction with water are major contributors to acid rain because polluted air usually contains particulate matter that catalyse the oxidation.
2SO₂(g) + O₂ (g) + 2H₂O(I) 2H₂SO₄(aq)
4NO₂(g) + O₂(g) + 2H₂O(I) 4HNO₃(aq)

Ammonium salts are also formed and can be seen as an atmospheric haze (aerosol of fine particles). Aerosol particles of oxides or ammonium salts in raindrops result in wet deposition. S02 is also absorbed directly on both solid and liquid ground surfaces and is thus deposited as dry-deposition. Acid rain is harmful for agriculture, trees and plants as it dissolves and washes away nutrients needed for their growth. It causes respiratory ailments in human beings and animals. When acid rain falls and flows as ground water to reach rivers, lakes etc. it affects plants and animal life in aquatic ecosystem. It corrodes water pipes resulting in the leaching of heavy metals such as. iron, lead arid copper into the drinking water. Acid rain damages buildings and other structures made of stone or metal. The Taj Mahal in India has been affected by acid rain.

2. Particulate pollutants
The term particulate refers to finely divided solid or liquid particles suspended in air. The particulates usually present in atmosphere are fly ash, soot, dust, metal particles, asbestos dust, solid hydrocarbons, smoke, sulphuric acid and nitric acid mists.

Minute living organisms such as fungi, moulds, algae etc. dispersed in air are called viable particulates. Particles formed either by breakdown of larger materials or by condensation of minute particles are called non-viable particulates, eg: Mists, fumes, smoke, dust etc.

Smog
The word smog is derived from smoke and fog.
This is the most common example of air pollution that occurs in many cities throughout the world. There are two types of smog:

a) Classical smog
occurs in cool humid climate. It is a mixture of smoke, fog and sulphur dioxide. Chemically it is a reducing mixture and so it is also called as reducing smog.

b) Photochemical smog
occurs in warm, dry and sunny climate. The main components of the photochemical smog result from the action of sunlight on unsaturated hydrocarbons and nitrogen oxides produced by automobiles and factories. Photochemical smog has high concentration of oxidising agents and is, therefore, called as oxidising smog. Formation of photochemical smog – When fossil fuels are burnt, a variety of pollutants are emitted into the earth’s troposphere. Two of the pollutants that are emitted are hydrocarbons (unburnt fuels) and nitric oxide (NO). When these pollutants build up to sufficiently high levels, a chain reaction occurs from their interaction with sunlight in which NO is converted into nitrogen dioxide (NO₂). This NO_{2, in turn,} absorbs energy from sunlight and breaks up into nitric oxide and free oxygen atom.

Ozone is a toxic gas and both NO₂ and O₃ are strong oxidising agents and can react with the unburnt hydrocarbons in the polluted air to produce chemicals such as formaldehyde, acrolein and peroxyacetyl nitrate (PAN).

Effects of photochemical smog
Both ozone and PAN act as powerful eye irritants. Ozone and nitric oxide irritates the nose and throat and their high concentration causes headache, chest pain, dryness of the throat, cough and difficulty in breathing. Photochemical smog leads to cracking of rubber and extensive damage to plant life. It also causes corrosion of metals, stones, building materials, rubber and painted surfaces. Catalytic converters are used in the automobiles, which prevent the release of nitrogen oxide and hydrocarbons to the atmosphere. Certain plants e.g., Pinus, Juniparus, Quercus, Pyrus and Vitis can metabolise nitrogen oxide and therefore, their plantation could help in this matter.

Stratospheric Pollution
Formation and Breakdown of Ozone

The main reason of ozone layer depletion is believed to be the release of chlorofluorocarbon compounds (CFCs), also known as freons. These compounds are non-reactive, non-flammable, non-toxic organic molecules and therefore used in refrigerators, air conditioners, in the production of plastic foam and by the electronic industry for cleaning computer parts etc. Once CFCs are released in the atmosphere, they mix with the normal atmospheric gases and eventually reach the stratosphere. In stratosphere, they get broken down by powerful UV radiations, releasing chlorine free radical.

The chlorine radicals are continuously regenerated and cause the breakdown of ozone. Thus, CFCs are transporting agents for continuously generating chlorine radicals into the stratosphere and damaging the ozone layer. Antarctica reported about depletion of ozone layer commonly known as ozone hole over the South Pole. It was found that a unique set of conditions was responsible for the ozone hole. In summer season, nitrogen dioxide and methane react with chlorine monoxide and chlorine atoms forming chlorine sinks, preventing much ozone depletion, whereas in winter, special type of clouds called polar stratospheric clouds are formed over Antarctica. These polar stratospheric clouds provide surface on which chlorine nitrate formed gets hydrolysed to form hypochlorous acid. It also reacts with hydrogen chloride produced as per reaction to give molecular chlorine.

When sunlight returns to the Antarctica in the spring, the sun’s warmth breaks up the clouds and HOCl and Cl₂ are photolysed by sunlight, as given in reactions.

Water Pollution
Causes of Water Pollution
(i) Pathogens:
The most serious water pollutants are the disease causing agents called pathogens. Pathogens include bacteria and other organisms that enter water from domestic sewage and animal excreta. Human excreta contain bacteria such as Escherichia coli and Streptococcus faecalis which cause gastrointestinal diseases.

Dissolved oxygen (DO) in water:
The concentration of dissolved oxygen in water is of vital importance to aquatic life. Growth of fish is inhibited if the dissolved oxygen level in waterfalls below 6 ppm. Pollution causes decrease in DO level.

Dissolved oxygen (DO) in water is consumed by micro-organisms to oxidise organic matter in sewage. Deoxygenation of water may also take place by the bio-oxidation of nitrogeneous matter.

Biochemical oxygen demand (BOD) :
It is the amount of dissolved oxygen required by microorganisms to oxidise organic and inorganic matter present in polluted water. It is generally expressed in ppm (parts per million). ‘Clean water1 would have a BOD value less than 5 ppm while highly contaminated water (say, river water) could have a BOD value 17 ppm or more.

The amount of oxygen in ppm that would be required to oxidise all the contaminants in water is called chemical oxygen demand (COD).

Nowadays most of the detergents available are biodegradable. However, their use can create other problems. The bacteria responsible for degrading biodegradable detergent feed on it and grow rapidly. While growing, they may use up all the oxygen dissolved in water. The lack of oxygen kills all other forms of aquatic life such as fish and plants. Fertilizers contain phosphates as additives. The addition of phosphates in water enhances algae growth. This process in which nutrient enriched water bodies support a dense plant opulation, which kills animal life by depriving it of oxygen and results in subsequent loss of biodiversity is known as Eutrophication.

International Standards for Drinking Water
The International Standards for drinking water are given below and they must be followed.

Fluoride:
For drinking purposes, water should be tested for fluoride ion concentration. Its deficiency in drinking water is harmful to man and causes diseases such as tooth decay etc. Soluble fluoride is often added to drinking water to bring its concentration upto 1 ppm or 1 mg dm^-3. The F^– ions make the enamel on teeth much harder by converting hydroxyapatite, [3(Ca₃(PO₄)₂.Ca(OH)₂], the enamel on the surface of the teeth, into much harder fluorapatite, [3(Ca₃(PO₄)₂.CaF₂]. However, F- ion concentration above 2 ppm causes brown mottling of teeth. At the same time, excess fluoride (over 10 ppm) causes harmful effect to bones and teeth, as reported from some parts of Rajasthan.

Lead:
Drinking water gets contaminated with lead when lead pipes are used for transportation of water. The prescribed upper limit concentration of lead in drinking water is about 50 ppb. Lead can damage kidney, liver, reproductive system etc.

Sulphate:
Excessive sulphate (>500 ppm) in drinking water causes laxative effect, otherwise, at moderate levels it is harmless.

Nitrate:
The maximum limit of nitrate in drinking water is 50 ppm. Excess nitrate in drinking water can cause disease such as methemoglobinemia (‘blue baby’ syndrome).

Soil Pollution
Any factor which destroys the quality, texture and mineral content of the soil or which disturbs the biological balance of the organisms in the soil is referred to as soil pollutant.

Soil pollution has adverse effect on plant growth. Soil pollution is mainly due to

• Indiscriminate use of fertilizers, pesticides etc.
• Dumping of waste materials.
• Deforestation.

Large number of pesticides are used to save plants from pests, rats, insects, fungi etc. Chlorinated hydrocarbons, malathion, aldrin, DDT, BHC etc. are some substances used extensively in agriculture. Herbicides are used to kill weeds. Sodium chlorate (NaClO₃), sodium arsenite (Na₃AsO₃) etc. are widely used as herbicides. Inorganic arsenic compounds are toxic to mammals. Fungicides are important because they counter the growth of fungi. Organo-mercury compounds are used to fungicides. However, these compounds break down in the soil causing harmful effects on human beings. Dumping of paper, plastics and other toxic substances in the soil creates serious problems. A large number of heavy metals get deposited in the soil around smelting industries. These effluents, in the long run, pollute the soil.

Control of environmental pollution
Both industrial and domestic wastes need treatment for safe disposal, (i) Recycling of wasters not only saves the cost on raw materials but also reduces waste disposal costs. Collection and recycling of glass, metal scrap, plastics etc. are some examples of industrial recycling, (ii) Sewage water is filtered to remove large solids and then allowed to settle. Solids can settle as a sludge at the bottom while oil, grease etc. float at the surface which can be skimmed off. The sludge is dried and incinerated at high temperature (above 1000°C) in presence of oxygen. Sewage sludge can be degraded by anaerobic digestion by micro-organisms.

Green Chemistry
Introduction
Green chemistry is a way of thinking and is about utilising the existing knowledge and principles of chemistry and other sciences to reduce the adverse impact on environment. Green chemistry is a production process that would bring about minimum pollution or deterioration to the environment. The byproducts generated during a process, if not used gainfully, add to the environmental pollution. Such processes are not only environmentally unfriendly but ‘ also cost-ineffective. The waste generation and its disposal both are economically unsound. Utilisation of existing knowledge base for reducing the chemical hazards along with the developmental activities is the foundation of green chemistry. It is well-known that organic solvents such as benzene, toluene, carbon tetrachloride etc., are highly toxic. One should be careful while using them.

Green Chemistry In Day-To-Day Life
i) Dry Cleaning of Clothes
Tetra chlroroethene (Cl₂C=CCl₂) was earlier used as solvent for dry cleaning. The compound contaminates the ground water and is also a suspected carcinogen. Replacement of halogenated solvent by liquid CO₂ will result in less harm to ground water. These days hydrogen peroxide (H₂O₂) is used for the purpose of bleaching clothes in the process of laundry, which gives better results and makes use of lesser amount of water.

ii) Bleaching of Paper
Chlorine gas was used earlier for bleaching paper. These days, hydrogen peroxide (H₂O₂) with suitable catalyst, which promotes the bleaching action of hydrogen peroxide, is used.

iii) Synthesis of Chemicals
Ethanal (CH₃CHO) is now commercially prepared by one step oxidation of ethene in the presence of ionic catalyst in aqueous medium with a yield of 90%.

Students can Download Chapter 13 Hydrocarbons Notes, Plus One Chemistry Notes helps you to revise the complete Kerala State Syllabus and score more marks in your examinations.

Kerala Plus One Chemistry Notes Chapter 13 Hydrocarbons

Introduction
The compounds formed by carbon and hydrogen are called Hydrocarbons.

Classification
Hydrocarbons are broadly classified into two main classes. Open chain hydrocarbons and closed chain or cyclic hydrocarbons.

Open chain hydrocarbons are hydrocarbons containing open chain of carbon atoms in their molecules. The chain may be straight or branched. These compounds are also called aliphatic hydrocarbons. They are further divided into three classes: Alkanes, alkenes, and alkynes.

Closed chain or cyclic hydrocarbons are hydrocarbons having closed chains or righs in their molecules. They are further divided into two categories: Alicyclic hydrocarbons and aromatic hydrocarbons. Alicyclic hydrocarbons contain ring or closed chain of carbon atoms, but resemble aliphatic compounds in most of their properties. Aromatic hydrocarbons contain at least one benzene ring which is a hexagonal ring of carbon atoms with three double bonds in alternate positions.

ALkanes
Alkanes are saturated aliphatic hydrocarbons containing only carbon-carbon single bonds in their molecules. They can be represented by the general formula c_n H_2n+2 Where n may have values 1, 2, 3, ………… Since they are relatively inert towards most of the common reagents, they are called paraffins (Latin : parum = Little, affinis = affinity)

Nomenclature
Nomenclature of alkanes has already been discussed in Unit =12 ‘

Preparation:
Petroleum and natural gas are the main sources of alkanes. However, alkanes can be prepared by following methods:
1. From unsaturated hydrocarbons
Dihydrogen gas adds to alkenes and alkynes in the presence of finely divided catalysts like platinum, palladium or nickel to form alkanes. This process is called hydrogenation. These metals adsorb dihydrogen gas on their surfaces and activate the hydrogen – hydrogen bond.

Platinum and palladium catalyse the reaction at room temperature but higher temperature and pressure are required.

2. From alkyl halides
i) Alkyl halides (except fluorides) on reduction with zinc and dilute hydrochloric acid give alkanes.

ii) Alkyl halides on treatment with sodium metal in dry ether (free from moisture) solution give higher alkanes. This reaction is known as Wurtz reaction and is used for the preparation of higher alkanes containing even number of carbon atoms.

3. From carboxylic acids
i) Sodium salts of carboxylic acids on heating with soda lime (mixture of sodium hydroxide and calcium oxide) give alkanes containing one carbon atom less than the carboxylic acid. This process of elimination of carbon dioxide from a carboxylic acid is known as Decarboxylation.

Sodium ethanoate

ii) Kolbe’s electrolytic method:
An aqueous solution of sodium or potassium salt of a carboxylic acid on electrolysis gives alkane containing even number of carbon atoms at the anode.
2CH₃COO^–Na⁺ +2H₂O
Sodium acetate
-Electrolysis
CH₃ – CH₃ +2CO₂ +H₂ +2NaOH
The reaction is supposed to follow the following path:

Methane cannot be prepared by this method.

Properties
Physical properties:
Alkanes are almost non-polar molecules because of the covalent nature of C-C and C-H bonds and due to very little difference of electronegativity between carbon and hydrogen atoms. They possess weak van der Waals’ forces. Due to the weak forces, the first four members, C₁ to C₄ are gases, C₅ to C₁₇ are liquids and those containing 18 carbon atoms or more are solids at 298 K. It is generally observed that in relation to solubility of substances in solvents, polar substances are soluble in polar solvents, whereas the non-polar ones in non-polar solvents i.e., like dissolves like. There is a steady increase in boiling point with increase in molecular mass.

Chemical properties
As already mentioned, alkanes are generally inert towards acids, bases, oxidising and reducing agents. However, they undergo the following reactions under certain conditions.

1. Substitution reactions
One or more hydrogen atoms of alkanes can be replaced by halogens, nitro group, and sulphonic acid group. Halogenation takes place either at higher temperature (573-773 K) or in the presence of diffused sunlight or .ultraviolet light. Lower alkanes do not undergo nitration and sulphonation reactions. These reactions in which hydrogen atoms of alkanes are substituted are known as substitution reactions. As an example,chlorination of methane is given below:

Halogenation

It is found that the rate of reaction of alkanes with halogens is F₂ > Cl₂ > Br₂ > l₂. Rate of replacement of hydrogens of alkanes is :3° > 2° > 1 °. Fluorination is too violent to be controlled, lodination is very slow and a reversible reaction. It can be carried out in the presence of oxidizing agents like HIO₃ or HNO₃.
CH₄ +I₂ \(\rightleftharpoons \) CH₃I+HI
HIO₃ +5HI → 3l₂ +3H₂O
Halogenation is supposed to proceed via free radical chain mechanism involving three steps namely initiation, propagation, and termination as given below:

Mechanism
i) Initiation :
The reaction is initiated by homolysis of chlorine molecule in the presence of light or heat. The Cl-Cl bond isweakerthantheC-C and C-H bond and hence is easiest to break.

ii) Propagation:
It involves the following steps

Two such steps given below explain how more highly halogenated products are formed.

iii) Termination :
The reaction stops after some time due to consumption of reactants and / or due to the following side reactions:
The possible chain terminating steps are :

Though in (c), CH₃-Cl, the one of the products is formed but free radicals are consumed and the chain is terminated. The above mechanism helps us to understand the reason for the formation of ethane as a byproduct during chlorination of methane.

2. Combustion

3. Controlled oxidation

iv) Ordinarily alkanes resist oxidation but alkanes having tertiary H atom can be oxidized to corresponding alcohols by potassium permanganate.

4. Isomerisation
n-Alkanes on heating in the presence of anhydrous aluminum chloride and hydrogen chloride gas isomerise to branched-chain alkanes. Major products are given below. Minor products are generally not reported in organic reactions.

5. Aromatization
n-Alkanes having six or more carbon atoms on heating to 773 K at 10-20 atmospheric pressure in the presence of oxides of vanadium, molybdenum or chromium supported over alumina get dehydrogenated and cyclised to benzene and its homologues. This reaction is known as aromatization or reforming.

6. Reaction with steam

7. Pyrolysis
Higher alkanes on heating to higher temperature decompose into lower alkanes, alkenes etc. Such a decomposition reaction into smaller fragments by the application of heat is called pyrolysis or cracking.

Preparation of oil gas or petrol gas from kerosene oil or petrol involves the principle of pyrolysis.

Conformations
The C-C bonds in alkanes are sigma bonds. Carbon atoms connected through sigma bonds in alkanes can undergo rotation about the bond axis. As a result of this rotation, a large number of different spatial arrangements of atoms of groups attached to the carbon atoms are possible. These different spatial arrangements are called conformations. Thus, the different spatial arrangements of atoms in a molecule which arise due to free rotation about carbon-carbon single bond are called conformations.

Conformations of ethene:
Ethane molecule (C₂H₆) contains a carbon-carbon single bond with each carbon atom attached to three hydrogen atoms. Considering the ball and stick model of ethane, keep one carbon atom stationary and rotate the other carbon atom around the C-C axis. This rotation results into infinite number of spatial arrangements of hydrogen atoms attached to one carbon atom with respect to the hydrogen atoms attached to the other carbon atom. These are called conformational isomers (conformers). There are two extreme cases. One such confirmation in which hydrogen atoms attached to two carbons are as closed together as possible is called eclipsed conformation and the other in which hydrogens are as far apart as possible is known as the staggered conformation. Any other intermediate conformation is called a skew conformation. lt may be remembered that in all the conformations, the bond angles and the bond lengths remain the same. Eclipsed and the staggered conformations can be represented by Sawhorse and Newman projections.

1. Sawhorse projection
C-C bond is represented by straight line. Upper end of the line is slightly tilted. The front carbon is shown by the lower end of the line and the rear carbon is represented by upper end. The angle between C-H bonds is 120°. Sawhorse projections of eclipsed and staggered conformations are depicted below.

2. Newman projections
In this projection, the molecule is viewed at the C-C bond head on. The carbon atom nearer to the eye is represented by a point. Three hydrogen atoms attached to the front carbon atom are shown by three lines drawn at an angle of 120° to each other. The rear carbon atom (the carbon atom away from the eye) is represented by a circle and the three hydrogen atoms are shown attached to it by the shorter lines drawn at an angle of 120° to each other.

Relative stability of conformations:
As mentioned earlier, in staggered form of ethane, the electron clouds of carbon-hydrogen bonds are as far apart as possible. Thus, there are minimum repulsive forces, minimum energy and maximum stability of the molecule. On the other hand, when the staggered form changes into the eclipsed form, the electron clouds of the carbon-hydrogen bonds come closer to each other resulting in increase in electron cloud repulsions. To check the increased repulsive forces, molecule will have to possess more energy and thus has lesser stability. As already mentioned, the repulsive interaction between the electron clouds, which affects stability of a conformation, is called torsional strain.

Magnitude of torsional strain depends upon the angle of rotation about C-C bond. This angle is also called dihedral angle or torsional angle. Of all the conformations of ethane, the staggered form has the least torsional strain and the eclipsed form, the maximum torsional strain. Thus it may be inferred that rotation around C-C bond in ethane is not completely free. The energy difference between the two extreme forms is of the order of is very small.

Alkenes
Alkenes are unsaturated hydrocarbons containing a carbon-carbon double bond. Acyclic alkenes with one double bond have the general formula C_nH_2n. Thus they are isomeric with cycloalkanes. They are also known as olefins (Greek word olefiant = oil forming) because the lower members of this family form oily products when treated with chlorine.

We have already seen that a carbon-carbon double bond consists of a sigma bond and a pi-bond. A pi-bond is weaker than a sigma – bond because sidewise overlapping takes place to smaller extent. It should be noted that a double bond is shorter than a single bond. Again, the C-H bond length in ethene is slightly shorter than that in alkanes because it is formed by the overlap of sp² hybrid orbitals of carbon.

Isomerism in Alkenes
Alkenes show chain isomerism, position isomerism and geometrical isomerism,
i) Chain Isomerism :
The simplest alkene which can exhibit isomerism is butene (C₄ H₈). It exists as two chain isomers.

ii) Position isomerism :
Position isomers of alkenes differ in the position of double bond.

iii) Geometrical isomerism :
The isomers which have the same structural formula but differ in spatial arangement of atoms or groups about the double bond are called geometrical isomers and the phenomenon is known as geometrical isomers of but 2 ene are given below.

The geometrical isomer with similar groups on the same side of the double bond is called the cis isomer while the other isomer with similar groups on opposite side is called the trans isomer. Because of this reason geometrical isomerism is also called cistrans isomerism.

Hindered rotation about carbon-carbon double bond is responsible for the existence of geometrical isomers. As we know, C=C consists of a sigma bond and a pi bond. Due to the presence of pi bond, the groups or atoms attached to doubly bonded carbon atoms cannot rotate about it. They can do so only by breaking the pi-electron cloud.

Preparation
1) From alkynes:
Partially deactivated palladised charcoal is known as Lindlar’s catalyst is used Alkenes thus obtained are having cis geometry. However, alkynes on reduction with sodium in liquid ammonia form trans alkenes.

2) From alkyl halides:
Alkyl halides (R-X)on heating with alcoholic potash eliminate one molecule of halogen acid to form alkenes. This reaction is known as dehydrohalogenation i.e., removal of halogen acid. This is example of β-elimination reaction, since hydrogen atom is eliminated from the β-carbon atom(carbon atom next to the carbon to which halogen is attached).

It is observed that for halogens, the rate is:iodine > bromine > chlorine, while for alkyl groups it is: tert > secondary > primary.

3) From vicinal dihalides :
Dihalides in which two halogen atoms are attached to two adjacent carbon atoms are known as vicinal dihalides. Vicinal dihalides on treatment with zinc metal lose a molecule of ZnX₂ form an alkene. This reaction is known as dehalogenation.
CH₂Br – CH₂Br + Zn CH₂ = CH₂ + ZnBr₂

4) From alcohols by acidic dehydration:
Alcohols on heating with concentrated sulphuric acid form alkenes with the elimination of one water molecule. Since a water molecule is eliminated from the alcohol molecule in the presence of an acid, this reaction is known as acidic dehydration of alcohols. This reaction is also the example of β-elimination reaction since-OH group takes out one hydrogen atom from the β-carbon atom.

Chemical Properties
1) Addition of dihydrogen:
Alkenes add up one molecule of dihydrogen gas in the presence of finely divided nickel, palladium or platinum to form alkanes

2) Addition of halogens :

3) Addition of hydrogen halides:
Hydrogen halides (HCl, HBr,HI) add up to alkenes to form alkyl halides. The order of reactivity of the hydrogen halides is HI > HBr> HCI. Like addition of halogens to alkenes, addition of hydrogen halides is also an example of electrophilic addition reaction. Addition reaction of HBr to symmetrical alkenes Addition reactions of HBr to symmetrical alkenes (similar groups attached to double bond) take place by electrophilic addition mechanism.

Addition reaction of HBr to unsymmetrical alkenes (Markovnikov Rule)
There are two possible products in the following reaction

Markovnikov, a Russian chemist made generalisations about such reactions. It to frame a rule called Markovnikov rule. The rule states that negative part of the addendum (adding molecule) gets attached to that carbon atom which possesses lesser number of hydrogen atoms. Thus according to this rule, product I i.e., 2- bromopropane is expected. In actual practice, this is the principal product of the reaction.

Anti Markovnikov addition or peroxide effect or Kharash effect
In the presence of peroxide, addition of HBr to unsymmetrical alkenes like propene takes place contrary to the Markovnikov rule. This happens only with HBr but not with HCI and HI. This addition reaction was observed by M.S. Kharash and F.R. Mayo in 1933. This reaction is known as peroxide or Kharash effect or addition reaction anti to Markovnikov rule.

4) Addition of sulphuric acid:
Cold concentrated sulphuric acid adds to alkenes in accordance with Markovnikov rule to form alkyl hydrogen sulphate by the electrophilic addition reaction.

5) Addition of water :
In the presence of a few drops of concentrated sulphuric acid alkenes react with water to form alcohols, in accordance with the Markovnikov rule.

6) Oxidation:
Alkenes on reaction with cold,dilute, aqueous solution of potassium permanganate (Baeyer’s reagent) produce vicinal glycols. Decolorisation of KMnO. solution is used as a test for unsaturation.

b) Acidic potassium permanganate or acidic potassium dichromate oxidises alkenes to ketones and/or acids depending upon the nature of the alkene and the experimental conditions.

7) Ozonolysis:
Ozonolysis of alkenes involves the addition of ozone molecule to alkene to form ozonide, and then cleavage of the ozonide by Zn- H20 to smaller molecules. This reaction is highly useful in detecting the position of the double bond in alkenes or other unsaturated compounds.

8) Polymerisation:
You are familiar with polythene bags and polythene sheets. Polythene is obtained by the combination of large number of ethene molecules at high temperature, high pressure and in the presence of a catalyst. The large molecules thus obtained are called polymers. This reaction is known as polymerisation. The simple compounds from which polymers are made are called monomers. Other alkenes also undergo polymerisation.

Alkynes
Like alkenes, alkynes are also unsaturated hydrocarbons. They contain at least one triple bond between two carbon atoms. The number of hydrogen atoms is still less in alkynes as compared to alkenes or alkanes. Their general formula is C_nH_2n-2 Alkynes show chain isomerism and position isomerism.

i) Chain isomerism :
Alkynes having five or more carbon atoms show chain isomerism due to different structures of the carbon chain. For example, the chain isomers of C₅H₈ are

ii) Position isomerism:
Butyne and higher alkynes show position isomerism. For example, the position isomers of butyne are

Structure Of Triple Bond
Each carbon atom of ethyne has two sp hybridised orbitals. Carbon-carbon sigma (σ)bond is obtained by the head-on overlapping of the two sp hybridised orbitals of the two carbon atoms. The remaining sp hybridised orbital of each carbon atom undergoes overlapping along the internuclear axis with the 1s orbital of each of the two hydrogen atoms forming two C-H sigma bonds. H-C-C bond angle is of 180°. Each carbon has two unhybridised p orbitals which are perpendicular to each other as well as to the plane of the C-C sigma bond. The 2p orbitals of one carbon atom are parallel to the 2p orbitals of the other carbon atom, which undergo lateral or sideways overlapping to form two pi (π) bonds between two carbon atoms. Thus ethyne molecule consists of one C-C σ bond, two C-H σ bonds, and two C-C π bonds. The strength of C ≡ C bond is more than those of C=C bond and C-C bond.

Preparation
1. From calcium carbide:

2. From vicinal dihalides:

Properties

Physical properties
Physical properties of alkynes follow the same trend of alkenes and alkanes. First three members are gases, the next eight are liquids and the higher ones are solids. All alkynes are colourless. Ethyene has characteristic odour. Other members are odourless. Alkynes are weakly polar in nature. They are lighter than water and immiscible with water but soluble in organic solvents like ethers, carbon tetrachloride, and benzene. Their melting point, boiling point and density increase with increase in molar mass.

Chemical Properties
Acidic Character Of Alkyne

A. Acidic character of alkyne:
The acidic character of acetylene and other l-alkynes can be explaned on the basis of the sp hybridisation state of the triply bonded carbon atom. We know that an electron in s-orbital is more tightly held than that in a p-orbital because s-electrons are more close to the nucleus. Now, sp hybridised orbital has greater s-character (50%) as compared to sp² (33%) and sp³ (25%) hybrid orbitals. Due to large s-character, the electrons in sp hybridised orbitals are held more tightly by the nucleus. Thus, sp hybridised carbon is more electronegative than sp² or sp³ hybridised carbon atoms. Due to this, the hydrogen atom attached to sp hybridised carbon atom develops slight positive charge and is acidic in character.

B. Addition reactions:

The addition product formed depends upon stability of vinylic cation. Addition in unsymmetrical alkynes takes place according to Markovnikov rule. Majority of the reactions of alkynes are the examples of electrophilic addition reactions. A few addition reactions are given below:
(i) Addition of dihydrogen

(ii) Addition od halogens

(iii) Addition of hydrogen halides :
Two molecules of hydrogen halides (HCl, HBr,HI) add to alkynes to form gem dihalides (in which two halogens are attached to the same carbon atom)

(iv) Addition of water:

(v) Polymerisation
(a) Linear polymerisation:
Under suitable conditions, linear polymerisation of ethyne takes place to produce polyacetylene or polyethyne which is a high molecularweight polyene containing repeating units . of (CH = CH – CH = CH) and can be represented as —(CH = CH – CH = CH)_n — Under special conditions, this polymer conducts electricity.

Thin-film of polyacetylene can be used as electrodes in batteries. These films are good conductors, lighter and cheaper than the metal conductors.

(b) Cyclic polymerisation:

Aromatic Hydrocarbons
Aromatic compounds are a class of compounds having characteristic stability in spite of having double bonds in their structure. Simple aromatic compounds contain one or more benzene rings. They are known as benzenoid aromatic compounds. There is yet another series of compound which exhibit aromatic properties but lack benzene like structures. They are called non-benzenoid aromatic compounds. Heterocyclic compounds such as pyrrole, thiophene etc. are simple examples of non-benzenoid aromatic compounds. Structures of some simple arenes (benzenoid) are given below.

Resonance and stability of benzene
According to resonance theory, benzene is a resonance hybrid of the two Kekule structures I & II shown below.

These two structures I & II are known as canonical structures of benzene. The actual structure of benzene is intermediate between the two Kekule structures and is referred to as the resonance hybrid. As a result of resonance, all carbon-carbon bond lengths in benzene become equal and lie in between C=C and C-C bond length.

Further, a resonance hybrid will always be more stable than any of the contributing (or canonical) structures. Thus the actual molecule of benzene is more stable than either of the two Kekule structures. The difference between the energy of the most stable contributing structure and the energy of the resonance hybrid is known as resonance energy. Resonance energy of benzene has been found to be about 150 kJ mol^-1.

Aromaticity
Benzene was considered as parent ‘aromatic’ compound. Now, the name is applied to all the ring systems whether or not having benzene ring, possessing following characteristics.
i) Planarity
ii) Complete delocalisation of the π electrons in the ring
iii) Presence of (4n + 2)π electrons in the ring where n is an integer (n = 0, 1,2,…).

This is often referred to as Hiickel Rule.
Some examples of aromatic compounds are given below:

Preparation of Benzene
Benzene is commercially isolated from coal tar. However, it may be prepared in the laboratory by the following methods.
i) Cyclic polymerisation of ethyne: (Section 13.4.4)
ii) Decarboxylation of aromatic acids:
Sodium salt of benzoic acid on heating with sodalime gives benzene.

iii) Reduction of Phenol

Properties

Physical Properties
Aromatic hydrocarbons are non- polar molecules and are usually colourless liquids or solids with a characteristic aroma. You are also familiar with naphthalene balls which are used in toilets and for preservation of clothes because of unique smell of the compound and the moth repellent property. Aromatic hydrocarbons are immiscible with water but are readily miscible with organic solvents. They bum with sooty flame.

Chemical properties
Arenes are characterised by electrophilic substitution reactions. However, under special conditions they can also undergo addition and oxidation reactions.

Electrophilic substitution reactions
The common electrophilic substitution reactions of arenes are nitration, halogenation, sulphonation, Friedel Craft’s alkylation and acylation reactions in which attacking reagent is an electrophile (E+)
i) Nitration:

ii) Halogenation:
Arenes react with halogens in the presence of a Lewis acid like anhydrous FeCl₃, FeBr₃ or AlCl₃ to yield haloarenes.

iii) Sulphonation:

iv) Friedel-Crafts alkylation reaction:

v) Friedel-Crafts acylation reaction:
The reaction of benzene with an acyl halide or acid anhydride in the presence of Lewis acids (AlCl₃) yields acyl benzene

If excess of electrophilic reagent is used, further substitution reaction may take place in which other hydrogen atoms of benzene ring may also be successively replaced by the electrophile. For example, benzene on treatment with excess of chlorine in the presence of anhydrous AlCl₃ in dark yields hexachlorobenzene (C₆Cl₆).

a) Generation of electrophile E^A :
During chlorination, alkylation and acylation of benzene, an-hydrous AlCl₃, being a Lewis acid helps in generation of the elctrophile Cl^⊕, R^⊕. RC^⊕O (acylium ion) respectively by combining with the attacking reagent.

In the case of nitration, the electrophile, nitronium ion, NO₂⁺ is produced by transfer of a proton (from sulphuric acid) to nitric acid in the following manner:

It is interesting to note that in the process of generation of nitronium ion, sulphuric acid serves as an acid and nitric acid as a base.
Thus, it is a simple acid-base equilibrium,

b) Formation of Carbocation (arenium ion):
Attack of electrophile results in the formation of σ – complex or arenium ion in which one of the carbon is sp³ hybridised and the arenium ion gets stabilised by resonance. Sigma complex or arenium ion loses its aromatic character because delocalisation of electrons stops at sp³ hybridised carbon.

The areneium ion gets stabilised by resonance:

c) Removal of proton:

Addition reactions

Combustion:

Directive Influence Of A Functional Group In Monosubstituted Benzene
In benzene all the six hydrogen atoms are equivalent. There fore, replacement of anyone hydrogen atom by a substituent gives a single monosubstituted derivative of benzene. However, when a monosubstituted product is converted into a disubstituted benzene derivative, three different isomers can be obtained. They are,

Carcinogenicity And Toxicity
Benzene and polynuclear hydrocarbons containing more than two benzene ring fused together are toxic and said to posses cancer producing (carcinogenic) property. They are formed due to the incomplete combustion of organic materials like tobacco, coal, and petroleum.