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Q: What are fossil fuels, how they formed ?

Ans:

About 90% of the energy used to sustain our way of life comes from fossil fuels. Coal, natural gas and petroleum are called fossil fuels.

Natural gas is a mixture of low molecular weight hydrocarbons. Mainly it contains methane.

Petroleum is a liquid mixture of mainly hydrocarbons. When these fuels are burnt, they produce carbon dioxide, water and heat. It has been estimated that the concentration of carbon dioxide in the atmosphere has increased up to 20% by the end of twentieth century.

Q: What are hydrocarbons, how they are classified?

Ans:

Hydrocarbons:

The organic compounds which contain only two elements, carbon and hydrogen are called hydrocarbons.

There are several kinds of hydrocarbons. They are classified according to the type of bonding between the carbon atoms.

Types:

Carbon dioxide and other gases produce a greenhouse effect About 25 billion tons of carbon dioxide is released into the atmosphere each year, 22 billion tons of it comes from the burning of fossil fuels. About 15 billion tons per year is removed by the plants. Thus 10 billion tons of carbon dioxide remains in the air. This is causing global warming.

Caron dioxide and other gases in the air let the sun rays in, to warm the surface of earth. When the earth tries to radiate this heat back into the space, molecules of these gases trap this energy.

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Q: What are alkanes, draw draw electronic dot & cross structure for simple alkanes?

Ans:

Alkanes

Alkanes are saturated hydrocarbons.

They have general formula CnH2n+2.

Each carbon atom forms four bonds and each hydrogen atom forms only one bond.

Simplest alkane molecule that is possible is CH4. It is called methane. Methane is the main component of natural gas.

Electron dot and cross structure for methane, ethane propane is as follows.

Q. Give methods for the preparation of alkanes?

Ans:

1. By Hydrogenation of alkenes and alkynes

Addition of hydrogen molecule across carbon-carbon multiple-bonds is called hydrogenation.

Hydrogenation takes place in presence of finally divided nickel at 200 – 300o C and high pressure. Hydrogenation can also be done in presence of Pt or Pd at room temperature.

Sources of Alkanes 1. The main sources of alkanes are

petroleum and natural gas. 2. Methane forms about 85% of

natural gas. 3. All the alkanes are obtained

commercially by the fractional distillation of crude petroleum.

4. Marsh gas is formed by the bacterial decay of vegetable matter contains mostly methane.

5. Fuel gases obtained from coal gas contain alkanes in small amounts.

6. Methane occurs in gobar gas, sewage gas and biogas which are formed by the decomposition of cattle dung, excreta and plant wastes.

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Complete the following reactions

CH3 – CH = CH2 + H2 CH3-CH2-CH3

CH3 – C C + 2H2 CH3-CH2-CH3

2. By the reduction of alkyl halides

When an alkyl halide is treated with Zn in presence of an aqueous acid, an alkane is produced. Usually aqueous solution of HCl or CH3COOH is used.

Zn reacts with aqueous acid to liberate atomic hydrogen called nascent hydrogen. Nascent hydrogen reduces alkyl halide. Addition of nascent hydrogen is called reduction.

Self-Assessment Exercise 12.4

Complete the following reactions.

(a) CH3 – CH2 + 2[H] CH3-CH3 + HCl

Cl

(b) CH3 – CH – CH3 + 2[H] CH3-CH2-CH3+ HCl

Cl

Q: What are physical properties of alkanes?

Ans:

Physical Properties of Alkanes

1. They are useful solvents. For instance hexane is used to extract vegetable oils from Corn, Soya beans, Cotton seed etc.

Self-Assessment Exercise 12.3

Zn

HCl (aq)

Zn

H2SO4 (aq)

Ni

200-300°C

Ni

200-300°C

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2. Alkanes containing up to four carbon atoms are colourless, odourless gases. 3. Alkanes containing five to seventeen atoms are colourless, odourless liquids. 4. Higher alkanes are solids which are also colourless and odourless. 5. They are nonpolar, therefore, they are insoluble in water but soluble in organic

solvents. 6. The density of alkanes increases gradually with the increase of molecular size. 7. The melting and boiling points of alkanes increase regularly with the increase of

molecular sizes. This is because of increase of attractive forces between the molecules of alkanes.

8. The alkanes become more viscous as their molecular sizes increase. 9. Alkanes become less flammable, i.e. more difficult to burn with the increase of

molecular sizes. Q: What are chemical properties of alkanes, give an account of them?

Ans: Although unreactive towards ionic substances, but they some times give substitution reactions

Halogenation : Alkanes readily react with halogens in sunlight. The reaction of an alkane and a halogen is a substitution reaction “A reaction in which one or more hydrogen atoms of a saturated compound are replaced with some other atoms (like halogen) is called a substitution reaction”

In direct sunlight, reaction is explosive and carbon is deposited.

In diffused sunlight, a series of reactions take place and at each step one hydrogen atom is substituted by halogen atoms, so that all the hydrogen atoms are substituted one by one by halogen atoms

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The chlorination of methane usually produces a mixture of products. The trends in reactivity of halogen with alkanes are as follows

F2 > Cl2 > Br2 > I2

Fluorine reacts explosively; chlorine reacts slowly in dark at room temperature but rapidly in strong sun light. Bromine is less reactive than chlorine and requires high temperature or strong sun

light. Iodine is essentially unreactive.

12.1.2.2 Combustion

A reaction of a substance with oxygen or air that causes the rapid release of heat and the appearance of a flame is called combustion.

Complete combustion of alkanes produces carbondioxide, water and heat. Most of

them burn with blue flame.

For example, following reaction occurs when natural gas is burned.

The lighter alkanes are widely used as fuels. This is because:

(i) Their combustion can be controlled. (ii) They produce large amount of heat per gram. (i) They are cheap and readily available.

Incomplete combustion occurs in presence of limited supply of oxygen. Incomplete

combustion of methane gives CO, C and H2O.

Uses of methane

(i) as domestic fuel (Sui gas). (ii) as a fuel for automobiles (CNG). (iii) to manufacture urea fertilizer.

Q: What are alkenes, give structural and condensed formulas of different alkenes?

Ans: Alkenes

Alkenes have one or more double bond between carbon atoms.

They have the general formula CnH2n.

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When two carbon atoms share two pairs of electrons, they form a double bond between the carbon atoms.

These compounds are also named as Olefins in Latin (means oil forming.)

Some Important Alkenes & their Formulae

Q: What are different methods for the preparation of alkenes?

General Methods of Preparation of Alkenes Alkenes are prepared by the following methods.

1. By Dehydration of Alcohols

Dehydration means loss of water. Alcohols dehydrate when their vapour are passed over heated alumina.

Commercial USES of Ethylene

Ethylene or ethene is the most important commercial organic chemical. It is used in the manufacture of polythene, one of the most familiar plastics. It is also converted to ethylene glycol which is used as antifreeze in auto mobile radiators

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H H H H

H – C – C – H C = C + H–OH

H OH H H

Ethanol Ethene

Phosphoric acid (H3PO4) and phosphorus pentaoxide (P4O10) can also be used as catalyst for dehydration of alcohols.

Concentrated Sulphuric acid is also used for dehydration.

CH3 – CH2 CH2 = CH2 + H2O

OH

Note that in dehydration reaction -OH group is removed from one carbon and H – atom from the adjacent carbon atom. Two such carbon atoms form double bond. Such a reaction is called elimination reaction.

2. By dehydrohalogenation of alkyl halides

Dehydrohalogenation means loss of hydrogen halide. Alkyl halides on heating with alcoholic potassium hydroxide undergo dehydrohalogenation.

CH3 – CH2 + KOH CH2 = CH2 + KCl + H2O

Cl

Complete the following reactions

1. CH3 – CH2 – CH2 CH3-CH=CH2 + H2O

OH

2. CH3 – CH2 – CH2 + KOH CH3-CH=CH2 + H2O+ KBr

Br

Al2O3

340-450°C

Conc.H2SO4

140-170°C

Self-Assessment Exercise 12.6

Conc.H2SO4

170°C

Alcohol

Alcohol

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Physical Properties of Alkenes

Physical Properties of Alkenes (i) The first member of the alkenes is ethene. It is a colourless gas with pleasant odour. (ii) Alkenes are nonpolar, therefore, they are insoluble in water but soluble in organic solvents. (iii) The first member of the series ethene is slightly less dense than air. (iv) Alkenes are flammable hydrocarbons. On complete combustion, they form carbon dioxide and water with release of energy. However, their flame is smokier than alkanes having a similar number of carbon atoms. (v) Their melting and boiling points gradually increase with the increase of molecular sizes of the compounds in the series

Chemical Properties of Alkenes

Reaction with halogens

Chlorine and bromine add to the double bond. One chlorine atom becomes attached with one carbon and the one with other carbon atom.

H H H H

C = C + Cl – Cl H – C – C –H

H H Cl Cl

Such a type of reaction is called addition reaction. Alkenes react with bromine water in the same way.

Ethene (colourless) (Reddish-brown) 1,2-Dibrome ethane (colourless)

Bromine is a reddish-brown liquid and the product is colourless. When bromine water is added to an alkene, the red-brown color disappears. The decolourization of bromine solution is frequently used as a simple test for the presence of unsaturation. Alkynes also give this reaction.

Science titbits

Alkenes occur widely in nature. Ripening fruits and vegetables give off ethene which helps in further ripening. So artificially ethene is used to hasten the normal ripening process. For example 1 kg of tomatoes can be ripened by exposure to 0.1mg of ethene for 24 hours. The red color of tomatoes is due to an alkene called Lycopene.

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Reaction with KMnO4

When an alkene is treated with dilute alkaline aqueous solution of KMnO4 (1%) addition of two hydroxyl groups occurs across the double bond. Baeyer’s test. Alkenes decolourize the pink colour of acidified dilute solution of potassium permanganate because the double bond electrons react with MnO4

- ion, which further goes on to form MnO2 and ethene glycol (1,2-ethanediol). Such as, there is addition of 2 two ‘hydroxyl group’ at the double bond.

Ethylene glycol is used as an anti-freeze.

Complete the following reactions

1. CH3 – CH = CH2 + Br2

2. CH3 – CH = CH2 + KMnO4 + H2O

3. CH3 – CH = CH2 + Cl2

Uses of Ethene (Ethylene)

Ethene is used: (i) for artificial ripening of fruits; (ii) as a general anaesthetic; (iii) for manufacture of polythene: Polythene is a plastic material used in packaging, toys, bags, etc; (iv) as a starting material for the manufacture of a large number of compounds such as ethylene oxide, ethyl alcohol, ethylene glycol, diethyl ether, etc.; ethylene oxide is used as a fumigant, ethylene glycol is used as an anti-freeze, diethyl ether and ethyl alcohol are used as solvents and (v) for making poisonous mustard gas which is used in chemical warfare.

Q: What are alkynes, Draw their structural formulas?

Alkynes

Hydrocarbons which have at least one triple bond between carbon atoms are called alkynes.

Self-Assessment Exercise 12.7

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Those with one triple bond have the general formula CnH2n-2.

Structure:

Ethyne also called acetylene is the simplest member of alkyne family. In ethyne the two carbon atoms share three pairs of electrons. This means the carbon atoms are joined by a triple bond

Q: write methods for preparation of alkynes?

Ans: General Methods of Preparation of Alkynes

1. By Dehydrohalogenations of vicinal dihalides

Dehydrohalogenation of Vicinal Dihalides When a vicinal dihalide is heated with alcoholic KOH, two hydrogen atoms along with two halogen atoms are removed from two adjacent carbon atoms with the formation of a triple bond between the adjacent carbons:

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2. By Dehalogenation of Tetrahalides.

Tetra halides on treatment with Zn dust undergo dehalogenation forming an alkyne.

Cl Cl

H – C – C – H + Zn heat HC = CH + ZnCl2

Cl Cl Cl Cl

1,1,2,2-Tetrachloroethane 1,2-Dichloroethene

HC = CH + Zn HC ≡ CH + ZnCl2

Cl Cl Ethyne

Properties of Alkynes

Like alkenes, alkynes are also unsaturated hydrocarbons.

The first three members are gases, next eight members are liquids and higher members are solids.

They are non-polar and dissolve readily in organic solvents.

Ethyne has garlic like odour.

Alkynes are reactive compounds due to presence of a triple bond. Alkynes undergo addition reaction across the triple bond.

Chemical properties of Alkynes:

1. Addition of Halogens Alkynes add two molecules of halogens.

Cl Cl

HC = CH + Cl2 (aq) H – C – C – H

Cl Cl Cl Cl

Tetrachloroethane

Cl Cl

HC ≡ CH + Cl2 (aq) HC = CH

Ethyne

1,2 - Dichlorethene

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Write chemical reaction of ethyne and bromine.

2. Reaction with KMnO4

Alkynes do not react with dilute alkaline aqueous solution of KMnO4. However, they are oxidized by strong alkaline solution of KMnO4 to give oxalic acid. First four hydroxyl groups add across the triple bond. Oxidation with KMnO4 . Ethyne is oxidized by alkaline KMnO4. And four hydroxyl groups add to the triple bond, such as:

This intermediate product eliminates water molecules to form glyoxal, which is further oxidized to form oxalic acid.

Uses of Ethyne (Acetylene)

Uses of Acetylene (i) Acetylene produces oxy-acetylene flame with oxygen. It is a highly exothermic reaction. Heat released is used for welding purposes. (ii) Acetylene is used to prepare other chemicals, such as alcohols, acetaldehyde and acids. (iii) It is used for the ripening of fruits. (iv) It is used for the manufacturing of polymer products like polyvinyl chloride, polyvinyl acetate and synthetic rubber like neoprene. (v) It is polymerized to form benzene, which is used as raw material to form a variety of organic compounds.