CHAPTER-12

CHEMISTRY SSC-II                       CHAPTER-12

HYDROCARBONS

LECTURE# 1

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

It has been estimated that the concentration of carbon dioxide in the atmosphere has increased up to 20% by the end of twentieth century.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.

Alkanes:

  • Alkanesare saturated hydrocarbons.
  • They have general formula CnH2n+2.Each carbon atom forms four bonds and each hydrogen atom forms only one bond.
  • The 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.

General Methods of Preparations of Alkanes

  1.  
  2. By Hydrogenation of alkenes and alkynes

Hydrogenation means addition of molecular hydrogen in alkenes and alkynes. , This reaction is carried out in the presence of nickel catalyst at 250 °C to 300 °C. However, in the presence of catalyst platinum or palladium, the reaction takes place at room temperature, such as:

Reduction of Alkyl Halides

  • Reduction means addition of nascent hydrogen.
  • In fact, it is a replacement of a halogen atom with a hydrogen atom. This reaction takes place in the presence of Zn metal and HCl.

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

Physical Properties of Alkanes.

1. Alkanes form a homologous series of compounds. First four members of the series are gases. The alkanes consisting of C5 to C10 are liquids while higher members of the series are solids.

2. They are nonpolar, therefore, they are insoluble in water but soluble in organic solvents.

3. The density of alkanes increases gradually with the increase of molecular size.

4. 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.

5. The alkanes become more viscous as their molecular sizes increase.

6. Alkanes become less flammable, i.e. more difficult to burn with the increase of molecular sizes

Chemical Reactions

Halogenation

Alkanes give only substitution reactions. 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. These reactions are a characteristic property of alkanes. Alkanes react fairly with halogens in diffused sunlight only. In dark there is no 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

Combustion

Alkanes burn in the presence of excess of air or oxygen to produce a lot of heat, carbon dioxide and water. This reaction takes place in automobile combustion engines, domestic heaters and cooking appliances. It is highly exothermic reaction and because of it alkanes are used as fuel.

In the limited supply of oxygen, there is incomplete combustion. As a result, carbon monoxide is produced that creates suffocation and causes death.

Uses of Methane and Ethane

(i) Natural gas that is chiefly methane is used as domestic fuel.

(ii) Compressed natural gas (CNG) is used as automobile fuel.

(iii) These gases are used in the manufacture of chemicals such as carbon black, methyl alcohol, ethyl alcohol, chloroform, carbon tetrachloride, formaldehyde and acetaldehyde.

(iv) to manufacture urea fertilizer

Alkenes

  • have one or more double bond between carbon atoms.
  • They have the general formula CnH2n. When two carbon atoms share two pairs of electrons, they form a double bond between the carbon atoms

General Methods of Preparation of Alkenes

  • By Dehydration of Alcohols

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

Concentrated Sulphuric acid is also used for dehydration.

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.

By dehydrohalogenation of alkyl halides

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

Note that removal of H and halogen takes place from two adjacent carbon atoms.

Complete the following reactions.

Properties of Alkenes

  • Alkenes are unsaturated hydrocarbons. First three members i.e. ethene, propene and butene are gases
  •  C5-C15 members are liquids and the higher members are solids.
  • They are insoluble in water but soluble in organic solvents such as alcohol etc.
  • The two carbons atoms forming double bond are joined to only three atoms. Since a carbon atom can join to four atoms. So other molecules can attack at this site of double bond.

Reaction with halogens

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

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.

Reaction with KMnO4When an alkene is treated with dilute alkaline aqueous solution of KMnO4 (1%) addition of two hydroxyl groups occurs across the double bond. The pink colour of KMnO4 solution is discharged during the reaction. This reaction is used as a test for the presence of an alkene and is known as Baeyer’s test.

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

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

H – C  C – H

Draw structural formula for

  • 1-Butyne                (b)  2-Butyne

General Methods of Preparation of Alkynes

By Dehydrohalogenations of vicinal dihalides: A vicinal dihalide has two halogen atoms on adjacent carbon atoms.            

Vicinal dihalide on treatment with alcoholic potassium hydroxide eliminates two molecules of hydrogen halides from adjacent carbon atoms. Removal of two molecules forms a triple bond between two carbon atoms. Reaction occurs in two steps.

  • By Dehalogenation of Tetrahalides.

Tetra halides on treatment with Zndust undergo dehalogenation forming an alkyne.

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 Reactions

Addition of HalogensAlkynes add two molecules of halogens.       

Addition of Halogen

Chlorine and bromine adds to acetylene to form tetrachloroethane and tetrabromoethane, respectively. When bromine water is added to acetylene, red-brown colour of bromine water is discharged rapidly

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.

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

Uses of Ethyne (Acetylene)

  • In oxy-acetylene torch for welding and cutting metals.
  • For ripening of fruits.
  •  For the manufacture of polyvinyl acetate (PVA), polyvinyl chloride (PVC), polyvinyl ethers and rubber.

Alkenes for fruit ripening

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.(C40H56).

Oxyacetylene flame:Acetylene is used in oxy-acetylene torches for cutting and welding metals. Such torches can produced temperature as high as 3000º C.

EXERCISE

Q. No. 2: Give short answers.

(i)         Give three examples of unsaturated hydrocarbons. [ethene, ethyne, propyne etc]

(ii)        Draw electron dot and cross structure for ethene.

  • Draw structural formulas of an alkane, an alkene and an alkyne containing five carbon atoms.
  • How can you differentiate ethane from ethene?(Bromine water test)
  • What do you mean by dehydration reaction? Give one example

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

Al2O3
340-450°C
  • How can you convert
  • ethene into ethane

(hydrogenation reaction)

  • methane into carbontetrachloride

 (reaction of methane with chlorine)

  • ethene into glycol

[Reaction ethene with KMnO4]

  • ethyl chloride into ethane

[Reduction of alkyl halide in presence of Zn/HCl]

  • ethyl bromide into ethene

4.         Write a chemical equation to show the preparation of an alkane from an alkene and an alkyne

[By Hydrogenation of alkenes and alkynes]

5.         Write a chemical equation to show the preparation of ethene from dehydration of an alcohol and dehydrohalogenation of alkyl halides.

Al2O3

6.         Write a chemical equation to show the preparation of ethyne from dehalogenation of 1,2 – dihalide and a tetrahalide.

7.         Write chemical equations showing reaction of KMnO4 with ethene and ethyne. [seeLec. 2 & 3]

8.         List some industrial uses of ethene and ethyne.

Q: Importance of halogenated hydrocarbons? Ans:      Many halogenated hydrocarbons have important commercial uses. Methyl chloride is a gas at room temperature. Dichloromethane, trichloromethane and tetra chloromethane are liquids. These three liquids can be used as solvents for grease, oils and other organic substances. Chloroform is used as an anesthetic. Tetrachloromethane has carcinogenic effects at high concentration.

9.         Explain why a systematic method of naming chemical compounds is necessary.

Millions of organic compounds exist. To understand, recognize and classify these compounds, systematic naming of organic compounds is necessary. Organic chemists began in the last century to devise a system of naming organic compounds that depends on their structure. An international body, the International Union of Pure and Applied chemistry (IUPAC) constantly reviews the rules for naming organic compounds. IUPAC system of naming organic compounds is based on the following principle.Each different organic compound should have a different name.

Draw electron dot and cross structure for

Propane,Propyne,      Propene

11.    Write chemical equations for the preparation of propene from

      (a)  CH3 CH2 – CH2 – OH

(b) CH3 – C  CH

12.   Write down structural formulas for the products which are formed when 1-butene is reacted with

     (a)   H2 /Ni

(b)       dilute alkaline aqueous KMnO4 solution

(c) bromine water

(d)       chlorine

Br2
Conc.H2SO4
Ethylalcohol
C
D
(
ii
I
I
)
Propene                 
alcohol
Br2
A    
B
KOH
(
I
i
)

13.       Identify A, B, C, D in the following reactio

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.

Q: Alkenes in ripening of fruits?

Q: Alkenes and alkynes in polymer synthesis?

Alkenes are starting materials for the synthesis of many valuable materials, especially polymers. Some alkenes and alkynes serve as starting materials for synthesis. For instance ethene and ethyne are used to synthesize a number of polymers. Polymers are made from smaller molecules such as ethene. Look around you, you will find polymers everywhere. Your clothes, carpets, curtains, towels, sheets, floor tiles, furniture, toys etc. are polymers made from ethene and ethyne. Even in your car, the dash board, seats, tyres, floor mat, ceiling, are also made of polymers. Hydrocarbons are also used as raw materials for the synthesis of synthetic rubber, plastic, films, adhesives, drugs and dyes. In the field of medicine, body replacement parts are made from polymers.

Q: What is future of polymers?  In future we will have artificial bones that can stimulate bone growth. We can also expect to have artificial lungs as well as artificial hearts. Industries are day and night busy in synthesizing marvelous new products. Is there anything that can replace petroleum as raw material for making plastics and other polymers.