Saturated Hydrocarbons

 Unit 14 Saturated Hydrocarbons

Classification of Hydrocarbons

The organic compounds made up of hydrogen and carbon atoms only are called hydrocarbons. These are considered the parent organic compounds. All other organic compounds are considered to be derived from them by the replacement of one or more hydrogen atoms by other substituents.

With the development of organic chemistry, the number of organic compounds became very large. To simplify and systematize the study of organic chemistry, all known hydrocarbons are broadly divided into two main classes based on the nature of their carbon skeleton.

Hydrocarbons

Acyclic compounds                                                   Cyclic compounds

Alkanes        Alkenes          Alkynes          Aliphatic                    Aromatic

                                                             Compounds                Compounds

1.      Acyclic or Open Chain Compounds

These are hydrocarbons containing carbon atoms linked together in an open-chain form. The carbon chain may be straight chain or branched. E.g.,

CH₃CH₂CH₂CH₂CH₃                                     CH₃CHCH₂CH₃

                                                                               CH₃

Pentane                                               2-methylbutane

(Straight chain)                                               (Branched)

The open-chain compounds are further classified as alkanes, alkenes and alkynes.

 

[A]       Alkanes:

These are the simplest hydrocarbons which contain carbon-carbon single bonds. These are also called saturated hydrocarbons as these cannot add up more hydrogen. E.g. Methane (CH₄), Ethane (CH₃CH₃), Propane (CH₃CH₂CH₃), 2-methyl propane (CH₃)₂CHCH₃ etc.

 

[B] Alkenes

These are unsaturated hydrocarbons which contain at least one carbon-carbon double bond. E.g.,

Ethene (CH₂=CH₂)                   Propene (CH₂=CHCH₃)

But-2-ene (CH₃CH=CHCH₃)     Buta-1,3-diene (CH₂=CHCH=CH₂) etc.

 

[C] Alkynes

These are also unsaturated hydrocarbons containing at least one carbon-carbon triple bond. E.g.,

2.      Cyclic or closed chain or ring compounds

The hydrocarbons that contain one or more rings of carbon atoms are called cyclic compounds. These are further divided into the following two categories.

[A] Aliphatic Cyclic Hydrocarbons (alicyclic)

These are closed-chain hydrocarbons but resemble open-chain hydrocarbons in many properties. These are further classified into cycloalkanes, cycloalkenes and cycloalkynes. e.g.,

[B] Aromatic hydrocarbons

The term aromatic comes from the Greek word aroma: meaning sweet smell. So, the organic compounds having a sweet smell were originally called aromatic hydrocarbons. Later, it was found that planar cyclic compounds having (4n+2) π electrons in a conjugated system (i.e. alternate single and double bonds) are aromatic compounds. e.g.,

15.2 Alkanes (Saturated hydrocarbons)

Alkanes are the simplest hydrocarbons having carbon-carbon single bonds.  These are called saturated hydrocarbons as these cannot add up more hydrogen. Also called Paraffins as are unreactive.

General formula: C_nH_2n+2

General methods of preparations

1.      From alkenes and alkynes (by catalytic hydrogenation)

Unsaturated hydrocarbons (alkenes and alkynes) undergo addition with hydrogen in the presence of finely divided Nickel (Ni), Platinum (Pt) or Palladium (Pd) as a catalyst at a temperature of about 200-300⁰C producing alkanes.

2.      From haloalkanes

a.      Reduction

Haloalkanes are reduced to alkanes with Lithium aluminium hydride (LiAlH₄). e.g.,

1.       How would you prepare Methane from Chloromethane?                  1

b.     Wurtz reaction

When a haloalkane is heated with sodium in dry ether, an alkane with a double number of carbon atoms is obtained, this is called the Wurtz reaction.

2.       What is Wurtz’s reaction? Give an example.                                     2

3.       Write a chemical reaction when Bromoethane is heated with sodium metal in the presence of dry ether.          2

4.       Identify A and give its IUPAC name

                                                   2

5.       What product do you expect when 2-bromopropane is heated with Na/dry ether?       1

6.       A haloalkane X if heated with sodium metal in the presence of dry ether produces 2,3-dimethylbutane as the major product. Identify X and write the chemical reaction.                              2

7.       A Haloalkane ‘X’ undergoes a Wurtz reaction to give 2,3-dimethylbtane as the major product. Identify ‘X’ and write the chemical reaction.                                                                 2

8.      From Sodium salt of carboxylic acids

Alkanoic acid               Sodium alkanoate

By soda lime decarboxylation

The sodium salt of carboxylic acid (sodium alkanoate) is heated with soda and lime (i.e. NaOH and CaO) an alkane with one less carbon is obtained, e.g.,

9.       Write an example of a decarboxylation reaction.                              1

10.   What is the decarbonylation reaction? Give one example.                      2

11.   How would you prepare methane from Sodium ethanoate?                  1

12.   How would you prepare methane from CH₃COONa?                            1

13.   What happens when sodium acetate is heated with soda-lime?               1

14.   What products are obtained when CH₃COOH is allowed to react with NaOH/CaO?  1

15.   Predict the major products of the following reactions:

                                                       

16.   Give a suitable method for the conversion of Ethanoic acid into methane.

Chemical properties

Alkanes are highly unreactive organic compounds, because of strong and nonpolar carbon-carbon bonds. Even carbon-hydrogen bonds are only very slightly polar. So these do not undergo reactions easily. But these undergo a few reactions in the presence of heat or light.

1.      Substitution reactions

Halogenation

When alkanes are treated with a limited amount of halogens in the presence of sunlight, halogen atoms successively replace the hydrogen atoms. E,g.,

e.g.,

The iodination of alkanes is reversible. The yield is increased in presence of conc. HNO₃.

Nitration

Alkanes when boiled with concentrated nitric acid yield nitroalkanes. Introducing the nitro (-NO₂) group in an organic compound is called nitration.

R-H + conc. HNO₃                                              R-NO₂ + H₂O

Alkanes                                         Nitroalkanes

R-H + OH- NO₂                R-NO₂ + H₂O

A mixture of all possible nitroalkanes is obtained by C-C bond fission.

 

      Sulphonation

Branched-chain alkanes and higher alkanes with 6 or more carbon atoms undergo sulphonation with concentrated and fuming sulphuric acid giving alkane sulphonic acid.

2.      Oxidation(Combustion)

Alkanes are quite inert towards common oxidizing agents and do not decolourise alkaline KMnO₄. However, when burnt in excess air, alkanes are completely oxidized into CO₂ and H₂O with the evolution of a large amount of energy. This is called combustion.

C_nH_2n+2 + 3(n+1)/2O₂                                     n CO₂ + (n+1) H₂O

CH₄ + 3 O₂                         CO₂ + 2 H₂O

C₂H₆ + 7/2 O₂                     2 CO₂ + 3 H₂O

2 C₂H₆ + 7 O₂                    4 CO₂ + 6 H₂O + energy

In a limited supply of air, some CO and even Carbon are formed.

Conversion

17.   Convert Methane to Ethane.                                                              2

Solution:

1. Which represents a cyclic alkane?

a.       C₃H₆                        b. C₃H₈                             c. C₈H₁₀                           d. C₈H₁₂

2. The general formula of alkanes is

a.       C_nH_2n

b.       C_nH_2n+1

c.        C_nH_2n+2

d.       C_nH_2n-2

3. Which of the following formula represents alkanes

a.       C₁₀H₂₀

b.       C₇H₁₆

c.        C₅₈

d.       C₉H₁₀

4. Alkanes are also known as

a.       Olefins

b.       Paraffin

c.        Acetylene

d.       Both a and b

5.       Sodium ethanoate with soda lime gives

a.       Methane        b. Ethane               c. Propane             d. Butane

6.       Wurtz reaction using Bromoethane yields

a.       2-bromobutane

b.       2-methylpropane

c.        Ethane

d.       Butane

7.       Which of the following compounds cannot be synthesized by the Wurtz reaction?

a.       Ethane            b. Butane               c. Hexane              d. Methane

8. Which of the following compounds cannot be synthesized by the Wurtz reaction?

a.       Ethane

b.       Methane

c.        Butene

d.       Both b and c

9.       Lower alkanes easily undergo

a.       Halogenation

b.       Nitration

c.        Sulphonation

d.       Aromatization

10.    When a hydrogen atom of an alkane is replaced by a halogen atom, the reaction is known as

a.       Addition         b. Oxidation          c. Substitution       d. reduction

11.    A saturated hydrocarbon is shown by C_nH₁₀. The value of n in this compound is

a.       2                      b. 4                         c. 5                          d. 6

 

This material is not sufficient for your study. Revise thoroughly from the textbook too.