ALKANES

ALKANES 

The alkanes are a homologous series of saturated aliphatic hydrocarbons with a general molecular formula of CnH2n+2.  The are the only saturated hydrocarbons because they contain only single bonds. The are found mainly in petroleum. The carbon atoms in the alkanes exhibit sp³ hybridization. There is a gradual change in the physical properties of the members from the gaseous state (for the lighter members) to the liquid state and then the solid state for the heavier compounds.

The first ten members of the series are.

1.       methane          CH₄

2.       Ethane             C₂H₆

3.       Propane           C₃H₈

4.       Butane             C₄H₁₀

_5.         Pentane           C₅H₁₂

6.        Hexane           C₆H₁₄

7.        Heptane          C₇H₁₆

8.        Octane            C₈H₁₈

9.        Nonane           C₉H₂₀

10.     Decane            C₁₀H₂₂

Chemical properties 

The alkanes undergo only two reactions, combustion and substitution reactions

1. Combustion Reaction: - this is a general property for all hydrocarbons. The alkanes burn in air or oxygen to yield Carbon (iv) oxide and water.

    CH_4(g) + O_2(g) → CO_2(g) + H₂O_(g)

2. Substitution Reaction: - Substitution reaction is the only other reactions that the alkanes can undergo because they are saturated.

A Substitution reaction is a reaction where one element (atom) replaces one or more of the hydrogen atoms in an alkane molecule.

METHANE: - This is the first and the simplest member of the alkane family. It has a molecular formula of CH₄ and a structural formula of

              H
               ׀
       H − C − H
               ׀
               H

It is found naturally in swamps or swampy areas when vegetations and dead organic matter decompose. It is also one of the major components in natural gas

Laboratory Preparation 

Methane is prepared in the laboratory by the action of sodium ethanoate on soda- lime.  (soda-lime is sodium hydroxide that has been slaked with lime CaO).

Soda- lime is preferred to caustic soda because 

1. It is not Deliquescent and 

2. It does not attack glass.

                        Laboratory Preparation of methane 

            CH₃COONa(S) + NaOH(s) → Na₂CO_3(s) + CH_4(g)  

Physical properties 

i.  It is a colourless gas. 

ii. It is less dense than air.

iii. It is insoluble in water.

Chemical properties 

I. Combustion: - methane burns with a pale blue flame in air or oxygen

CH_4(g) + O_2(g) → CO_2(g) + H₂O_(l) 

2. Substitution reaction: - Methane combines with Chlorine to produce various products.

The reaction is faster in the presence of light (photochemical reaction) and it occurs in stages.

I. CH_4(g) + Cl_2(g) → CH₃Cl + HCl

                             Mono chloromethane

II. CH₃Cl + Cl_2(g) → CH₂Cl₂ + HCl

                             dichloromethane

III. CH₂Cl₂ + Cl_2(g) → CHCl₃ + HCl

                            trichloromethane

IV.  CHCl₃ + Cl_2(g) → CCl₄ + HCl

                             tetrachloromethane

Uses of Methane 

I. It is used mainly as fuel sometimes mixed with other fuels

2. It is used to make hydrogen

3. It is used to make carbon black

4. When refined it is can be used as a rocket fuel

OBJECTIVE QUESTIONS 

1. What is the IUPAC name of the compound with this structure 

         H     H    H     H
         |        |      |        |   
H— C — C — C — C — H
         |        |       |         | 
         H      |      H       H
                  | 
          H —C—H
                  | 
                 H 
a. 3-methylbutane

b. 3-methylpentane

c.2-methylbutane 

d.2-methylpropane 

2.  Which of the following compounds is the structural isomer of the compound above?

a. 2,2-dimethylpropane 

b. 2-ethylpropane

c. 1,2-methylbutane 

d. 2-methylpentane

3. The energy value of petrol can be determined by a 

a. bomb calorimeter

b. catalytic cracker

c. fractionating column

d. thermometer

4.  C8H18 will undergo the following reactions except 

a.  Cracking 

b. Combustion 

c. Substitution

d. Addition

5. One of the products of pentane in excess air 

a. pentanol 

b. pentene

c. nitrogen (II) oxide 

d. carbon (IV) oxide 

6. The gas produced when a mixture of sodium propanoate and soda lime is heated is 

a. Methane.

b. Pentane

c.. Ethane

d. Butene

7. The general molecular formula of alkanes is
A. CₙH₂ₙ
B. CₙH₂ₙ₊₂
C. CₙH₂ₙ₋₂
D. CₙHₙ

8. Which of the following is an alkane?
A. C₂H₄
B. C₃H₆
C. C₄H₁₀
D. C₂H₂

9. The IUPAC name of CH₃CH₂CH₂CH₃ is
A. Propane
B. Butane
C. Pentane
D. Butene

10. Methane can be prepared in the laboratory by heating sodium ethanoate with
A. concentrated H₂SO₄
B. soda lime
C. aqueous NaOH
D. dilute HCl

11. The major product formed when methane reacts with chlorine in the presence of sunlight is
A. chloromethane
B. chloroethene
C. methanol
D. carbon tetrachloride

12. The reaction between an alkane and chlorine in the presence of ultraviolet light is known as
A. addition
B. substitution
C. elimination
D. neutralization

13. Complete combustion of an alkane produces
A. carbon and hydrogen
B. carbon monoxide and water
C. carbon dioxide and water
D. carbon dioxide and hydrogen

14. Which of the following is a characteristic property of alkanes?
A. They readily undergo addition reactions.
B. They are generally saturated hydrocarbons.
C. They contain at least one double bond.
D. They readily react with bromine water.

15. How many hydrogen atoms are present in one molecule of pentane?
A. 10
B. 11
C. 12
D. 14

16. The cracking of long-chain alkanes is important industrially because it produces
A. only alcohols
B. shorter-chain alkanes and alkenes
C. only carboxylic acids
D. only aromatic hydrocarbons

THEORY QUESTIONS 

1. Methane reacts with chlorine under certain conditions to produce tetrachloromethane 

i.). State the condition for the reaction 

ii). Name the type of reaction

iii). Give two uses of methane 

iv). Name one natural source of methane.