Alkanes

 

Alkanes General Formula:  C_nH_2n+2

 

Reactions of Alkanes

 

Alkanes are extremely unreactive. Here are the 3 reasons why:

 

1. C-C and C-H bonds are non-polar and cannot be attracted to nucleophiles.

 

2. C-C has a low electron density (does not have an electron rich centre) so it is NOT attracted to electrophiles.

 

3. The bond enthalpy of C-C and the C-H bonds are TOO HIGH.

 

Therefore... Alkanes undergo: Free Radical Substitution.

 

Definition of a free radical: A species with an unpaired electron.

 

The mechanism for the reaction of alkanes with a free radical is shown below:

 



 

 

 

Why can a mixture of products be formed?

 

1. Because different products can be formed in the termination step.

 

2. Several isomers of a product can be made (e.g. the halogenoalkane)

 

3. Multi-substitution of the hydrogen, e.g. in CH₄.

 

 

 

Alkanes can also react with halogens to form halogenoalkanes.

 

E.g.

C₄H₁₀ + I₂ → C₄H₉I + HI

C₆H₁₄ + Br₂ → C₆H₁₃Cl + HCl

 

 

Quick Quiz:

 

1. What is a nucleophile? (1)

2. Why does the boiling point increase as the Mr increases in the straight chain alkanes? (1)

3. What type of fission takes place when the bond in Cl₂ breaks? (1)

4. State the condition necessary for the fission of the bond in Cl₂? (1)

5. Write a balanced equation for the complete combustion of propane? (1)

 

Answers:

 

1. An electron pair donor.

2. There are more Van-der-Waals forces because there is more surface contact.

3. Homolytic fission

4. UV light

5. C₃H₈ + 5O₂ → 3CO₂ + 4H₂O