Question 1

Why are alkanes relatively unreactive?

Accepted Answer

Alkanes are unreactive because they contain strong C-C and C-H sigma bonds which require a large amount of energy to break. Also, these bonds are essentially non-polar, so they do not attract electrophiles or nucleophiles.

Question 2

What is the difference between homolytic and heterolytic fission?

Accepted Answer

In homolytic fission, a covalent bond breaks and each atom gets one of the bonding electrons, forming two radicals (e.g., $Cl_2 \rightarrow 2Cl\cdot$). In heterolytic fission, the bond breaks and one atom gets both bonding electrons, forming a positive and a negative ion (e.g., $H-Cl \rightarrow H^+ + Cl^-$).

Question 3

Why does free-radical substitution produce a mixture of products?

Accepted Answer

Several factors lead to a mixture. Firstly, substitution can occur at any position on the carbon chain, leading to isomers (e.g., 1-chloropropane and 2-chloropropane). Secondly, further substitution can occur, where a second hydrogen on the same molecule is replaced (e.g., forming dichloromethane from chloromethane). Finally, different termination steps produce various side-products (e.g., two alkyl radicals joining).

Question 4

What is the economic importance of cracking?

Accepted Answer

Cracking is vital for the oil industry. It converts the surplus of long-chain hydrocarbons from fractional distillation, which have low demand, into more valuable products. These include shorter-chain alkanes for petrol (gasoline) and alkenes, which are essential starting materials for the production of polymers (plastics) and other chemicals.

Alkanes

What you need to know

Alkanes: Building Blocks and Reactions