15. Halogens

Question 1

reactions (reagents and conditions) by which halogenoalkanes can be produced:

Answer 1

(a) the free-radical substitution of alkanes by Cl2 or Br2 in the presence of ultraviolet light, as exemplified
by the reactions of ethane

(b) electrophilic addition of an alkene with a halogen, X2, or hydrogen halide, HX(g), at room temperature

(c) substitution of an alcohol, e.g. by reaction with
1. HX(g);
2. KCl and concentrated H2SO4 or concentrated H3PO4;
3. PCl3 and heat;
4. PCl5
5. SOCl2

Question 2

classify halogenoalkanes

Answer 2

primary
secondary
tertiary

Question 3

nucleophilic substitution reactions:

Answer 3

(a) the reaction with NaOH(aq) and heat to produce an alcohol

(b) the reaction with KCN in ethanol and heat to produce a nitrile

(c) the reaction with NH3 in ethanol heated under pressure to produce an amine

(d) the reaction with aqueous silver nitrate in ethanol as a method of identifying the halogen present as
exemplified by bromoethane

Question 4

elimination reaction

Answer 4

elimination reaction with NaOH in ethanol and heat to produce an alkene as exemplified by bromoethane- mechanism

Question 5

describe the SN1 and SN2 mechanisms

Answer 5

SN1 and SN2 mechanisms of nucleophilic substitution in halogenoalkanes including the
inductive effects of alkyl groups

Question 6

describe the SN1 and SN2 reactions

Answer 6

primary halogenoalkanes tend to react via the SN2 mechanism;

tertiary halogenoalkanes via
the SN1 mechanism;

secondary halogenoalkanes by a mixture of the two, depending on structure.

Question 7

describe and explain the different reactivities of halogenoalkanes

Answer 7

with particular reference to the relative
strengths of the C–X bonds as exemplified by the reactions of halogenoalkanes with aqueous silver
nitrates)