State two features of a homologous series.

Ethane, a member of the homologous series of alkanes, can react with bromine.

Explain the free-radical mechanism of this reaction, including any necessary reaction conditions.

same functional group;

same general formula;

(successive members) differ by CH₂;

similar chemical properties;

gradation in physical properties;

Do not accept “same” instead of “similar”, or vice versa.

Initiation:

\({\text{B}}{{\text{R}}_2}\xrightarrow{{{\text{UV/hf/hv}}}}2{\text{Br}} \bullet \);

Reference to UV light or high temperature must be included.

Propagation:

\({\text{Br}} \bullet  + {{\text{C}}_2}{{\text{H}}_6} \to {{\text{C}}_2}{{\text{H}}_5} \bullet  + {\text{HBr}}\);

\({{\text{C}}_2}{{\text{H}}_5} \bullet  + {\text{B}}{{\text{r}}_2} \to {{\text{C}}_2}{{\text{H}}_5}{\text{Br}} + {\text{Br}} \bullet \);

Termination:

\({\text{Br}} \bullet  + {\text{Br}} \bullet  \to {\text{B}}{{\text{r}}_2}/{{\text{C}}_2}{{\text{H}}_5} \bullet  + {\text{Br}} \bullet  \to {{\text{C}}_2}{{\text{H}}_5}{\text{Br}}/{{\text{C}}_2}{{\text{H}}_5} \bullet  + {{\text{C}}_2}{{\text{H}}_5} \bullet  \to {{\text{C}}_4}{{\text{H}}_{10}}\);

Accept representation of radical without • (eg, Br, C₂H₅) if consistent throughout mechanism.

Penalize reference to heterolytic fission once only.

Award [0] to any mechanism involving ions.

Accept further bromination.

Award [3 max] if initiation, propagation and termination are not stated or are incorrectly labelled for equations.

Accept correct description of processes without equations.

This question tended to be well answered. The most common errors were that some candidates stated that homologous series have the same empirical formula, and that the difference between “the same” and “similar” were confused. The knowledge of the free radical substitution was very good, with the three processes of initiation, propagation and termination quoted often, although some only gave the first and last, the equations for propagation were most likely to be incorrect.

This question tended to be well answered. The most common errors were that some candidates stated that homologous series have the same empirical formula, and that the difference between “the same” and “similar” were confused. The knowledge of the free radical substitution was very good, with the three processes of initiation, propagation and termination quoted often, although some only gave the first and last, the equations for propagation were most likely to be incorrect.