(i)     State the meaning of the term isomers.

(ii)     Deduce the structural formulas of 2-bromobutane and 1-bromo-2-methylpropane, and identify each molecule as primary, secondary or tertiary.

(i)     Explain why alkanes have low reactivity.

(ii)     Outline the meaning of the term homolytic fission.

(iii)     Describe the meaning of the symbol \({\text{Br}} \bullet \).

(iv)     State an equation for the reaction of ethane with bromine.

(v)     Explain the reaction of ethane with bromine using equations for the initiation step, two propagation steps and one termination step.

(i)     Identify a suitable catalyst for this reaction.

(ii)     But-2-ene can be converted to 2-bromobutane and then to butan-2-ol as follows:

\({\text{C}}{{\text{H}}_3}{\text{CH}}\)=\({\text{CHC}}{{\text{H}}_3}\xrightarrow{I}{\text{C}}{{\text{H}}_3}{\text{CH(Br)C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_3}\xrightarrow{{II}}{\text{C}}{{\text{H}}_3}{\text{CH(OH)C}}{{\text{H}}_{\text{2}}}{\text{C}}{{\text{H}}_{\text{3}}}\)

Identify the reagent(s) and conditions necessary for each of the steps I and II.

Step I:

Step II:

(i)     compounds with the same molecular formula but different arrangements of atoms / compounds with the same molecular formula but different structural formulas;

(ii)     \({\text{C}}{{\text{H}}_{\text{3}}}{\text{CH(Br)C}}{{\text{H}}_{\text{2}}}{\text{C}}{{\text{H}}_{\text{3}}}\);

secondary/ 2°;

\({\text{C}}{{\text{H}}_2}{\text{(Br)CH(C}}{{\text{H}}_3}{\text{)C}}{{\text{H}}_3}\);

primary/ 1°;

Accept full or condensed structural formulas.

(i)     relatively strong bonds/high bond enthalpies so large amounts of energy must be provided for a reaction to occur;

non-polar bonds so not susceptible to attack / no C=C so no addition reactions;

(ii)     bond breaks and one electron (from the covalent bond) goes to each atom / OWTTE;

(iii)     bromine (free) radical / a bromine atom (with an unpaired electron);

(iv)     \({\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_3} + {\text{B}}{{\text{r}}_2} \to {\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{Br}} + {\text{HBr}}\);

Accept further substitution.

(v)     Initiation:

\({\text{B}}{{\text{r}}_2} \to {\text{2Br}} \bullet \);

Essential condition:

UV/sunlight/hf/hv;

Propagation:

\({\text{Br}} \bullet  + {\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_3} \to  \bullet {\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_3} + {\text{HBr}}\);

\({\text{B}}{{\text{r}}_2} +  \bullet {\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_3} \to {\text{BrC}}{{\text{H}}_2}{\text{C}}{{\text{H}}_3} + {\text{Br}} \bullet \);

Accept other valid propagation reactions.

Termination:

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

Accept other valid termination reactions.

(i)     (concentrated) sulfuric acid/ \({{\text{H}}_2}{\text{S}}{{\text{O}}_4}\) / phosphoric acid/ \({{\text{H}}_3}{\text{P}}{{\text{O}}_4}\);

Acid must be named or formula given.

(ii)     step I:

HBr/hydrogen bromide;

gaseous / anhydrous / inert/non-polar solvent;

step II:

sodium hydroxide/NaOH / potassium hydroxide/KOH;

aqueous (solution) / dilute / warm / heat / reflux;

This was the least popular question in section B, but was generally chosen by those who were a little more “expert”. Part (a) was answered well and hydrogen atoms were rarely missing as they have been in other examination sessions. It is accepted that the answer box for (ii) would have been better without lines in it.

In part (b)(i), candidates found it difficult to explain why alkanes have low reactivity – it is always more difficult to explain a negative – and homolytic fission was not well explained in (ii). There were many good answers to (iii), (iv) and (v). In (b)(iv), candidates were asked to state an equation so further substitution was accepted.

Part (c) caused more trouble. The catalyst was often mis-identified in (i) (potassium dichromate seemed to be a common choice). In (ii), the condition for using HBr was rarely given (although it is accepted that it is rarely given in the texts) and the reagent for step II was sometimes given as hydroxide or \(^ - {\text{OH}}\).