Halogenoalkanes: Chemistry 9701 (Cambridge International AS & A Level)

The table shows approximate average bond enthalpies for three carbon-halogen bonds:

Equal small volumes of 1-chlorohexane, 1-bromohexane and 1-iodohexane are each heated under reflux with an excess of hot aqueous sodium hydroxide, and the rate at which each is hydrolysed is compared.

Which statement correctly ranks the reaction rates and links this to the bond enthalpy data?

1-Chloropentane, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_2\text{Cl}$, is reacted separately under three different sets of conditions.

(a) 1-Chloropentane is heated under reflux with aqueous sodium hydroxide, $\text{NaOH(aq)}$.

(i) Give the structural formula and name of the organic product. [1]

(ii) Describe, in words, the mechanism of this reaction, explaining why this particular mechanism operates for a primary halogenoalkane. [3]

(b) 1-Chloropentane is instead heated under reflux with potassium cyanide dissolved in ethanol, $\text{KCN}$ in $\text{C}_2\text{H}_5\text{OH}$.

Give the structural formula and name of the organic product, and explain why its carbon chain contains one more carbon atom than 1-chloropentane. [2]

(c) 1-Chloropentane is instead heated in a sealed tube with an excess of concentrated ammonia dissolved in ethanol.

Name the organic product formed, and explain why an excess of ammonia is used. [3]

The table gives approximate C–Cl bond data for two compounds:

1-Chlorooctane and chlorobenzene are separately heated under reflux with aqueous sodium hydroxide. 1-Chlorooctane is readily hydrolysed under these conditions, but chlorobenzene shows no detectable reaction.

Which statement correctly uses the bond data to explain this difference in reactivity?

Four bromoalkanes are:

A: 1-bromobutane, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{Br}$ B: 2-bromobutane, $\text{CH}_3\text{CH}_2\text{CH}(\text{Br})\text{CH}_3$ C: 2-bromo-2-methylpropane, $(\text{CH}_3)_3\text{CBr}$ D: 1-bromo-2-methylpropane, $(\text{CH}_3)_2\text{CHCH}_2\text{Br}$

Which bromoalkane is classified as tertiary, and therefore reacts with aqueous sodium hydroxide almost entirely by the SN1 mechanism?

2-Bromo-2-methylpropane, $(\text{CH}_3)_3\text{CBr}$, is heated under reflux with aqueous sodium hydroxide.

(a) Give the structural formula and name of the organic product. [1]

(b) Describe, in words, the two-step SN1 mechanism by which this reaction proceeds. [4]

(c) Explain, in terms of the inductive effect of alkyl groups, why the intermediate formed in step 1 is stable enough to exist, and why this makes SN1 the favoured mechanism for a tertiary substrate. [2]

(d) State whether the rate of the rate-determining step depends on the concentration of $\text{OH}^-(aq)$, and explain your answer. [1]

Bromoethane, $\text{CH}_3\text{CH}_2\text{Br}$, can be converted into two different organic products depending only on the solvent used with sodium hydroxide.

(a) State the reagent and conditions needed to convert bromoethane into ethanol, and give the balanced equation for this reaction. [2]

(b) State the reagent and conditions needed to convert bromoethane into ethene, and give the balanced equation for this reaction. [2]

(c) Explain, in terms of the role played by the hydroxide ion, why the same ion, $\text{OH}^-$, can bring about two chemically different types of reaction depending on the solvent. [3]

(d) Describe, in words, the elimination mechanism that converts bromoethane into ethene. [2]

Equal small volumes of 1-chlorobutane, 1-bromobutane and 1-iodobutane are each added to separate test tubes of aqueous silver nitrate dissolved in ethanol (ethanol is used because halogenoalkanes do not dissolve well in water alone), and the mixtures are warmed gently in a water bath. The time taken for a precipitate to appear is recorded for each. 1-Fluorobutane is not normally included in this experiment, because no precipitate is observed even after prolonged warming.

Approximate C–X bond enthalpies: C–F $\approx 484$ kJ mol⁻¹, C–Cl $\approx 338$ kJ mol⁻¹, C–Br $\approx 276$ kJ mol⁻¹, C–I $\approx 238$ kJ mol⁻¹.

(a) Give the ionic equation for the precipitation step common to all three halides once the halide ion, $\text{X}^-$, has been released by hydrolysis. [1]

(b) State the colour of the precipitate formed from each of 1-chlorobutane, 1-bromobutane and 1-iodobutane. [3]

(c) Explain why 1-iodobutane produces its precipitate fastest, and 1-chlorobutane produces its precipitate slowest, of the three halides tested. [2]

(d) Explain why no precipitate is observed for 1-fluorobutane under these conditions. [2]

1-Bromobutane, 2-bromobutane and 2-bromo-2-methylpropane are all reacted separately with aqueous sodium hydroxide.

(a) State which mechanism, SN1 or SN2, is followed almost exclusively by 1-bromobutane, and which is followed almost exclusively by 2-bromo-2-methylpropane. [2]

(b) Explain why 2-bromobutane, unlike the other two compounds, reacts by a mixture of both the SN1 and SN2 mechanisms. Refer to both a steric factor and an electronic (carbocation stability) factor in your answer. [4]

(c) A student increases the concentration of $\text{OH}^-(\text{aq})$ used with 2-bromobutane. Explain the effect this has on the relative proportion of product formed via the SN2 pathway compared with the SN1 pathway. [3]

A single, optically pure enantiomer of 2-bromobutane, $\text{CH}_3\text{CH}_2\text{CH}(\text{Br})\text{CH}_3$, is heated under reflux with aqueous sodium hydroxide. Because the carbon bonded to bromine is secondary, this substrate is known to react through a mixture of the SN1 and SN2 mechanisms operating alongside each other, rather than through only one of them.

Which statement correctly predicts and explains the optical activity of the butan-2-ol formed?

Bromoethane and bromobenzene are separately heated under reflux with aqueous sodium hydroxide. Bromoethane is readily hydrolysed to ethanol; bromobenzene shows no detectable reaction even after prolonged heating.

(a) Describe, in terms of orbitals, how a lone pair on the bromine atom in bromobenzene interacts with the aromatic ring's pi system, and state the resulting effect on the length and strength of the C–Br bond compared with the C–Br bond in bromoethane. [3]

(b) Explain how this interaction reduces the electron deficiency (partial positive charge) at the ring carbon bonded to bromine, and use this to explain why $\text{OH}^-$ cannot attack that carbon effectively. [3]

(c) Suggest why a mechanism analogous to SN1, proceeding via a carbocation, is also not a feasible alternative pathway for bromobenzene. [2]