Question

Draw the structural formula of the enol formed in each alkyne hydration reaction; then draw the structural formula of the carbonyl compound with which each enol is in equilibrium. (a) \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{5} \mathrm{C} \equiv \mathrm{CH}+\mathrm{H}_{2} \mathrm{O} \frac{\mathrm{HgSO}_{4}}{\mathrm{H}_{2} \mathrm{SO}_{4}}\) (an enol) \(\longrightarrow\) (b) \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{5} \mathrm{C} \equiv \mathrm{CH} \frac{\text { 1. }(\text { sia })_{\mathrm{BH}}}{\text { 2. } \mathrm{NaOH} / \mathrm{H}_{2} \mathrm{O}_{2}}\) (an enol) \(\longrightarrow\)

Short answer

Answer: The carbonyl compound in part (a) is an aldehyde with the structural formula: \(CH_3(CH_2)_5CHO\). The carbonyl compound in part (b) is a ketone with the structural formula: \(CH_3(CH_2)_4CH_2COCH_3\).

Step-by-step solution

Part (a): Identify Reagents

In part (a), the reagents are: \(CH_3(CH_2)_5C\equiv CH\), water \((H_2O)\), and a catalyst \((HgSO_4/H_2SO_4)\).

Part (a): Determine Type of Hydration

The catalyst \((HgSO_4/H_2SO_4)\) indicates that the reaction follows Markovnikov's rule. This means that the oxygen from water will attack the less substituted carbon in the alkyne, forming an enol.

Part (a): Draw the Structural Formula of the Enol

Following Markovnikov's rule, the structural formula of the enol formed in this reaction is: \(CH_3(CH_2)_5CHOH-C\equiv CH\).

Part (a): Draw the Structural Formula of the Carbonyl Compound

The enol is in equilibrium with a carbonyl compound through keto-enol tautomerism. The structural formula of the carbonyl compound is: \(CH_3(CH_2)_5CHO\) (an aldehyde).

Part (b): Identify Reagents

In part (b), the reagents are: \(CH_3(CH_2)_5C\equiv CH\), and a two-step process using \((\text{sia})_{\mathrm{BH}}\) and \(NaOH/H_2O_2\).

Part (b): Determine Type of Hydration

The use of the two-step process \((\text{sia})_{\mathrm{BH}}\) followed by \(NaOH/H_2O_2\) indicates that the reaction is an anti-Markovnikov hydration. This means that the oxygen from water will attack the more substituted carbon in the alkyne, forming an enol.

Part (b): Draw the Structural Formula of the Enol

Following the anti-Markovnikov rule, the structural formula of the enol formed in this reaction is: \(CH_3(CH_2)_5C\equiv CHOH\).

Part (b): Draw the Structural Formula of the Carbonyl Compound

The enol is in equilibrium with a carbonyl compound through keto-enol tautomerism. The structural formula of the carbonyl compound is: \(CH_3(CH_2)_4CH_2COCH_3\) (a ketone).