Question

Alkenes can be hydrated to form alcohols by (1) hydroboration followed by oxidation with alkaline hydrogen peroxide and (2) acid-catalyzed hydration. Compare the product formed from each alkene by sequence (1) with those formed from (2). (a) Propene (b) cis-2-Butene (c) trans-2-Butene (d) Cyclopentene (e) 1 -Methylcyclohexene

Short answer

Question: Compare the products formed from hydrating the following alkenes using (1) hydroboration followed by oxidation with alkaline hydrogen peroxide and (2) acid-catalyzed hydration: (a) propene, (b) cis-2-butene, (c) trans-2-butene, (d) cyclopentene, and (e) 1-methylcyclohexene. Answer: (a) Propene: (1) 1-propanol, (2) 2-propanol (b) cis-2-Butene: (1) 2-butanol, (2) 2-methylpropanol (c) trans-2-Butene: (1) 2-butanol, (2) 2-methylpropanol (d) Cyclopentene: (1) cyclopentanol, (2) 1-methylcyclopentanol (e) 1-Methylcyclohexene: (1) 2-methylcyclohexanol, (2) 1-methylcyclohexanol

Step-by-step solution

(a) Propene - Sequence 1: Hydroboration-Oxidation

Hydroboration-oxidation (HB-O) is an anti-Markovnikov addition reaction. In the case of propene, borane (BH3) reacts with propene to form an alkylborane intermediate. Then, alkaline hydrogen peroxide oxidizes the intermediate to form the alcohol. The product for sequence 1 is 1-propanol.

(a) Propene - Sequence 2: Acid-Catalyzed Hydration

Acid-catalyzed hydration is a Markovnikov addition reaction. The water molecule is added to the double bond of the propene in the presence of an acid catalyst like sulfuric acid. The product for this reaction is 2-propanol.

(b) cis-2-Butene - Sequence 1: Hydroboration-Oxidation

The HB-O reaction on cis-2-butene results in the formation of 2-butanol, following an anti-Markovnikov rule. The reaction proceeds with syn addition of borane and oxidation with hydrogen peroxide without changing the stereochemistry of the substituents.

(b) cis-2-Butene - Sequence 2: Acid-Catalyzed Hydration

The acid-catalyzed hydration of cis-2-butene results in the formation of 2-methylpropanol. As an electrophilic addition reaction, it follows the Markovnikov rule. There is no stereochemistry control on the final product.

(c) trans-2-Butene - Sequence 1: Hydroboration-Oxidation

Similar to the previous reaction, the HB-O reaction on trans-2-butene results in the formation of 2-butanol, following an anti-Markovnikov rule. The reaction proceeds with syn addition and no change in stereochemistry.

(c) trans-2-Butene - Sequence 2: Acid-Catalyzed Hydration

Acid-catalyzed hydration of trans-2-butene also results in the formation of 2-methylpropanol. There is no stereochemistry control in this reaction.

(d) Cyclopentene - Sequence 1: Hydroboration-Oxidation

Upon performing HB-O on cyclopentene, we obtain cyclopentanol as the final product. The hydroxyl group is added on the less substituted carbon due to the anti-Markovnikov rule.

(d) Cyclopentene - Sequence 2: Acid-Catalyzed Hydration

Acid-catalyzed hydration reaction on cyclopentene produces 1-methylcyclopentanol. The hydroxyl group is added to the more substituted carbon, according to the Markovnikov rule.

(e) 1-Methylcyclohexene - Sequence 1: Hydroboration-Oxidation

HB-O on 1-methylcyclohexene gives 2-methylcyclohexanol. The anti-Markovnikov rule is followed, and the hydroxyl group is added to the less substituted carbon.

(e) 1-Methylcyclohexene - Sequence 2: Acid-Catalyzed Hydration

Acid-catalyzed hydration of 1-methylcyclohexene produces 1-methylcyclohexanol. The hydroxyl group is added to the more substituted carbon, following the Markovnikov rule.