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

Compare the products formed from the hydration of various alkenes through the hydroboration-oxidation sequence and the acid-catalyzed hydration sequence. For propene, hydroboration-oxidation forms 1-propanol, while acid-catalyzed hydration forms 2-propanol. For cis-2-butene and trans-2-butene, hydroboration-oxidation forms 2-butanol, while acid-catalyzed hydration forms 3-butanol. For cyclopentene, hydroboration-oxidation forms cyclopentan-1-ol, while acid-catalyzed hydration forms cyclopentan-2-ol. Finally, for 1-methylcyclohexene, both sequences form 1-methylcyclohexan-1-ol. In conclusion, hydroboration-oxidation mainly forms anti-Markovnikov products, while acid-catalyzed hydration mainly forms Markovnikov products. The exception is 1-methylcyclohexene, for which both sequences result in the same product.

Step-by-step solution

Hydroboration-oxidation product for propene

During hydroboration, the boron atom attaches to the less substituted carbon due to steric hindrance. Upon oxidation with hydrogen peroxide and a hydroxide ion, we replace the boron atom with an OH group, forming an alcohol following anti-Markovnikov's rule. The product is 1-propanol.

Acid-catalyzed hydration product for propene

Under acidic conditions, the water molecule adds to the double bond, using the rule of Markovnikov's addition. It forms a carbocation on the more substituted carbon, and the final product is 2-propanol. (b) cis-2-Butene

Hydroboration-oxidation product for cis-2-Butene

For cis-2-butene, the hydroboration reactions leads to the anti-Markovnikov alcohol, and oxidation yields 2-butanol.

Acid-catalyzed hydration product for cis-2-Butene

The acid-catalyzed hydration proceeds through Markovnikov addition, forming a carbocation on the more substituted carbon, resulting in the product 3-butanol. (c) trans-2-Butene

Hydroboration-oxidation product for trans-2-Butene

For trans-2-butene, hydroboration followed by oxidation yields the same product as in cis-2-butene, as only the position of substituents matter - 2-butanol.

Acid-catalyzed hydration product for trans-2-Butene

In the case of trans-2-butene, the product of acid-catalyzed hydration is also the same as in cis-2-butene, since only the position of the substituents matter, which is 3-butanol. (d) Cyclopentene

Hydroboration-oxidation product for cyclopentene

When cyclopentene undergoes hydroboration-oxidation, the resulting product is cyclopentan-1-ol, the anti-Markovnikov product.

Acid-catalyzed hydration product for cyclopentene

When cyclopentene undergoes acid-catalyzed hydration, the product is cyclopentan-2-ol, the Markovnikov product. (e) 1-Methylcyclohexene

Hydroboration-oxidation product for 1-Methylcyclohexene

1-Methylcyclohexene, after undergoing hydroboration-oxidation, produces 1-methylcyclohexan-1-ol, as the OH group attaches to the less substituted carbon.

Acid-catalyzed hydration product for 1-Methylcyclohexene

1-Methylcyclohexene, after acid-catalyzed hydration, yields the same product as in Step 1 following Markovnikov's rule, which is 1-methylcyclohexan-1-ol. In conclusion, each sequence leads to different products in most cases - hydroboration-oxidation leads to anti-Markovnikov products, whereas acid-catalyzed hydration leads to Markovnikov products. For 1-Methylcyclohexene, both sequences result in the same product.