Question

Which of the following can be used to reduce a ketone to a secondary alcohol? \((\mathrm{A}) \quad \mathrm{CrO}_{3}\) (B) \(\mathrm{KMnO}_{4}\) (C) \(\quad \mathrm{LiAlH}_{4}\) (D) \(\quad \mathrm{Ag}_{2}\)

Short answer

(C) \(\mathrm{LiAlH}_{4}\)

Step-by-step solution

- Understand the question

The task is to identify which reagent can reduce a ketone to a secondary alcohol. A ketone has the structure R-CO-R', and reducing it converts the C=O double bond to a C-O single bond with the addition of hydrogen atoms.

- Analyze each option

Evaluate each reagent to determine if it can reduce a ketone to a secondary alcohol.

- Review (A) \mathrm{CrO}_{3}

\(\mathrm{CrO}_{3}\) is a strong oxidizing agent and is typically used to oxidize alcohols to ketones or carboxylic acids, not to reduce ketones to secondary alcohols.

- Review (B) \mathrm{KMnO}_{4}

\(\mathrm{KMnO}_{4}\) is another strong oxidizing agent commonly used to oxidize alkenes, alcohols, and other compounds, but not for reducing ketones.

- Review (C) \mathrm{LiAlH}_{4}

\(\mathrm{LiAlH}_{4}\) is a strong reducing agent and is often used to reduce carbonyl compounds like ketones to secondary alcohols.

- Review (D) \mathrm{Ag}_{2}\)

\(\mathrm{Ag}_{2}\) is not a common reducing agent for ketones to secondary alcohols. Silver compounds are usually involved in oxidation reactions or forming mirror surfaces.

- Conclusion

After evaluating all the options, it is clear that \(\mathrm{LiAlH}_{4}\) (option C) is the reagent that can reduce a ketone to a secondary alcohol.

Key concepts

Secondary Alcohol Synthesis

Synthesizing secondary alcohols typically involves the reduction of a ketone. Ketones have the structure R-CO-R', where R and R' are alkyl or aryl groups. In the reduction process, the carbon-oxygen double bond (C=O) is converted to a carbon-oxygen single bond (C-OH), resulting in the addition of hydrogen atoms. This transformation is essential in organic chemistry and is often used in the synthesis of pharmaceuticals and fragrances.

Reducing Agents

Reducing agents are substances that donate electrons to another compound, reducing the oxidation state of that compound. They are vital in various chemical reactions, including the reduction of ketones to secondary alcohols. Different reducing agents have varying strengths and specificities:

• Strong reductants: These include agents like lithium aluminum hydride (LiAlH4) and sodium borohydride (NaBH4), which are commonly used to reduce carbonyl compounds to alcohols.
• Weak reductants: These agents, like hydrogen gas in the presence of a catalyst, are less reactive and may only reduce certain types of compounds.

Understanding the reactivity and selectivity of these reductants is crucial for choosing the right one for your reaction.

LiAlH4

Lithium aluminum hydride (LiAlH4) is one of the most robust and commonly used reducing agents in organic chemistry. It displays exceptional reactivity and is capable of reducing a wide range of carbonyl compounds:

• Ketones: It effectively reduces ketones to secondary alcohols, making it suitable for synthesizing these compounds in labs.
• Aldehydes: LiAlH4 can also reduce aldehydes to primary alcohols.
• Esters and Carboxylic acids: Even these less reactive compounds can be converted to alcohols using LiAlH4.

When using LiAlH4, it’s essential to handle it with care under anhydrous conditions, as it reacts violently with water and air moisture. Proper safety protocols should always be followed.