Question

\(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH}_{2}-\mathrm{CN} \stackrel{\mathrm{R}}{\longrightarrow} \mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH}_{2}-\mathrm{CHO}\) The reagent R used for the above conversion is (a) \(\mathrm{NaBH}_{4}\) (b) \(\mathrm{H}_{2} / \mathrm{Pd}-\mathrm{BaSO}_{4}\) (c) \(\mathrm{Na} / \mathrm{liq} \mathrm{NH}_{3}\) (d) \(\mathrm{SnCl}_{2} / \mathrm{HCl}\)

Short answer

Answer: \(\mathrm{SnCl}_{2} / \mathrm{HCl}\)

Step-by-step solution

Identify the initial and final products

We are given the following conversion reaction: $\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH}_{2}-\mathrm{CN} \stackrel{\mathrm{R}}{\longrightarrow} \mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH}_{2}-\mathrm{CHO}$ The reactant is benzyl cyanide, and the product is benzaldehyde.

Analyze the given reagents

Four reagents are given as options: (a) \(\mathrm{NaBH}_{4}\): Sodium borohydride is a mild reducing agent. It mainly reduces aldehydes and ketones to alcohols. (b) \(\mathrm{H}_{2} / \mathrm{Pd}-\mathrm{BaSO}_{4}\): This combination of hydrogen gas and palladium supported on barium sulfate is Lindlar's catalyst. It mainly reduces alkynes to cis-alkenes (hydrogenation of triple bonds to double bonds). (c) \(\mathrm{Na} / \mathrm{liq} \mathrm{NH}_{3}\): Sodium in liquid ammonia is a strong reducing agent. It is mainly used for the reduction of alkynes to trans-alkenes (formation of double bonds). (d) \(\mathrm{SnCl}_{2} / \mathrm{HCl}\): Tin(II) chloride in hydrochloric acid is a strong reducing agent. It is capable of reducing nitrile groups (CN) to aldehyde groups (CHO).

Identify the correct reagent for the conversion

In our exercise, the desired conversion is from benzyl cyanide (nitrile group) to benzaldehyde (aldehyde group). Based on the analysis in step 2, only option (d) is capable of carrying out this conversion. Therefore, the correct reagent is: (d) \(\mathrm{SnCl}_{2} / \mathrm{HCl}\)

Key concepts

Benzyl Cyanide to Benzaldehyde Conversion

Organic chemistry often involves the transformation of one functional group into another, which is vital for synthesizing various compounds. Specifically, converting benzyl cyanide (a compound featuring a nitrile group, \textbf{-CN}) to benzaldehyde (which contains an aldehyde group, \textbf{-CHO}) is a common reaction in the world of organic chemistry.

The process involves choosing a suitable reducing agent that is capable of converting the \textbf{-CN} group into an \textbf{-CHO} group without affecting the other parts of the molecule. In the given exercise, the correct reagent for this conversion is tin chloride (\textbf{SnCl\(_2\)}) in the presence of hydrochloric acid (HCl). This reagent system efficiently reduces the nitrile to an aldehyde, an essential reaction in both lab and industry-scale organic synthesis.

Understanding this conversion is critical for students as it showcases the precision required in selecting reagents when targeting specific functional group transformations.

Use of Reducing Agents in Organic Chemistry

The role of reducing agents in organic chemistry cannot be overstated. These substances facilitate the gain of electrons by organic molecules in various reactions, converting higher oxidation state functional groups to lower states. Choosing the correct reducing agent is crucial based on the desired reaction outcome. For example, sodium borohydride (\textbf{NaBH\(_4\)}) is typically used for reducing ketones and aldehydes to alcohols. In contrast, complex hydrides such as lithium aluminum hydride (\textbf{LiAlH\(_4\)}) are more powerful and can reduce carboxylic acids and esters in addition to the functionalities reduced by sodium borohydride.

In the particular scenario of converting benzyl cyanide to benzaldehyde, a stronger reducing agent that can cleave the carbon-nitrogen triple bond is necessary. \textbf{SnCl\(_2\)}/HCl is a potent reducing agent, capable of selectively reducing the nitrile group while preserving the rest of the molecule's integrity. This highlights the importance of understanding the strength and selectivity of reducing agents in organic synthesis.

Identification of Organic Reagents

Identification of the correct organic reagents is a fundamental skill in organic chemistry. It involves not just remembering what each reagent does but also understanding the mechanisms by which they react with different functional groups. For students, getting acquainted with reagent characteristics such as reactivity, selectivity, and compatibility with various groups can be immensely helpful.

Using the example from the textbook solution, to achieve the transformation of benzyl cyanide to benzaldehyde, \textbf{SnCl\(_2\)}/HCl is identified as the most suitable reagent. Knowing that this reagent is capable of reducing nitrile groups to aldehyde groups while \textbf{NaBH\(_4\)}, \textbf{H\(_2\)}/Pd-BaSO\(_4\), and \textbf{Na}/liquid NH\(_3\) are not, demonstrates the importance of reagent identification based on their chemical properties and reactions. This knowledge is essential not only for solving textbook problems but also for practical applications in research and chemical industries.