Question

Draw a structural formula for the product formed by treating butanal with each reagent. (a) \(\mathrm{LiAlH}_{4}\) followed by \(\mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{NaBH}_{4}\) in \(\mathrm{CH}_{3} \mathrm{OH} / \mathrm{H}_{2} \mathrm{O}\) (c) \(\mathrm{H}_{2} / \mathrm{Pt}\) (d) \(\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}{ }^{+}\)in \(\mathrm{NH}_{3} / \mathrm{H}_{2} \mathrm{O}\) (e) \(\mathrm{H}_{2} \mathrm{CrO}_{4}\), heat (f) \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}, \mathrm{HCl}\) (g) \(\mathrm{Zn}(\mathrm{Hg}) / \mathrm{HCl}\) (h) \(\mathrm{N}_{2} \mathrm{H}_{4}, \mathrm{KOH}\) at \(250^{\circ} \mathrm{C}\) (i) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) (j) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NHNH}_{2}\)

Short answer

Draw the structural formula for the product formed when butanal reacts with each of the following reagents: (a) LiAlH4 followed by H2O: CH3CH2CH2CH2OH (b) NaBH4 in CH3OH/H2O: CH3CH2CH2CH2OH (c) H2/Pt: CH3CH2CH2CH2OH (d) Ag(NH3)2+ in NH3/H2O: CH3CH2CH2COOH (e) H2CrO4, heat: CH3CH2CH2COOH (f) HOCH2CH2OH, HCl: CH3CH2CH2CH(OC2H5)2 (g) Zn(Hg)/HCl: CH3CH2CH2CH3 (h) N2H4, KOH at 250°C: CH3CH2CH2CH3 (i) C6H5NH2: CH3CH2CH2CH=N-C6H5 (j) C6H5NHNH2: CH3CH2CH2CH=N-NHC6H5

Step-by-step solution

(a) Reaction with \(\mathrm{LiAlH}_4\) followed by \(\mathrm{H}_2\mathrm{O}\)

\(\mathrm{LiAlH}_4\) is a reducing agent that readily reduces aldehydes to primary alcohols. The reaction proceeds by nucleophilic attack of a hydride ion on the carbonyl group of butanal. Then, \(\mathrm{H}_2\mathrm{O}\) is used to quench and protonate the alkoxide intermediate, yielding the final product. The structural formula for the product is CH3CH2CH2CH2OH.

(b) Reaction with \(\mathrm{NaBH}_4\) in \(\mathrm{CH}_3\mathrm{OH}/\mathrm{H}_2\mathrm{O}\)

\(\mathrm{NaBH}_4\) is a milder reducing agent than \(\mathrm{LiAlH}_4\), but it is also capable of reducing aldehydes to primary alcohols. The reaction proceeds similarly to (a), and the structural formula for the product in this case is also CH3CH2CH2CH2OH.

(c) Reaction with \(\mathrm{H}_2/\mathrm{Pt}\)

Treatment with \(\mathrm{H}_2/\mathrm{Pt}\) will reduce the carbonyl group, and the product will be the same as in (a) and (b). The structural formula is CH3CH2CH2CH2OH.

(d) Reaction with \(\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}{ }^{+}\) in \(\mathrm{NH}_{3}/\mathrm{H}_{2}\mathrm{O}\)

The given reagent, also known as Tollen's reagent, oxidizes aldehydes such as butanal into carboxylic acids. The structural formula for the obtained product is CH3CH2CH2COOH.

(e) Reaction with \(\mathrm{H}_2\mathrm{CrO}_4\), heat

This reagent is an oxidizing agent that transforms aldehydes into carboxylic acids, similar to (d). Upon heating butanal with \(\mathrm{H}_2\mathrm{CrO}_4\), the oxidation takes place, and the product is CH3CH2CH2COOH.

(f) Reaction with \(\mathrm{HOCH}_2\mathrm{CH}_2\mathrm{OH},\mathrm{HCl}\)

The reagent is an acetal protecting group. The reaction forms an acetal, which protects the carbonyl group from further reactions. The structural formula for the product is CH3CH2CH2CH(OC2H5)2.

(g) Reaction with \(\mathrm{Zn}(\mathrm{Hg})/\mathrm{HCl}\)

This reagent, also called Clemmensen reduction, is used to reduce carbonyl groups in aldehydes to methylene groups. The structural formula for the product in this case is CH3CH2CH2CH3.

(h) Reaction with \(\mathrm{N}_2\mathrm{H}_4,\mathrm{KOH}\) at \(250^{\circ}\mathrm{C}\)

This reaction is similar to (g) and performs the Wolff-Kishner reduction, which reduces aldehydes to alkanes. The structural formula for the product in this case is CH3CH2CH2CH3.

(i) Reaction with \(\mathrm{C}_6\mathrm{H}_5\mathrm{NH}_2\)

The reagent is aniline, a nucleophile, which can react with the carbonyl group of butanal to form imines. The structural formula for the product in this case is CH3CH2CH2CH=N-C6H5.

(j) Reaction with \(\mathrm{C}_6\mathrm{H}_5\mathrm{NHNH}_2\)

The reagent is phenylhydrazine, another nucleophile. The reaction with butanal leads to the formation of phenylhydrazones. The structural formula for the product in this case is CH3CH2CH2CH=N-NHC6H5.

Key concepts

Butanal Reactions

When encountering butanal, a four-carbon aldehyde, in organic chemistry, students often need to predict its behavior with various reagents. Butanal has a range of reactions due to its functional group, the terminal carbonyl (C=O). For example, butanal can be reduced to butanol using strong reducing agents like Lithium Aluminum Hydride (\texttt{LiAlH}\(_4\)) or Sodium Borohydride (\texttt{NaBH}\(_4\)), which involves the addition of a hydride ion to the carbonyl carbon, followed by the acquisition of a proton from water. This is the essence of aldehyde reduction.

Butanal can also undergo oxidation, where it reacts with oxidizing agents like Tollen's reagent or chromic acid (\texttt{H}\(_2\)\texttt{CrO}\(_4\)) to form butanoic acid. Here, the aldehyde is transformed by gaining an oxygen atom and losing two hydrogen atoms. Acid-catalyzed reactions might lead to acetal formation, a common way to protect the carbonyl group in further synthetic sequences. Butanal is a versatile molecule, demonstrating the reactivity inherent to carbonyl compounds.

Aldehyde Reduction

Aldehyde reduction is a fundamental transformation in organic chemistry that converts aldehydes to primary alcohols. This reaction is essential as it allows chemists to increase the molecular complexity by adding hydrogen atoms in a controlled manner. The key to this transformation is the transfer of hydride ions, which are negatively charged hydrogen atoms, from reducing agents to the electrophilic carbon atom of the carbonyl group.

Common reducing agents include \texttt{LiAlH}\(_4\), used in stronger reductions, and \texttt{NaBH}\(_4\), preferred for gentler reductions. The process involves two main steps: the nucleophilic attack by the hydride ion forming an alkoxide, and the subsequent protonation by water to yield the alcohol. This reaction not only increases the hydrogen content of the molecule but also removes the oxygen from the carbonyl group, demonstrating the versatile manipulations possible with aldehydes.

Carbonyl Compound Chemistry

Carbonyl compounds, characterized by a carbon-oxygen double bond, form a significant group in organic chemistry, with aldehydes and ketones as primary members. Their chemistry is rich and varied, leading to a wide array of reactions and products. The carbonyl carbon is electrophilic due to the oxygen's higher electronegativity, which makes it a target for nucleophilic attack.

In the context of aldehydes like butanal, they can be oxidized to carboxylic acids, reduced to alcohols, or undergo nucleophilic addition reactions to form hemiacetals, acetals, and imines, among others. Each reagent interacts differently with the carbonyl compound, dictating the outcome of the reaction. For instance, in nucleophilic addition reactions, the incoming nucleophile can change the double bond to a single bond and adds new groups to the carbon skeleton. Organic chemistry of carbonyl compounds is foundational, and mastering their reactions opens up a world of synthesis possibilities in both laboratory settings and nature.