Question

Predict the products formed when cyclohexanecarbaldehyde reacts with following reagents. (i) PhMgBr and then \(\mathrm{H}_{3} \mathrm{O}^{+}\) (ii) Tollens' reagent (iii) Semicarbazide and weak acid (iv) Excess ethanol and acid (v) Zinc amalgam and dilute hydrochloric acid

Short answer

(i) Cyclohexylmethanol with a phenyl group; (ii) Cyclohexanecarboxylic acid; (iii) Cyclohexanecarbaldimine-N-semicarbazone; (iv) Cyclohexanecarbaldehyde diethyl acetal; (v) Cyclohexylmethane.

Step-by-step solution

Grignard Reaction with PhMgBr

When cyclohexanecarbaldehyde reacts with PhMgBr (phenylmagnesium bromide, a Grignard reagent), the Grignard reagent will attack the carbonyl carbon of the aldehyde. This results in the formation of a new carbon-carbon bond, producing a secondary alcohol. After hydrolysis with \(\mathrm{H}_{3} \mathrm{O}^{+}\), you obtain cyclohexylmethanol with a phenyl group attached to the alpha carbon, \(\mathrm{C}_{6}\mathrm{H}_{11}\mathrm{CH}(\mathrm{OH})\mathrm{Ph}\).

Reaction with Tollens' Reagent

Tollens' reagent is used to oxidize aldehydes to carboxylic acids. Here, cyclohexanecarbaldehyde oxidizes to cyclohexanecarboxylic acid, \(\mathrm{C}_{6}\mathrm{H}_{11}\mathrm{COOH}\).

Formation of Semicarbazone

When cyclohexanecarbaldehyde reacts with semicarbazide in the presence of a weak acid, a condensation reaction occurs, forming a semicarbazone. The product will be cyclohexanecarbaldimine-N-semicarbazone, \(\mathrm{C}_{6}\mathrm{H}_{11}\mathrm{CH} = \mathrm{NCONHNH}_{2}\).

Acetal Formation with Ethanol and Acid

Excess ethanol in the presence of acid will convert cyclohexanecarbaldehyde into an acetal. The product is cyclohexanecarbaldehyde diethyl acetal, \(\mathrm{C}_{6}\mathrm{H}_{11}\mathrm{CH}(\mathrm{OC}_{2}\mathrm{H}_{5})_{2}\).

Clemmensen Reduction

Zinc amalgam and hydrochloric acid (HC\(\ell\)), used in Clemmensen reduction, will reduce the aldehyde group in cyclohexanecarbaldehyde to a methylene group, yielding cyclohexylmethane, \(\mathrm{C}_{6}\mathrm{H}_{11}\mathrm{CH}_{3}\).

Key concepts

Grignard Reaction

The Grignard reaction is a crucial method in organic chemistry that involves the formation of a carbon-carbon bond. When cyclohexanecarbaldehyde reacts with PhMgBr, a Grignard reagent, the nucleophilic carbon atom in the reagent attacks the electrophilic carbon atom in the aldehyde's carbonyl group. This addition reaction creates a new C-C bond. After the intermediate is formed, it undergoes hydrolysis with water and acid (\(\mathrm{H}_{3}\mathrm{O}^{+}\)) to yield a secondary alcohol. The final product is cyclohexylmethanol with a phenyl group attached, which indicates a successful addition across the carbonyl group.

• Grignard reagents are reactive organometallic compounds.
• They facilitate the formation of alcohols and other complex molecules.
• Control over reaction conditions leads to different types of products.

This reaction is often employed in synthesis because it allows for direct insertion of carbon atoms into organic frameworks, pivotal for building carbon skeletons in larger molecules.

Tollens' Reagent

Tollens' reagent is a gentle oxidizing agent primarily used to test for aldehydes. Upon reacting with an aldehyde like cyclohexanecarbaldehyde, it facilitates oxidation to produce a corresponding carboxylic acid, cyclohexanecarboxylic acid in this example. The reagent is distinctive for forming a silver mirror on the test tube's surface due to the reduction of silver ions to metallic silver when an aldehyde is oxidized.

• It is an aqueous solution of silver nitrate (AgNO₃) in ammonia.
• Primarily oxidizes aldehydes; ketones usually do not react.
• This test specifically distinguishes aldehydes from ketones.

Tollens' test is not only crucial for identifying aldehydes but also serves as a classic lab experiment to demonstrate oxidation-reduction reactions.

Semicarbazone Formation

The formation of semicarbazones is an example of a condensation reaction where an aldehyde or ketone reacts with semicarbazide. During this reaction, cyclohexanecarbaldehyde condenses with semicarbazide in the presence of a weak acid to form cyclohexanecarbaldimine-N-semicarbazone. These derivatives are stabilizing products that make it possible to identify or characterize the parent carbonyl compound through melting point determination.

• Semicarbazones are useful for purifying and isolating aldehydes and ketones.
• This reaction offers a method for deriving stable solid forms of volatile carbonyl compounds.
• The process involves the substitution of the carbonyl oxygen with a nitrogen-containing group.

Semicarbazone formation is particularly beneficial in biochemical research for stabilizing labile carbonyl substrates and aiding in structure elucidation.

Acetal Formation

Acetal formation is a vital reaction in organic synthesis, often used to protect aldehyde or ketone carbonyl groups during multi-stage synthesis. In the presence of acid and excess ethanol, cyclohexanecarbaldehyde reacts to form cyclohexanecarbaldehyde diethyl acetal. This reaction is reversible and involves several steps: protonation of the carbonyl oxygen, nucleophilic attack by ethanol, and elimination of water.

• Acetals are stable only under non-acidic conditions.
• They are widely used as protecting groups during chemical synthesis.
• The formation of acetals is an equilibrium process.

Reversibility of acetal formation is exploited in protective group strategies, where acetals can protect reactive aldehyde groups from reacting during other parts of a synthetic procedure.

Clemmensen Reduction

Clemmensen reduction is an essential technique in organic chemistry used to reduce ketones and aldehydes to alkanes. Using zinc amalgam and hydrochloric acid, cyclohexanecarbaldehyde is reduced to cyclohexylmethane. This process entails the complete removal of the carbonyl group, transforming it into a methylene group.

• Useful for compounds stable under acidic conditions.
• Replaces the carbonyl group with methylene, removing oxygen functionality.
• A valuable tool for synthesizing hydrocarbons from carbonyl compounds.

Clemmensen reduction is favorable when other reduction methods might affect compound stability, making it a standard choice for transforming carbonyl groups without affecting other functional groups.