Most Important Aldehydes And Ketones MCQs with Answers

Acetone is prepared by dry distillation of calcium acetate.

The industrial method uses 500°C, while the laboratory method for formaldehyde uses 300°C.

Air is drawn through methyl alcohol with the help of a suction pump.

This reaction specifically shows the laboratory preparation of formaldehyde.

The carbonyl group has a σ-bond and a π-bond, allowing it to undergo addition reactions. Most reagents react with the carbonyl group by adding to it.

As oxygen is more electronegative, it tends to attract the electrons to itself, making the carbonyl group a polar group.

The carbon atom in the carbonyl group has a partial positive charge and is electrophilic.

Most of the reactions of the carbonyl group are considered to be nucleophilic addition reactions, which are the characteristic reactions of carbonyl compounds.

In these reactions, the negative part of the reagent combines with the electrophilic carbon of the carbonyl group.

A base catalyst increases the nucleophilic character of the reagent.

An acid catalyst promotes the nucleophilic attack by increasing the positive character (electrophilic character) of the carbonyl carbon atom.

The positive part of the reagent, which is usually hydrogen, goes to the oxygen atom of the carbonyl group.

Whether the addition is base-catalysed or acid-catalysed, the adduct is the same.

A base-catalysed nucleophilic addition reaction will take place with a strong nucleophilic reagent. The base reacts with the reagent and generates the nucleophile.

Most reactions of the carbonyl group are nucleophilic addition reactions, which are characteristic of carbonyl compounds.

The negative part of the reagent (nucleophile) combines with the electrophilic carbon of the carbonyl group.

A base catalyst increases the nucleophilic character of the reagent, making it a stronger nucleophile.

An acid catalyst promotes nucleophilic attack by increasing the electrophilic character(positive character) of the carbonyl carbon atom through protonation of the carbonyl oxygen.

Aldehydes and ketones react with hydrogen cyanide to form cyanohydrins.

The addition of HCN to carbonyl compounds is usually catalysed by a strong base like NaOH or K₂CO₃. The base produces a cyanide ion (CN⁻), which is a strong nucleophile.

In the first step of base-catalysed addition of HCN, the base reacts with HCN to generate the strongly nucleophilic cyanide ion (CN⁻).

Grignard reagents react with aldehydes and ketones to form addition products which, on hydrolysis, yield alcohols.

Formaldehyde reacts with Grignard reagents to yield primary alcohols.

Any aldehyde other than formaldehyde reacts with Grignard reagents to yield secondary alcohols.

Ketones react with Grignard reagents to yield tertiary alcohols.

Aldehydes and ketones react with a saturated solution of sodium bisulphite (NaHSO₃) to form bisulphite addition products.

The original aldehyde or ketone can be regenerated from the bisulphite addition product by heating with an acid or alkali. This property is used in the separation and purification of carbonyl compounds.

Bisulphite addition products are generally crystalline solids. This reaction is particularly useful for the separation and purification of low molecular weight aldehydes and ketones from non-carbonyl compounds.

Condensation reaction is a reaction where two or more molecules combine with the elimination of a simple molecule like water, ammonia, or an alcohol.

Aldehydes and ketones react with ammonia derivatives, G-NH₂, to form compounds containing the group, C=N-G and water.

This reaction is known as a condensation reaction or addition-elimination reaction because water is lost after addition occurs.

The reaction of aldehydes and ketones with ammonia derivatives is acid catalysed.

Ketones are less reactive than aldehydes due to two reasons: (1) steric hindrance: ketones have two bulky alkyl groups, increasing hindrance to nucleophilic attack; (2) electronic effects: two alkyl groups in ketones release electrons, decreasing the positive charge on the carbonyl carbon and making it less electrophilic compared to aldehydes.

Formaldehyde (HCHO) is the most reactive aldehyde/ketone towards nucleophilic addition because it has the least steric hindrance (two hydrogen atoms attached to the carbonyl carbon) and no electron-donating alkyl groups to reduce the electrophilicity of the carbonyl carbon. Reactivity decreases as steric hindrance and electron donation increase.

Aldehydes are reduced to primary alcohols.

Ketones are reduced to secondary alcohols.

Aldehydes and ketones can be reduced to alcohols by sodium borohydride (NaBH₄) or lithium aluminum hydride (LiAlH₄).

The addition of hydrogen across the carbonyl group in the presence of a catalyst like nickel or platinum is called catalytic hydrogenation.

The reduction of aldehydes and ketones is a type of addition reaction, specifically the addition of hydrogen.

Lithium aluminum hydride (LiAlH₄) is a more powerful reducing agent than sodium borohydride (NaBH₄) and can reduce carboxylic acids and their derivatives, whereas NaBH₄ is selective for aldehydes and ketones.

Acetone is a ketone; ketones reduce to secondary alcohols. 2-propanone reduces to 2-propanol.

Ethanal is an aldehyde; aldehydes reduce to primary alcohols. Ethanal reduces to ethanol.

This equation shows the addition of two hydrogen atoms to an aldehyde to form a primary alcohol.

The reduction of aldehydes and ketones using sodium and ethanol is known as the Bouveault-Blanc reduction.

Aldehydes are readily oxidized to carboxylic acids containing the same number of carbon atoms.

Aldehydes are easily oxidized because they contain a hydrogen atom attached to the carbonyl carbon. Ketones lack this hydrogen atom, making them more resistant to oxidation.

Aldehydes can be oxidized by strong oxidizing agents like acidified potassium dichromate (K₂Cr₂O₇/H₂SO₄) or potassium permanganate (KMnO₄).

Tollens' reagent is a mild oxidizing agent that oxidizes aldehydes but generally not ketones, making it useful for distinguishing between them.

When an aldehyde is warmed with Tollens' reagent, a shining silver mirror is formed on the inner side of the test tube, or a black precipitate of silver may appear.

Tollens' reagent is an ammoniacal solution of silver nitrate.

Fehling's solution is a deep blue alkaline solution of cupric ions. When warmed with an aldehyde, the blue color disappears, and a red precipitate of cuprous oxide (Cu₂O) is formed.

Fehling's solution B is an alkaline solution of sodium potassium tartrate (Rochelle salt). Fehling's solution A is an aqueous solution of copper sulfate.

Under vigorous conditions, ketones undergo oxidation by the breaking of C-C bonds adjacent to the carbonyl group, forming a mixture of carboxylic acids with fewer carbon atoms than the original ketone.

The haloform reaction (e.g., iodoform test) is used to detect the presence of methyl ketones (compounds containing a CH₃-C(=O)- group) or secondary alcohols that can be oxidized to methyl ketones.