Chemical Reactions of Aldehydes and Ketones: Aldehydes and ketones are the two functional groups that share a lot of similarities. They both contain the polar carbonyl group \(\left({ > {\text{C}} = {\text{O}}} \right);\) hence, they are polarised and have a \(\delta + \) charge on the carbon atom and a \(\delta – \) charge on the oxygen atom. Thus, due to the structural similarity, aldehydes and ketones have many common reactions for both the carbonyl compounds. Let’s learn about the chemical reactions of these compounds.

1. Nucleophilic Addition Reaction

a. Mechanism

A nucleophile is a nucleus seeking negatively charged species, whereas an electrophile is an electron seeking positively charged species. In nucleophilic addition reaction, the electron-rich nucleophile attacks the electron-deficient electrophiles.

In aldehydes and ketones, the carbon atom of the polar carbonyl group acts as the electrophile. A nucleophile attacks the carbon atom of the polar carbonyl group, which acts as an electrophile from a direction approximately perpendicular to the plane of \({\text{s}}{{\text{p}}^2}\) hybridised orbitals of the carbonyl carbon atom. This leads to the change in the hybridisation of the carbon atom from \({\text{s}}{{\text{p}}^2}\) to \({\text{s}}{{\text{p}}^3}\) with the formation of a tetrahedral alkoxide intermediate. The intermediate so formed captures a proton from the reaction medium to form an electrically neutral product. The entire process results in the addition of \({\text{N}}{{\text{u}}^ – }\) and \({{\text{H}}^ + }\) across the carbon-oxygen double bond.

b. Reactivity

In nucleophilic addition reactions, aldehydes are generally more reactive than ketones due to steric and electronic reasons.

1. The presence of two relatively bulky groups in ketones hinders the approach of the nucleophile to the electrophilic carbonyl carbon atom. However, such steric hindrance is not present in aldehydes as aldehydes have only one such substituent.
2. Electronically, the two alkyl groups in ketones reduce the electrophilicity of the carbonyl carbon more effectively than aldehydes with only one alkyl group.

c. Some important examples of nucleophilic addition and nucleophilic addition-elimination reactions:

1. Addition of hydrogen cyanide

Aldehydes and ketones react with \({\text{HCN}}\) (hydrogen cyanide) to yield cyanohydrins that are useful synthetic intermediates. It is a base catalysed reaction that generates cyanide ion \(\left({{\text{C}}{{\text{N}}^ – }} \right).\) As \({\text{C}}{{\text{N}}^ – }\) is a stronger nucleophile, it readily adds to carbonyl compounds to yield the corresponding cyanohydrin.

2. Addition of sodium hydrogen sulphite:

Aldehydes and ketones react with Sodium hydrogen sulphite to form corresponding addition products. For aldehydes, the position of the equilibrium lies largely to the right-hand side. However, for most ketones, the position of the equilibrium lies largely to the left-hand side due to steric reasons. The addition compound formed by the addition of hydrogen sulphite to aldehydes and ketones is water-soluble. It can be converted back to the original carbonyl compound by treating it with dilute mineral acid or alkali. Therefore, these are useful for the separation and purification of aldehydes.

3. Addition of Grignard reagents:

Aldehydes and ketones react with Grignard reagents to form alcohols. It is a two-step reaction. In the first step of the reaction, the Grignard reagent adds to the carbonyl group through a nucleophilic addition reaction. This results in an adduct formation. In the second step, hydrolysis of the adduct takes place that yields alcohol.

The reaction of Grignard reagents with methanal or formaldehyde \(\left({{\text{HCHO}}} \right)\) produces primary alcohol; however, secondary alcohols are produced with other aldehydes. The reaction of Grignard reagents with ketones yields tertiary alcohols.

4. Addition of Alcohols

Aldehydes react with one molecule of monohydric alcohol in the presence of dry hydrogen chloride to yield an alkoxy alcohol intermediate. This intermediate is known as hemiacetals, which react with one more alcohol molecule to give a gem-dialkoxy compound known as acetal.

Ketones react with ethylene glycol in the presence of dry hydrogen chloride to form cyclic products known as ethylene glycol ketals.

Dry hydrogen chloride protonates the oxygen of the carbonyl group, which in turn increases the electrophilicity of the carbonyl carbon. This facilitates the nucleophilic attack of ethylene glycol. Both acetals and ketals are hydrolysed with aqueous mineral acids to yield corresponding aldehydes and ketones, respectively.

5. Addition of ammonia and its derivatives

Ammonia and its derivatives \({{\text{H}}_2}{\text{N}} – {\text{Z}}\) (\({\text{Z=}}\) Alkyl, aryl, \({\text{OH,}}{{\text{C}}_6}{{\text{H}}_5}{\text{NH}},{\text{NHCON}}{{\text{H}}_2}\) Etc.) add to the carbonyl group of aldehydes and ketones. It is an acid catalysed reversible reaction. The equilibrium favours the product formation due to rapid dehydration of the intermediate to form \( > {\text{C}} = {\text{N}} – {\text{Z}}.\)

2. Reduction

1. Reduction to alcohols

Aldehydes and ketones are reduced to the corresponding alcohols by catalytic hydrogenation. In this process, addition of hydrogen takes place in the presence of catalysts such as platinum, palladium, or nickel. This reaction also takes place in the presence of sodium borohydride \(\left({{\text{NaB}}{{\text{H}}_4}} \right)\) or lithium aluminium hydride \(\left({{\text{LiAl}}{{\text{H}}_4}} \right).\) Aldehydes yield primary alcohols, whereas ketones give secondary alcohols.

2. Reduction to hydrocarbons

Clemmensen reduction

In Clemmensen reduction, the carbonyl group of aldehydes and ketones is reduced to \({\text{C}}{{\text{H}}_2}\) group on treatment with zinc amalgam and concentrated hydrochloric acid.

Wolff-Kishner reduction

In Wolff-Kishner reduction, the carbonyl group of aldehydes and ketones is reduced to \({\text{C}}{{\text{H}}_2}\) group. This reaction takes place when carbonyl compounds are treated with hydrazine followed by heating with sodium or potassium hydroxide in a high boiling solvent such as ethylene glycol.

3. Oxidation

Aldehydes and ketones undergo oxidation reactions differently. Aldehydes readily undergo oxidation reactions with common oxidising agents like nitric acid, potassium permanganate, potassium dichromate, etc., to form carboxylic acids. Even mild oxidising agents, such as Tollens’ reagent and Fehlings’ reagent, also oxidise aldehydes.

Ketones generally require strong oxidising agents and elevated temperatures to undergo oxidation reactions. Their oxidation involves carbon-carbon bond cleavage to form a mixture of carboxylic acids with fewer carbon atoms than the parent ketone.

1. Tollen’s test

Tollen’s test is also known as the silver mirror test. It is an ammoniacal silver nitrate solution that is prepared as follows-

Aldehydes react with Tollens’ Reagent to form a precipitate that appears like a silver mirror on the walls of the test tube. The tollens’ reagent oxidises aldehydes to their corresponding carboxylic acid, which reduces the oxidation state of silver from \( + 1\) to its elemental form. The reaction occurs in an alkaline medium. Generally, ketones do not respond to this test.
\(\underset{{{\text{An}}\,{\text{aldehyde}}}}{\mathop {{\text{RCHO}}\left({{\text{aq}}} \right)}} + 2{\text{Ag}}\left({{\text{N}}{{\text{H}}_3}}\right)_2^ + \left({{\text{aq}}} \right) + 3{\text{O}}{{\text{H}}^ – }\left({{\text{aq}}}\right) \to {\text{RCO}}{{\text{O}}^ – }\left({{\text{aq}}} \right) + \underset{{{\text{Free}}\,{\text{silver}}}}{\mathop {2{\text{Ag}}\left({\text{s}}\right)}} + 4{\text{N}}{{\text{H}}_3}\left({{\text{aq}}} \right) + 2{{\text{H}}_2}{\text{O}}\)

2. Fehling’s test

Fehling’s solution is a mixture of two solutions. Fehling’s solution \({\text{A + }}\) Fehling’s solution \({\text{B}}{\text{.}}\)
Fehling’s solution A contains \(7\,{\text{g}}\) of hydrated copper sulphate in \(100\,{\text{ml}}\) of water.
Fehlings solution \({\text{B}}\) contains \(24\,{\text{g}}\) of \({\text{KOH}}\) and \(34.6\,{\text{g}}\) of potassium sodium tartrate in \(100\,{\text{ml}}\) water.
\({\text{R}} – {\text{CHO + 2C}}{{\text{u}}^{2 + }} + 5\overline {\text{O}} {\text{H}} \to {\text{RCO}}\overline {\text{O}} + \underset{{{\text{Red}} – {\text{brown}}\,{\text{ppt}}}}{\mathop {{\text{C}}{{\text{u}}_2}{\text{O}}}} + 3{{\text{H}}_2}{\text{O}}\)

3. Oxidation of methyl ketones by haloform reaction:

Methyl ketones are aldehydes and ketones with at least one methyl group linked to the carbonyl carbon atom. These methyl ketones are oxidised by sodium hypohalite to sodium salts of corresponding carboxylic acids. These carboxylic acids have one carbon atom less than that of the parent carbonyl compound. The methyl group is converted to haloform. A carbon-carbon double bond, if present in the molecule, is not affected by this oxidation. Iodoform reaction with sodium hypoiodite is also used for detection of \({\text{C}}{{\text{H}}_3}{\text{CO}}\) group or \({\text{C}}{{\text{H}}_3}{\text{CH}}\left({{\text{OH}}}\right)\) the group which produces \({\text{C}}{{\text{H}}_3}{\text{CO}}\) group on oxidation.

4. Reactions Due to Alpha Hydrogen

The hydrogen atom present on the carbon atom adjacent to the carbonyl carbon of the aldehyde and ketone is called \(\alpha \)-hydrogens of aldehydes and ketones. The \(\alpha \)-hydrogen atoms of aldehydes and ketones are acidic in nature and undergo a number of reactions. The strong electron-withdrawing effect of the carbonyl group and resonance stabilisation of the conjugate base accounts for the acidity of \(\alpha \)-hydrogen atoms of carbonyl compounds.

1. Aldol condensation:

Carbonyl compounds like aldehydes and ketones that contain at least one \(\alpha \)-hydrogen undergo a reaction in the presence of dilute alkali as a catalyst. This reaction results in the formation of \(\beta \)-hydroxy aldehydes (aldol) or \(\beta \)-hydroxy ketones (ketol), respectively and is known as the Aldol condensation reaction.

Mechanism:

Step-I: A carbanion (i.e., enolate ion) is formed when the hydroxide ion from alkali removes a proton from the \(\alpha \) – carbon of one molecule of ethanal.

Step-II: A nucleophilic addition of enolate ion to the carbonyl carbon of the second molecule of ethanal takes place to produce an alkoxide ion.

Step-III: The alkoxide ion formed in step II takes up a proton from water to form β-hydroxy aldehyde (aldol).

The name aldol corresponds to the two functional groups, aldehyde, and alcohol, present in the products. And the name ketol corresponds to the functional group ketone and alcohol in the products. The aldol and ketol readily lose water to give \(\alpha ,\beta \)-unsaturated carbonyl compounds; hence, it is a condensation reaction. For ketones, the general name aldol is used due to its similarity with aldehydes.
Aldehydes and ketones which do not contain any \(\alpha \) – hydrogen atom such as \({\text{HCHO}}{\left({{\text{C}}{{\text{H}}_3}} \right)_3}{\text{CHO}},{{\text{C}}_6}{{\text{H}}_5}{\text{CHO}},\) etc., do not undergo an aldol condensation reaction.

2. Cross aldol condensation:

Cross aldol condensation is the aldol condensation that occurs between two different aldehydes and/or ketones. If both of them contain α-hydrogen atoms, it gives a mixture of four products. This is illustrated below by the aldol reaction of a mixture of ethanal and propanal.

Ketones can also be used as one component in the cross aldol reactions.

Cross aldol condensation is useful when any one of the carbonyl compounds does not have \(\alpha \)-hydrogen and cannot undergo self-condensation. For example, benzaldehyde can be used with other aldehydes and ketones containing \(\alpha \)-hydrogen.

Mechanism of crossed-aldol condensation reaction

Cross aldol condensation between acetaldehyde (ethanal) and benzaldehyde is shown as below:

Step-I:

In this step, the hydroxide ion from alkali removes a proton from the \(\alpha \) – carbon of ethanal to give a carbanion (i.e., enolate ion).

Step-II:

In this step, the nucleophilic addition of enolate ion to the carbonyl carbon of benzaldehyde occurs to produce an alkoxide ion.

Step-III:

In this step, the alkoxide ion takes up a proton from water to form \(\beta \)-hydroxy aldehyde (aldol).

5. Other Reactions

1. Cannizaro reaction

Aldehydes and ketones that do not have an \(\alpha \)-hydrogen atom undergo self oxidation and reduction (disproportionation) reaction on heating with concentrated alkali. In this reaction, one aldehyde molecule is reduced to alcohol while another is oxidised to carboxylic acid salt.

2. Electrophilic substitution reaction:

Aromatic aldehydes and ketones undergo electrophilic substitution at the ring in which the carbonyl group acts as a deactivating and meta-directing group.

Summary

Aldehydes and ketones exhibit similar chemical properties due to the presence of the carbonyl group. The carbonyl group is polar in nature, which means the shared pair of electrons between carbon and oxygen is more inclined towards oxygen. This is because oxygen being more electronegative than carbon, pulls the shared pair of electrons towards itself and develops a slight negative charge over itself and the carbonyl carbon atom develops a slight positive charge over itself.

This carbon atom acts as an electrophile and accounts for the reactivity of carbonyl compounds. In this article, we learned the different chemical reactions of aldehyde and ketones. We also learned some important name reactions such as Clemmensen’s reduction, Wolff-Kishner reduction, Tollen’s and Fehling’s test, Aldol reaction, and cross aldol reaction.

Frequently Asked Questions

Q.1. What are the 2 steps in the nucleophilic addition reaction of aldehydes and ketones?
Ans: Step 1: Addition of a nucleophile to the electrophilic carbonyl carbon atom. In this step, \({\text{C-Nu}}\) is formed, and \({\text{C-O}}\left( \pi \right)\) bond is cleaved, which results in a negatively charged oxygen atom.
Step 2: Protonation takes place, which results in the formation of the \({\text{O-H}}\) bond.

Q.2. Why do aldehydes and ketones undergo nucleophilic addition reactions?
Ans: In carbonyl compounds, the carbon-oxygen bond is polar. Oxygen is more electronegative than carbon. It pulls the shared pair of electrons towards itself and develops a slight negative charge over itself; this results in a slight positive charge over the carbonyl carbon atom. This carbon atom acts as an electrophile and accounts for the nucleophilic addition reaction of carbonyl compounds. In ketones, the positive charge of the carbonyl carbon is stabilised by the adjacent alkyl groups.

Q.3. Which is more electrophilic: aldehyde or ketone?
Ans: Aldehydes are more electrophilic than ketones. This is because the electron-donating nature of alkyl groups stabilises the partial positive charge of carbonyl carbon in ketones.

Q.4. Does benzaldehyde give Cannizzaro?
Ans: Carbonyl compounds that do not contain \(\alpha \)-hydrogens undergo a Cannizzaro reaction. Benzaldehyde has no \(\alpha \)-hydrogens; hence, it will undergo the Cannizzaro reaction.

Q.5. What is needed for Cannizzaro reaction?
Ans: Aldehydes and ketones that do not have an \(\alpha \)-hydrogen atom undergo self oxidation and reduction (disproportionation) reaction on heating with concentrated alkali. In this reaction, one aldehyde molecule is reduced to alcohol while another is oxidised to carboxylic acid salt.