Reaction of Aldehydes and Ketones

Imagine walking into your favorite bakery. Smell the aromas of freshly baked goods wafting in the air. You are hit by the sweet, comfortable smells of vanilla, cinnamon, and melting butter that appeal to you, that make your mouth water. Ever thought what chemical reactions can be attributed to these mouth-watering odors? Much of it deals with aldehydes and ketones, two broad categories of organic compounds critical in the flavors and fragrances industry.

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This Story also Contains

1. Key Concept and Definitions
2. Reduction and Oxidation Reactions
3. Intermolecular and Intramolecular Aldol Condensation
4. Intermolecular and Intramolecular Cannizzaro Reaction
5. Relevance and Applications
6. Some Solved Examples
7. Summary

The case of vanillin, a major constituent of vanilla extract and an aldehyde by nature is illustrative in this context. Of equal importance is the fact that diacetyl belongs to the class of ketones and happens to be responsible for the smell of butter. The types of such compounds also play a great role in the pharmaceutical and chemical industries. These are the building blocks for a vast array of products: perfumes, plastics, medications, and dyes. In this paper, we will discuss aldehydes and ketones with their various reactions.

Key Concept and Definitions

Aldehydes and ketones are organically simply examples of classes of organic compounds characterized by the presence of a carbonyl group, C=O. They simply differ in how this carbonyl group is placed. The carbonyl is connected to at least one hydrogen atom and one carbon atom; thus, aldehydes are highly reactive. By contrast, the carbonyl in ketones is attached to two carbon atoms, which normally makes them less reactive than aldehydes. These, together with other compounds, make up the backbone of organic chemistry and take part in numerous reactions that form a basis for synthesizing hosts of complex molecules. Aldehydes and ketones can be divided into some primary chemistry—nucleophilic addition reactions, reduction and oxidation reactions, and some condensation reactions. In nucleophilic addition, the nucleophile is an electron-rich species that initiates an attack on the electrophilic carbon in the carbonyl group. Special interest in this reaction exists for aldehydes and ketones because of the polarization of the carbonyl group. One of the most typical examples is the addition of hydrogen cyanide to form cyanohydrins. Those are the intermediates during the synthesis of amino acids.

(i) Mechanism of nucleophilic addition reactions:

A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of sp² hybridized orbitals of carbonyl carbon. The hybridization of carbon changes from $s p^2$ to $s p^3$in this process and a tetrahedral alkoxide intermediate is produced. This intermediate captures a proton from the reaction medium to give the electrically neutral product. The net result is the addition of $\mathrm{Nu}^{-}$and $\mathrm{H}^{+}$ across the carbon-oxygen double bond as shown in the figure below.

(ii) Reactivity

Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two relatively large substituents in ketones hinders the approach of nucleophiles to carbonyl carbon than in aldehydes having only one such substituent. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl carbon more effectively than in former.

Reduction and Oxidation Reactions

Aldehydes and ketones are reduced into primary and secondary alcohols respectively. Some of the possible reagents for doing this are:$\mathrm{NaBH}_4, \mathrm{LiAlH}_4$, etc. The other way they differ is by their oxidation; that is, aldehydes can be oxidized to carboxylic acids with mild oxidizing agents but ketones are often resistant to oxidation because there is no hydrogen attached directly to the carbonyl carbon.

Reduction to hydrocarbons:

The carbonyl group of aldehydes and ketones is reduced to the CH₂ group on treatment with zinc amalgam and concentrated hydrochloric acid (Clemmensen reduction) hydrazine hydrazone or with hydrazine followed by heating with sodium or potassium hydroxide in a high boiling solvent such as ethylene glycol (Wolff-Kishner reduction).

Oxidation

Aldehydes differ from ketones in their oxidation reactions. Aldehydes are easily oxidized to carboxylic acids on treatment with common oxidizing agents like nitric acid, potassium permanganate, potassium dichromate, etc. Even mild oxidizing agents, mainly Tollens’ reagent and Fehlings’ reagent also oxidize aldehydes.

Intermolecular and Intramolecular Aldol Condensation

It is an important reaction involving two aldehydes or ketones in the presence of a base, whereby the reaction first proceeds via a β-hydroxy aldehyde or ketone and then undergoes subsequent dehydration to form α,β-unsaturated carbonyl compounds. Aldol condensations can be divided into two, namely, intermolecular aldol condensation between two molecules of the same or different compounds and intramolecular aldol condensation inside one molecule, whereby the process leads to ring formation. The next step ensues.

It is the condensation taking place when two different aldehydes or two different ketones or one aldehyde and one molecule of ketone both contain -H atoms. A number of products due to self-condensation and cross-condensation are obtained. The reaction occurs as follows.

For example,

Intermolecular and Intramolecular Cannizzaro Reaction

The base-induced disproportionation of aldehydes not α-enolizable in the Cannizzaro reaction follows the equation below. It involves the formation of alcohol plus carboxylic acid. In the intermolecular version, two aldehyde molecules are involved. The intramolecular Cannizzaro reaction takes place in a single molecule and often forms interesting cyclic structures.

The aldehydes and ketones undergo several reactions due to the acidic nature of $\alpha$-H, which in turn is due to the strong electron-withdrawing effect of the (C=O) group and resonance stabilization of the conjugate base. When two molecules of the same aldehyde or ketone containing $\alpha$-H atom condense together in the presence of dilute alkalies, such as $\mathrm{NaOH}, \mathrm{KOH}, \mathrm{K}_2 \mathrm{CO}_3, \mathrm{Na}_2 \mathrm{CO}_3$, or at least $2 \mathrm{H}-$-atoms to give a molecule of aldol or ketol, it is called aldol condensation. On heating, it loses a molecule of $\mathrm{H}_2 \mathrm{O}$ to give a molecule of $\alpha_2 \beta$-unsaturated aldehyde or ketone.

For example,

When two different aldehydes lacking α-H atom are reacted in the presence of a strong base, they undergo disproportionation or redox reaction to give a molecule of alcohol and salt of an acid. Alcohol is obtained from the less reactive aldehyde and acid salt is obtained from the more reactive aldehyde. In this reaction, OH^- attacks at the C of (C=O) group of more reactive aldehyde and gives an adduct anion from which H^- ion is transferred to the less reactive aldehyde. It gives acid ions from more reactive aldehyde and alcohol from less reactive aldehyde.

For example,

Two molecules of the same aldehyde lacking α-H atom undergo disproportionation or redox reaction in the presence of a strong base to give a molecule of alcohol and a molecule of the salt of an acid.

For example,

Relevance and Applications

The chemistry of aldehydes and ketones doesn't need to remain bench-bound; on the contrary, it has far wider applications in the real world. One such example is nucleophilic addition reactions, which find an application in drug manufacture for a host of different pharmaceuticals, from antibiotics to those used against depression. Many aldehydes and ketones are in demand in the fragrance industry because each of the huge ranges possesses unique smells. Aldol condensations are greatly useful in fine chemical syntheses and the production of polymers and food additives. While itself a rare process in industrial application, still finds its use in the synthesis of individual alcohols and acids applied in organic synthesis the Cannizzaro reaction. These, however, act as stepping stones to advanced studies in the domains of organic chemistry, catalysis, and materials science and open up avenues for research in the development of new synthetic pathways and environmentally friendly chemical processes.

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Some Solved Examples

Example 1The increasing order of the rate of HCN addition to compound A-D is

A. HCHO
B. $\mathrm{CH}_3 \mathrm{COCH}_3$
C. $\mathrm{PhCOCH}_3$
D. PhCOPh

1)A<B<C<D

2)D<B<C<A

3) D<C<B<A

4)C<D<B<A

Solution:

As we learned in nucleophilic addition at Acyl carbon,

Aldehydes are more reactive than ketones due to steric hindrance and the +I effect of an alkyl group.

Also, benzylic aldehydes or ketones are less reactive than their non-benzylic counterparts.

Thus the rate of addition of HCN is given as

D<C<B<A

Hence, the answer is the option (3).

Example 2 Which of the following derivatives of alcohols is unstable in an aqueous base?

1)

2)

3)

4)

Solution:

Hydrolysis of ester occurs in a basic medium due to it is unstable in an aqueous base.

Therefore, the correct option is (1).

Example 3 The increasing order of the reactivity of the following compounds in nucleophilic addition reaction is :

Propanal, Benzaldehyde, Propanone, Butanone

1)Benzaldehyde < Butanone < Propanone < Propanal

2) Butanone < Propanone < Benzaldehyde < Propanal

3)Propanal < Propanone < Butanone < Benzaldehyde

4)Benzaldehyde < Propanal < Propanone < Butanone

Solution:

Rate of Nucleophilic Addition-

1. Aldehyde > Ketone

2. Aliphatic aldehyde > Aromatic aldehyde

Order : Butanone < Propanone < Benzaldehyde < Propanal

Summary

Aldehydes and ketones are very important in everyday life and advanced scientific research. Their reactions include nucleophilic addition, reduction and oxidation, aldol condensation, and Cannizzaro reaction. These are the roots of many synthetic routes to whole arrays of chemicals, from pharmaceuticals and perfumes to plastics, and even food additives. During the course of their study, new methodologies for the synthesis of such complex molecules will be developed in an efficient and sustainable way.