Aldehydes and Ketones

Introduction

The organic compounds having C=O bonds are known as the carbonyl group. Aldehydes and ketones are the subclasses of this group. In aldehydes, the carbonyl group is attached to either two oxygen atoms or one hydrogen atom and one carbon-containing group. In ketones, the carbonyl group is attached to two carbon-containing groups, which may be the same or different aryl or an alkyl group. 

Structure of Aldehydes and Ketones

The C=O bonds in both aldehydes and ketones consist of both the sigma as well as pi bonds. As per the molecular orbital theory, one of the Sp2 hybridised orbitals of the carbon overlap with the p orbital of the oxygen atoms, forming the C-O Sp2-p sigma bonds. The remaining two sp2 orbitals of carbon atoms form an additional sigma bond by either overlapping with the 1st orbital or with the two H atoms, as seen in the case of the Formaldehyde. 

Physical Properties

1. Both of them are polar in nature due to the presence of C=O bonds. 
2. Despite having a polar nature, they do not form H, as they do not contain any H atom directly attached to the O atom. 
3. The lower members of both groups are soluble in water. 
4. All the aldehydes and the ketones are fairly soluble in organic solvents like benzene, methanol, chloroform, etc. 

Methods of Preparation

Preparation of Aldehydes: 

1. From Alcohols: 

Oxidising agents: 

Oxidation of primary and secondary alcohols through oxidising agents like CrO3, K2CR2O7, etc. gives either aldehydes or ketones.

RC=OH → RHCOH 

2. From Dehydration of Alcohol: 

This approach is suitable for volatile alcohols and has a wide range of applications in the industry. Alcohol vapours are passed through heavy metal catalysts (Ag or Cu) in this approach. Aldehydes and ketones are produced by primary and secondary alcohols, respectively. 

3. From Hydrocarbons: 

Ozonolysis: 

➢ Hydration of Alkenes: 

Additions of water to Ethyne in the presence of H2SO4 and HgSO4 give acetaldehyde. This reaction is popularly known as the hydroboration oxidation mechanism. 

RR’=R’’ → RR’R’’OH 

4. Acyl Chloride: 

On reaction with H2, Pd- BaSO4, the acyl chloride is converted into corresponding Aldehydes. The reaction is popularly known as the Rosenmund reaction. 

RC=OCl → RCHO 

5. From Nitriles and Esters: 

Nitriles and Esters are reduced to the corresponding imine, stannous chloride, etc. in the presence of reagents like hydrochloric acid. Later on, through hydrolysis, it is converted into aldehydes. 

This is popularly known as the Stephens reaction. 

RCN++HCl SnCl2→ RCH ≡NH → RCHO 

Preparation of Ketones: 

1. Acyl Chloride: 

In this process, dialkyl cadmium is prepared first by the reaction of the Grignard reagent and CdCl2. Then, this is made to react with the acyl chloride. This gives aldehyde as one of the major products.

2. From Nitriles: 

Nitrile ethers on treatment with the Grignard reagent and is followed by hydrolysis, giving the ketone as a major product. 

RC≡ N+ PMgX → RC=OP 

3. From Benzene or Substituted Benzenes: 

Benzene and its substitutes on reaction with acid chlorides in the presence of anhydrous AlCl3  give the corresponding ketone group. The reaction is also known as the Friedel craft reaction. 

Different Types of Chemical Reactions

As both the aldehydes and ketones have a very similar group, they usually show very similar chemical and physical properties. 

• A nucleophile, such as CN-, can attack the carbonyl carbon and exploit its bond pair to form a new carbon – nucleophile’ link, while two electrons are transferred to the most electronegative oxygen atom. 
• An alkoxide ion is formed as a result of this. Carbon hybridization shifts from sp2 to sp3 hybridised orbital state. 

➢ Reaction with Grignard reagent: 

Reacting a Grignard reagent with an aldehyde gives secondary alcohol as the major product. 

➢ Reduction reactions: 

The aldehydes and ketones through various reducing agents are converted to either the corresponding alcohol or the hydrocarbon, depending upon the strength of the reducing agent and whether it is a controlled or uncontrolled reaction. 

Hydride reagents are the most effective at reducing aldehydes and ketones. 

LiAlH4 and NaBH4 are reducing agents that provide 4 x H- (hydride ion).

To make H-C-O-H, two H atoms are inserted across the C=O. 

Alcohols are formed when hydride combines with the carbonyl group, C=O, in aldehydes or ketones. 

The type of resulting alcohol is determined by the substituents on the carbonyl. 

Methanal (formaldehyde) is reduced to methanol. 

• Primary alcohols are formed by reducing aldehydes (both are present as a terminating group) 
• Secondary alcohols are formed when ketones are reduced. 

The acidic work-up uses a simple acid-base reaction to transform an intermediate metal alkoxide salt into the required alcohol. 

➢ Oxidation reaction: 

The aldehydes and ketones on reaction with corresponding oxidising agents usually give carboxylic acid. The aldehydes are easily oxidised through nitric acid, potassium permanganate, etc., whereas the ketones need very powerful oxidising agents. 

Tollen’s Reagent: 

The procedure of the Tollens test is as follows: 

Step 1: Combine aqueous silver nitrate and aqueous sodium hydroxide in a mixing bowl. 

AgNO3+NaOH2AgOH→AgOH+NaHO3→Ag2O+H2O

Step 2: Add aqueous ammonia drop by drop until the precipitated silver oxide dissolves completely. 

Ag2O+4NH3+H2O→2Ag (NH3) +2OH−

In the reaction, the silver ion is reduced to metallic silver. This gives the bright mirror-like silver appearance on the test tube. 

Fehling Solution Test

What is the Fehling solution?

A solution is used in Fehling’s test, which is normally made fresh in laboratories. Initially, the solution is in the form of two independent solutions, Fehling’s A and Fehling’s B, which are labelled as such. 

➢ Fehling’s A contains copper (II) sulphate solution.  

➢ Fehling’s B is a clear liquid containing potassium sodium tartrate and a strong alkali. Potassium sodium tartrate is also known as Rochelle salt. 

During the test, solutions A and B are each made and stored separately. 

Procedure

The operation can be carried out in the following manner: 

• Fill a dry and cleaned test tube with the given sample. 
• Some distilled water should be kept in a separate tube. 
• The content of the test tube is now to be added to Fehling’s solution. 
• The tubes must be kept submerged in water. 
• Make observations and keep track of any red precipitation that develops. 

Notably, the result is affirmative if a reddish-brown precipitate forms, while the result is negative if there is no evidence of such a change. 

4. Reaction Due to Alpha Hydrogen: 

➢ Aldol condensation: 

Aldol (as the name suggests, aldehyde + alcohol) reaction — In the presence of NaOH or KOH, the carbonyl group reacts with another Carbonyl group to generate -hydroxy aldehyde or ketone. 

The format of the reaction that takes place is as follows: 

R2 C=O +R2C=O → RC=C=OR 

➢ Cannizzaro reaction: This is a type of disproportionation reaction. It involves the reaction of two molecules of aldehydes or ketones which do not have any kind of alpha H atoms. For example: 

2C6H5CHO + KOH → C6H5CH2OH + C6H5COOK

5. Electrophilic substitution reaction: 

The aromatic aldehydes and ketones undergo substitution reactions like sulphonation, nitration, halogenation, etc. As the aldehydic group is meta directing as well as electron-withdrawing, they are a deactivating group. 

RCHO → NO2-R-CHO

Uses of Aldehydes

• Formaldehyde is a flammable gas. Formalin is made from a 40 percent solution in water and is used to preserve biological materials. 
• Embalming, tanning, preparing glues and polymeric materials, as well as germicides, insecticides, and fungicides for plants, all require formaldehyde. It’s also used in photography and drug testing. 
• When formaldehyde reacts with phenol, Bakelite is formed, which is utilised in plastics, coatings, and adhesives. 
• Acetaldehyde is primarily used to make acetic acid and pyridine derivatives. 
• Perfumes, cosmetics, and colours all include benzaldehyde. It’s used in cuisine to give it an almond flavour, and it’s also a bee repellent.
  

Uses of Ketones:

1. The main characteristic of Ketones, which makes them very important in Industrial and Laboratory usage, is that they are excellent solvents.
2. They find a wide range of applications in textile industries, manufacturing plastics and varnishes, paint removers, etc.
3. Some complex compounds of Ketones are used in manufacturing explosives, tannings, etc.

Conclusion 

The class of organic compounds which have C=O bonds is known as carbonyl compounds. Both the groups have pi as well as sigma bonds in their structure. Both of them are polar in nature due to the presence of the C=O bond. Chemically, the aldehydes are more reactive compared to the ketones. Both of them can be prepared by oxidation of alcohols (K2Cr2O7, CrO3, etc.), dehydration of alcohols, ozonolysis, the Rosenmund reaction, etc. They mostly show nucleophilic addition, substitution, reduction reactions, etc. While the carbonyl group having alpha H atoms gives Aldol condensation, the groups having no alpha H atoms give Cannizzaro reaction.