(34) Carbonyl Compounds | Introduction, Structure, Method of Preparation | Organic Chemistry 01
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Carbonyl Compounds
The compounds containing a carbonyl group (the -C=O group) are called carbonyl compounds.
The carbonyl group, C=O, is probably the most important functional group in organic chemistry. These compounds are an integral part of organic chemistry and their primary members are called aldehydes, ketones, and carboxylic acids.
In other words, A carbonyl group is a functional group with the formula C=O that is formed of a carbon atom double-bonded to an oxygen atom and is divalent at the carbon atom.
Structure of Carbonyl Group
The carbonyl carbon ( –C=O) is sp2 hybridised and the carbon–oxygen bond is similar to carbon–carbon double bond in alkenes. The carbonyl carbon forms three σ bonds using their three sp2 hybridised orbital. One of the sigma bonds is formed with oxygen and the other two with hydrogen and carbon (in aldehydes) or with two carbons (in ketones). All the three 'σ' bonded atoms lie on the same plane as shown in image 1.
The fourth valence electron of carbon remains in its unhybridized ‘2p’ orbital which lies perpendicular to the plane and overlaps with the 2p orbital of oxygen to form a carbon–oxygen p bond. The oxygen atom evidently has two nonbonding pairs of electrons, which will occupy its remaining two p-orbitals. Oxygen, the second most electronegative atom, attracts the shared pair of electrons between the carbon and oxygen towards itself. Hence, the bond is polar. This polarisation plays a major role in the reaction of aldehydes and ketones.
Examples of Organic Carbonyl Compounds
These include carbamates, urea, and also the derivatives of phosgene, acyl chlorides chloroformates, carbonate esters, lactones, thioesters, lactams, isocyanates, and hydroxamates.
Aldehydes, Ketones and Carboxylic Acids
When the carbonyl group is linked to one alkyl or aryl group and one hydrogen atom, the resulting carbonyl compound is called an aldehyde. The general formula of an aldehyde is R-CHO. Ketones are carbonyl compounds in which the carbonyl carbon is linked to two alkyl or aryl groups. Their chemical formulae can be generalized to R-(C=O)-R’. When the carbonyl carbon is bonded to one alkyl/aryl group and one OH group, the resulting carbonyl compound can be classified as carboxylic acid and its chemical formula can be generalized to R-COOH.
Preparation of Aldehyde and Ketone
Aldehydes synthesis:
1. Oxidation of 1o alcohols
2. Oxidation of methylaromatics
3. Reduction of acid chlorides
Ketones synthesis:
1. Oxidation of 2o alcohols
2. Friedel-Crafts acylation
3. Coupling of R2CuLi with acid chloride
Properties of Carbonyl Compounds
a. Some properties of carbonyl compounds are given below:
b. These are to be polar in nature. They exhibit both positive and negative charges in slight form. Hence, these are said to be polar molecules.
c. These compounds are reported to be insoluble in water but sometimes they dissolve other forms of polar molecules.
d. These are known to be chemically reactive compounds. These undergo nucleophilic addition and substitution reactions.
Chemical Reactions of Carbonyl Compounds
The carbon atom of the carbonyl group is said to be electrophilic in nature as they tend to attract electron-rich compounds. Aldehydes and ketones undergo a variety of reactions which leads to the formation of different products. The most common reactions are nucleophilic addition reactions, which lead to the formation of alcohols, alkenes, diols and cyanohydrins. These also undergo nucleophilic addition reactions.
Carbonyl Reduction: This reaction is a process in which the carbonyl groups are reduced by the hydride reagents such as the LiAlH4 and NaBH4 with baker’s yeast, or by the process of catalytic hydrogenation.
Carbonyl alkylation: This is a process in which the carbonyl compounds are alkylated with the use of organometallic compounds like Grignard reagents, organolithium reagents, acetylides, etc.
Carbonyl Alpha-Substitution Reaction: This kind of substitution reaction involves the substitution of the atom of α hydrogen by an electrophile.
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