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Chapter 12: Aldehydes, Ketones, and Carboxylic Acids
In the previous Unit, you studied organic compounds with functional groups containing a carbon-oxygen single bond.
In this Unit, we will study the organic compounds containing a carbon-oxygen double bond (>C=O) called the carbonyl group, which is one of the most important functional groups in organic chemistry.
In aldehydes, the carbonyl group is bonded to carbon and hydrogen while in the ketones, it is bonded to two carbon atoms.
The carbonyl compounds in which the carbonyl group is bonded to oxygen are known as carboxylic acids, and their derivatives (e.g. esters, anhydrides) while compounds, where carbon is attached to nitrogen and to halogens, are called amides and acyl halides respectively.
Methanal is a gas at room temperature. Ethanol is a volatile liquid. Other aldehydes and ketones are liquid or solid at room temperature.
The boiling points of aldehydes and ketones are higher than hydrocarbons and ethers of comparable molecular masses.
It is due to weak molecular association in aldehydes and ketones arising out of the dipole-dipole interactions.
Also, their boiling points are lower than those of alcohols of similar molecular masses due to the absence of intermolecular hydrogen bonding.
The following compounds of molecular masses 58 and 60 are ranked in order of increasing boiling points.
In the chemical industry aldehydes and ketones are used as solvents, starting materials, and reagents for the synthesis of other products.
Formaldehyde is well known as a formalin (40%) solution used to preserve biological specimens and to prepare bakelite (a phenol-formaldehyde resin), urea-formaldehyde glues, and other polymeric products.
Acetaldehyde is used primarily as a starting material in the manufacture of acetic acid, ethyl acetate, vinyl acetate, polymers, and drugs.
Benzaldehyde is used in perfumery and in dye industries. Acetone and ethyl methyl ketone are common industrial solvents.
Many aldehydes and ketones, e.g., butyraldehyde, vanillin, acetophenone, camphor, etc. are well known for their odors and flavors.
Carbon compounds containing a carboxyl functional group, –COOH are called carboxylic acids.
The carboxyl group consists of a carbonyl group attached to a hydroxyl group, hence its name carboxyl. Carboxylic acids may be aliphatic (RCOOH) or aromatic (ArCOOH) depending on the group,
alkyl or aryl, attached to carboxylic carbon.
A large number of carboxylic acids are found in nature. Some higher members of aliphatic carboxylic
acids (C12 – C18) known as fatty acids occur in natural fats as esters of glycerol.
Carboxylic acids serve as starting material for several other important organic compounds such as anhydrides, esters, acid chlorides, amides, etc.
| Author | NCERT |
| Language | English |
| No. of Pages | 32 |
| PDF Size | 1777 KB |
| Category | Chemistry |
| Source/Credits | ncert.nic.in |
NCERT Solutions Class 12 Chemistry Chapter 12 Aldehydes, Ketones, and Carboxylic Acids
Question 12.1: What is meant by the following terms? Give an example of the reaction in
each case.
(i) Cyanohydrin
(ii) Acetal
(iii) Semicarbazone
(iv) Aldol
(v) Hemiacetal
(vi) Oxime
(vii) Ketal
(vii) Imine
(ix) 2,4-DNP-derivative
(x) Schiff’s base
Solution:
(i) Cyanohydrin:
Cyanohydrin is the organic compound with the formula RR’C(OH)CN, where alkyl or aryl groups maybe R and R ‘.
In the presence of excess sodium cyanide (NaCN) as a precursor to using cyanohydrin, aldehydes and ketones react with hydrogen cyanide (HCN). Such reactions are classified as reactions to cyanohydrins.
The useful synthetic intermediates are Cyanohydrins.
(ii) Acetal:
The gem−dialkoxy alkanes at the terminal carbon atom consisting of two groups of alkoxy are called Acetal. One bond is connected to an alkyl group, and the other to a hydrogen atom.
The general structure of an acetal.
Hemiacetals are formed in the presence of dry HCl gas when aldehydes are treated with two monohydric alcohol equivalents. In addition, hemiacetals react with another alcohol molecule to yield acetal.
(iii) Semicabarbazone:
Semicarbazone comes from aldehydes and ketones, which is formed by the condensation reaction between a ketone or aldehyde and semicarbazide.
Semicarbarzones serve to identify and characterize aldehydes and ketones.
(iv) Aldol:
An aldol is a β-hydroxy aldehyde or ketone. It is produced in the presence of a base by the condensation reaction of two molecules of the same or a single molecule, each of two different aldehydes or ketones.
(v) Hemiacetal:
Hemiacetals are α−alkoxyalcohols
The general structure of a hemiacetal.
In the presence of dry HCl gas, aldehyde reacts with one single molecule of monohydrated alcohol.
(vi) Oxime:
General formula – RR′CNOH
Where R is an organic side chain and R′ is either an organic side or hydrogen side chain. If R′ is an organic side chain, then ketoxime is known, and if R′ is H, it is called aldoxime.
Aldehydes or ketones form oximes when treated with hydroxylamine in a weakly acidic medium.
(vii) Ketal:
Ketals are gem−dialkoxyalkanes in which one carbon atom is comprised of two groups of alkoxy within the chain. The other two carbon atom bonds are related to two alkyl groups.
The general structure of a ketal.
A cyclic product is produced in the presence of dry HCl gas, called ethylene glycol ketals, when ketones react with ethylene glycol.
(viii) Imine:
Imines are chemical compounds that have a double bond between carbon-nitrogen.
The reaction of aldehydes and ketones with ammonia or 1º-amines forms imine derivatives
(ix) 2, 4−DNP−derivative:
2, 4−dinitrophenylhydragenes are 2, 4−DNP−derivatives that are produced in a weakly acidic medium when aldehydes or ketones react with 2, 4−dinitrophenylhydrazine.
The 2, 4−DNP derivatives are used to identify and characterize aldehydes and ketones.
(x) Schiff’s base:
Schiff’s base is a chemical compound that contains a double bond between carbon-nitrogen and the nitrogen atom connected to an aryl or alkyl group-but, not hydrogen. They have the general R1R2C formula= NR3. Thus, it’s an imine. It’s named for Hugo Schiff, a scientist.
Aldehydes and ketones in the presence of a residue of acid on treatment with primary aliphatic or aromatic amines yield a base of a Schiff.
Aldehydes, Ketones, And Carboxylic Acids NCERT With Solution PDF Free Download