 |
Introduction |
| |
This chapter contains oxygen compounds in which oxygen is bonded to a carbon atom through a double bond. The functional unit >C = O, present in these compounds is called carbonyl group. We limit our study now to compounds having an acyl group (R - C = O) is bonded to hydrogen, carbon, oxygen, halogens and nitrogen. These compounds are known as carbonyl compounds and are grouped into families of aldehydes, ketones, carboxylic acids and their derivatives. |
|
Nomenclature of Aldehydes and Ketones |
| |
The IUPAC names of open chain aliphatic aldehydes and ketones are derived from the names of the corresponding alkanes by replacing the ending -e with -al and -one respectively. |
|
Structure of the Carbonyl Group |
| |
The aldehydes and ketones have carbonyl group as the functional group. The carbonyl carbon sp2 hybridized and thus has three sp2 hybrid orbitals and one unhybridized p-orbital. It uses sp2 hybrid orbitals to form three s-bonds one with oxygen atom and remaining two with two other atoms or groups (R or H). All these three sigma bonds lie in the same plane at an angle of 120o. |
|
Isomerism in Aldehydes and Ketones |
| |
Aldehydes with 4 or more carbon atoms and ketones with five or more carbon atoms show chain isomerism.
Examples: C4H8O has the isomers.
 |
|
Physical Properties |
| |
Methanol is a gas at room temperature. Ethanol is a volatile liquid 294K. Other aldehyde and ketones are liquids or solids at room temperature. Lower aldehydes have sharp pungent odors. As the size of the molecule increases, the odor becomes less pungent and more fragrant. |
|
General Methods of Preparation from Alcohol and Acid Chlorides |
| |
Aldehydes and ketones are generally prepared by oxidation of primary and secondary alcohols respectively. Common oxidising agents are KMnO4, K2Cr2O7 and CrO3. Strong oxidising agents oxidise the aldehyde produced by the oxidation of a primary alcohol to carboxylic acid. |
|
General Methods of Preparation from Hydrocarbons and Nitriles |
| |
Alkenes react with ozone to form ozonides which on subsequent reductive cleavage with zinc dust and water or H2/Pd give aldehydes, Ketones or a mixture of both depending on the substitution pattern of the alkene.
 |
|
Chemical Reactions - Nucleophilic Addition Reactions |
| |
Aldehydes and Ketones are highly reactive compounds. They undergo similar reactions because of the presence of a carbonyl functional group in both of them. |
|
Reduction Reactions of Aldehydes and Ketones |
| |
Aldehydes and ketones can be reduced to a variety of compounds under different conditions with different reducing agents. |
|
Oxidation Reactions of Aldehydes and Ketones |
| |
Aldehydes differ from ketones in their oxidation reactions. Aldehydes are easily oxdised to carboxylic acids on treatment with common oxidising agents like nitric acid, potassium permanganate, potassium dichromate etc. |
|
Reactions of Aldehydes and Ketones Involving a - Hydrogen |
| |
Two molecules of an aldehyde or a ketone having at least one a-hydrogen atom condense in the presence of a dilute acid to give a b-hydroxyaldehyde or b-hydroxy - ketone. |
|
Cannizaro Reaction |
| |
Aldehydes which do not have an a-hydrogen atom undergo self oxidation and reduction (i.e., disproportionation) reaction on treatment with concentrated alkali. In this reaction, one molecule of the aldehyde is reduced to alcohol and another is oxidised to carboxylic acid salt. |
|
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. |
|
Some Commercially Important Aldehydes and Ketones |
| |
It is manufactured by dehydrogenation or air oxidation of methanol in the presence of heated metallic copper or silver catalyst or by oxidation of methane with air in the presence of various metallic oxide catalysts. |
|
Carboxylic Acids and its Derivatives - Introduction |
| |
Carbon compounds containing a carboxyl functional group -COOH are called carboxylic acids. A carboxyl group is constituted of two groups - a carbonyl group
 and a hydroxyl group -OH. Carboxylic acids may be aliphatic (R-COOH) or aromatic (Ar-COOH) depending on whether -COOH group is attached to aliphatic alkyl chain or aryl groups respectively. |
|
Nomenclature |
| |
Since carboxylic acids are amongst the earliest organic compounds to be isolated from nature, a large number of them are known by their common names. The common names end with the suffix-ic acid and have been derived from Latin or Greek names of their natural sources. For e.g., formic acid (HCOOH) was first obtained from red ants (formica means ant), acetic and (CH3COOH) from vinegar (acetum means vinegar) butyric acid (CH3CH2CH2COOH) from rancid butter (butyrum means butter) caproic acid (CH3CH2CH2CH2COOH) from goats (caper means goat). The positions of the substitutes are indicated by Greek letters a, b, g, d etc the a-carbon being the one directly attached to the carboxyl group, b - the next and so on. |
|
Structure of the Carboxyl Group |
| |
Electron and neutron diffraction studies show that carboxyl group has a planar structure. Thus the carboxylic carbon and the two oxygen atoms in carboxylic group are sp2 hybridized. The carboxylic carbon forms are (sp2 - sp2) s - bond with two oxygen atoms. One the oxygen atoms form (sp2 - s) s - bond with a hydrogen atom. One half filled unhybridized p-orbital of the carbon and the unhybridized p-orbital of the oxygen atom undergo sideways overlap. This results in the formation of the delocalized p-electron cloud. This is confirmed by the C-O single bond length in formic acid being shorter than the C-O bond length in ethanol. |
|
Physical Properties |
| |
The first three aliphatic acids are colorless liquids with pungent smell. The next six are oily liquids with an odor of rancid butter. The higher members are colorless, odorless, waxy solids. Benzoic acid is a crystalline solid. |
|
Methods of Preparation of Carboxylic Acids |
| |
Primary alcohols are readily oxidised to carboxylic acids with common oxidising agents such as potassium permanganate (KMnO4) in neutral, acidic or alkaline media. |
|
Reaction of Carboxylic Acids - Acidity |
| |
Since carboxyl group consists of carbonyl and hydroxyl groups, it is expected that acids show reactions due to both these groups. However the reactions expected of these groups are modified as a result of interaction between these groups due to close proximity. |
|
Reaction of Carboxylic Acids - Reduction |
| |
Carboxylic acids are reduced to primary alcohols on treatment with lithium aluminium hydride or better with diborane. Here -COOH group is reduced to -CH2OH. |
|
Reactions of Carboxylic Acids - Substitutions |
| |
Carboxylic acids having an a-hydrogen are halogenated at the a-position on treatment with chlorine or bromine in the presence of small amount of red phosphorus to give a-halocarboxylic acids. This reaction is known as Hell - Volhard Zelinsky reaction. |
|
Some Commercially Important Carboxylic Acids |
| |
It occurs in a variety of biting and stingins plants and insects. It is manufactured by the reaction of carbomonoxide with sodium hydroxide under pressure at 473 K. Sodium methanoate is obtained which is acidified and distilled to give the acid. |
|
Functional Derivatives Of Carboxylic Acids - Introduction |
| |
Replacement of hydroxyl group in carboxylic acids with a halogen, carboxylate, alkoxy or amino group gives functional derivatives of carboxylic acid known as acyl halides, acid anhydrides, esters or amides respectively. |
|
Acid Chlorides |
| |
These are derivatives of carboxylic acids in which hydroxyl (-OH) part of carboxyl group in which hydroxyl (-OH) part of carboxyl group is replaced by halo group. The most reactive compounds among acyl halides are chloro compounds. |
|
Amides |
| |
Acid amides may be regarded as the derivatives of carboxylic acids in which -OH part of the carboxylic group is replaced by the -NH2 group. |
|
Nomenclature of Acyl Halides |
| |
Acyl halides (sometimes also called acid halides) are named by identifying the acyl group (RCO) and then the halide. The name of the acyl group is derived from the name of the corresponding -ic acid by replacing the ending -ic acid by -yl or carboxylic acid by -carbonyl. |
|
Acid Anhydrides |
| |
Acid anhydrides are called symmetrical when the two acyl groups are identical when the two acyl groups are identical and if the two acyl groups are different it is said to be unsymmetrical. |
|
Esters - Introduction |
| |
RCOOR' are named after the corresponding carboxylic acids by replacing the ending -ic acid with -ate and preceding this with the name of the alkyl or aryl group attached to the oxygen atom. |
|
Structures of Functional Groups Present in Carboxylic Acid Derivatives |
| |
The structure of the functional groups in acyl halide, acid anhydride, ester and amide are similar to that of the carboxyl group. Due to the presence of lone pairs of electrons at the halogen, oxygen and nitrogen atoms, resonance is possible in these derivatives just like that in carboxylic acids. |
|
Physical Properties |
| |
Being polar in nature, the acid derivatives have higher boiling points than hydrocarbons of comparable molecular masses. Acid chlorides, anhydrides and esters have nearly the same boiling points as the aldehydes and ketones of comparable molecular masses. Their boiling points are lower than that of carboxylic acids of comparable molecular masses, due to the absence of hydrogen bonding in acid derivatives. |
|
Reactions and Comparative Reactivity of Acid Derivatives |
| |
The reactions of carboxylic acids and their derivatives involve substitution of the group L with nucleophiles and are known as nucleophilic acyl substitution reaction. |
|
Acyl Halides |
| |
The most important acyl halides are acyl chlorides because they are more easily prepared, more stable and less expensive. |
|
Reduction |
| |
Acyl halides are reduced to primary alcohols with LiAlH4 and NaBH4.
 |
|
Acid Anhydrides |
| |
Acid anhydrides are considered to be derived from carboxylic acids by the removal of a molecule of water from two molecules of acid. So acid anhydrides can be prepared by heating carboxylic acid in the presence of P2O5 in dehydrating agent. |
|
Esters - Preparation |
| |
Esters are prepared by the acylation of alcohols or phenols.
 |
|
Amides - Preparation |
| |
Amides are generally prepared by the reaction of acyl chlorides or anhydrides with ammonia or amines.
 |
|
Amides - Reactions |
| |
Amides are feeble bases. The lone pair of electrons on the nitrogen atom is responsible for the basic character. This lone pair of electrons on nitrogen atom is involved in resonance with the carbonyl group (structure II). Thus the electron pair of nitrogen is not easily available for protonation. Consequently the basic character is considerably decreased. |
|
Summary |
| |
Aldehydes, ketones, carboxylic acids and their functional derivatives (aryl halides, acid anhydrides, esters and amides) contain a carbonyl group and are highly polar molecules. |
