Structure of carboxyl group:
The carboxyl group represent a planar arrangement of atoms. In – COOH group, the
centre carbon atom and both the oxygen atoms are in sp2 hybridisation. The three sp2 hybrid orbitals of the carbon atom overlap.
The two sp2 – hybridised orbitals of the carboxyl carbon overlap with one sp2 hybridised orbital of each oxygen atom while the third sp2 hybridised orbital of carbon overlaps with either a s -orbital of H – atom or a sp2 – hybridised orbital of C –atom of the alkyl group to form three s- bonds. Each of the two oxygen atoms and the carbon atom are left with one unhybridised p – orbital which is perpendicular to the s- bonding skeleton.
All these three p – orbitals being parallel overlap to form a π- bond which is partly delocalized between carbon and oxygen atom on one side, and carbon and oxygen of the OH group on the other side. In other words, RCOOH may be represented as a resonance hybrid of the following two canonical structures.
R - C
O :
O - H
R - C
O :
O - H
The carboxylic carbon is less electrophilic than carbonyl carbon because of the possible resonance structure. i.e., delocalisation of lone pair electrons from the oxygen in hydroxyl group.
XII U12-Carbonyl Compounds.indd 169 2/19/2020 5:15:14 PM
| Malonic acidHO O C-CH -CO OHPropanedioic acid2 | – | prop | ane | 1, 3 – dioicacid |
|---|---|---|---|---|
| Succinic acidHO O C-(CH ) -CO OHButanedioic acid2 2 | – | but | ane | 1, 4 – dioicacid |
| Glutaric acidHO O C-(CH ) -CO OHPentanedioic acid2 3 | – | pent | ane | 1,5 –dioicacid |
| Adipic acidHO O C-(CH ) -CO OHHexanedioic acid2 4 | – | hex | ane | 1,6 – dioicacid |
170