An ester functional group contains a carbonyl group with a second oxygen atom single bonded to the carbonyl carbon and also single bonded to another carbon atom. A general ester structure has an R group bonded to the carbonyl carbon atom and another R group bonded to the second oxygen. It is a functional group that is found in the middle of a molecule.
The ester functional group’s carbon atom is sp2 hybridized with a trigonal planar local geometry. Its carbonyl oxygen is sp hybridized, and one of its unhybridized 2p atomic orbitals forms the π bond with the carbon’s unhybridized 2p atomic orbital. This oxygen also has two lone pairs: one occupies a sp hybrid orbital; the other occupies a 2p atomic orbital that is perpendicular to the π bond. The second oxygen (non-carbonyl oxygen) is sp2 hybridized and has a bent local geometry. It also has two lone pairs, one in a sp2 hybrid orbital, the other in the unhybridized 2p atomic orbital.
As shown in the preceding figure, the 2p lone pair on the non-carbonyl O is aligned parallel to the 2p orbitals that form the π bond. This leads to some delocalization of the lone pair electron densities, which can be expressed by resonance structures:
While the resonance structure on the right makes only a fairly minor contribution to the description of the ester molecule, that structure is helpful in understanding the ester’s chemical and physical properties. For example, the -COO- ester group is planar, and the non-carbonyl C-O bond is not as freely rotatable as a typical single bond. Moreover, an ester’s reactivity is quite different from that of a ketone or an ether, and hence an ester is a distinct functional group.
The odors of ripe bananas and many other fruits are due to the presence of esters.