Structure and Properties
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Structure and Properties

The functional group for carboxylate esters is the carboxylate group, -COO-. It is similar to a carboxylate ion group except that instead of being ionic, it is covalently bonded to a carbon atom. It is similar to a carboxylic acid group but instead of a hydrogen atom bonded to one of the oxygen atoms, there is another carbon atom.

Structure and hybridization of carboxylate ester group. [64str11.JPG]
This carbon atom has sp2 hybridization. Therefore this carbon atom has three sigma bonds to three atoms lying in a flat plane and forms a pi bond to one of the oxygen atoms. That shows up as a double bond in the structural formula.

 

These esters with a carboxylate group are usually referred to simply as esters. However, there are other kinds of esters, such as phosphate esters, sulfonate esters and others. So be careful, when you hear someone talking about esters; they probably mean carboxylate esters, but they might mean something else.

The carboxylate group is a polar group but it is necessarily surrounded by carbon chains. Thus, ester molecules are only slightly polar with some dipole-dipole bonding between them. Thus, the small ones are generally liquid at room temperature and the very large ones are solids. The solubility in water is fairly typical of oxygen containing organic compounds.

Ball-and-stick model of an ester. [64mod01.JPG]
The small ones are soluble in water and the larger ones are not. Again the brown atom in these space-filling models are carbon atoms in sp2 hybridization.

Space-filling models of large and small esters. [64mod02.JPG]

 

Esters are responsible for many of the aromas and flavors with which we are familiar. In order for a chemical to have a smell, it must be able to reach our noses. Usually molecules reach our noses by vaporizing. For something to continue to have a smell the molecules must vaporize slowly, otherwise the smell will dissipate as quickly as the molecules do. Anyway, the ability of esters to vaporize and interact with sensors in our noses has a lot to do with the way we live our lives.

You will get a better view of the shape of an ester molecule and the location of the carboxylate group within the molecule when you take a look at the model on display in the lab area.

 

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