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8.2: Background

Difficulty Level: At Grade Created by: CK-12
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This is the structure of an ester. The “R” represents the rest of the compound.

Esters are chemical compounds that contain an ester group, denoted as R-COO-R'. It is produced by the condensation (formation of water) of a carboxylic acid and an alcohol. The general chemical equation for an esterification reaction is:

Alcohol + Carboxylic Acid ⇔ Water + Ester

This diagram illustrates what happens during an esterification reaction.

In an esterification reaction, adding a dehydrating agent, such as sulfuric acid, that only removes the water without reacting with the other components will push the reaction more towards the products side. In this case, the products would be water and ester. This would greatly increase the effectiveness of the esterificiation reaction.

An important physical property of many esters is their solubility in both fats and water. Esters are soluble in water because their polar oxygen-carbon bonds are capable of participating in hydrogen bonding. The negatively-charged oxygen in an ester molecule will be attracted to the positively-charged hydrogen in a water molecule. Small esters are extremely soluble in water; however, the larger the ester molecule, the less soluble it is in water. When the ester chains get to be too large, the hydrocarbon parts of the ester start to disrupt the hydrogen bonds amongst the water molecules. It is more energetically favorable for the water molecules to stay bonded together via hydrogen bonding than it is for the water and ester to mix. On the other hand, the hydrocarbon chains make esters soluble in fats. Since the hydrocarbon portions of the ester are nonpolar, these parts are capable of mixing with nonpolar fats.

Two special types of esters are acetates and lactones. Acetates are esters that are formed from acetic acid or vinegar. Lactones are cyclic esters. Many examples of lactones are found in oak trees and other plants and help give flavor to whiskey.

Esters are very popular compounds in both nature and everyday life. They typically have a sweet smelling fragrance that is responsible for giving many fruits their sweet scents. Raspberries, grapes, and apples, among others, get their smells from various esters. In addition to fruits, there are many other familiar scents that are attributed to esters. For example, some nail polish removers get their scents from an ester called ethyl acetate, and rum’s scent is from an ester called ethyl propionate. The cosmetics, fragrance, and food industries all rely heavily on esters for their distinct scents.

Esters are named by the chemicals that they are created from. It is very similar to the process of naming salts. The alkyl group (R’), which is always attached to an oxygen atom, is the first portion of the ester’s two-word name. The second portion is by taking the acid and replacing the acid name with “–oate.”

The name of this ester is pentyl ethanoate.

An example of this would be pentyl ethanoate. The alkyl group that makes up the first portion of the ester's two-word name is pentyl. The acid, which contains a double bonded oxygen, is ethanoic acid. Adding the prefix “-oate” to the acid name gives you the second part of the ester name.

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