Ester Group: Structure & Overview

Instructor: Felicia Fullilove
In this lesson, you will learn about the structure of an ester. You will also learn about some common uses for esters in chemistry and your everyday life.

What Is an Ester?

An ester is a common organic functional group. In organic chemistry, functional groups are defined as a specific arrangement of atoms within a molecule. You can think of functional groups as chemical identifiers; just as people with brown hair can be described as brunettes, molecules can be described by their functional groups. An ester functional group has a carbon that is double bonded to an oxygen (a carbonyl) and single bonded to an O-R (an alkoxy). Here, O represents oxygen and R represents an alkyl or aryl group.


Ester Structure

Esters come in all shapes and sizes. They can be as small as allyl hexanoate (pineapple odor) and as large as a long-chain triglyceride, such as soybean oil. Small, low molecular weight esters are commonly known for their sweet odor and flavor. Chemists typically use them for the production of perfumes and soaps. Large, high molecular weight esters are typically known for their insolubility in water and are oily, like fish and vegetable oil.

Ester Examples

While esters may be small or large, the atoms directly connected to the central carbon, the carbonyl carbon, sit in the same plane, 120° degrees away from each other. Because of this arrangement, this portion of an ester molecule is flat.

Ester Geometry
Ester Geometry

As you may have noticed, the oxygen of the carbonyl and the alkoxy group have unshared electrons. These unshared electrons contribute to the structure of the ester. The unshared electrons on the alkoxy can interact with the carbonyl carbon resulting in electrons being kicked up to the carbonyl oxygen. This type of interaction is known as electron delocalization. This is represented by a resonance structure.

Resonance Structures of an Ester

A resonance structure is two or more forms of a molecule that have the same connectivity but different electron distribution. In our case, both structures represent an ester. The ester, in turn, balances between both structures. These two structures influence the stability and reactivity of an ester.

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