Core Concepts in Ester Functional Groups
In this tutorial you will learn about the basic properties and structure of an ester functional group. You will also learn about esterification and its mechanism.
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What is an Ester Functional Group?
In organic chemistry, esters are a common functional group. The basic structure of an ester consists of a carbon single bonded to carbon, double bonded to oxygen, and single bonded to oxygen. For the molecule below, the simple structure of an ester group is shown.
The basic chemical formula of an ester is R-COOR’. The R groups denote the rest of the molecule not included in the functional group.
What is an Ester Bond?
An ester bond is a linkage between an atom that is double bonded to an oxygen atom bearing any alkyl or aryl group. Ester bonds are found in ester functional groups. In addition, the bond is generally covalent and is essential in the formation of lipids.
These ester bonds are important when it comes to lipids in our bodies. When glycerol (alcohol) combines with fatty acid molecules, it allows for three esters to be produced. This process is what causes ester bonds to form.
Properties of Esters
In comparison to alcohols and carboxylic acids, esters tend to be polar less allowing for them to have lower boiling points. Due to the polar property of esters, they can participate in hydrogen bonding as hydrogen bond acceptors. Their ability to participate in hydrogen bonding depends on the length of the alkyl chains attached to the ester functional group.
Naming Esters
The name of an ester derives from the parent alcohol and parent acid. While simple esters can be named after their common names, all esters can be named using their systematic IUPAC name. The name is based on the acid followed by the suffix “-oate”.
When an ester is formed from a common carboxylic acid, there are substitutions for the ester’s IPUAC name.
- “ethanoate” = “acetate”
- “methanoate” = “formate”
- “propanoate” = “propionate”
- “butanoate” = “butyrate”
Examples of Ester Functional Groups
Esters are common in nature. Esters generally have pleasant odors and are responsible for the characteristic fragrances of fruits and flowers. Flavoring agents and perfumes contain natural and synthetic esters.
Fats and vegetable oils are long-chain fatty acids and glycerol that have esters. In addition, esters of phosphoric acids have the utmost importance to life. Some common examples of esters and their names are in the list below.
- methyl methanoate/formate (HCOOCH3)
- methyl ethanoate/acetate (CH3COOCH3)
- ethyl ethanoate/acetate (CH3COOCH2CH3)
Esterification
What is esterification?
Fischer esterification is the most common reaction to make simple esters and involves mixing a carboxylic acid and an alcohol with a small amount of catalytic acid. The reaction can be driven to completion through refluxing (presence of lots of energy). Excessive heat is essential for esterification since it takes a lot of energy to remove the “-OH” group from a carboxylic acid to generate an ester.
Esterification Mechanism
- Firstly, the esterification reaction begins with the protonation of the carbonyl oxygen on the carboxylic acid. The oxonium ion created becomes stabilized by resonance (the ability to move electrons to another atom).
- Subsequently, the alcohol attacks the carbonyl carbon, pushing the electrons onto oxygen. As a result, there is the formation of a tetrahedral intermediate.
- Following that, the oxonium ion of the tetrahedral intermediate loses a proton to produce a neutral tetrahedral intermediate.
- At either of the hydroxyl groups, protonation occurs and generates a good leaving group (H2O).
- Consequently, water becomes dissociated as a good leaving group as the carbonyl is re-formed.
- Lastly, a proton is shed to yield a neutral ester as the reaction product.
