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Ester Group Structure & Overview

Maram Ghadban, Felicia Fullilove
  • Author
    Maram Ghadban

    A freelance tutor equipped with a bachelor's of science in chemical engineering. Graduated from the American University of the Middle East with a GPA of 3.87, performed a number of scientific primary and secondary research. Tutored university level students in various courses in chemical engineering, math, and art. Has experience tutoring middle school and high school level students in science courses.

  • Instructor
    Felicia Fullilove
Learn about esters and their various physical properties. Understand the role of esters as a functional group. Learn about the structural formula of esters. Updated: 01/02/2022

What is an Ester?

Esters are organic chemical compounds whose primary structure is made of carbons and hydrogens. These compounds are produced through the reaction of molecules that contain the hydroxyl group, -OH, with molecules that contain the carboxyl group, -COOH. The hydroxyl group is a functional group that is made of an oxygen atom that is sharing a single bond with a hydrogen atom. The carboxyl group is also a functional group, it is comprised of two functional groups; a hydroxyl group, -OH, and a carbonyl group, -C=O. The carboxyl, -COOH, is made of one carbon that is sharing a double bond with an oxygen atom, and a single bond with the hydroxyl's oxygen.

The orientation and the atom arrangements of esters are presented in figure 1. Figure 1 shows the general structure for esters, which involves a carbonyl, -C=O, bonding to a side chain, R, and an alkoxy group, -OR. The side chains, R, are carbon-based groups; they can be any carbon imitating groups, like alkyls and aryls. The alkoxy group is simply an alkyl group bonding to an oxygen atom.


Figure 1: Esters are contain a carbonyl, a carbon-based side chain and an alkoxy group

Esters general structure


To summarize, an ester molecule is made of the following parts:

  • A carbonyl group, -C=O.
  • A side chain, R.
  • An alkoxy group, -OR.

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.

Properties of Esters

Esters are polar molecules; their polarity surpasses that of ethers. Polarity is the consequence of the uneven electron distribution throughout the atom, mainly due to the discrepancies between the electronegativity of the constituting atoms.

Reviewing the image in figure 1 shows that esters have two hydrogen bonds acceptors, which are electronegative atoms that are capable of forming hydrogen bonds with neighboring hydrogen containing molecules. The hydrogen bonds acceptors in esters, are the oxygen atoms in the carbonyl and in the alkoxy groups. Esters are capable of forming hydrogen bonds with other compounds that contain hydrogens, but they cannot form hydrogen bonds with themselves; they do not contain any hydrogen donors.

Esters limitation in forming hydrogen bonds is one of the main reasons why the boiling points of these compounds are intermediate.

The solubility of esters varies; whether or not esters are soluble in water, is dependent on the length of the hydrophobic parts. Large esters, with high molecular weights, are not water miscible; their large hydrophobic parts cannot be dissolved in water. The main reason why some esters, small ester compounds, are water miscible is because of the esters' ability to form hydrogen bonds with the water molecules.

Esters are volatile organic molecules; they have the tendency to evaporate easily. Esters are colorless compounds that have interesting odors. Small ester molecules have pleasant fruity scents, and sweet flavor. They are often used in making soaps and perfumes. Large ester molecules are quite oily, and are known to be insoluble in water.

Ester Structural Formula

The ester compound structure was previewed in figure 1. Figure 2 shows the ester structure of ethyl acetate. The structure of ethyl acetate is simple; it contains the following parts:

  • The carbonyl group {eq}-C=O {/eq}.
  • A methyl side chain {eq}-CH_3 {/eq}.
  • An ethoxy side chain {eq}-OCH_2CH_3 {/eq}.


Figure 2: The structural formula of a simple ester compound, ethyl acetate

Ester structural formula

The structural formula for all esters can be represented by R-CO-OR. The carbonyl center is {eq}sp^2 {/eq} hybridized; the bond angle between the side chains and the carbonyl center is estimated to be equal to 120 {eq}^{\circ} {/eq}. This hybridization and bond angle is shared by all esters, no matter how different their side chains are. The oxygen atoms in esters have two pairs of free electrons. In total, the number of free electron pairs in esters are four; two coming from the oxygen contributing to the carbonyl bond -C=O, and two coming from the oxygen from the ethoxy group -OR. The presence of free electrons makes esters capable of delocalizing their electrons throughout their constituent atoms. Esters have multiple resonance structures because of that, which are structures that illustrate the possible molecular structures due to electron delocalization.

Figure 3: The oxygens in esters have two pairs of free electrons

Ester structural formula

Ester Functional Group

Ester, which is R-CO-OR, is the functional group for esters. As mentioned in the previous sections, the ester functional group is made of three parts, a central carbonyl group, an alkyl group, and an alkoxy group. Esters are formed by reacting carboxylic acids, R-COOH, with alcohols, R-OH. Carboxylic acids can be thought of as the parent compounds of esters. The nomenclature of esters is basically the combination of the parent compounds name; the names of the carboxylic acid and the alcohol. An example, is the ester formed by the reaction of tert-butyl alcohol and butanoic acid. The resulting ester name follows the format: alcohol name, carboxylic acid name. The correct systematic name for such an ester is tert-butyl butanoate. Notice that the "-ic" acid suffix was substituted with "-ate."


Figure 4: Tert-butyl butonate structure

Esters functional group


Ester
Ester

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
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
resonance

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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Additional Info

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
Ester

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
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
resonance

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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Frequently Asked Questions

What is the functional group of ester?

Ester is the functional group of esters. The ester functional group is composed of three parts, which are:

  • The carbonyl group.
  • The alkyl side chain.
  • The alkoxy side chain.

The format of the ester functional group is, R-CO-OR.

How do you draw an ester structure?

The ester structural formula is, R-CO-OR, where, R, is an alkyl group, the, C=O, is the carbonyl group, and, -OR, is the alkoxy group. The angles between the bonds are equal to 120 degrees. The structure of esters can be drawn by following the, R-CO-OR, format.

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