Ester is a very common functional group, that takes part in variety of organic reactions. The term “ester” was first introduced by a German chemist, Leopold Gmelin, in the first half of 19th century.
Ester is basically a chemical compound derived from parent acid and parent alcohol. In general, an ester bond contains one carbon atom doubly bonded to one oxygen atom and singly bonded to another oxygen molecule, further connected with alkyl or aryl group. Ester is denoted as R-COOR1.
In this article, “ester bond structure” formation, structure with some other detailed facts and some frequently asked questions about ester bond are described briefly.
Ester Bond Formation
Formation of an ester bond is generally a condensation reaction (elimination of water molecule).
Ester bond formation reaction proceed with a parent acid and a parent alcohol as reactant. Acid catalyst must be present to accelerate the forward reaction. Concentrated sulphuric acid (H2SO4) is used as acid catalyst but dry hydrogen chloride in gaseous state may be used in some cases. This method is called Esterification Reaction.
RCOOH+ R1OH ⇌ RCOOR1 + H2O
Alcohols are converted to its corresponding esters in the above reaction. But this esterification reaction is not applicable for that OH group directly attached to the benzene group that is phenol group. Phenols are comparatively more acidic than the alkyl or allyl alcohol. Due to the greater acidity, it reacts with carboxylic acid very slowly and the reaction becomes unstable for the purpose of preparing.
This reaction of forming ester is reversible and relatively slow process because the newly formed ester and water can react to form the corresponding alcohol and acid.
In the esterification reaction, the reactivity of carboxylic acid is enhanced by the presence of acid catalyst which protonates the oxygen doubly bonded with carbon (carbonyl carbon).
Mechanism of Esterification Reaction
Mechanism of esterification is described below-
1.Formation of Cation:
Parent carboxylic acid takes up a proton from the acid catalyst.
2. Nucleophilic attack:
The parent alcohol acts as a nucleophile and attacks the carbon center of acid combined carboxylic acid.
3.Proton Transfer:
Proton (H+) is transferred from the hydroxyl group of acid to the hydroxyl group of alcohol and a good leaving group (OH2) is formed
4. Pi bond formation:
The lone pair of the oxygen comes from the hydroxyl group is donated to make a pi bond with the carbon. Then the good leaving group is eliminated as water molecule (H2O) from the intermediate formed by the combination of carboxylic acid, acid catalyst and alcohol. As a result ester linkage is formed.
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To know more please follow: Peptide Bond vs Ester Bond: Comparative Analysis And Facts
What is an Ester Bond?
Ester bond is a linkage between an atom which is doubly bonded with an oxygen atom, and the oxygen atom bearing any alkyl or aryl group.
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Ester bond is generally a covalent bond and very much essential in formation of variety of lipids. In lipid, condensation reaction to form an ester takes place between glycerol and a fatty acid. In phospholipids, one fatty acid present in the triglyceride is substituted by a group containing phosphate (PO43-).
Image Credit: Wikimedia Commons
To know more please check: 5+ Double Bond Examples: Detailed Insights And Facts
Esters can form hydrogen bonds with their oxygen atom (donor atom) with the hydrogen atom (acceptor atom) of water molecule. Esters are weak electrophilic due to its carbonyl centre. Thus it can participate in nucleophilic substitution or any other nucleophilic chemical reaction. Due to the electrophilic character of carbonyl carbon, the C-H bond adjacent to the carbonyl centre is mild acidic in nature, (pka=25) but it undergoes deprotonation reaction in presence of strong base like NaOH, KOH etc.
Ester linkages have one carbonyl center. It gives rise to bond angle almost 1200 and attain a planar structure and sp2 hybridization. Esters are not structurally rigid group of molecules like amide linkage. Esters are preferred to attain s-cis conformation rather than E or s-trans conformation due to dipole -dipole interaction and hyperconjugation effect.
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In ester the bond rotation about C-O-C bond is allowed as this bond possesses low energy barrier. Due to less rigidity, melting point of ester is generally low and volatility is high with comparing to alcohols and ethers having similar molecular weight. Polarity of esters is greater than ethers but lesser than alcohols.
To know more please follow: 4 Hydrogen Bond Examples : Detailed Insights And Facts
Identification of esters can be done through gas chromatography due to its volatile nature. Vibrational frequency (wave number) of C=O bond (νc=o) is almost 1730-1750 cm-1. This frequency can be changed due to the changing of functional groups attached to it. For example, presence of any group having conjugation with the carbonyl carbon may decrease the wave number almost 30 cm-1.
Though carboxylic acids have some unpleasant bad smell esters have a characteristic pleasant fruity smell. For this smell, esters are used as fragrances in essential oils, perfumes. Esters perform as for a broad array of plastics, resins, plasticizers etc. and one of the well known synthetic lubricants in the commercial market. Polymer of esters that is polyesters are important plastics. Phosphodiester linkage is the back bone of DNA molecule.
The hydrolysis of ester bond takes place in presence of strong bases like Potassium hydroxide (KOH) is called saponification reaction. This reaction is an important formation procedure of soaps, fats and oils etc.
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To know more please go through: SN1 mechanism: Detailed Insights And Facts
Frequently Asked Questions (FAQ)
Some FAQs about ester linkage is answered below-
Are ester linkages acidic are basic?
Answer: Esters are mainly neutral compound. In chemical reaction like hydrolysis, “splitting with water” the alkoxide group (OR1) is substituted by another group.
Are esters polar molecule?
Answer: Yes, esters are polar molecule. But their boiling and melting points are lesser than the corresponding carboxylic acids and alcohols.
What can break an ester bond?
Answer: Hydrolysis with a strong base can break ester bond. In hydrolysis, hydroxide ion (OH–), the nucleophile, attacks the carbonyl carbon center and ester bond is cleaved.
Is ester soluble in water?
Answer: Due to the hydrogen bonding with water molecules, esters having low molecular weight are soluble in water, but the solubility is not so high.
Also Read:
- Adenine and uracil bond
- Peptide bond vs ester bond
- Disulfide bond structure
- Peptide bond vs phosphodiester bond
- What is a disulfide bond
- Peptide bond vs disulfide bond
- Peptide bond formation 2
- Peptide bond formation
- Is o2 a triple bond
- Is peptide bond a hydrogen bond
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I am Aditi Ray, a chemistry SME on this platform. I have completed graduation in Chemistry from the University of Calcutta and post graduation from Techno India University with a specialization in Inorganic Chemistry. I am very happy to be a part of the Lambdageeks family and I would like to explain the subject in a simplistic way.
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