Carboxylic acids are the organic compounds having the (-COOH) functional group attached directly to the carbon atom. Whereas, esters are organic compounds with a (-COOR) functional group. Esters are derivatives of carboxylic acids and are formed by a condensation reaction between carboxylic acids (-COOH) and alcohols (-OH) in a process called esterification.
|Carboxylic acids are weak acids.||Esters are usually neutral species.|
|The general formula for carboxylic acids is RCOOH.||The general formula for esters is RCOOR.|
|While naming carboxylic acids, a suffix (-oic) is added at the end of the name e.g ethanoic acid, propanoic acid, etc.||While naming esters, a suffix (-ate) is used at the end of its name e.g ethyl ethanoate, methyl propanoate, etc.|
|They have an unpleasant odor.||They have a sweet smell.|
|They are soluble in water because of the ability to form hydrogen bonds.||They are slightly soluble in water because of less difference in electronegativities of constituent atoms (less polarity).|
|They have relatively high boiling points (because of hydrogen bonding).||They have low boiling points because of weak intermolecular forces.|
|They are present in many food items e.g vinegar etc.||Esters are mostly used in perfumes and flavors.|
The carboxylic group (-COOH) is named so because it is a combination of carbonyl (C=O) and hydroxyl (-OH) groups. They are called acids because of their acidic properties. Carboxylic acids are widely spread in nature. Countless natural products are based on carboxylic acids and their derivatives. They are the parent compounds of many derivatives like acyl chlorides, acid anhydrides, amides, and esters, etc. They are often referred to as fatty acids as many of these compounds are obtained from the hydrolysis of animal and vegetable fats.
Some examples of carboxylic acids and their occurrence are listed:
- Formic acid (found in insect stings).
- Acetic acid (used as food flavoring e.g. vinegar).
- Butyric acid (butter).
- Benzoic acid (found in strawberries, apples, yogurt).
- Stearic acid (tallow).
- Palmitic acid (aka Palm oil).
- Caproic acid (goat milk).
Esters are derivatives of carboxylic acids and are obtained in a condensation reaction. The molecules of alcohol and carboxylic acid condense together to form ester and water (a condensation byproduct). This process is known as esterification.
Fischer esterification is one of the most widely used industrial methods for the production of esters.
RCOOH + ROH ⇌ RCOOR + H2O
The forward reaction is esterification whereas the reverse reaction is hydrolysis.
They are active ingredients in medication and flavoring agents. As a part of naturally occurring fats and oil, esters form a part of our daily diet.
Examples of esters
Some examples of esters and their occurrence are listed:
- Methyl ethanoate is a clear liquid commonly present in apples, bananas, and grapes, etc.
- Ethyl butyrate is found in pears and pineapples.
- Ethyl hexanoate is responsible for fruity aromas in apples, strawberries, and blackberries, etc.
- Ethyl acetate is present in yeast and sugar cane.
- Ethyl formate is naturally found in the bodies of ants and stings of bees.
- Ethyl octanoate is identified in various grape species.
- N-Propyl ethanoate is a colorless liquid used as fragrances as plant metabolites.
- Octyl ethanoate is found in the citrus family e.g. oranges, grapefruits, etc.
Nomenclature of carboxylic acids and esters
Carboxylic acids are better known by their common names rather than systematic or IUPAC names e.g. acetic acid, benzoic acid, oxalic acid, and malonic acid, etc. On the other hand, esters are named systematically by the IUPAC system. They are named alkyl alkanoates where alkyl refers to the alcoholic part while alkanoate refers to the carboxylic part of molecules.
Here is a quick overview of how carboxylic acids are named according to the IUPAC system:
- Select the longest carbon chain containing the carboxyl group.
- Change the ‘e’ of the corresponding alkane with ‘oic’ and acid.
- Start numbering from the carbon of the carbonyl group.
- Since the carboxylic functional group (-COOH) is always at the end, therefore, its position is never mentioned.
- Substituents are named and their position is indicated as well.
- Aromatic carboxylic acids generally have specific names while others are derivatives of benzoic acid.
Similarly, esters are often named by the IUPAC naming system although they have trivial names too.
- The letter ‘e’ of the corresponding alkane from carboxylic acids in esterification is changed with ‘ate’.
Carboxylic acids are classified into two main classes. Aliphatic carboxylic acids are further classified into two subclasses.
- Aliphatic carboxylic acids
- Open chain aliphatic carboxylic acids
- Closed chain aliphatic carboxylic acids
- Aromatic carboxylic acids
Aliphatic carboxylic acids
When the carboxylic group is bonded to an alkyl group or a hydrogen atom, they are called aliphatic carboxylic acid.
Aliphatic carboxylic acids are further classified, based on the type of structures.
- Open chain aliphatic carboxylic acids
Molecules having a carboxylic group at the end of the chain are called open chain carboxylic acids. For example, methanoic acid, ethanoic acid, pentanoic acid, dodecanoic acid, and octadecanoic acid etc.
- Closed chain aliphatic carboxylic acids
Molecules having a carboxylic group bonded to the alkyl ring are called closed chain carboxylic acid. For example, cyclopentane carboxylic acid and cyclohexane carboxylic acid, etc.
Aromatic carboxylic acid
When the (-COOH) group is bonded to an aromatic ring, they are called aromatic carboxylic acids. For example, benzoic acid and phthalic acid, etc.
There are no significant types of esters.
- Lower molecular weight carboxylic acids are volatile liquids.
- Higher molecular weight carboxylic acids are solid at room temperature.
- They have an unpleasant odor.
- They are polar because of the presence of (C=O) and (-O-H) bonds.
- They exist as dimers because of strong hydrogen bonding.
- Aliphatic carboxylic acids are soluble in water while aromatic ones are insoluble.
- Lower members of carboxylic acids are colorless liquids with an unpleasant smell.
- Their solubility generally decreases with the increase in molecular masses.
- Boiling points are usually high due to hydrogen bonding.
- Boiling points increase with the increase in the chain length.
The boiling point of formic acid (HCOOH) is 100 0C, for acetic acid (CH3COOH) is 118 0C, and for propionic acid (C2H5COOH) is 151 0C.
- Esters are colorless, usually volatile organic compounds.
- They have a pleasant smell.
- They have low melting and boiling points as compared to carboxylic acids.
- Lower members of esters (first few members) are fairly soluble in water.
- Higher members of esters are insoluble in water.
The boiling point for ethyl formate (HCOOC2H5) is 54 0C and for methyl butyrate (C3H7COOCH3) is102 0C.
Acidity of carboxylic acid and esters
Carboxylic acids are acidic in nature. They turn blue litmus red. They are Bronsted Lowery acids due to their proton donor nature. Although they are weak when compared to mineral acid, they are much stronger acids than several organic acidic compounds, like phenol, etc.
The relative acidities of organic compounds are given below.
RCOOH > ArOH > HOH > ROH > Alkynes > Alkanes
Carboxylic acids have a pKa value of ‘5’ while alcohols have ’16’. This means that carboxylic acids are 11 times stronger than alcohols.
Similarly, the acidity of aromatic acids depends upon the nature of the substituents on the ring. These substituents can either be electron withdrawing or electron donating species.
Hybridization has a great influence on the strength of acids as well. Increasing the ‘s’ character (from sp3 to sp2) of the orbital of an atom increases electronegativity and thus, the acidic strength increases.
Esters are usually neutral species.
A carboxylic acid is considered to be a resonance hybrid of two structures that are not equivalent. The negative charge of carboxylate ion is delocalized between two electronegative oxygen atoms in a resonance stabilized structure.
The structure of resonance hybrid carboxylic acid is as follows.
Resonance stabilized structures are very stable conjugate bases making carboxylic acids great acids while esters lack this property.
- Carboxylic acids are used in pharmaceutical drugs.
- Usage as organic solvents is their common application.
- They are used in the food industry.
- Nowadays, they are being applied as food preservatives.
- They are used in the manufacturing of soap (saponification).
- They are used in the production of polymers and polyesters.
- Esters are used in flavoring food items.
- They are constituents of perfumes and cosmetics.
- Polyesters are used to make plastic.
- The making of surfactants by esters is very common these days.
- Polyesters can be used in making fibers.
- Being nonpolar organic compounds, esters are applied as nail polish removers and paint removers.
- Esters are used as organic solvents e.g. ethyl acetate.
What is the difference between an ester and a carboxylic acid?
- Carboxylic acids are acidic while esters are neutral species.
- They have a difference in their general formulae.
- They have differences in odors.
- Carboxylic acids are more reactive than esters.
What is common between carboxylic acids and esters?
- Carboxylic acids and esters are used as organic solvents.
- Both have carbonyl groups.
- Both decompose to give alkene and carbon monoxide at 200 0C in the presence of a Pd catalyst.
How do you separate carboxylic acids and esters?
Esters can be separated through fractional distillation. Whereas, carboxylic acids can be separated by using bicarbonate ions because they only neutralize carboxylic acid. They can also be separated through the fractional distillation method.
Are esters more reactive than carboxylic acids?
Carboxylic acids are more reactive than esters due to the presence of (C=O) and (O-H) bonds which are much reactive than only (C=O) bonds of esters.
Why do esters smell sweet?
Esters have weak intermolecular forces. They evaporate easily and can be sensed via smell. They have specific bond arrangements which bind with our nasal sensors and create sensations of fruits and sweets.
Why do esters float?
Esters float as they are not very soluble in water. They form a separate layer above water owning to their lower density than water.
What ester smells like bananas?
Esters are present in different fruits. Banana contains amyl acetate ester which is also named isopentyl acetate. Bananas also have methyl acetate (methyl ethanoate) which gives them a characteristic banana smell.
How do carboxylic acids and esters differ in chemical structure?
Carboxylic acids and esters differ in chemical structures. Esters have one extra R (alkyl) group that is electron donating making ester’s α-carbon less electrophilic in nature. Whereas, carboxylic acid’s α-carbon is more electrophilic. Acids also have strong hydrogen bonding so they behave differently in many aspects.
Which is a more acidic, carboxylic acid or ester?
Carboxylic acids are more acidic than esters because acidic strength depends upon the stability of the conjugate base. Carboxylic acids have a more stable conjugate base than esters. That’s why acids are more acidic than esters.
Examplify carboxylic acid vs ester by boiling points?
Carboxylic acids have relatively higher boiling points than esters because they have strong hydrogen bonding. While esters have no hydrogen bonding at all.
The boiling point of formic acid (HCOOH) is 100 0C whereas methyl formate (HCOOCH3) is 31.8 0C.
Carboxylic acid and esters are less reactive to Nu than aldehydes or ketones. Why?
Carboxylic acids and esters are less reactive toward Nu (nucleophiles) because their carbonyl group is not readily available as in aldehydes or ketones. Aldehydes or ketones have trigonal planar geometry so Nu can attack from any side of the plane of the carbonyl groups.
Why are carboxylic acid and ester not considered in a carbonyl group though they have a C=O structure?
Carboxylic acids and esters are not considered in the carbonyl group family because they have unlike chemical behavior than aldehydes and ketones. In carbonyl compound, the carbon atom is electrophilic in nature while in carboxylic acid and esters, the carbon atom is less electrophilic because of resonance. This makes them behave differently than aldehydes and ketones.
What two functional groups react to form an ester?
Esters are derived through a reaction between carboxylic acid and alcohol through an esterification reaction.
RCOOH + ROH ⇌ RCOOR + H2O
This reaction is a reversible one which gives esterification in forward direction and hydrolysis in the backward one.
Why do we add sodium carbonate at the end of esterification reactions? Can sodium carbonate be used?
Yes, we can add sodium carbonate or sodium bicarbonate in esterification reactions to remove unreacted acid. Na2CO3 neutralizes the excess acid and converts it into a soluble salt. This process is known as the washing of esters, this way pure products are obtained.
Why is dicarboxylic acid stronger than monocarboxylate?
Dicarboxylic acids are stronger than monocarboxylic acids because their conjugate base is more stable than the conjugate base of monocarboxylic acids. This stability is due to the presence of two functional groups and a di-resonating structure.
Can two carboxylic acids react to form an ester?
Ester can not be made from the reaction between two carboxyl groups. This is because carboxyl groups condense to form acid anhydrides by condensation reaction. The carboxyl group has to react with an alcoholic group to make esters.
What is the difference between fatty acids and carboxylic acid?
All fatty acids are carboxylic acid but all carboxylic acids are not fatty acids because molecules having (-COOH) are called carboxylic acid while fatty acids are long-chain carboxylic acids that are obtained by hydrolysis of fats and oil.
What happens when a carboxylic acid is heated?
When a carboxylic acid is heated this leads to decarboxylation of acids.
Heating a carboxylic acid leads to the removal of carbon dioxide. This phenomenon is known as decarboxylation.
RCOOH → RH + CO2
Why do carboxylic acids have a higher boiling point than alcohols?
They have higher boiling points than alcohol because of strong intermolecular forces. Hydrogen bonding is much stronger in carboxylic acids due to the presence of two highly negative oxygen atoms as compared to alcohols which have only one. This makes acids higher in boiling points than alcohols.
Why are acid anhydrides more reactive than esters?
Reactivity depends upon the nature and stability of leaving group.
Acid anhydrides are more reactive than esters because they are less stable having two electron withdrawing carbonyl groups (cross conjugation). In the case of anhydrides, leaving group is a carboxylic acid. While in esters, leaving group is a hydroxide (OH–) which is not a good leaving group.
Why does LiAlH4 reduce esters amides or carboxylic acids while NaBH4 cannot reduce them?
LiAlH4 is a strong reducing agent as compared to NaBH4. That is why, it cannot reduce esters, amides, or carboxylic acids. Another reason is that NaBH4 is a selective reducing agent which can only reduce aldehyde or ketones.
What is the difference between ester and ether?
The main difference between ester and ether is they have different functional groups. Esters have a carbonyl group and have two oxygen atoms attached directly to the α-carbon while ethers have only one oxygen atom that is the linkage between two alkyl groups.
Which has the higher boiling point and why carboxylic acid or amide?
Carboxylic acids have high boiling points as compared to amides because they have strong hydrogen bonding. The reason behind this strong hydrogen bonding is the presence of more electronegative atoms, i.e. oxygen atoms in carboxylic acids. Amides have nitrogen atoms (Nitrogen atoms have less electronegativity than oxygen atoms).
How is the cyanide group converted into the carboxylic group?
The cyanide group is hydrolyzed easily when treated with water at 100 0C to yield carboxylic acids and ammonia.
RCN + 2H2O → RCOOH + NH3
This is a very slow reaction but can be catalyzed by acid or base.
Why do aldehydes and ketones have a lower boiling point than carboxylic acid and alcohols?
The boiling point is a physical phenomenon that depends upon the strength of intermolecular forces. Carboxylic acids and alcohols have much strong hydrogen bonding thus, they have high melting and boiling points.
Does alcoholic KOH react with carboxylic acid?
Alcoholic KOH is alkaline in nature, it dissolves to form (OH–) ion which acts as a strong nucleophile.
ClCH2COOH + KOH → HOCH2COOH + KCl
Why is amide the least reactive of the carboxylic acid derivative?
Amides are the least reactive derivative of carboxylic acid because in amides, (NH–2) is the leaving group. It is a weak conjugate base so, a poor leaving group. Also, there is a prominent resonance effect that stabilizes the carbonyl group. This stability of the carbonyl group and bad leaving tendency of (NH–2) makes amides the least reactive carboxylic acid derivatives.
Why are peroxy acids weaker than carboxylic acids?
Peroxy acids are weaker than carboxylic acid because resonance is dominant in carboxylic acid as compared to peroxy acid. They have less stable anions and have their lone pair at only one oxygen atom making them less stable conjugate base. The less stable conjugate bases do not last long in ionic form hence, peroxy acids are weaker than carboxylic acids.
Why is benzoic acid a stronger acid than cyclohexane carboxylic acid?
Benzoic acid is a stronger acid because of its more stable conjugate base, being resonance stabilised conjugate base . The resonance of electrons spreads over a large area compared to cyclohexane carboxylic acid. Hence, benzoic acid is more susceptible to lose protons in the solution.
Why are fatty acids referred to as carboxylic acids?
Fatty acids are referred to carboxylic acids having higher molecular weights. Fatty acids have carboxylic (COOH) functional groups and are solids at room temperature, hence, are known as fatty acids.