All of us have come across the word fatty acids. What are those? They are members of the group carboxylic acids. Fatty acids are the higher members, from C12-C18, of aliphatic carboxylic acids found in natural fats in the form of esters of glycerol. A carboxylic group is nothing but a carbon compound containing a carbonyl group along with a carboxyl group, hence the name carboxylic acids. Carboxylic are widespread in nature. Moreover, it is the precursor for many essential organic compounds such as acid chlorides, esters, anhydrides, amides, etc.
A carboxyl group (COOH) comprises a carbonyl group (>C=O) and a hydroxyl group (−OH). Carboxylic acids can easily release protons and thus, demonstrate the acidic behaviour. Carboxyl group acts as the functional group part of carboxylic acids. Carboxylic acids can be either aliphatic or aromatic in nature depending on alkyl or aryl group present with the carboxylic carbon.
Browse more Topics Under Aldehydes Ketones Carboxylic Acids
- Chemical Reactions and Uses of Carboxylic Acids
- Methods of Preparation of Carboxylic Acids
- Nomenclature and Structure of Carbonyl Group
- Nomenclature and Structure of Carboxyl Group
- Nucleophilic Addition Reaction
- Physical properties of Aldehydes, Ketones and Carboxylic Acids
- Preparation of Aldehydes
- Preparation of Aldehydes and Ketones
- Preparation of Ketones
- Reactions due to Alpha-Hydrogen
- Uses of Aldehydes and Ketones
Acidic Property of Carboxylic Acids
Carboxylic Acids are weaker than sulphonic acids and mineral acids. Mineral acids include H2SO4, HNO3, HCl. However, carboxylic acids are stronger in comparison to phenols and alcohols.
Comparison of Strength of Carboxylic acids with Alcohols
Carboxylic Acids are stronger acids in comparison to alcohols. The reason for such a behaviour is the conjugate base of the carboxylic acids achieves stabilization by resonance. In this case, the conjugate base, the product remaining after hydrogen removal from the conjugate base, will be carboxylate ion.
However, carboxylate ion stabilization is possible by resonance. Therefore, it becomes easy for carboxylic acids to release a proton thereby readily forming a carboxylate ion.
However, in case of alcohols, the alkoxide ion (conjugate base of alcohol group) cannot readily achieve a resonance stabilized structure. Thus, it is less stable. Therefore, we can say that alkoxide ion cannot easily release proton for forming a less stable conjugate base. Hence, alcohol is less acidic.
Comparisons of Acidic Behavior of Carboxylic Acids and Phenols
Carboxylic Acids are more acidic than phenol. In case of phenols, phenoxide ion or C6H5O− act as the conjugate base of phenol. The resonance structure of a phenoxide ion will be
From the third, fourth and fifth resonance structure, we can notice that the less electronegative carbon atom bears the negative charge. Hence, it does not help in resonance stabilization of phenoxide ion. Thus, the contribution of these structures can be neglected.
Therefore, if we compare the first and second structure of phenoxide ion with a carboxylate ion, the negative charge present on the oxygen atom of the phenoxide ion undergoes localization, that is it remains in the same oxygen atom. However, the carboxylate ion undergoes delocalization of the negative charge. Hence, the carboxylate ion becomes more resonance stabilized. Therefore, carboxylic acids are stronger in nature than phenols.
Substitution Effect on the Acidic Property of Carboxylic Acids
The “I groups” or the electron attracting groups is responsible for increasing the acidity of carboxylic acids. The increasing order of electron attracting group with respect to −I-effect are
+I Group or Electron Releasing Group is responsible for decreasing the acidity of carboxylic acids. The increasing order with respect to +I Effect is
Ortho Effect on Carboxylic Acids
The benzoic acids with ortho substitution demonstrate more acidic property than the normal benzoic acid regardless of the nature of the substituent group, whether the substituent is electron withdrawing or electron donating group. For instance, o-toluic acid exhibit more acidic property in comparison to benzoic acid. Let us study the ortho-effect in one such compound.
Ortho-Effect in the Compound “Aminobenzoic Acid”
An amino group exhibit weaker –I effect but it demonstrates a stronger +R effect. This is the reason amino acids exhibit less acidic property than benzoic acids. In fact, o-aminobenzoic acid has lesser acidic property in comparison to benzoic acid. Now we have to understand the reason behind such a behaviour.
We know that –COOH group exhibit acidic nature whereas –NH2 exhibit basic nature. Therefore, the nitrogen present in the amino group will develop a bond with hydrogen present in the carboxyl group. This, in turn, will lead to the formation of a zwitterion. A zwitterion is a neutral molecule which contains both positive as well as negative charges.
Therefore, the ortho-effect will gradually be reduced to an extent where the compound will become weaker than a benzoic acid.
Chemical Reactions of Carboxylic Acid
The carboxylic acid (carbon compound containing carboxyl group) will undergo a number of chemical reactions. Let us study one by one.
Reaction with Metals
The reaction of carboxylic acids with metals such as K, Na, Mg, Ca leads to the formation of the corresponding salts. In the reaction process, a proton will be released from the carboxyl group of the carboxylic acid where the metal substation will occur. The reaction liberates H2 gas.
Reaction with Alkalies
The reaction of carboxylic acids with alkalies will lead to the formation of corresponding salts and water.
Reaction with Carbonates and Bicarbonates
Carboxylic acids undergo decomposition reaction with carbonates and bicarbonates leading to the formation of respective salts, water, and carbon dioxide gas.
This reaction can also help in testing the presence of carboxyl group. Carboxylic acids undergo reaction with a saturated solution of sodium bicarbonate produce effervescence due to the release of CO2 gas. However, most phenols do not release effervescence with an aqueous NaHCO3 solution. Thus the reaction of bicarbonate with carboxylic acids helps in distinguishing between phenols and carboxylic acids.
Acid Chlorides Formation
Carboxylic Acids react with thionyl chloride (SOCl2), Phosphorus pentachloride (PCl5), or Phosphorus pentachloride to form the respective acid chlorides. Refer below to understand the reaction.
Formation of Esters (Esterification)
Warming carboxylic acids with alcohols in the presence of a concentrated sulphuric acid or dry hydrochloric acid produces esters having a fruity smell.
In this reaction concentrated sulphuric acid act as the dehydrating agent. The reaction is an example of an equilibrium reaction. Hence, the ester is distilled to continue shifting the reaction in the forward direction.
Formation of Amide Compounds
Treatment of carboxylic acids with ammonia thereby forming ammonium salts. Ammonium salts further upon heating lose a water molecule leading to the formation of amides.
Distillation of soda lime (NaOH + CaO) with sodium salts of carboxylic acids result in decarboxylation reaction thereby forming alkanes.
Formation of Anhydrides
Two molecules of carboxylic Acids undergo heating with a dehydrating agent like phosphorus pentoxide leading to the formation of acid anhydrides.
HVZ Reaction or Hell-Volhard Zelinsky Reaction
Carboxylic Acids react with chlorine molecule (Cl2) or bromine molecule (Br2) to form α-substituted carboxylic acids. The reaction takes place in the presence of red phosphorus. This reaction refers to as H.V.Z reaction or Hell-Volhard Zelinsky Reaction.
Formic Acid does not undergo this reaction because it does not contain alkyl group. It is important to note that bromination will only take place at the α-position. Moreover, the reaction will cease after replacement of all the α-hydrogens by the bromine atoms. However, chlorination will occur initially at the α-position and the replacement will occur in hydrogen atom by chlorine atoms then the replacement will move further along the chain.
Electrophilic Substitution Reactions
Aromatic carboxylic acids undergo different types of electrophilic substitution reaction like nitration, sulphonation, and halogenation. Carboxyl group (-COOH) is electron withdrawing group. Thus, the reaction will occur at the meta-position. The carboxyl group deactivates. Therefore, the reaction will only occur under vigorous conditions.
Friedel Crafts Reactions
This group will not undergo Friedel-Crafts Reactions because the carboxyl group is strong electron attracting group. Thus, benzene ring will be deactivated. Hence, it will not undergo alkylation and acylation.
Uses of Carboxylic Acids
- Carboxylic acid acts as a disinfectant.
- Simplest carboxylic acid “formic acid” acts as reducing agent in textile treatments.
- Acetic acid, member of the carboxylic acid group, helps in the production of esters and cellulose plastics.
- Acetic acid acts as the precursor for the formation of an ester of salicylic acid which is used for aspirin production.
- Palmitic acid and stearic acid finds its use in the manufacturing of soaps, pharmaceuticals, candles, cosmetics, protective coating, etc.
- Stearic acid also helps in rubber manufacturing processes.
- Acrylic acid acts as an ester and helps in the production of polymers or acrylates. Similarly, methacrylic acid undergoes polymerization to form Lucite.
- Moreover, oleic acid, a type of carboxylic acids, helps in manufacturing soaps and detergents. Additionally, it is also used in textiles.
Solved Examples for You
Question: Identify the carboxylic acid having the highest boiling point.
- Hexanoic Acid
- Heptanoic Acid
- Nonanoic Acid
- Decanoic Acid
Solution: Option 4 (Decanoic Acid). Intermolecular forces of molecules affect the boiling point of a solution. Thus, the ability of remain together increases with the increase in the intermolecular forces. Therefore, more energy is needed to break the compounds. In the above question, every option contains carboxyl functional group.
Thus, every option has hydrogen bonds. However, the major difference is in the number of carbons present in the chain. With the increase in the number of carbons, the molecular weight increases thereby increasing the Van Der Walls molecular forces. Therefore, this will further increase the heat energy required for breaking the bond.
Among all the options, decanoic acid has longest carbon chain. Hence, it will require more energy to break the bond and separate the molecules. Thus, the decanoic acid will have the highest boiling point.