Chemistry

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Chemical Properties of Carboxylic Acids

Carboxylic acid do not give the characteristic reaction of carbonyl group image 1 as given by the aldehydes and ketones. as the carbonyl group of carboxylic acid is involved in resonance:

The reactions of carboxylic acids can be classified as follows:

(A) Reactions involving cleavage of O – H bond.
(B) Reactions involving cleavage of C – OH bond.
(C) Reactions involving – COOH group.
(D) Substitution reactions involving hydrocarbon part.

(A) Reactions involving cleavage of O – H bond.

1. Reactions with Metals

Carboxylic acid react with active metals like Na, Mg, Zn etc to form corresponding salts with the liberation of hydrogen.

Example

2. Reaction with Alkalies

Carboxylic acid reacts with alkalies to neutralise them and form salts.

Example

3. Reaction with Carbonates and Bicarbonates (Test for Carboxylic Acid Group)

Carboxylic acids decompose carbonates and bicarbonates evolving carbondioxide gas with effervescence.

Example

4. All Carboxylic Acids Turn Blue Litmus Red

(B) Reactions involving cleavage of C-OH bond

1. Reactions with PCl₅, PCl₃ and SOCl₂

Example

2. Reactions with Alcohols (Esterification)

When carboxylic acids are heated with alcohols in the presence of conc. H₂SO₄ or dry HCl gas, esters are formed. The reaction is reversible and is called esterification.

Example

Mechanism of Esterification:

The Mechanism of esterifiation involves the following steps.

(C) Reactions involving – COOH group

1. Reduction

(i) Partial Reduction to Alcohols

Carboxylic acids are reduced to primary alcohols by LiAlH₄ or with hydrogen in the presence of copper chromite as catalyst. Sodium borohydride does not reduce the – COOH group.

Example

(ii) Complete Reduction to Alkanes

When treated with HI and red phosphorous, carboxylic acid undergoes complete reduction to yield alkanes containing the same number of carbon atoms.

Example

2. Decarboxylation

Removal of CO₂ from carboxyl group is called as decarboxylation. Carboxylic acids lose carbon dioxide to form hydrocarbon when their sodium salts are heated with soda lime (NaOH and CaO in the ratio 3:1)

Example

3. Kolbe’s Electrolytic Decarboxylation

The aqueous solutions of sodium or potassium salts of carboxylic acid on electrolysis gives alkanes at anode. This reaction is called kolbes electrolysis.

Sodium formate solution on electrolysis gives hydrogen

4. Reactions with Ammonia

Carboxylic acids react with ammonia to form ammonium salt which on further heating at high temperature gives amides.

Example

5. Action of Heat in the Presence of P₂O₅

Carboxylic acid on heating in the presence of a strong dehydrating agent such as P₂O₅ forms acid anhydride.

Example

(D) Substitution Reactions in the Hydrocarbon Part

1. α – Halogenation

Carboxylic acids having an α – hydrogen are halogenated at the α – position on treatment with chlorine or bromine in the presence of small amount of red posphorus to form α halo carboxylic acids. This reaction is known as Hell – Volhard – Zelinsky reaction (HVZ reaction). The α – Halogenated acids are convenient starting materials for preparing α – substituted acids.

2. Electrophilic Substitution in Aromatic Carboxylic Acids

Aromatic carboxylic acid undergoes electrophilic substitution reactions. The carboxyl group is a deactivating and meta directing group. Some common electrophilic substitution reactions of benzoic acid are given below.

(i) Halogenation

(ii) Nitration

(iii) Sulphonation

(iv) Benzoic acid does not undergo friedal crafts reaction. This is due to the strong deactivating nature of the carboxyl group.

(E) Reducing Action of Formic Acid

Formic acid contains both an aldehyde as well as an acid group. Hence, like other aldehydes, formic acid can easily be oxidised and therefore acts as a strong reducing agent

(i) Formic acid reduces Tollens reagent (ammonical silver nitrate solution) to metallic silver.

(ii) Formic acid reduces Fehlings solution. It reduces blue coloured cupric ions to red coloured cuprous ions.

Tests for Carboxylic Acid Group

1. In aqueous solution carboxylic acid turn blue litmus red.
2. Carboxylic acids give brisk effervescence with sodium bicarbonate due to the evolution of carbon-di-oxide.
3. When carboxylic acid is warmed with alcohol and Con H₂SO₄ it forms an ester, which is detected by its fruity odour.

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Physical Properties of Carboxylic Acids

1. Aliphatic carboxylic acid upto nine carbon atoms are colour less liquids with pungent odour. The higher members are odourless wax like solids.

2. Carboxylic acids have higher boiling point than aldehydes, ketones and even alcohols of comparable molecular masses. This is due to more association of carboxylic acid molecules through intermolecular hydrogen bonding.

In fact, most of the carboxylic acids exist as dimer in its vapour phase.

3. Lower aliphatic carboxylic acids (up to four carbon) are miscible with water due to the formation of hydrogen bonds with water. Higher carboxylic acid are insoluble in water due to increased hydrophobic interaction of hydrocarbon part. The simplest aromatic carboxylic acid, benzoic acid is insoluble in water.

4. Vinegar is 6 to 8% solution of acetic acid in water. Pure acetic acid is called glacial acetic acid. Because it forms ice like crystal when cooled. When aqueous acetic acid is cooled at 289.5 K, acetic acid solidifies and forms ice like crystals, where as water remains in liquid state and removed by filtration. This process is repeated to obtain glacial acetic acid.

• Carboxylic acids have high boiling points compared to other substances of comparable molar mass. Boiling points increase with molar mass.
• Carboxylic acids having one to four carbon atoms are completely miscible with water. Solubility decreases with molar mass.

Carboxylic acids are soluble in water. Carboxylic acids do not dimerise in water, but forms hydrogen bonds with water. Carboxylic acids are polar and due to the presence of the hydroxyl in the carboxyl group, they are able to form hydrogen bonds with water molecules.

The solubility of compounds containing the carboxyl functional group in water depends on the size of the compound. The smaller the compound (the shorter the R group), the higher the solubility. The boiling point of a carboxylic acid is generally higher than that of water.

Larger carboxylic acids are solids with low melting points. There are a great many aromatic carboxylic acids, which are all crystalline solids. Carboxylic acids can form intermolecular hydrogen bonds and thus have relatively high melting and boiling points compared to other organic compounds that cannot hydrogen bond.

• Carboxyl group comprises electronegative oxygen double bond to a carbon atom.
• A compound comprising a carboxyl group will possess a high melting point, hydrophilic centres, and boiling point.

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Methods of Preparation of Carboxylic Acids

Some important methods for the preparation of carboxylic acids are as follows:

1. From Primary Alcohols and Aldehydes

Primary alcohols and aldehydes can easily be oxidised to the corresponding carboxylic acids with oxidising agents such as potassium permanganate (in acidic or alkaline medium), potassium dichromate (in acidic medium)

Example

2. Hydrolysis of Nitriles

Nitriles yield carboxylic acids when subjected to hydrolysis with an acid or alkali.

Example

3. Acidic Hydrolysis of Esters

Esters on hydrolysis with dilute mineral acids yield corresponding carboxylic acid

Example

4. From Grignard Reagent

Grignard reagent reacts with carbon dioxide (dry ice) to form salts of carboxylic acid which in turn give corresponding carboxylic acid aftr acidifiation with mineral acid.

Example

Formic acid cannot be prepared by Grignard reagent since the acid contains only one carbon atom.

5. Hydrolysis of Acylhalides and Anhydrides

(a) Acid chlorides when hydrolysed with water give Carboxylic acids.

Example

(b) Acid anhydride when hydrolysed with water give corresponding carboxylic acids.

6. Oxidation of Alkyl Benzenes

Aromatic carboxylic acids can be prepared by vigorous oxidation of alkyl benzene with chromic acid or acidic or alkaline potassium permanganate. The entire side chain is oxidised to – COOH group irrespective of the length of the side chain.

Example

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Structure of Carboxyl Group:

Th carboxyl group represent a planar arrangement of atoms. In – COOH group, the centre carbon atom and both the oxygen atoms are in sp hybridisation. The three sp² hybrid orbitals of the carbon atom overlap.

The two sp² – hybridised orbitals of the carboxyl carbon overlap with one sp² hybridised orbital of each oxygen atom while the third sp² hybridised orbital of carbon overlaps with either a s – orbital of H – atom or a sp² – hybridised orbital of C – atom of the alkyl group to form three s – bonds. Each of the two oxygen atoms and the carbon atom are left with one unhybridised p – orbital which is perpendicular to the s – bonding skeleton.

All these three p – orbitals being parallel overlap to form a π – bond which is partly delocalized between carbon and oxygen atom on one side, and carbon and oxygen of the OH group on the other side. In other words, RCOOH may be represented as a resonance hybrid of the following two canonical structures.

The carboxylic carbon is less electrophilic than carbonyl carbon because of the possible resonance structure. i.e., delocalisation of lone pair electrons from the oxygen in hydroxyl group.

Carboxyl group is a functional organic compound. In this structure of a carboxyl group, a carbon atom is attached to an oxygen atom with the help of a double bond. The carboxyl group ionizes and releases the H atom present in the hydroxyl group part as a free H⁺ ion or a proton.

Carboxylic acid, any of a class of organic compounds in which a carbon (C) atom is bonded to an oxygen (O) atom by a double bond and to a hydroxyl group (- OH) by a single bond. A fourth bond links the carbon atom to a hydrogen (H) atom or to some other univalent combining group.

The Carboxyl group contains a double bond of electronegative oxygen to a carbon atom. As a result, the polarity of a bond will increase. A compound containing a carboxyl group should possess hydrophilic centres with a high melting point and boiling point.

Carboxyl groups are functional groups with a carbon atom double-bonded to an oxygen atom and single bonded to a hydroxyl group. Ionized carboxyl groups act as acids, require less energy and are more stable. Electron sharing between oxygen atoms on ionized carboxyl groups increases stability.

A carboxyl group (COOH) is a functional group consisting of a carbonyl group (C=O) with a hydroxyl group (O-H) attached to the same carbon atom. Carboxylic acids are a class of molecules which are characterized by the presence of one carboxyl group.

When deprotonated, carboxylate anions are extremely stable due to resonance. This enables carboxyl groups to be influential components of fatty acids and amino acids, which can be further reacted to generate esters, proteins, lipids, and alcohols within the body.

A carboxyl group (COOH) is a functional group consisting of a carbonyl group (C=O) with a hydroxyl group (O-H) attached to the same carbon atom. Carboxyl groups have the formula -C(=O)OH, usually written as -COOH or CO₂H.

Carboxyl groups are commonly found in amino acids, fatty acids, and other biomolecules. An example of a less hydrophilic group is the carbonyl group (C=O), an uncharged but polar (contains partial positive and partial negative charges) functional group.

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Iupac Nomenclature of Carboxylic Acids

The IUPAC name of a carboxylic acid is derived from that of the longest carbon chain that contains the carboxyl group by dropping the final – e from the name of the parent alkane and adding the suffix – oic followed by the word “acid.” The chain is numbered beginning with the carbon of the carboxyl group.

Carboxylic acids are named by counting the number of carbons in the longest continuous chain including the carboxyl group and by replacing the suffix – ane of the corresponding alkane with – anoic acid.

For molecules with two carboxylic acid groups the carbon chain in between the two carboxyl groups (including the carboxyl carbons) is used as the longest chain; the suffix – dioic acid is used. For molecules with more than two carboxylic acid groups, the carboxyl groups are named as carboxylic acid substituents.

Carboxylic acids are the most common type of organic acid. A carboxylic acid is an organic acid that contains a carboxyl group (C(=O)OH) attached to an R-group. The general formula of a carboxylic acid is R-COOH or R-CO₂H, with R referring to the alkyl, alkenyl, aryl, or other group.

Carboxylic acids are commonly identified by their trivial names. They often have the suffix – ic acid. IUPAC-recommended names also exist; in this system, carboxylic acids have an -oic acid suffix. For example, butyric acid (C₃H₇CO₂H) is butanoic acid by IUPAC guidelines.

Carboxylic acids occur in many common household items.

• Vinegar contains acetic acid
• Aspirin is acetylsalicylic acid
• Vitamin C is ascorbic acid
• Lemons contain citric acid, and
• Spinach contains oxalic acid.

Carboxylic acids are weak acids because they only partially ionise in solution. Their solutions do not contain many hydrogen ions compared to a solution of a strong acid at the same concentration.

Carboxylic acids are very important biologically. The drug aspirin is a carboxylic acid, and some people are sensitive to its acidity. Carboxylic acids that have very long chains of carbon atoms attached to them are called fatty acids. As their name suggests, they are important in the formation of fat in the body.

Carboxylic acids are soluble in water. Carboxylic acids do not dimerise in water, but forms hydrogen bonds with water. Carboxylic acids are polar and due to the presence of the hydroxyl in the carboxyl group, they are able to form hydrogen bonds with water molecules.

Aspirin is both an aromatic carboxylic acid (red oval) and a phenyl ester of acetic acid (blue oval). While esterification will convert the carboxylic acid group to a methyl ester, transesterification (exchange of one alcohol portion of an ester for another alcohol) to afford methyl acetate 4 and methyl salicylate 3.

A carboxylic acid is an organic compound that contains a carboxyl group (C(=O)OH) attached to an R-group. The general formula of a carboxylic acid is R-COOH, with R referring to the alkyl group. Important examples include the amino acids and fatty acids.