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Carboxylic Acid Reactions

The carboxylic are tantalizing class of bio renewable chemicals but the toxicity is limiting the activities to some extent. The series of reactions that are covered under carboxylic group are mostly due to the presence of the hydroxyl group present in one end. The other end contains an alpha carbon which gives the acid a saturated look if the carbon is saturated and in case the alpha carbon is part of the aromatic ring then the molecule is called an aromatic acid.

Carboxylic acid are similar to alcohols and water in that they all contain O-H group which is responsible for hydrogen bonding and carboxylic acids are an extreme example of hydrogen bonding and these hydrogen bonding are so strong that the acid molecules are bound together in dimers.

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Carboxylic Acid Reactions Summary

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Whenever an acid reacts with a base there is a production of salt and water and carboxylic acid is no exception. It produces carboxylate salt and a water molecule. The O-H of the acid is not present in carboxylate and have reacted to form part of the water molecule which is a by-product of the reaction.

When two molecules of carboxylic acid reacts an acid anhydride is formed and along with that a water molecule is also produced. An acid anhydride contains two carbonyl groups separated by an oxygen and one carbon atom is attached to each carbonyl carbon.

Carboxylic acids substituted by electron donating groups are less acidic as their carboxylate ions are destabilized. The extent of dissociation of a carboxylic acid in a buffered solution of a given pH can be calculated with Henderson Hasselbalch equation.

General reactions of Carboxylic acids include:
  • loss of acidic proton,
  • nucleophilic acyl substitution at the carbonyl group
  • substitution on the alpha carbon
  • reduction
Carboxylic acid reactions with alcohol

Esters contain a C – O – C linkage in which one of the carbon atoms belongs to a carbonyl C=O group. Every ester molecule contain a Carboxylic acid residue and an alcohol residue.

A residue is the part of a reactant molecule that remains when it has been incorporated into a product. These residues reflect how esters are formed by the reaction of a carboxylic acid with an alcohol in the presence of the catalyst H+.

Apart from the ester this double bond replacement reaction produces a water molecule.

The esters so formed by the reaction between Carboxylic acid and alcohol are named by first listing the name of the alkyl group contained in the alcohol residue.

The second part of the name is formed by changing the ending on the name of the Carboxylic acid that provides the Carboxylic acid residue from ‘ic’ to ‘ate’.

$CH_{3}CH_{2}COOH + CH_{3} OH \leftarrow (H+) \rightarrow CH_{3}CH_{2}COOCH_{3} + H_{2}O$

Propanoic acid reacting with methyl alcohol in presence of $H_2SO_4$ at 170 C gives methyl propanoate (ester) and water.

$CH_{3}CH_{2}COOH + CH_{3}CH_{2}OH \leftarrow (H+) \rightarrow CH_{3}CH_{2}COOCH_{2}CH_{3} + H_{2}O$

Propanoic acid reacting with ethanol in presence of H2SO4 at 170 C gives di ethyl ester or ethyl propanoate and water.
Salient points of the reaction:
  • Hydrogen bonds do not form between esters because they have no covalent bond joining a hydrogen atom to an oxygen atom.
  • The forces which hold one ester to another are dipole – dipole interaction and London force as these non-covalent forces are weaker than hydrogen bonds.
  • Ester formation is reversible process which means that in presence of H+ Carboxylic acid plus an alcohol will react to yield an ester plus water and an ester plus water will react to give Carboxylic acid and an alcohol.
  • The Le Chatelier’s principle is applied which drive the reaction forward as an excess of Carboxylic acid and / or alcohol is used. The reverse reaction is favoured when an excess of ester and / or water are used.

Reactions of Alkanoic Acids

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Alkanoic acids or Carboxylic acids take part in very typical acidic reactions.

  • The Alkanoic acids are neutralised by alkalis and form salt and water only.

$CH_{3}COOH + NaOH \rightarrow CH_{3}COONa + H_{2}O$

  • The Alkanoic acids react with the carbonates and form a salt, carbon di oxide and water

$2 CH_{3}COOH + CaCO_{3} \rightarrow (CH_{3}COO)_{2} Ca + CO_{2} + H_{2}O$

  • The Alkanoic acids react with reactive metals like magnesium and form salt and hydrogen gas

$2 CH_{3}COOH + Mg \rightarrow (CH3COO)_2 Mg + H_2$

  • The Alkanoic acids react with alcohol to form esters and water

Carboxylic Acid Preparation and Properties

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The Carboxylic acids are commonly used as the starting material for the preparation of the other acid derivative.

Any of the acid derivatives can be hydrolysed to the Carboxylic acid by reacting with water under suitable conditions. The acid or base catalysis is necessary for the less reactive derivatives.

  • Acyl chlorides and anhydrides are guarded against water as they react vigorously with water to produce Carboxylic acids. The reaction is not useful commercially as acyl chloride and anhydride is usually prepared from the acid. However, the hydrolysis reaction is occasionally used for the preparation of a Carboxylic acid if the acyl chloride or anhydride is available from some other source.
  • Esters can be hydrolysed to Carboxylic acids under either acidic or basic conditions. Under acidic conditions the mechanism is the reverse of Fischer esterification mechanism. The acid and the ester have comparable reactivities and hence some method must be used to drive the equilibrium toward the desired product or the Carboxylic acid. This applies with Le Chatelier’s principle which helps in shifting the direction of the reaction equilibrium.

  • The Carboxylic acid group can form hydrogen bonds with water molecules which result in Carboxylic acids dissolving well in water, provided that their carbon chains are of short lengths.
  • The acids have much higher melting temperature than alkanes of similar relative molecular mass as they form hydrogen bonds with one another in solid state.
  • The Carboxylic acids till Butanoic acid are completely soluble in water.
  • The Carboxylic acid molecule has the tendency to form hydrogen bond with another Carboxylic acid molecule in liquid or gaseous state and form a dimer.
  • With the increase in length of Carboxylic acid molecule acids dissolve better in organic solvents.
  • The smell of Carboxylic acid is typical as acetic acid is vinegar and Butanoic has rancid smell.
  • The Carboxylic acid group is polarised and Cd+ is therefore likely to be attacked by nucleophile while the Od- may be attacked by positively charged species like H+.
  • The Cd+ tends to attract electrons from C-OH bond which reduces the d+ character of this cation atom and makes it less easily attacked by nucleophiles than carbonyl carbon in aldehydes and ketones.
  • The oxygen of the –OH group in turn attracts electrons from OH bond, weakening this bond and allow the loss of hydrogen as an H+ ion.
  • The carbonyl carbon is susceptible to nucleophile attack
  • The –OH group easily loses a proton, making it acidic. 

Polarisation of Carboxylic Acid
The Polarisation of Carboxylic Acid

Acidity of Carboxylic Acids

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The carboxylic acids considered to be weak acids as most of the aromatic and unsubstituted carboxylic acids fall within the range of $10^{-5}$ to $10_{-4}$.

The value of Ka for acetic acid is 1.74 $\times$ 10$^{-5}$ with a pKa of 4.76

$CH_{3}COOH + H_{2}O \leftrightarrow CH_{3}COO^{-} + H_{3}O^{+}$ 

Ka $\frac{([CH_{3} COO^{-} ][H_{3} O^{+}])}{(CH_{3} COOH)}$  =  1.74 $\times$ 10$^{(-5)}$

The pKa = 4.76

The substitution at the alpha carbon of an atom or group of atoms of higher electronegativity than carbon further increase the acidity of carboxylic acids by inductive effect.

The acidity of acetic acid and Chloroacetic acid are compared below.

 $CH_3COOH$   $ClCH_2COOH$ 
  Name  Acetic acid   Chloroacetic acid   Di Chloroacetic acid   Tri Chloroacetic acid 
 pKa  4.76  2.86  1.48  0.70

The electronegative atoms on the carbon adjacent to carboxyl group increase acidity because they pull electron density from away from the O-H bond and hence facilitate ionisation of the carboxyl group and making it a stronger acid.

When carboxylic acids are dissolved in an aqueous solution the form of the carboxylic acid present depends upon the pH of the solution in which it is dissolved.

When the pH of the solution is equal to the pKa of the carboxylic acid the acid and its conjugate base are present in equal concentrations.
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