Properties of Carboxylic acids

PHYSICAL PROPERTIES

Some important physical properties of carboxylic acids are given below,

1. Solubility

As the size of the alkyl group increases, the solubility of the acid decreases and polarity is reduced.

2. Boiling points

Due to intramolecular hydrogen bonding dimerization of acid takes place and boiling point of carboxylic acid is higher than expected.

3. Melting points

The melting points of aliphatic carboxylic acids do not show a regular pattern. The first ten members show a alteration effect, i.e. the melting point of an acid containing even number of carbon atoms is higher than the next lower and next higher homologues containing odd number of carbon atoms.

Illustration 1. On the basis of H-bonding explain that the second ionization constant K₂ for fumaric acid is greater than for maleic acid.

Solution: We know that H-bonding involving acidic H has an acid weakening effect and H-bonding in conjugate base has an acid strengthening effect.

Both dicarboxylic acids have two ionisable hydrogen atoms. Considering second ionization step.

Since the second ionisable H of the Maleate ion participates in H-bonding more energy is needed to remove this H because the H-bond must be broken. The maleate mono anion is, therefore, the weaker acid.

(i) Carbon-oxygen bond length in formic acid are 1.24 Å and 1.36 Å but in sodium formate both the carbon-oxygen bonds have same value i.e.1.27 Å.

(ii) Acetic acid in the vapour state shows a relative molecular weight of 120.

CHEMICAL REACTIONS

The characteristic chemical behavior of carboxylic acids is, of course, determined by their functional group, carboxyl, –COOH. This group is made up of a carbonyl group

(C = O) and a hydroxyl group (–OH). As we shall see, it is the –OH that actually undergoes nearly every reaction. Loss of H⁺, or replacement by another group but it does so in a way that is possible only because of the effect of the C = O.

Carboxylic acids can also show various types of reactions.

(i) Removal of H⁺ (due to cleavage of O $-$ H bond) by reaction with a base (at 'a' above)

(ii) C $-$ O bond breaks at b (by PCl₅, PCl₃, SOCl₂, NH₃/D)

(iii) Nucleophilic attack at point (c) in carboxyl carbon (Ester formation)

Reaction in which OH is replaced by $-$NH₂, Cl is S_N type

(iv) Halogenation at $\alpha$ - C by P/Br₂ at point d (Hell – Volhard Zelinsky reaction)

(v) Oxidation of $\alpha$ - methylene group by SeO₂

\begin{align} RC{{H}_{2}}COOH\xrightarrow[{}]{Se{{O}_{2}}}R\overset{\overset{O}{\mathop{||}}\,}{\mathop{C}}\,COOH+{{H}_{2}}O+Se \\ \,\,\,\,\,\,\,\,\alpha -keto\,\,acid \\\end{align}

Some reactions are summarized below:

Reactions of RCOOH

1. Acidity and salt formation

Acidity of Carboxylic Acids

The acidity of a carboxylic acid is due to the resonance stabilization of its anion.

Because of the resonance, both the carbon oxygen bond in the carboxylate anion have identical bond length. In the carboxylic acid, these bond lengths are no longer identical.

The acidity of carboxylic acid depends very much on the substituent attached to – COOH group. Since acidity is due to the resonance stabilization of anion, substituent causing stabilization of anion increases acidity whereas substituent causing destabilization of anion decrease acidity. For example, electron withdrawing group disperses the negative charge of the anion and hence makes it more stable causing increase in the acidity of the corresponding acid, on the other hand, electron-releasing group increases the negative charge on the anion and hence makes it less stable causing the decrease in the acidity. In the light of this, the following are the orders of a few substituted carboxylic acids.

(a) Increase in the number of Halogen atoms on $\alpha$-position increases the acidity, eg.

Cl₃CCOOH > Cl₂CHCOOH > ClCH₂COOH > CH₃COOH

(b) Increase in the distance of Halogen from COOH decreases the acidity e.g.

This is due to the fact that inductive effect decreases with increasing distance.

(c) Increase in the electronegativity of halogen increases the acidity.

FCH₂COOH > BrCH₂COOH > ICH₂COOH

Illustration 2. Which one of the following would be expected to be most highly ionised in water?

(A) CH₂Cl–CH₂–CH₂COOH (B) CH₃CHCl –CH₂–COOH

(C) CH₃–CH₂–CHCl–COOH (D) CH₃CH₂–CCl₂ –COOH

Solution: (D)

SALT FORMATION

Carboxylic acids are weak acids and their carboxylate anions are strong conjugate bases and are slightly alkaline due to the hydrolysis of carboxylate anion compared to other species, the order of acidity and basicity of corresponding conjugate bases are as follows:

Acidity RCOOH > HOH > ROH > HC $\equiv$ CH > NH₃ > RH

Basicity RCOO^– < HO^– < RO^– < HC $\equiv$ C^– < $N{H}_2^{-}$< R^–

1. The carboxylic acids react with metals to liberate hydrogen and are soluble in both NaOH and NaHCO₃ solutions. For example.

2CH₃COOH + 2Na $\to$ 2CH₃COO^–Na⁺ + H₂

CH₃COOH + NaOH $\to$ CH₃COO^–Na⁺ + H₂O

CH₃COOH + NaHCO₃ $\to$ CH₃COO^–Na⁺ + H₂O + CO₂

Examples:

\begin{align}  2C{{H}_{3}}COOH+Zn\xrightarrow[{}]{{}}{{\left( C{{H}_{3}}COO \right)}_{2}}Zn+{{H}_{2}} \\  Acetic\,acidZinc\,\,acetate \\ \end{align}

\begin{align}  C{{H}_{3}}{{\left( C{{H}_{2}} \right)}_{10}}COOH+NaOH\xrightarrow[{}]{{}}C{{H}_{3}}{{\left( C{{H}_{2}} \right)}_{10}}CO{{O}^{-}}N{{a}^{+}}+{{H}_{2}}O \\  lauric\,acidSodium\,laurate \\ \end{align}

2. Conversion into functional derivatives

(a) Conversion into acid chlorides

Examples:

Illustration 3. Benzoyl chloride is prepared from benzoic acid by

(A) Cl₂,$h\nu$ (B) SO₂Cl₂

(C) SOCl₂ (D) Cl₂, H₂O

Solution: (C)

(b) Conversion into esters

Conversion into Esters (Esterification):

Carboxylic acid on reacting with alcohols in presence of dehydrating agent (H₂SO₄ or dry HCl gas) gives esters. The reaction is known as esterification.

This reaction is reversible and the same catalyst, hydrogen ion, that catalyzes the forward reaction, esterification, necessarily catalyzes the reverse reaction hydrolysis.

The equilibrium is particularly unfavourable when phenols (ArOH) are used instead of alcohol; yet if water is removed during the reaction, phenolic esters [RCOOAr] are obtained in high yield.

The presence of bulky group near the site of reaction, whether in alcohol or in the acid, slows down esterification (as well as its reverse, hydrolysis).

Reactivity CH₃OH > 1° > 2° > 3°

In esterification HCOOH>CH₃COOH>RCH₂COOH > R₂CHCOOH > R₃CCOOH

The Mechanism of the Esterification Reaction:

The step in the mechanism for the formation of an ester from an acid and an alcohol are the reverse of the steps for the acid-catalyzed hydrolysis of an ester, the reaction can go in either direction depending on the conditions used. A carboxylic acid does not react with an alcohol unless a strong acid is used as a catalyst, protonation makes the carbonyl group more electrophilic and enables it to react with the alcohol, which is a weak nucleophile.

Examples:

Illustration 4. Assign a structure to each compound indicated by a letter in the following equations.

Solution:

(c) Conversion into amides

Example:

Illustration 5. Discuss the reason for that a characteristic reaction of aldehydes and Ketones is one of nucleophilic addition while Acyl compounds yield Nucleophilic substitution product.

Solution:

The initial step in both reactions involves nucleophilic addition at the carbonyl carbon atom. It is after the initial nucleophilic attack has taken place that the two reactions differ. The tetrahedral intermediate formed from an aldehyde or ketone usually accepts a proton to form a stable addition product. By contrast, the intermediate formed from an acyl compound usually eliminates a leaving group: this elimination leads to regeneration of the carbon oxygen double bond and to a substitution product. The overall process in the case of acyl substitution occurs, therefore, by a nucleophilic addition–elimination mechanism. Acyl compounds react as they do so because they all have good leaving groups attached to the carbonyl carbon atom.

3. Reduction

$R-COOH\stackrel{LiAl{H}_4}{\to }R-\underset{1^{0}Alcohol}{C{H}_{2}OH}$.

Examples:

4. Substitution in alkyl or aryl group

Halogenation of Aliphatic Acids (Hell-Volhard-Zelinsky Reaction)

In the presence of phosphorus, aliphatic carboxylic acids react smoothly with chlorine or bromine to yield a compound in which $\alpha$-hydrogen has been replaced by halogen.

$C{H}_3-COOH\stackrel{C{l}_2\overrightarrow{\leftarrow }\overrightarrow{\leftarrow }/,P}{\to }Cl-C{H}_2-COOH\stackrel{C{l}_2,/\overrightarrow{\leftarrow }P}{\to }C{l}_{2}CH-COOH\stackrel{C{l}_2,/\overrightarrow{\leftarrow }\overrightarrow{\leftarrow }P}{\to }C{l}_{3}CCOOH$

The function of the phosphorus is ultimately to convert a little of the acid into acid halide so that it is the acid halide, not the acid itself, that undergoes this reaction.

P + X₂ $\to$ PX₃

R – CH₂ – COOH + PX₃ $\to$ RCH₂ – COX

The halogen of these halogenated acid undergoes nucleophilic displacement and elimination same as it does in the simple alkyl halides. Halogenation is therefore the first step in the conversion of a carboxylic acid into many important substituted carboxylic acid.

Examples:

(b) Ring substitution in aromatic acids:

$-$COOH deactivates and directs incoming electrophilic to meta position.

Example:

Illustration 6. Hydrolysis of a compound A (C₇H₃Cl₅) gives an acid B of the formula C₇H₄Cl₂O₂. Decarboxylation of acid yields a neutral substance (C), the nitration of which forms only one mono derivative (D). Identify A, B, C

and D.

Solution: