Chemical Reactions and Carboxylic Acids: Class 12 NCERT Guide

Introduction to Carboxylic Acids and Their Functional Group

Carboxylic acids are organic compounds containing the carboxyl group ($-COOH$). This group consists of a carbonyl ($C=O$) and a hydroxyl ($-OH$) attached to the same carbon atom. The presence of this group imparts acidic properties and unique chemical reactivity to these compounds.

Common examples include acetic acid ($CH_3COOH$) and formic acid ($HCOOH$). Carboxylic acids are widely studied in Class 12 NCERT Chemistry due to their importance in both biological systems and industrial applications.

Key features:

• Polar nature due to $-COOH$ group
• Ability to form hydrogen bonds
• Acidity caused by release of $H^+$ ion

Understanding the structure helps explain their reactions and derivatives.

Acid-Base Reactions of Carboxylic Acids

Carboxylic acids behave as acids because they can donate a proton ($H^+$) from the $-COOH$ group. This leads to several important reactions:

• Reaction with metals: Carboxylic acids react with active metals like sodium or magnesium to produce hydrogen gas and metal carboxylate salts.

Example: $$2CH_3COOH + 2Na \rightarrow 2CH_3COONa + H_2 \uparrow$$

• Reaction with bases: They form carboxylate salts by neutralization.

Example: $$CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O$$

These reactions are fundamental and frequently asked in Class 12 exams. The salt formation is important for industrial uses of carboxylic acids.

Reduction of Carboxylic Acids to Primary Alcohols

Carboxylic acids can be reduced to primary alcohols using strong reducing agents such as lithium aluminium hydride ($LiAlH_4$). This reaction is significant in organic synthesis and is covered in detail in NCERT.

Reaction: $$R-COOH \xrightarrow{LiAlH_4} R-CH_2OH$$

For example, acetic acid reduces to ethanol: $$CH_3COOH \xrightarrow{LiAlH_4} CH_3CH_2OH$$

This reaction does not occur with milder reducing agents like sodium borohydride ($NaBH_4$), highlighting the strength of $LiAlH_4$.

Worked Example: Reduce butanoic acid to its corresponding alcohol.

Solution: $$CH_3CH_2CH_2COOH \xrightarrow{LiAlH_4} CH_3CH_2CH_2CH_2OH$$

This forms butanol, a primary alcohol.

Formation of Acid Chlorides and Their Importance

Carboxylic acids react with reagents like phosphorus pentachloride ($PCl_5$) or thionyl chloride ($SOCl_2$) to form acid chlorides, which are more reactive derivatives.

General reaction: $$R-COOH + PCl_5 \rightarrow R-COCl + POCl_3 + HCl$$

Acid chlorides are useful intermediates in organic synthesis because they react readily with nucleophiles to form esters, amides, and other derivatives.

Comparison Table:

Understanding acid chlorides helps in mastering the synthesis of various carboxylic acid derivatives.

Esterification: Formation of Esters from Carboxylic Acids

Esterification is a key reaction where carboxylic acids react with alcohols in the presence of an acid catalyst (usually concentrated sulfuric acid) to form esters and water. This is a reversible reaction known as Fischer esterification.

Reaction: $$R-COOH + R'-OH \xleftrightarrow[H_2O]{H^+} R-COOR' + H_2O$$

Example: Acetic acid reacts with ethanol to form ethyl acetate: $$CH_3COOH + C_2H_5OH \xleftrightarrow[H_2O]{H_2SO_4} CH_3COOC_2H_5 + H_2O$$

Activity Suggestion: Prepare ethyl acetate in the lab by heating acetic acid and ethanol with concentrated sulfuric acid and observe the fruity smell.

Esters have pleasant smells and are used in perfumes and flavorings.

Decarboxylation of Carboxylic Acids

Decarboxylation is the removal of the carboxyl group ($-COOH$) as carbon dioxide ($CO_2$) from carboxylic acids upon heating with soda lime (a mixture of sodium hydroxide and calcium oxide).

Reaction: $$R-COOH + NaOH \xrightarrow{CaO, \Delta} R-H + Na_2CO_3$$

For example, sodium salt of benzoic acid on heating gives benzene: $$C_6H_5COONa + NaOH \xrightarrow{CaO, \Delta} C_6H_6 + Na_2CO_3$$

This reaction is useful for preparing hydrocarbons from carboxylic acids and is an important concept in Class 12 Chemistry.