Substitution Reactions in the Hydrocarbon Part
This section emphasizes two significant types of substitution reactions pertaining to carboxylic acids, specifically halogenation and aromatic electrophilic substitution.
1. Halogenation
Carboxylic acids with an alpha-hydrogen can undergo halogenation, where chlorine or bromine reacts in the presence of red phosphorus, leading to the formation of alpha-halocarboxylic acids. This process is formally recognized as the Hell-Volhard-Zelinsky reaction. The significance lies in modifying the acidity and reactivity of the carboxylic acid by adding halogen atoms at critical positions that affect biological and chemical properties.
2. Ring Substitution
Aromatic carboxylic acids are involved in electrophilic substitution reactions, where they act as deactivating and meta-directing groups during the substitution process. Notably, these acids do not participate in Friedel-Crafts reactions due to the nature of the carboxylic group, which renders the aromatic ring less reactive by forming a complex with the catalyst (usually aluminum chloride). This highlights how substituents can significantly influence the reactivity pattern of aromatic compounds.
In conclusion, understanding substitution reactions helps evaluate the chemical behavior of carboxylic acids and their derivatives in synthetic applications and organic reactions.