Carboxylic acids-part I (4)

Definition: Carboxylic acids are organic compounds characterized by the presence of a carboxyl group. A carboxyl group is the combination of a carbonyl group ( $C=O$ ) and a hydroxyl group ( $-OH$ ) attached to the same carbon atom.
Acidity: Compounds containing the carboxyl group are distinctly acidic, which is the defining chemical property leading to their name.

General Formula: The general formula for carboxylic acids is $C_nH_{2n}O_2$ , where $n \ge 1$ .
Aliphatic Carboxylic Acids: These consist of an alkyl group ( $R$ ) bonded to the carboxyl group. The general structural formula is $RCOOH$ .
Aromatic Carboxylic Acids: These consist of an aryl group ( $Ar$ ) bonded to the carboxyl group. The general structural formula is $ArCOOH$ .
Substituent Classification: Carboxylic acids are classified based on the nature of the substituent bonded directly to the carboxyl carbon. An aliphatic acid has an alkyl group, while an aromatic acid has an aryl group.

Historical Origins: Many aliphatic carboxylic acids have been known for centuries. Their common names typically reflect their original historical or natural sources. * Formic acid: Extracted from ants (Latin: formica). * Acetic acid: Isolated from vinegar (Latin: acetum, meaning "sour").
Greek Lettering System: In common nomenclature, the positions of substituents are designated using Greek letters ( $\alpha, \beta, \gamma, \delta$ , etc.). * The carbon atom immediately adjacent to the carboxyl carbon is the $\alpha$ -carbon. * The carbon following the $\alpha$ -carbon is the $\beta$ -carbon, and so on.
Specific Examples (Common Names): * $\alpha$ -chloropropionic acid: A propionic acid chain with a chlorine atom on the second carbon (the $\alpha$ position). * $\gamma$ -aminobutyric acid: A butyric acid chain with an amino group ( $NH_2$ ) on the fourth carbon (the $\gamma$ position).

Parent Alkane Selection: The IUPAC name is derived from the name of the alkane corresponding to the longest continuous chain of carbon atoms that includes the carboxyl group.
Suffix Replacement: The final "-e" in the alkane name is replaced by the suffix "-oic acid."
Numbering Rules: The carbon chain is numbered starting with the carboxyl carbon atom as number 1. This numbering determines the positions of all substituents along the chain.
Comparative Examples (IUPAC vs. Common): * $HCOOH$ : IUPAC name is methanoic acid; common name is formic acid. * $CH_3COOH$ : IUPAC name is ethanoic acid; common name is acetic acid. * 4-aminobutanoic acid: IUPAC name for $\gamma$ -aminobutyric acid. * 3-phenylpentanoic acid: An IUPAC name; also known as $\beta$ -phenylvaleric acid. * 3-methylbutanoic acid: IUPAC name for isovaleric acid.

Unsaturated Acids: If the acid contains a double bond, it is named using the parent alkene name. The final "-e" is replaced by "-oic acid." * Numbering starts at the carboxyl carbon. * The location of the double bond is indicated by a number. * Stereochemical descriptors such as cis/trans or $E$ / $Z$ are applied as they are for other alkenes.
Examples of Unsaturated Acids: * (E)-4-methylhex-3-enoic acid: A six-carbon chain with a double bond starting at C3 and a methyl group at C4. * trans-3-phenyl-2-propenoic acid: Also known as (E)-3-phenylprop-2-enoic acid or cinnamic acid.
Cyclic Acids: If the $-COOH$ group is bonded to a ring structure, the compound is named by stating the name of the ring followed by the suffix "-carboxylic acid." * The ring is numbered starting at the carbon atom bonded to the carboxyl group (C1).

Benzoic Acid Derivatives: Aromatic acids are generally named as derivatives of benzoic acid ( $Ph-COOH$ ).
Substituent Positions: The prefixes ortho- ( $o-$ ), meta- ( $m-$ ), and para- ( $p-$ ) may be used for disubstituted rings.
Numeric Locants: Numbers are used if there are more than two substituents on the aromatic ring.
Named Examples: * Benzoic acid: The simplest aromatic carboxylic acid. * p-aminobenzoic acid: Benzoic acid with an amino group in the para position. * o-hydroxybenzoic acid: Commonly known as salicylic acid. * p-methylbenzoic acid: Commonly known as p-toluic acid.

Naming Convention: Aliphatic dicarboxylic acids are named by adding the suffix "-dioic acid" to the name of the parent alkane.
Parent Chain: The parent alkane is the longest continuous chain containing both carboxyl groups.
Numbering: The chain is numbered starting from the carboxyl carbon that results in the lower numbers for substituents.
Examples: * 3-bromohexanedioic acid: A six-carbon chain with two carboxyl groups and a bromine at C3. * 2-methyl-3-phenylpentanedioic acid: A five-carbon chain with two carboxyl groups, a methyl at C2, and a phenyl group at C3.