Comprehensive Study Notes on Carboxylic Acids and Nomenclature

Chapter nine focuses on the carbonyl group ( $C=O$ ).
The variable "R" attached to the carbonyl can represent a variety of structures: - A chain of carbons. - A ring structure. - A single hydrogen atom ( $H$ ).
The carboxylate group is defined as a carbonyl group ( $C=O$ ) that also has a hydroxyl group ( $OH$ ) attached to the same carbon.
This specific combination ( $COOH$ or $CO_2H$ ) defines a carboxylic acid.

The nomenclature is based on the length of the longest continuous carbon chain including the carboxyl carbon.
Naming Rule: Identify the parent alkane name, drop the final "-e," and add the suffix "-oic acid."
Parent Alkanes to remember: - 1 Carbon: Methane ( $CH_4$ ) - 2 Carbons: Ethane ( $C_2H_6$ ) - 3 Carbons: Propane ( $C_3H_8$ ) - 4 Carbons: Butane ( $C_4H_{10}$ - 5 Carbons: Pentane ( $C_5H_{12}$ - 6 Carbons: Hexane ( $C_6H_{14}$
Specific Examples: - 1 Carbon ( $HCOOH$ ): Methanoic acid (pronounced "Meth-a-no-ic"). - 2 Carbons ( $CH_3COOH$ ): Ethanoic acid (pronounced "Eth-a-no-ic"). - 3 Carbons: Propanoic acid (The transcript mentions "Propylene-oic" as a phonetic exploration, but corrects to Propanoic). - 4 Carbons: Butanoic acid. - 5 Carbons: Pentanoic acid. - 6 Carbons: Hexanoic acid.
The carboxyl carbon is always designated as carbon number one because it is the most important functional group in the structure for naming purposes.

A carboxylic acid acts as an acid because it is a proton donor (based on Chapter six definitions).
A proton is a hydrogen ion ( $H^+$ ), which is what remains after a hydrogen atom loses its electron.
In a structure like Methanoic acid, there may be multiple hydrogens (one on the carbon and one on the oxygen), but only one is acidic.
The Acidic Proton: The specific hydrogen atom attached to the oxygen in the carboxylate group ( $R-COO-H$ ) is the one that is released. Hydrogens attached directly to carbons (like those in alkanes) do not get released.
Weak Acid Nature: All carboxylic acids are weak acids, meaning they do not dissociate completely in water. They instead set up a chemical equilibrium.

Identify the longest continuous chain indicated by the parent name (e.g., Hexane = 6 carbons).
Draw the carbon backbone.
Add the functional group (the carbonyl and the hydroxyl) to the first carbon. - Adding just the carbonyl ( $C=O$ ) creates an aldehyde (e.g., Hexanal). - Adding the hydroxyl ( $OH$ ) to that same carbon creates the carboxylic acid (e.g., Hexanoic acid).
Attach substituents (the "arms and legs") to the numbered carbons based on the prefix.

3,3-dichloropentanoic acid: - Start with a 5-carbon chain (Pentane). - Carbon 1 is the carboxylic acid group. - At Carbon 3, attach two chlorine ( $Cl$ ) atoms.
3-isopropylhexanoic acid: - Start with a 6-carbon chain (Hexane). - Carbon 1 is the carboxylic acid group. - At Carbon 3, attach an isopropyl group.

Benzoic Acid: This is a specialized name that must be memorized. It consists of a benzene ring attached to a carboxylic acid group.
Formation Transition: - Benzene + Carbonyl ( $C=O$ ) = Benzaldehyde. - Benzaldehyde + Hydroxyl ( $OH$ ) on the same carbon = Benzoic acid.
3,5-dibromobenzoic acid: - Start with the benzoic acid structure. - The carbon where the carboxyl group is attached is always Carbon 1. - Count clockwise or counter-clockwise to find positions 3 and 5, then attach bromine ( $Br$ ) atoms to those locations.
2-hydroxybenzoic acid: - Contains a benzoic acid base with a hydroxyl group ( $OH$ ) at the second position. - Note: If the carboxyl group were removed, a benzene ring with an $OH$ would be called Phenol. However, because the carboxylate group is higher in priority, the molecule is named as a derivative of benzoic acid rather than a phenol.

When a carboxylic acid is placed in water, it reacts as a proton donor: - $R-COOH + H_2O ⇌ R-COO^- + H_3O^+$
The formation of the hydronium ion ( $H_3O^+$ ) confirms the acidic reaction.
The resulting negative ion (the conjugate base) is called a carboxylate ion.
Naming the Ion (The "o-ate" rule): Change the "-oic acid" suffix to "-oate ion." - Ethanoic acid → Ethanoate ion. - Methanoic acid → Methanoate ion. - Propanoic acid → Propanoate ion. - Benzoic acid → Benzoate ion.
Structure of the Ion: To draw the ion, copy the acid's structure exactly but remove the acidic hydrogen and add a negative charge to the oxygen.
The "oate" ion is a recurring and critically important concept throughout Chapter 9.