Amines, Amides, Esters, Acid Halides

This section covers the remaining major compound classes you need for the MCAT: amines, amides, esters, and acid halides. These four groups are especially important because amides and esters form the backbone of proteins (peptide bonds are amide linkages) and lipids (triglycerides are triesters), while amines and acid halides appear throughout biochemical and synthetic pathways.

Naming Amines (-NH2, -NHR, -NR2)

Amines contain a nitrogen atom bonded to one, two, or three carbon groups (plus enough hydrogens to complete nitrogen’s three bonds).

IUPAC suffix: “-amine”
IUPAC prefix (when not the principal group): “amino-”

Steps:

1. Find the longest chain that includes the carbon bonded to nitrogen
2. Replace “-e” with “-amine”
3. Number the chain so the nitrogen-bearing carbon gets the lowest locant

Examples:

• Methanamine: CH3NH2. One carbon + -amine = methanamine (common name: methylamine)
• Ethanamine: CH3CH2NH2 (common name: ethylamine)
• Propan-2-amine: CH3CH(NH2)CH3 (common name: isopropylamine)

Secondary and Tertiary Amines

When nitrogen has two or three carbon groups, the additional groups are named with the prefix “N-” (capital N, to distinguish them from carbon-chain locants):

• N-methylethanamine: CH3NHCH2CH3. The parent chain is ethane with an -amine. The second group on nitrogen is a methyl, designated “N-methyl.”
• N,N-dimethylmethanamine: (CH3)3N. The parent is methanamine. Two additional methyl groups on nitrogen: “N,N-dimethyl.” (Common name: trimethylamine.)

Common Naming for Amines

In the common naming system, name all alkyl groups attached to nitrogen in alphabetical order, then add “-amine”:

• Diethylamine = (CH3CH2)2NH
• Triethylamine = (CH3CH2)3N
• Ethylmethylamine = CH3CH2NHCH3

Naming Amides (-CONH2)

Amides have a carbonyl group bonded directly to a nitrogen. They are carboxylic acid derivatives - formed when the -OH of a carboxylic acid is replaced by -NH2 (or -NHR or -NR2).

IUPAC suffix: “-amide”
IUPAC prefix (when not the principal group): “carbamoyl-”

Steps:

1. Name the parent carboxylic acid
2. Drop “-oic acid” (or “-ic acid” for common names) and add “-amide”

Examples:

• Methanamide: HCONH2. From methanoic acid, drop “-oic acid,” add “-amide” = methanamide (common name: formamide)
• Ethanamide: CH3CONH2. From ethanoic acid = ethanamide (common name: acetamide)
• Propanamide: CH3CH2CONH2

For N-substituted amides, use the “N-” prefix just as with amines:

• N-methylethanamide: CH3CONHCH3
• N,N-dimethylmethanamide: HCON(CH3)2 (common name: DMF, dimethylformamide - a very common solvent)

Naming Esters (-COOR)

Esters have a carbonyl bonded to an oxygen that is itself bonded to a carbon group. They are formed from a carboxylic acid and an alcohol.

IUPAC suffix: “-oate” (as part of a two-word name)

Steps:

1. Name the alkyl group attached to the oxygen (the “alcohol” part)
2. Name the parent carboxylic acid, dropping “-ic acid” and adding “-ate”
3. Write as two words: [alkyl group] [parent acid-ate]

Examples:

• Methyl ethanoate: CH3COOCH3. The oxygen-attached group is methyl. The acid parent is ethanoic acid, so the ester is methyl ethanoate (common name: methyl acetate).
• Ethyl methanoate: HCOOCH2CH3. The oxygen-attached group is ethyl. The acid parent is methanoic acid = ethyl methanoate (common name: ethyl formate).
• Ethyl propanoate: CH3CH2COOCH2CH3.

Esters in Biology

Esters are everywhere in biochemistry:

• Triglycerides (fats and oils): Three fatty acid chains ester-linked to glycerol
• Phospholipids: Fatty acids ester-linked to glycerol with a phosphate head group
• Aspirin: An ester of salicylic acid and acetic acid
• DNA backbone: Phosphodiester bonds link nucleotides

When an MCAT passage mentions “ester hydrolysis” or “saponification,” it is describing the breaking of the ester bond with water (hydrolysis) or base (saponification), regenerating the acid and alcohol components.

Naming Acid Halides (-COX)

Acid halides (also called acyl halides) have a carbonyl bonded to a halogen atom. They are the most reactive carboxylic acid derivatives.

IUPAC naming: Replace “-ic acid” of the parent carboxylic acid with “-yl halide.”

Examples:

• Ethanoyl chloride: CH3COCl. From ethanoic acid, replace “-ic acid” with “-yl chloride” = ethanoyl chloride (common name: acetyl chloride)
• Propanoyl bromide: CH3CH2COBr
• Methanoyl chloride: HCOCl (common name: formyl chloride) - although this compound is unstable

Carboxylic Acid Derivatives - The Big Picture

All four compound classes in this section (amides, esters, acid halides, and anhydrides) are derived from carboxylic acids by replacing the -OH group with something else:

This pattern - same core carbonyl, different group attached - is the foundation of nucleophilic acyl substitution reactions, one of the most heavily tested reaction types on the MCAT.