Organic Chemistry - Chapter 19: Amines

Classes of Amines

Primary ( $1^\circ$ ): This class of amine possesses a single alkyl group bonded directly to the nitrogen atom, represented by the general formula $\text{RNH}_2$ .
Secondary ( $2^\circ$ ): This class features two alkyl groups bonded to the nitrogen atom, represented by the general formula $\text{R}_2\text{NH}$ .
Tertiary ( $3^\circ$ ): This class has three alkyl groups bonded to the nitrogen atom, represented by the general formula $\text{R}_3\text{N}$ .
Quaternary ( $4^\circ$ ): These are quaternary ammonium salts where the nitrogen atom is bonded to four alkyl groups ( $\text{R}_4\text{N}^+$ ) and consequently bears a positive formal charge.

Specific Classification Examples

Primary ( $1^\circ$ ) Amines: * Cyclohexylamine: A cyclic hydrocarbon chain attached to an amino group. * Tert-butylamine: A tert-butyl group attached to an amino group, denoted as $\text{CH}_3\text{-C(CH}_3\text{)}_2\text{-NH}_2$ .
Secondary ( $2^\circ$ ) Amines: * $N$ -ethylaniline: An aniline derivative with an ethyl group on the nitrogen. * Piperidine: A saturated six-membered heterocyclic ring containing one nitrogen atom.
Tertiary ( $3^\circ$ ) Amines: * $N,N$ -diethylaniline: An aniline derivative with two ethyl groups on the nitrogen. * Quinuclidine: A bridged bicyclic tertiary amine.
Quaternary Ammonium Salts: * The nitrogen atom is attached to four alkyl groups. * The nitrogen atom is positively charged.

Nomenclature of Amines

Common Names: These are constructed by naming the alkyl groups bonded to the nitrogen atom in alphabetical order, followed by the suffix "-amine."
Amines as Substituents: When a molecule contains a functional group of higher priority (such as a carboxylic acid or an alcohol), the amine moiety is named as an "amino-" substituent.
IUPAC Names: * The naming convention is based on identifying the longest continuous carbon chain containing the nitrogen atom. * The terminal "-e" of the corresponding alkane name is replaced with the suffix "-amine."
Aromatic Amines: * Aromatic amines are defined by an amino group bonded directly to a benzene ring. * The parent compound is specifically named "aniline."

Heterocyclic Amines

Rule for Cyclic Amines: When naming heterocyclic amines, the nitrogen atom is systematically assigned position number 1.
Examples of Heterocycles: * Aziridine: A three-membered ring containing nitrogen. * Pyrrole: A five-membered aromatic ring with nitrogen and two double bonds. * Pyrrolidine: A saturated five-membered ring containing nitrogen. * $1$ -methylpyrrolidine (also known as $N$ -methylpyrrolidine). * Imidazole: A five-membered ring with two nitrogen atoms. * Indole: A bicyclic structure with a benzene ring fused to a pyrrole ring. * Pyridine: A six-membered aromatic ring with one nitrogen atom. * Piperidine: A saturated six-membered ring with one nitrogen atom. * $2$ -methylpyridine: Pyridine with a methyl group at the second position. * Pyrimidine: A six-membered aromatic ring with two nitrogen atoms at the 1 and 3 positions. * Purine: A bicyclic aromatic heterocycle consisting of a pyrimidine ring fused to an imidazole ring.

Structure and Basicity of Amines

Molecular Structure: * The nitrogen atom in an amine is $sp^3$ hybridized. * It possesses a lone pair of electrons in one of the $sp^3$ orbitals. * Due to the presence of the lone pair, the bond angles are slightly less than the ideal tetrahedral angle of $109.5^\circ$ .
Basicity Properties: * Amines act as bases because the lone pair of electrons on the nitrogen can accept a proton ( $H^+$ ) from an acid. * Aqueous solutions of amines are basic and will turn litmus paper blue. * Ammonia Basicity: Ammonia has a $pK_b = 4.74$ . * Alkyl Amine Basicity: Alkyl amines are generally stronger bases than ammonia due to the electron-donating effect of alkyl groups. * Solvation Effects: Increasing the number of alkyl groups around the nitrogen decreases the solvation of the resulting ammonium ion. Consequently, secondary and tertiary amines exhibit basicity similar to that of primary amines.
Stabilization: Alkyl group stabilization makes the nitrogen a more effective base than ammonia.

Reactivity of Amines

As a Nucleophile: Amines react with electrophiles. For example, the reaction of an amine ( $R\text{-NH}_2$ ) with methyl iodide ( $\text{CH}_3\text{I}$ ) results in the formation of a new $N\text{-C}$ bond as the amine attacks the electrophilic methyl carbon.
As a Proton Base: Amines react with proton acids ( $HX$ ) to form protonated ammonium salts ( $R\text{-NH}_3^+ X^-$ ).

Spectroscopy of Amines

Infrared (IR) Spectroscopy: * The $N\text{—}H$ stretching frequency occurs between $3200\,cm^{-1}$ and $3500\,cm^{-1}$ . * Primary ( $1^\circ$ ) amines typically show two distinct peaks in this region (symmetric and asymmetric stretching). * Secondary ( $2^\circ$ ) amines show only one peak.
Nuclear Magnetic Resonance (NMR) Spectroscopy: * Nitrogen is less electronegative than oxygen. As a result, the protons located on the $\alpha$ carbon atoms (the carbons directly attached to the nitrogen) are not as strongly deshielded as those on oxygen-bearing carbons. * Protons on the $\alpha$ carbon usually appear in the range of approximately $2$ to $3\,ppm$ on the $δ$ (ppm) scale.

Reactions with Ketones and Aldehydes

Primary ( $1^\circ$ ) Amines: React with ketones or aldehydes to form a carbinolamine (hemiaminal) intermediate, which then undergoes dehydration to produce derivatives: * Reaction with $R\text{-NH}_2$ ( $Y = H$ or alkyl) yields an imine. * Reaction with hydroxylamine ( $Y = OH$ ) yields an oxime. * Reaction with hydrazine or its derivatives ( $Y = NHR$ ) yields a hydrazone.
Secondary ( $2^\circ$ ) Amines: React with ketones or aldehydes to form an iminium salt, which can then lead to enamines (though specific enamine nomenclature was not explicitly detailed on the 2° amine slide, the iminium intermediate is the key species).

Alkylation of Amines

Reaction with Alkyl Halides: * Proceeds via an $S_N2$ mechanism. * Secondary alkyl halides are prone to giving elimination products instead of substitution. * The Problem of Multiple Alkylations: Amines are nucleophilic and can react multiple times with alkyl halides, leading to complex mixtures of primary, secondary, tertiary, and quaternary products.
Alkylations with Excess Ammonia: To favor the formation of a primary amine in good yields, a large excess of ammonia ( $NH_3$ ) is used. The excess ammonia is allowed to evaporate after the reaction is complete.
Exhaustive Alkylation: This process drives the reaction until the tetraalkylammonium salt ( $4^\circ$ salt) is formed. Mild basic conditions, such as using sodium bicarbonate ( $NaHCO_3$ ), are required to deprotonate intermediates and neutralize the acid produced during the reaction.

The Hofmann Elimination

Amines as Leaving Groups: Amino groups ( $\text{-NH}_2$ or $\text{-NHR}$ ) are very strong bases and therefore terrible leaving groups. To make them leave, they must be converted into quaternary ammonium salts.
Exhaustive Methylation: Methyl iodide ( $CH_3I$ ) is used to convert the amino group into a quaternary ammonium salt ( $R_4N^+$ ). The leaving group then becomes a neutral amine.
Elimination Process: * The quaternary ammonium salt is converted to a hydroxide salt. * Heating the hydroxide salt triggers a one-step, concerted E2 reaction.
Regioselectivity (Hofmann Product): Unlike the Zaitsev rule, the Hofmann elimination predominantly produces the least substituted alkene.

Synthesis of Amines

Reductive Amination: This is the most general method for synthesizing amines, capable of adding primary or secondary alkyl groups. * Secondary Amine Synthesis: Condensation of a ketone/aldehyde with a primary amine forms an $N$ -substituted imine (Schiff base), which is then reduced. * Tertiary Amine Synthesis: Condensation of a ketone/aldehyde with a secondary amine forms an iminium salt. Because these are unstable and rarely isolated, a reducing agent in the solution reduces them directly to a tertiary amine.
Acylation–Reduction: * Primary to Secondary: A primary amine is acylated using an acid chloride to form an amide (this prevents overacylation). The amide is then reduced using lithium aluminum hydride ( $LiAlH_4$ ) followed by hydrolysis to yield a secondary amine. * Secondary to Tertiary: Similarly, a secondary amine is acylated and then reduced with $LiAlH_4$ to produce a tertiary amine.
Reduction of Nitriles: * Nitriles ( $C \equiv N$ ) are excellent $S_N2$ nucleophiles. * Reduction of nitriles with hydrogen gas ( $H_2$ ) or $LiAlH_4$ converts the nitrile group into a primary amine.