12 Amines and Introductory Organic Synthesis

Understand how amines are classified, named, and transformed, then apply their reactivity to plan selective routes to target molecules.

Amine structure and classification

are ammonia derivatives in which one or more hydrogen atoms are replaced by carbon groups. Nitrogen typically has three bonds and a lone pair, which can accept a proton or donate an electron pair.

Classify an amine by counting the carbon groups attached directly to nitrogen—not by counting the substitutions on a neighboring carbon:

Primary: $\mathrm{RNH_2}$ , with one carbon group attached to nitrogen.
Secondary: $\mathrm{R_2NH}$ , with two carbon groups attached to nitrogen.
Tertiary: $\mathrm{R_3N}$ , with three carbon groups attached to nitrogen.
Quaternary ammonium ion: $\mathrm{R_4N^+}$ , with four carbon groups attached to nitrogen.

A typical amine nitrogen is trigonal pyramidal and can invert through a nearly planar arrangement. As a result, with three different carbon groups on nitrogen usually interconvert too rapidly for their enantiomers to be isolated separately. Quaternary ammonium ions have no lone pair and can be configurationally stable when all four groups differ.

Takeaway: Count the groups bonded directly to nitrogen; the lone pair is central to understanding amine behavior.

Naming

For a simple amine, choose the longest suitable carbon chain that contains the carbon bonded to nitrogen. Number the chain to give the amine the lowest possible locant, then use the suffix -amine. The names propan-1-amine and propan-2-amine distinguish the position of the amine group. Aniline is a common retained name for benzenamine.

When another functional group has naming priority, name $\mathrm{-NH_2}$ with the prefix amino-, as in 2-aminopropanoic acid. Use N- locants for groups attached directly to nitrogen: N-methylethanamine has an ethyl and a methyl group on nitrogen, while N,N-dimethylpropan-1-amine is tertiary. In cyclic , nitrogen is part of the ring; pyrrolidine and piperidine are common examples.

Takeaway: Use the -amine suffix when the amine is the principal group, and N- locants for substituents attached to nitrogen.

and protonation

An amine acts as a base when its nitrogen lone pair binds a proton. For a primary amine in water, the reversible reaction is:

\mathrm{RNH_2 + H_2O \rightleftharpoons RNH_3^+ + OH^-}

The conjugate-acid $\mathrm{p}K_\mathrm{a}$ indicates amine : a higher value corresponds to a stronger amine base. Simple alkylammonium ions typically have $\mathrm{p}K_\mathrm{a}$ values around $10$ to $11$ in water. Alkyl groups tend to increase electron density at nitrogen, but the observed order in water also depends on how well the ions are solvated.

Aniline is much less basic than an alkylamine because its nitrogen lone pair is delocalized into the aromatic ring and is less available to bind $\mathrm{H^+}$ . Electron-withdrawing groups on the ring generally reduce aniline , while electron-donating groups tend to increase it. Amides are also much less basic than because resonance delocalizes the nitrogen lone pair toward the carbonyl oxygen.

Protonation can help separate an amine from neutral compounds. Add aqueous acid to form a water-soluble ammonium salt, separate the aqueous layer, then add base to regenerate the neutral amine.

Takeaway: Resonance can make a lone pair less available for protonation, while reversible protonation can provide a useful separation method.

Reactions of

An amine's lone pair can form a bond to an electrophile, allowing the amine to act as a nucleophile. The product depends on the reaction partner and conditions.

Alkylation

An amine can react with an alkyl halide by an $\mathrm{S_N2}$ mechanism: the nitrogen lone pair forms a bond to the carbon as the leaving group departs. The initial product may still be nucleophilic and react again. Repeated alkylation can produce more highly alkylated and sometimes quaternary ammonium salts, so direct alkylation may be unsuitable when a single product is required.

Acylation

Primary and secondary react with acid chlorides or acid anhydrides to form amides. The amine attacks the carbonyl carbon, creating a tetrahedral intermediate. The intermediate then collapses and expels the leaving group; a base is commonly added to remove acid formed in the reaction. Tertiary lack an $\mathrm{N-H}$ bond and do not form neutral amides by this reaction.

In , an aldehyde or ketone reacts with ammonia or an amine, with loss of water, to form an imine or, under acidic conditions, an iminium ion. A reducing agent then adds hydrogen to the $\mathrm{C=N}$ bond to give an amine. The nitrogen reagent determines the product class: ammonia gives a primary amine, a primary amine gives a secondary amine, and a secondary amine gives a tertiary amine. Plan which group attached to nitrogen will come from the carbonyl compound and which will come from the starting amine.

Reactions of primary arylamines

A primary arylamine such as aniline reacts with nitrous acid under cold acidic conditions to form an . The diazonium group can then be replaced by groups such as chlorine, bromine, iodine, $\mathrm{CN}$ , or $\mathrm{OH}$ . This provides a route to aromatic compounds that may be difficult to make by direct substitution. Diazonium ions can also couple with activated aromatic rings to form azo compounds.

Takeaway: Match the reaction to the bond and product you need. Repeated alkylation can limit selectivity, while joins a carbonyl-derived carbon to nitrogen.

Selective synthesis and route planning

Choose a synthesis route by working backward from the target. Identify the bond that must be formed, consider whether the carbon skeleton must change, and check whether the reaction will be selective and compatible with other functional groups.

connects a carbonyl-derived carbon to nitrogen and is often more selective than direct amine alkylation.
makes a primary amine from a suitable primary alkyl halide. A phthalimide anion displaces the halide by $\mathrm{S_N2}$ , and subsequent hydrolysis releases the amine. The protected nitrogen avoids repeated alkylation during the bond-forming step.
Azide substitution followed by reduction can convert a suitable alkyl halide into a primary amine. Alkyl azides, especially low-molecular-weight ones, can be hazardous and require appropriate professional handling.
converts a primary amide into a primary amine with one fewer carbon atom:

\mathrm{RCONH_2 \longrightarrow RNH_2}

This route is useful when the target carbon skeleton is one carbon shorter than that of the available amide.

Example: from 2-phenylethanol to N-methyl-2-phenylethanamine

First, selectively oxidize the primary alcohol to phenylacetaldehyde. Then perform with methylamine and a suitable reducing agent, such as sodium triacetoxyborohydride:

\mathrm{C_6H_5CH_2CH_2OH \longrightarrow C_6H_5CH_2CHO \longrightarrow C_6H_5CH_2CH_2NHCH_3}

The first step converts an alcohol into an aldehyde. In the second step, methylamine adds to the aldehyde, an imine or iminium intermediate forms, and reduction gives the secondary amine. The route uses the carbonyl carbon to form the new carbon–nitrogen bond while avoiding uncontrolled repeated alkylation.

If creates a stereogenic carbon, as can occur with an unsymmetrical ketone, the planar carbonyl can be attacked from either face. Both configurations may result unless a chiral influence or resolution makes the reaction selective.

Takeaway: Work backward from the target, identify the bond to form, and check for carbon-skeleton changes and competing reactions before choosing a route.

Amine structure and classification

Naming amines

Basicity and protonation

Reactions of amines