BackAmines: Structure, Properties, and Biological Importance
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Chapter 16: Amines
16.1 Classifying Amines
Amines are organic derivatives of ammonia (NH3) in which one or more hydrogen atoms are replaced by organic groups. They are classified based on the number of organic groups attached to the nitrogen atom.
Primary (1°) amine: One organic group attached to nitrogen (RNH2).
Secondary (2°) amine: Two organic groups attached to nitrogen (R2NH).
Tertiary (3°) amine: Three organic groups attached to nitrogen (R3N).
Quaternary ammonium ion: Four organic groups attached to nitrogen, resulting in a positively charged ion (R4N+).
The organic groups may be alkyl or aryl, and can also be connected through a ring structure (heterocyclic amines).
The lone pair on the nitrogen atom is responsible for much of the chemistry of amines, including their basicity and ability to form quaternary ammonium ions.
Examples of alkyl and aryl amines:
16.2 Naming and Drawing Amines
Amines are named according to their structure and classification. The naming conventions differ for primary, secondary, and tertiary amines, as well as for aromatic amines.
Primary alkyl amines (RNH2): Name the alkyl group and add the suffix -amine.
Secondary and tertiary amines (non-heterocyclic): Use di- or tri- prefixes and the suffix -amine.
Complex secondary and tertiary amines: Name as N-substituted derivatives of a primary amine, with the largest group as the parent.
Amino group as a substituent: Use the prefix amino-.
Aromatic amines: Common names are often used (e.g., aniline).
Proteins are polymers of amino acids, which have the general structure H2N—CHR—COOH.
Worked Example 16.1
N,N-diethylbutylamine is a tertiary amine because three alkyl groups are bonded to the nitrogen atom.
Worked Example 16.2
The compound N-methylcyclohexylamine is a secondary amine, as two groups are bonded to the nitrogen atom.
Worked Example 16.3
The heterocyclic amine octahydroindolizine is a tertiary amine, with three carbon groups bonded to nitrogen.
16.3 Properties of Amines
Amines exhibit unique physical and chemical properties due to the lone pair of electrons on the nitrogen atom.
Basicity: Amines act as weak Brønsted-Lowry or Lewis bases.
Hydrogen bonding: Primary and secondary amines can form hydrogen bonds with each other and with water.
Boiling points: Primary and secondary amines have higher boiling points than alkanes of similar size, but lower than alcohols.
Solubility: Amines with up to six carbon atoms are appreciably soluble in water.
Odor and toxicity: Many amines have strong odors and can be irritating or toxic.
16.4 Heterocyclic Nitrogen Compounds
Heterocyclic amines are compounds in which nitrogen is part of a ring structure. These can be aromatic or nonaromatic and are common in biological molecules and drugs.
16.5 Basicity of Amines
Amines are weak bases in aqueous solution, forming hydroxide ions and substituted ammonium ions. The basicity depends on the structure of the amine.
Reaction with water:
Reaction with acids:
Alkylamine cations are named by replacing -amine with -ammonium.
Regeneration of amines: Ammonium ions with at least one hydrogen can react with bases to regenerate the amine.
Amines exist as ammonium ions in aqueous environments such as blood.
Relative basicity: Nonaromatic amines are stronger bases than ammonia; aromatic amines are weaker.
Nonaromatic Amines > Ammonia > Aromatic Amines
Worked Example 16.4
Ammonia and ethylamine both act as bases in water, accepting a hydrogen ion from water (the acid).
Worked Example 16.5
Histamine acetate reacts with potassium hydroxide to regenerate the free amine, water, and potassium acetate.
16.6 Amine Salts
Amine salts are ionic compounds formed from the reaction of amines with acids. They are generally odorless, white, crystalline solids with higher solubility than neutral amines.
Naming: Combine the names of the amine and the acid (e.g., methylamine hydrochloride).
Regeneration: Free amines can be regenerated from their salts by treatment with a base.
Quaternary ammonium ions: Have four organic groups attached to nitrogen, are always positively charged, and are not affected by pH changes.
Benzalkonium chlorides are quaternary ammonium salts with antimicrobial and detergent properties, used in surgical scrubs and instrument sterilization.
16.7 Amines in Plants: Alkaloids
Alkaloids are naturally occurring, nitrogen-containing compounds isolated from plants. They are usually basic, bitter, and often poisonous, serving as a defense mechanism for plants.
Coniine: Extracted from poison hemlock, toxic to humans and animals.
Atropine: Found in deadly nightshade, acts on the central nervous system, used medicinally to reduce digestive cramping and as an antidote to nerve gases.
Solanine: Found in potatoes and tomatoes; toxic at high levels, especially when potatoes turn green due to sunlight exposure.
Morphine and codeine: Alkaloids from the poppy plant, used as painkillers. Heroin is a synthetic derivative of morphine.
Summary Table: Classification of Amines
Type | General Formula | Example |
|---|---|---|
Primary (1°) | RNH2 | CH3CH2NH2 |
Secondary (2°) | R2NH | (CH3CH2)2NH |
Tertiary (3°) | R3N | (CH3CH2)3N |
Quaternary Ammonium | R4N+ | Acetylcholine |
Key Equations:
Base reaction:
Acid reaction:
Regeneration:
