Properties of Amines

Amines are more basic than ammonia. In general, secondary amines are more basic than primary amines, which are more basic than tertiary amines. Amines are capable of forming hydrogen bonds with water and therefore have strong intermolecular forces that result in higher melting points, boiling points, and water solubility.
Amines, organic compounds with one or more NR{\rm{N{-}R}} single bonds, are more basic than ammonia. Tertiary amines are considered stereogenic centers because the nitrogen atom is bonded to three different R groups and also has a lone pair. A stereogenic center is a single tetrahedral atom with four different groups. However, the nitrogen atom undergoes nitrogen inversion, a process in which the lone pair on the nitrogen rapidly inverts so that the molecule is not chiral. A chiral molecule is a molecule that does not have a plane of symmetry whose isomers cannot be interconverted by rotation or reflection. If an amine does not have a lone pair, it can no longer invert.

Nitrogen Inversion

A nitrogen inversion is comparable to an umbrella flipping up in the wind.
Anionic amines are the most basic of the amines, and the ammonium ion (NH4+) is the least basic of the amines. Amines are capable of forming hydrogen bonds with water and therefore have higher melting points, boiling points, and water solubility than many other functional groups. However, amines are not as soluble in water as alcohols with similar molecular weights.

Amines are basic, with slight changes in basicity depending upon the alkyl groups. Secondary amines are stronger bases, followed by primary amines, then tertiary amines, and lastly ammonia. This order is because of competition between electron donating group (EDG) effects of the alkyl groups and the steric hindrance of the alkyl groups. EDGs donate electron density to the rest of the compound. Steric hindrance is an effect through 3-D space because of the size of groups around an atom, such as an amine. Secondary amine structures are more basic because there are two electron donating groups, which increase basicity, but there is not too much steric hindrance, which decreases basicity.

When amines are deprotonated, they form strongly basic anions, called amides. Sodium amide (NaNH2) is a strong base, and the negatively charged amine group accepts protons easily. In contrast, the protonated amine conjugate acids, such as the ammonium ion (NH4+), are not basic. The ammonium ion is positively charged and is likely to donate its additional proton to another substance, making it a weak acid.

Chiral amines are used as resolving agents. A resolving agent is a chiral compound added to a racemic mixture to form diastereomeric salts. The salts are separated and separately converted back into the separate R and S enantiomers of the mixture. Resolving agents are used to separate racemic mixtures. Addition of an optically pure chiral amine to a racemic mixture of acids can be used to separate the R and S enantiomers of the acid.

Anilines are compounds that have an amino group bonded directly to a benzene ring. The basicity of anilines varies with the nature of the other substituents on the benzene ring.

  • Anilines are much less basic than amines.
  • Anilines with an electron donating group (EDG) on the ring are more basic than aniline.
  • Anilines with an electron withdrawing group (EWG) on the ring are less basic than aniline.
  • Aromatic heterocyclic amines with localized lone pairs, such as pyridine, are more basic than aniline.
  • Aromatic heterocyclic amines with localized lone pairs are less basic than alkyl amines.
  • Aromatic heterocyclic amines where the lone pair is delocalized into the aromaticity, such as pyrrole, are less basic than aniline.
  • Amides are less basic than all of these because the delocalization of nitrogen’s lone pair into the carbonyl produces resonance.

Electrostatic Potential Maps of Cyclic Compounds

Benzene is a very stable ring with delocalized charges, as shown by the electrostatic potential map. Indole contains a cyclic secondary amine and shows areas with more positive charges near the amine because indole is slightly acidic. The electrostatic potential map of pyridine shows a concentration of negative charge around the nitrogen, making pyridine basic.
Amines have characteristic patterns in spectroscopy. Infrared (IR) spectroscopy is a method of observing the absorption of infrared light through a compound as evidence of the functional groups present. It can be used to differentiate amines. Primary amines have two sharp, medium-intensity stretches between 3,300 cm–1 and 3,500 cm–1 in the IR spectrum because of the two hydrogen atoms on a primary amine (RNH2). Secondary amines will have one sharp, medium-intensity stretch in this same range because of the one hydrogen atom on a secondary amine (R2NH), but tertiary amines will not absorb in that range because there are no hydrogen atoms on a tertiary amine (R3N).

IR Spectra of Amphetamine

Amphetamine is a primary amine. Two medium-intensity peaks are found near the 3,300-3,500 cm-1 range.
Nuclear magnetic resonance (NMR) spectroscopy is a method of measuring the resonance, in parts per million (ppm) of protons or carbon-13 in a compound as evidence of its structure. Different structural components have resonance peaks at specific ppm. NMR can be used to distinguish between amines. The NH{\rm{N{-}H}} peaks show up somewhere between 0 and 5 ppm, and the hydrogen on the alpha carbon to an amine typically shows up between 3 and 4 ppm. The NH {\rm{N{-}H}} peak for a cyclic amine tends to appear between 2 and 5 ppm, and an amide NH{\rm{N{-}H}} appears between 5 and 9 ppm. The integration of the NH{\rm{N{-}H}} peak can help identify if it is a primary amine with 2H integration or secondary amine with 1H integration. The IR spectra provide information about the type of amine but not other structural features, while the NMR spectroscopy provides both information on the type of amine and structural information for the compound.

Methamphetamine NMR

The 1H NMR for methamphetamine shows a peak for the secondary amine (NH) at 3.32 ppm. The hydrogen atoms on the alpha carbons on either side of the amine have peaks at 3.11 and 3.26 ppm, which is the usual range for hydrogen on carbons next to an amine.