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23.1: Properties of amines

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  • Ammonia

    Amines are organic compounds which contain an

    Amines are organic compounds which contain and are often actually based on one or more atoms of nitrogen. Structurally amines resemble ammonia in that the nitrogen can bond up to three hydrogens, but amines also have additional properties based on their carbon connectivity. In an amine, one or more of the hydrogen atoms from ammonia are replaced by organic substituents like alkyl (alkane chain) and aryl (aromatic ring) groups.

    Another type of organic molecule contains nitrogen without being, strictly speaking, an amine: carboxylic acid derivatives containing a trivalent (three-bond) ammonia in ground state are actually amides instead of amines. Amides and amines have different structures and properties, so the distinction is actually very important. Organic-nitrogen compounds containing metals are also called amides, so if you see a molecule that has a nitrogen and either a carbonyl group or a metal next to that nitrogen, then you know that molecule should be an amide instead of an amine.

    Properties

    Types of amines

    Amines can be either primary, secondary or tertiary, depending on the number of carbon-containing groups that are attached to them. If there is only one carbon-containing group (such as in the molecule CH3NH2) then that amine is considered primary. Two carbon-containing groups makes an amine secondary, and three groups makes it tertiary. Utilizing the lone electron pair of nitrogen, it is sometimes energetically favored to use the nitrogen as a nucleophile and thus bind a fourth carbon-containing group to the amine. In this case, it could be called a quaternary ammonium ion.

    Primary Amine
    SecondaryAmine
    Tertiary Amine

    An organic compound with multiple amine groups is called a diamine, triamine, tetraamine and so forth, based on the number of amine groups (also called amino groups) attached to the molecule. The chemical formula for methylene diamine (also called diaminomethane), for example, would be as follows: H2N-CH2-NH2

    Aromatic amines

    Aromatic amines have the nitrogen atom directly connected to an aromatic ring structure. Due to its electron withdrawing properties, the aromatic ring greatly decreases the basicity of the amine – and this effect can be either strengthened or offset depending on what substituents are on the ring and on the nitrogen. The presence of the lone electron pair from the nitrogen has the opposite effect on the aromatic ring itself; because the nitrogen atom can “loan” electron density to the ring, the ring itself becomes much more reactive to other types of chemistry.

    Naming conventions

    For primary amines, where the amine is not the principal characteristic group, the prefix “amino-” is used. For example: 4-aminobenzoic acid where the carboxylic acid is the principal characteristic. Otherwise, the suffix “-amine” is used with with either the parent hybride or the R group substituent name. Example: ethanamine or ethylamine. Alternatively, the suffix “-azane” can be appended to the R group substituent name: Example: propylazane.

    For secondary, tertiary, and quarternary amines, the naming convention is a bit different, but the suffixes are the same. For symmetrical amines, the “di” or “tri” prefix is used depending on whether there are 2 or 3 substituents. For example, dipropylamine is a secondary amine, and triphenylamine is a tertiary amine. For asymmetric amines, the parent chain gets the “-amine” suffix. This name is then prefixed with “N-” (indicating the nitrogen bond) and the substituent group name, for each substituent, using alphabetic order for tertiary amides. For example, N-ethyl-N-methyl-propylamine, not N-methyl-N-ethyl-propylamine.

    To sum up:

    • as prefix: “amino-“
    • as suffix: “-amine”
    • the prefix “N-” shows substitution on the nitrogen atom (in the case of secondary, tertiary and quaternary amines)

    Systematic names for some common amines:

    methylamine

    Physical properties

    As one might readily guess, the inclusion of a heteroatom such as nitrogen in otherwise exclusively carbon and hydrogen molecules has quite an effect on the properties of amines as compared to alkanes.

    General properties

    Hydrogen bonding significantly influences the properties of primary and secondary amines as well as the protonated derivatives of all amines. Thus the boiling point of amines is higher than those for the corresponding phosphines (compounds containing phosphorus), but generally lower than the corresponding alcohols. Alcohols, or alkanols, resemble amines but feature an -OH group in place of NR2. Since oxygen is more electronegative than nitrogen, RO-H is typically more acidic than the related R2N-H compound.

    Methyl, dimethyl, trimethyl, and ethyl amines are gases under standard conditions. Most common alkyl amines are liquids, and high molecular weight amines are, quite naturally, solids at standard temperatures. Additionally, gaseous amines possess a characteristic ammonia smell, while liquid amines have a distinctive “fishy” smell.

    Most aliphatic amines display some solubility in water, reflecting their ability to form hydrogen bonds. Solubility decreases relatively proportionally with the increase in the number of carbon atoms in the molecule – especially when the carbon atom number is greater than six. Aliphatic amines also display significant solubility in organic solvents, especially in polar organic solvents. Primary amines react readily with ketone compounds (such as acetone), however, and most amines are incompatible with chloroform and also with carbon tetrachloride as solvent solutions.

    Aromatic amines have their lone pair electrons conjugated (“shared”) into the benzene ring, so their tendency to engage in hydrogen bonding is somewhat diminished. The boiling points of these molecules are therefore usually somewhat higher than other, smaller amines due to their typically larger size.
    They also often have relatively diminished solubility in water, although they retain their solubility in other organic solvents.

    Aromatically conjugated amines are often quite toxic and have the potential to be easily absorbed through the skin, so should always be treated as “hazardous”.

    Chirality

    Tertiary amines of the type NHRR’ and NRR’R” are not chiral: although the nitrogen atom bears four distinct substituents counting the lone pair, the lone pair of electrons can “flip” across the nitrogen atom and invert the other molecules. The energy barrier for just such a Walden inversion of the stereocenter with a lone pair of electrons is relatively low, e.g. ~7 kcal/mol for a trialkylamine, therefore it is difficult to obtain reliably chiral products using tertiary amines. Because of this low barrier, amines such as NHRR’ cannot be resolved optically and NRR’R” can only be resolved when the R, R’, and R” groups are constrained in cyclic structures. Quaternary amine structures, e.g. H3C-N+-RR’R”, are chiral and are readily optically resolved.

    Properties as bases

    Like ammonia, amines act as bases and are reasonably strong (see the provided table for some examples of conjugate acid Ka values). The basicity of amines varies by molecule, and it largely depends on:

    • The availability of the lone pair of electrons from nitrogen
    • The electronic properties of the attached substituent groups (e.g., alkyl groups enhance the basicity, aryl groups diminish it, etc.)
    • The degree of solvation of the protonated amine, which depends mostly on the solvent used in the reaction

    The nitrogen atom of a typical amine features a lone electron pair which can bind a hydrogen ion (H+) in order to form an ammonium ion — R3NH+. The water solubility of simple amines is largely due to the capability for hydrogen bonding that can occur between protons on the water molecules and these lone pairs of electrons.

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