Why amine acts as a base

Amines are derivatives of ammonia in which one or more of the hydrogens has been replaced by an alkyl or aryl group. The nomenclature of amines is complicated by the fact that several different nomenclature systems exist, and there is no clear preference for one over the others. When applied to amines these terms refer to the number of alkyl or aryl substituents bonded to the nitrogen atom , whereas in other cases they refer to the nature of an alkyl group.

The four compounds shown in the top row of the following diagram are all C 4 H 11 N isomers. This system names amine functions as substituents on the largest alkyl group.

The Chemical Abstract Service has adopted a nomenclature system in which the suffix -amine is attached to the root alkyl name. These CA names are colored magenta in the diagram. Finally, a common system for simple amines names each alkyl substituent on nitrogen in alphabetical order, followed by the suffix -amine.

These are the names given in the last row colored black. Many aromatic and heterocyclic amines are known by unique common names, the origins of which are often unknown to the chemists that use them frequently. Since these names are not based on a rational system, it is necessary to memorize them. There is a systematic nomenclature of heterocyclic compounds, but it will not be discussed here. Nature abounds with nitrogen compounds, many of which occur in plants and are referred to as alkaloids.

Structural formulas for some representative alkaloids and other nitrogen containing natural products are displayed below, and we can recognize many of the basic structural features listed above in their formulas.

These will be identified by pressing the " Show Structures " button under the diagram. Nitrogen atoms bonded to carbonyl groups, as in caffeine, also tend to be planar. In contrast, atropine, coniine, morphine, nicotine and quinine have stereogenic pyramidal nitrogen atoms in their structural formulas think of the non-bonding electron pair as a fourth substituent on a sp 3 hybridized nitrogen.

In quinine this nitrogen is restricted to one configuration by the bridged ring system. The other stereogenic nitrogens are free to assume two pyramidal configurations, but these are in rapid equilibrium so that distinct stereoisomers reflecting these sites cannot be easily isolated.

It should be noted that structural factors may serve to permit the resolution of pyramidal chiral amines. Because of the molecule's bridged structure, the nitrogens have the same configuration and cannot undergo inversion. The chloro aziridine can invert, but requires a higher activation energy to do so, compared with larger heterocyclic amines. It has in fact been resolved, and pure enantiomers isolated. An increase in angle strain in the sp 2 -hybridized planar transition state is responsible for the greater stability of the pyramidal configuration.

To see these features Click on the Diagram. Of course, quaternary ammonium salts, such as that in muscarine, have a tetrahedral configuration that is incapable of inversion. With four different substituents, such a nitrogen would be a stable stereogenic center. In the formula shown below a triple bond is counted as two double bonds. This molecular formula analysis may be extended beyond hydrocarbons by a few simple corrections.

These are illustrated by the examples in the table above, taken from the previous list of naturally occurring amines. Properties of Amines.

It is instructive to compare the boiling points and water solubility of amines with those of corresponding alcohols and ethers. The dominant factor here is hydrogen bonding , and the first table below documents the powerful intermolecular attraction that results from -O-H O- hydrogen bonding in alcohols light blue columns. Corresponding -N-H N- hydrogen bonding is weaker, as the lower boiling points of similarly sized amines light green columns demonstrate.

Like other amines, these compounds are basic. Such a compound is an alkaloid A nitrogen-containing organic compound obtained from plants that has physiological properties. Caffeine is a stimulant found in coffee, tea, and some soft drinks.

Its mechanism of action is not well understood, but it is thought to block the activity of adenosine, a heterocyclic base that acts as a neurotransmitter, a substance that carries messages across a tiny gap synapse from one nerve cell neuron to another cell. The effective dose of caffeine is about mg, corresponding to about two cups of strong coffee or tea. Nicotine acts as a stimulant by a different mechanism; it probably mimics the action of the neurotransmitter acetylcholine.

People ingest this drug by smoking or chewing tobacco. Its stimulant effect seems transient, as this initial response is followed by depression. Nicotine is highly toxic to animals. It is especially deadly when injected; the lethal dose for a human is estimated to be about 50 mg.

Nicotine has also been used in agriculture as a contact insecticide. Cocaine acts as a stimulant by preventing nerve cells from taking up dopamine, another neurotransmitter, from the synapse. High levels of dopamine are therefore available to stimulate the pleasure centers of the brain. After the binge, dopamine is depleted in less than an hour. This leaves the user in a pleasureless state and often craving more cocaine. Cocaine is used as the salt cocaine hydrochloride and in the form of broken lumps of the free unneutralized base, which is called crack cocaine.

In the first stage of the reaction, the ammonia acts as a Bronsted-Lowry base. With a small amount of ammonia solution, hydrogen ions are pulled off two water molecules in the hexaaqua ion. This produces a neutral complex - one carrying no charge. Because of the lack of charge, the neutral complex isn't soluble in water, and so you get a pale blue precipitate. The reaction is reversible because ammonia is only a weak base. That precipitate dissolves if you add an excess of ammonia solution, giving a deep blue solution.

The ammonia replaces four of the water molecules around the copper to give tetraamminediaquacopper II ions. The ammonia uses its lone pair to form a co-ordinate covalent bond dative covalent bond with the copper. It is acting as an electron pair donor - a Lewis base. The small primary amines behave in exactly the same way as ammonia. There will, however, be slight differences in the shades of blue that you get during the reactions.

With a small amount of methylamine solution you will get a pale blue precipitate of the same neutral complex as with ammonia. All that is happening is that the methylamine is pulling hydrogen ions off the attached water molecules. With more methylamine solution the precipitate redissolves to give a deep blue solution - just as in the ammonia case.

In the case of aliphatic R groups, the diazonium ions are extremely unstable, rapidly decomposing to give carbocations which undergo reaction with whatever nucleophiles may be present such as water. The reason this especially high level of reactivity is that dinitrogen, being thermodynamically highly stable, is an outstanding leaving group.

Positive charge on nitrogen is inherently not very favorable electronegative atom , but resonance stabilization makes this ion stable enough to form. Even when these diazonium ions are formed at ice bath temperatures, they lose nitrogen extremely quickly, forming a carbocation, which then reacts with available nucleophiles e. This permits the use of the aryldiazonium ions in reactions with substances supplied after the diazonium ion is generated.

In other words, the pi system of the N-N pi bond overlaps with the pi system of the benzene ring, providing delocalization of the positive charge onto the ortho and para positions of the benzene ring. Azo Compounds. However, they are relatively mild not highly reactive, but very selective electrophiles, because of their resonance stabilization.

As very mild and selective electrophiles, they do not react with benzene or toluene or even anisole methoxybenzene—normally considered a highly reactive aromatic. They do, however, reactive with aromatics which have the powerfully electron donating amine function. In particular, N,N-dimethylaniline reacts readily with aryl diazonium ions as shown below:.

Another common use for aryldiazonium ions is in the transformation of the amino group of aniline or a derivative of aniline to other functionality such as a halide or a nitrile function. This involves the addition of an appropriate salt containing the desired nucleophile to the cold, aqueous solution containing the diazonium ion and the allowing the temperature to ascend to room temperature.

In this way, the diazonium ion decomposes to the aryl carbocation, which then reacts with the appropriate nucleophile. However, when the potent leaving group is dinitrogen, even aryl systems can undergo an S N 1 substitution reaction. Consider the acid dissociation, in dilute aqueous solution, of ammonia and a representative primary, secondary, and tertiary amine: q Note that the strongest acid least positive pK a is ammonia.

See the indicated overlap in the orbital picture shown below: q This conjugation is only possible when the orbital external to the ring is in the benzylic-type position that is, on an atom directly attached to the ring.

The Hoffmann Elimination Reaction. Recall that alkyl halides except fluorides and alcohols in the presence of acid can undergo elimination reactions to give alkenes. In both of these systems, good leaving groups are present, thus permitting an E2 elimination or in some cases an E1 elimination. In the case of halides, the chloride, bromide, and iodide ions are good leaving groups. Recall that good leaving groups are weak bases. In the case of alcohols, the hydroxide ion, being a strong base, is a poor leaving group, but in acidic solution, when protonated, a good leaving group is generated water.

What about amines. If we should want to perform an elimination reaction on an amine to convert this functionality to an alkene function, could we do it? And how could we do it? Azo Compounds q Aryldiazonium ions are electron deficient and therefore are electrophilic.

In particular, N,N-dimethylaniline reacts readily with aryl diazonium ions as shown below: q The final product of this reaction is generically called an azo comound.