Why are no tertiary amines acetylated
13. Amines and their derivatives
Amines are derivatives of ammonia in which one to three hydrogen atoms have been replaced by alkyl or aryl groups. Accordingly, there are primary amines, secondary amines and tertiary amines:
A compound with four substituents on the N atom is called a quaternary ammonium salt. An amine with alkyl substituents is called an alkylamine. An amine with aryl substituents (aromatic rings) is called arylamine. The N atom in one Alkylamine is sp3-hybridized, but in one Arylaminesp2-hybridized.
13.1 Naming of the amines
As with other functional groups, one gets confused by the multitude of trivial names in the literature. It is best to name aliphatic amines after the Chemical abstractsSystem, i.e. it is considered to be Alkanamineswhere the word "amine" is added to the name of the alkane. The position of the functional group is indicated by the numbering of the carbon atom attached to it, as in the case of alcohols, e.g .:
In the case of secondary and tertiary amines, the largest alkyl substituent forms the eponymous stem of the alkanamine, the other groups attached to the N atom are identified by an N-prefixed to their name, e.g .:
13.2 Structure and physical properties of amines
The nitrogen atom in Alkylamines is sp3-hybridizes and therefore forms an almost regular tetrahedron. The substituents occupy three of the tetrahedron corners - the lone pair of electrons on the N atom points into the fourth. The nitrogen atom in Aryl amines however, is sp2-hybridized and therefore planar. The lone pair of electrons is now in a p orbital:
However, the tetrahedral geometry at the N atom in the alkylamine is not rigid, as it is rapid inversion can take place. For this movement, an energy barrier of about 21-29 KJ / mol has to be overcome in simple amines:
It is therefore not possible to obtain an isomerically pure, simple di- or trialkylated amine at RT. The inversion takes place very quickly at RT.
The special ability of alcohols to form hydrogen bonds is responsible for their high boiling points. We find the same properties in amines:
However, amines form weaker hydrogen bonds than alcohols. The boiling points of the amines are generally between those of the corresponding alkanes and alcohols. Simple amines are soluble in water and alcohols.
13.3 Basicity of amines
The chemistry of amines is dominated by their lone pair of electrons on the nitrogen atom. Because of this lone pair of electrons, amines have basic and nucleophilic Properties.
As expected, amines are much more basic than alcohols. Amines deprotonate water to a small extent, so that ammonium and hydroxide ions are formed:
One possibility to compare the relative basicity of amines is through the pKa Values of the corresponding ammonium ions:
|Surname||structure||pKa of the ammonium ion|
| Primary amines || Methylamine || MeNH2|| 10.64 || Ethylamine || CH3CH2NH2|| 10.75 || aniline || Ph-NH2|| 4.63 || Secondary amines || Dimethylamine || Me2NH || 10.73 || Diethylamine || Et2NH || 10.94 || Tertiary amines || Trimethylamine || Me3N || 9.79 || Triethylamine || Et3N || 10.79 |
We also see that Arylamines, like aniline, are much weaker bases than alkylamines. Why?
Obviously, the protonated form is now not as preferred as with the alkylamine. The reason for this can be found by comparing the structure of both forms:
Arylamines are more stable than alkylamines because there are several resonance structures for arylamines, in which the lone pair of electrons is delocalized over the aromatic ring (they are therefore "less accessible" for protonation). This resonance effect is lost during protonation. So we lose resonance stabilization, and therefore the protonated form is less preferred.
As a reminder: at the Amide the N atom is no longer basic at all. So amides are in terms of acid-base properties neutral :
The basic properties of amines can be used to purify amines:
13.4 Synthesis of amines
Alkylation of amines
Amines are nucleophilic: they react with haloalkanes to form alkali amines. However, the reaction is difficult to control. Multiply substituted products are formed, mostly even tetraalkylammonium salts.
For example :
Let us consider the alkylation of ammonia with 1-bromobutane. If we present equimolar amounts of the starting substances, the product is butylammonium bromide, which immediately exchanges a proton with the ammonia present. The small amounts of butylamine that are formed in this way compete with ammonia for the alkylating reagent. Further alkylation creates a dibutylammonium salt, which can give its proton to either of the two N-bases present, so that di-n-butylamine is formed. etc. This ultimately leads to a mixture of alkylammonium salts and alkanamines:
Reduction of nitriles
We have already seen that reduction with LiAlH4 or by hydrogenation with Pd / C catalyst and H2 Nitriles can be converted into amines:
Reduction of nitroarenes
The simplest method for the synthesis of arylamines is the reduction of the corresponding nitro aromatic. The nitro group reduction can be carried out under various conditions, e.g .:
13.5 reactions of amines
The chemical behavior of amines is essentially determined by the nucleophilicity of the nitrogen atom. This property has already been mentioned in previous sections, e.g .:
Amine + haloalkane
Amine + acid chloride
Amine + aldehyde / ketone
In the last two processes, tertiary amines are not reactive because they do not have a hydrogen atom on the nitrogen atom that could be split off:
The nucleophilic attack of an amine on a haloalkane creates an ammonium ion. In the case of a quaternary ion, no further alkylation is possible, since there are no longer any substitutable protons and the lone pair of electrons on the N atom is also not available.
Quaternary ammonium salts are unstable in the presence of strong bases because they can undergo bimolecular elimination (E2 response), which leads to the formation of an alkene. The base attacks the hydrogen atom in the ß-position to the nitrogen, and the neutral trialkylamine emerges together with its electron pair:
This reaction is called Hofmann elimination. It is similar to the acid-catalyzed dehydration of alcohols, in which water acts as a leaving group.
Eliminations are often found in nature:
13.6 Amines in Nature - Alkaloids
Nitrogen is contained in a large number of physiologically active compounds. Many known natural substances contain amine groups, and many other synthetic medicinally active substances also contain N atoms in the form of amine groups, e.g .:
Alkaloids are natural nitrogen-containing compounds that are mainly found in plants. The name is derived from the fact that all alkaloids show characteristic basic (alkali-like) properties, which are caused by the lone pair of electrons on the N atom. Many alkaloids have an extraordinarily strong pharmacological effect and are important drugs:
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