Properties of amines

PROPERTIES OF AMINES

Key Notes

Structure

Amines consist of an sp³ hybridized nitrogen linked to three substituents by σ bonds. The functional group is pyramidal in shape with bond angles of approximately  109°.  If  the  substituents  are  alkyl  groups,  the  amine  is aliphatic or an alkylamine. If one or more of the substituents is aromatic, the amine is aromatic or an arylamine. If the amine has only one alkyl or aryl substituent, it is defined as primary. If there are two such substituents, the amine is secondary, and if there are three such groups, the amine is tertiary.

Pyramidal inversion

Amines can be chiral if they have three different substituents. However, it is not possible to separate enantiomers since they can easily interconvert by pyramidal inversion. The process involves a planar intermediate where the nitrogen has changed from sp³  hybridization to sp²  hybridization and the lone pair of electrons are in a p orbital. Pyramidal inversion is not possible for chiral quaternary ammonium salts and enantiomers of these structures can be separated.

Physical Properties

Amines are polar compounds with higher boiling points than comparable alkanes. They have similar water solubilities to alcohols due to hydrogen bonding, and low molecular weight amines are completely miscible with water. Low molecular weight amines have an offensive fishy smell.

Basicity

Amines are weak bases which are in equilibrium with their ammonium ion in aqueous solution. The basic strength of an amine is indicated by its pK_b value. There are two main effects on basic strength. Alkyl groups have an inductive effect which stabilizes the ammonium ion and results in increased basicity. Solvation of the ammonium ion by water stabilizes the ion and increases basicity. The more hydrogen bonds which are possible between the ammonium ion and water, the greater the stability and the greater the basicity. The alkyl inductive effect is greates t for ammonium ions formed from tertiary amines, whereas the solvation effect is greatest for ammoniumions  formed  from  primary  amines.  In  general,  primary  and  secondary amines  are  stronger  bases  than  tertiary  amines.  Aromatic  amines  are weaker bases than aliphatic amines since nitrogen’s lone pair of electrons interacts with the π system of the aromatic ring, and is less likely to form a bond to aproton. Aromatic substituents affect basicity. Activating sub- stituents increase electron density in the aromatic ring which helps to stabi- lize the ammonium ion and increase basic strength. Deactivating groups have the opposite effect. Substituents capable of interacting with the aro-matic ring by resonance have a greater effect on basicity if they are at theortho or para positions.

Reactivity

Amines react as nucleophiles or bases since they have a readily available lone pair of electrons which can participate in bonding. Primary and sec-ondary amines can act as weak electrophiles or acids with a strong base, by losing an N–H proton to form an amide anion (R₂N^-).

Spectroscopic analysis

Evidence for primary and secondary amines include N–H stretching and bending absorptions in the IR spectrum as well as a D₂O exchangeable proton in the ¹H nmr spectrum.

Structure

Amines consist of an sp³ hybridized nitrogen linked to three substituents by three bonds. The substituents can be hydrogen, alkyl, or aryl groups, but at least one of the substituents has to be an alkyl or aryl group. If only one such group is present, the amine is defined as primary. If two groups are present, the amine is secondary. If three groups are present, the amine is tertiary. If the substituents are all alkyl groups, the amine is defined as being an alkylamine. If there is at least one aryl group directly attached to the nitrogen, then the amine is defined as an arylamine.

The nitrogen atom has four sp³ hybridized orbitals pointing to the corners of a tetrahedron in the same way as ansp³ hybridized carbon atom. However, one of the sp³ orbitals is occupied by the nitrogen’s lone pair of electrons. This means that the atoms in an amine functional group are pyramidal in shape. The C–N–C bond angles are approximately 109° which is consistent with a tetrahedral nitrogen. However, the bond angle is slightly less than 109° since the lone pair of electrons demands a slightly greater amount of space than a σ bond.

Pyramidal inversion

Since amines are tetrahedral, they are chiral if they have three different substituents. However, it is not possible to separate the enantiomers of a chiral amine since amines can easily undergo pyramidal inversion – a process which interconverts the enantiomers (Fig. 1). The inversion involves a change of hybridization where the nitrogen becomes sp² hybridized rather than sp³hybridized. As a result, the molecule becomes planar and the lone pair of elec-trons occupy a p orbital. Once the hybridization reverts back to sp³, the molecule can either revert back to its original shape or invert.

Although the enantiomers of chiral amines cannot be separated, such amines can be alkylated to form quaternary ammonium salts where the enantiomers can be separated. Once the lone pair of electrons is locked up in aσ bond, pyramidal inversion becomes impossible and the enantiomers can no longer interconvert.

Physical properties

Amines are polar compounds and intermolecular hydrogen bonding is possible for primary and secondary amines. Therefore, primary and secondary amines have higher boiling points than alkanes of similar molecular weight. Tertiary amines also have higher boiling points than comparable alkanes, but have slightly lower boiling points than comparable primary or secondary amines since they cannot take part in intermolecular hydrogen bonding.

However, all amines can participate in hydrogen bonding with protic solvents, which means that amines have similar water solubilities to comparable alcohols.

Low molecular weight amines are freely miscible with water. Low molecular weight amines have an offensive fish-like odor

Basicity

Amines are weak bases but they are more basic than alcohols, ethers, or water. As a result, amines act as bases when they are dissolved in water and an equilibrium is set up between the ionized form (the ammonium ion) and the unionized form (the free base; Fig. 2).

The basic strength of an amine can be measured by its pK_b value. The lower the value of pK_b, the stronger the base. The pK_b for ammonia is 4.74, which compares with pK_b values for methylamine, ethylamine, and propy-lamine of 3.36, 3.25 and 3.33, respectively. This demonstrates that larger alkyl groups increase base strength. This is an inductive effect whereby the ion is stabi-lized by dispersing some of the positive charge over the alkyl group (Fig. 3). This shifts the equilibrium of the acid base reaction towards the ion, which means that the amine is more basic. The larger the alkyl group, the more significant this effect.

Further alkyl substituents should have an even greater inductive effect and one might expect secondary and tertiary amines to be stronger bases than primary amines. This is not necessarily the case and there is no direct relationship between basicity and the number of alkyl groups attached to nitrogen. The inductive effect of more alkyl groups is counterbalanced by a solvation effect.

Once the ammonium ion is formed, it is solvated by water molecules – a stabilizing factor which involves hydrogen bonding between the oxygen atom of water and any N–H group present in the ammonium ion (Fig. 4). The more hydro-gen bonds which are possible, the greater the stabilization. As a result, solvation and solvent stabilization is stronger for alkylaminium ions formed from primary amines than for those formed from tertiary amines. The solvent effect tends to be more important than the inductive effect as far as tertiary amines are concerned and so tertiary amines are generally weaker bases than primary or secondary amines.

Aromatic amines (arylamines) are weaker bases than alkylamines since the orbital containing nitrogen’s lone pair of electrons overlaps with the π system of the aromatic ring. In terms of resonance, the lone pair of electrons can be used to form a double bond to the aromatic ring, resulting in the possibility of three zwit-terionic resonance structures (Fig. 5). (A zwitterion is a molecule containing a pos-itive and a negative charge.) Since nitrogen’s lone pair of electrons is involved in this interaction, it is less available to form a bond to a proton and so the amine is less basic.

The nature of aromatic substituent also affects the basicity of aromatic amines. Substituents which deactivate aromatic rings (e.g. NO₂, Cl, or CN) lower electron density in the ring, which means that the ring will have an electron-withdrawing effect on the neighboring ammonium ion. This means that the charge will be destabilized and the amine will be a weaker base. Substituents which activate the aromatic ring enhance electron density in the ring which means that the ring will have an electron-donating effect on the neighboring charge. This has a stabilizing effect and so the amine will be a stronger base. The relative position of aromatic substituents can be important if resonance is possible between the aromatic ring and the substituent. In such cases, the substituent will have a greater effect if it is at the ortho or para position. For example, para-nitroaniline is a weaker base than meta-nitroaniline. This is because one of the possible resonance structures for the para isomer is highly disfavored since it places a positive charge immediately nextto the ammonium ion (Fig. 6). Therefore, the number of feasible resonance struc-tures for the para isomer is limited to three, compared to four for the meta isomer. This means that the para isomer experiences less stabilization and so the amine will be less basic.

If an activating substituent is present, capable of interacting with the ring by resonance, the opposite holds true and the para isomer will be a stronger base than the meta isomer. This is because the crucial resonance structure mentioned above would have a negative charge immediately next to the ammonium ion and this would have a stabilizing effect..

Reactivity

Amines react as nucleophiles or bases, since the nitrogen atom has a readily available lone pair of electrons which can participate in bonding (Fig. 7). As a result, amines react with acids to form water soluble salts. This allows the easy separation of amines from other compounds. A crude reaction mixture can be extracted with dilute hydrochloric acid such that any amines present are protonated and dissolve into the aqueous phase as water-soluble salts. The free amine can be recovered by adding sodium hydroxide to the aqueous solution such that the free amine precipitates out as a solid or as an oil.

Amines will also react as nucleophiles with a wide range of electrophiles includ-ing alkyl halides, aldehydes, ketones, and acid chlorides.

The N–H protons of primary and secondary amines are weakly electrophilic or acidic and will react with a strong base to form amide anions. For example, diiso-propylamine (pKa ~40) reacts with butyllithium to give lithium diisopropylamide (LDA) and butane.

Spectroscopic analysis

Primary and secondary amines are likely to show characteristic absorptions due to  N–H  stretching  and  N–H  bending.  The  former  occurs  in  the  region 3500–3300 cm⁻¹, and in the case of primary amines two absorptions are visible. The absorptions tend to be sharper but weaker than O–H absorptions which can occur in the same region. N–H bending occurs in the region 1650–1560 cm⁻¹ for primary amines and 1580–1490 cm⁻¹  for secondary amines although the latter tend to be weak and unreliable. These absorptions occur in the same region as alkene and aromatic C=C stretching absorptions, and care has to be taken in assigning them.

Naturally, these absorptions are not present for tertiary amines. For aromaticamines,  an  absorption  due  to  Ar–N  stretching  may  be  visible  in  the  region1360–1250 cm⁻¹.

The ¹H nmr spectrum of a primary or secondary amine will show a broad sig- nal for the N–H proton in the region 0.5–4.5 ppm which will disappear from the spectrum if the sample is shaken with deuterated water. For aromatic amines this signal is typically in the range 3–6 ppm. The chemical shifts of neighboring groups can also indicate the presence of an amine group indirectly. For example, an N-methyl group gives a singlet near 2.3 ppm in the 1H spectrum and appears in the region 30–45 ppm in the ¹³C spectrum.

If the molecular ion in the mass spectrum has an odd number, this indicates that an odd number of nitrogen atoms are present in the molecule. This supports the presence of an amine but does not prove it, since there are other functional groups containing nitrogen. Amines undergo α-cleavage when they fragment (i.e. cleavage next to the carbon bearing the amine group.