Part 8. The chemistry of organic nitrogen compounds

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

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Part 8.2 Structure, classification and physical properties of aliphatic amines

Sub-index for this page

8.2.1 The structure and sub-classification of types of amines

8.2.2 The data table of the physical properties of aliphatic amines

8.2.3 Discussion of the physical properties of aliphatic amines - various comparisons

8.2.1 The structure and sub-classification of types of amines

 I've included some aromatic amines for comparison

Functional group of the homologous series	PRIMARY	SECONDARY	TERTIARY	Comments
AMINES				There are prim/sec/tert aliphatic (alkyl) or aromatic (aryl) amines.
aliphatic amine examples	ethylamine	N-ethylmethylamine	triethylamine	aliphatic amine examples
aromatic amine examples	phenylamine	diphenylamine	triphenylamine	aromatic amine examples

The types or sub-classification of amines

 

TOP OF PAGE and sub-index

8.2.2 The physical properties of aliphatic amines

Abbreviations used: mpt = melting point;  bpt = boiling point; sub. = sublimes, dec.= thermally decomposes

8.2.3 Discussion of the physical properties of aliphatic amines - various comparisons

Extra notes mainly based on the data table of aliphatic amines above

(a) Alternative names of linear primary aliphatic amines

1-aminopropane (propylamine), 1-aminobutane (butylamine) .... 1-aminodecane (decylamine)

also 1-propylamine, 1-butylamine .... 1-decylamine are also used

 

(b) Physical state of linear primary aliphatic amines at room temperature

C₁ and C₂ are colourless gas, C₃ to C₁₀ colourless liquids, C₁₂ onwards are white waxy solids.

They all have fishy or ammonia like odours.

Rotten fish smell of amines produced in their biodegradation !!!

The smell is partly due to putrescine and cadavarine , which are formed from the decarboxylation of two amino acids, when dead fish and other animal bodies rot, hence their 'lovely' names and rotten smell!

 

(c) Melting/freezing points of linear primary aliphatic amines

Higher than alkanes of similar size and shaped molecules due to extra intermolecular bonding from permanent dipole interactions including hydrogen bonding - more on this in (d).

They generally increase with molecular mass, but not a particularly steady trend.

 

(d) Boiling point trend for linear primary aliphatic amines

You get the expected steady incremental rise in boiling point for linear molecules for each extra CH₂ unit in the carbon

You expect a steady increase in boiling point for a homologous series like linear primary aliphatic amines as the there will be an incremental increase in the intermolecular force due to increasing instantaneous dipole - induced dipole attractions for every CH₂ unit added to the alkyl chain (data in the skeletal formula diagram above).

This is irrespective of the intermolecular force due to the permanent dipole - permanent dipole forces, including hydrogen bonding, both due to the polar bond (^δ-N-H^δ+) of the NH₂ amine group.

Any increase in the total intermolecular bonding, requires higher kinetic energy molecules for vapourisation, hence an increase in boiling point and enthalpy of vapourisation.

 

From the boiling point plots above, you can see compared with linear alkanes with a similar number of electrons, the boiling points of linear aliphatic amines are higher.

The instantaneous dipole - induced dipole forces will be similar in both sets of molecules.

However, amines have a polar bond (^δ-N-H^δ+) so the intermolecular forces are increased by permanent dipole - permanent dipole attractions including hydrogen bonding (N-H^δ+llll^δ-N-H) between the amine molecules (diagram below).

e.g. comparing the linear alkane butane (CH₃CH₂CH₂CH₃, 34 electrons) and the linear primary aliphatic amine propylamine (CH₃CH₂CH₂NH₂, 34 electrons)

butane : melting point -138^oC and boiling point -0.5^oC.

Intermolecular forces: Only instantaneous dipole - induced dipole attraction (Van der Waals forces only consist of these dispersion forces).

propylamine : melting point -83^oC and boiling point 49^oC.

The Van der Waals forces (intermolecular bonding forces) consist of instantaneous dipole - induced dipole plus permanent dipole - permanent dipole attractions including hydrogen bonding.

Despite having similar numbers of electrons, the permanent dipole results in propylamine having a considerably higher melting point and boiling point than butane from the increase in the total intermolecular forces (Van der Waals forces).

 

(e) Comparing boiling points of isomeric aliphatic amines based on the molecular formula C₃H₉N

(i) , propylamine (primary) bpt. 49^oC

The least compact molecule with the greater surface - surface intermolecular force contact, so has the highest boiling point.

(ii) ,   N-ethylmethylamine (secondary) bpt. 33^oC

The more compact molecule with the lesser surface - surface intermolecular force contact, so has a lower boiling point (decreased by 16^oC.

(i) and (ii) compare with linear butane having higher boiling point (-0.5^oC) than the branched 2-methylpropane (-11.7^oC).

(iii) ,  trimethylamine (tertiary) bpt. 4^oC

The most compact molecule of the three with the smallest surface - surface intermolecular force contact, so has the lowest boiling point (decreased by a further 29^oC).

BUT, note the greater decrease in boiling point is because in a tertiary amine, hydrogen bonding is not possible, but there is still polar ^δ-N-C^δ+ bonds, so the boiling point is still higher than that of 2-methylpropane because of the 'extra' permanent dipole - permanent dipole attractive force.

 

(e) Solubility of aliphatic amines in water

I had trouble getting reliable and constant solubility data for linear primary aliphatic amines.

The lower molecular mass aliphatic amines are very soluble in water (completely miscible) because the polar molecules can hydrogen bond with water molecules in the solvation process - unlike non-polar alkanes (see diagram below comparing propylamine and butane).

However, you don't seem to get a steady decrease in solubility of the higher aliphatic amines.

You expect the increasing disruption of the hydrogen bonds in water by the hydrophobic alkyl group of the amine to eventually have an effect on the solubility of the amine.

If the disruption of water - water hydrogen bonds is not compensated sufficiently by amine - water hydrogen bonds, solvation becomes energetically less favourable and the solubility is reduced.

However, although methylamine to pentylamine are fully miscible with water, at hexylamine the solubility is considerably reduced (see diagram below) in general decreases further with increase in carbon chain length.

In the case of alcohols and carboxylic acids, both hydrogen bonded with water you seem to get a much steadier trend in the decrease in solubility with increasing carbon chain length, but there is still a dramatic decrease after the lower members of these homologous series are no longer miscible with water.

See also section 7.10 for physical properties of aromatic amines

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