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Advanced Level Chemistry: Aliphatic amines: structure, classification, physical properties

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

Part 8.2 Structure, classification and physical properties of aliphatic amines


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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


INDEX of Organic Nitrogen Compound NOTES

All Advanced A Level Organic Chemistry Notes

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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 structural formula primary aliphatic aromatic amine molecular structure structural formula secondary aliphatic aromatic amine molecular structure structural formula tertiary aliphatic aromatic amine molecular structure There are prim/sec/tert aliphatic (alkyl) or aromatic (aryl) amines.
aliphatic amine examples structural formula ethylamine molecular structure

(c) doc b

ethylamine

structural formula N-ethylmethylamine molecular structure

(c) doc b

N-ethylmethylamine

structural formula triethylamine molecular structure

(c) doc b

triethylamine

aliphatic amine examples
aromatic amine examples structural formula phenylamine molecular structure

phenylamine

structural formula diphenylamine molecular structure

diphenylamine

structural formula triphenylamine molecular structure

triphenylamine

aromatic amine examples

The types or sub-classification of amines

structural formula of the types of amine structure of primary secondary tertiary quaternary ammonium ion molecular structure

A Primary amine has only one carbon atom from an alkyl or aryl group and two hydrogen atoms attached to the nitrogen atom of the amine group.

A Secondary amine has two carbon atoms from alkyl or aryl groups and one hydrogen atom attached to the nitrogen atom of the amine group.

A tertiary amine has three carbon atoms from alkyl or aryl groups and no hydrogen atoms attached to the nitrogen atom of the amine group.

There are also quaternary ammonium ion salts

These have four carbon atoms from an alkyl or aryl groups attached to the nitrogen atom of the original amine group.

e.g. the simplest is tetramethylammonium chloride, (CH3)4N+ Cl-

Note on bond angles

The three single covalent bond pairs (N-C or N-H) and the non-bonding electron pair orbital form a tetrahedral arrangement around the nitrogen atom of the amine group (or four covalent bonds in the quaternary ammonium cation. The three bonds form a pyramidal arrangement.

Therefore the bond angles e.g. R-N-H, R-N-R' etc. should be around 109o.

However, the lone pair - bond pair repulsion is greater than bond pair - bond pair repulsion, so the tetrahedral bond angle might a bit less than 109o, (the H-N-H bond angle is ~107o in ammonia), but take care ...

... in trimethylamine :N(CH3)3, the three alkyl groups are more bulky than hydrogen atoms and the C-N-C bond angle is ~111o.


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8.2.2 The physical properties of aliphatic amines

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

 

Name Structure mpt/oC bpt/oC Solubility g/100g water Comments
methylamine CH3NH2 -97 -6 ~miscible  
ethylamine CH3CH2NH2 -84 17 ~miscible  
propylamine CH3CH2CH2NH2 -83 49 ~miscible  
butylamine CH3CH2CH2CH2NH2 -51 78 ~miscible  
pentylamine CH3(CH2)3CH2NH2 -55 103 ~miscible  
hexylamine CH3(CH2)4CH2NH2 -19 130 1.2  
heptylamine CH3(CH2)5CH2NH2 -18 156 ~0.5  
octylamine CH3(CH2)6CH2NH2 0 180 <0.5  
nonylamine CH3(CH2)7CH2NH2 -1 202 0.13  
decylamine CH3(CH2)8CH2NH2 14 221 <0.4  
undecylamine CH3(CH2)9CH2NH2 17 242 very low  
dodecylamine CH3(CH2)10CH2NH2 28 259 <0.06  
Three for comparison          
propylamine CH3CH2CH2NH2 -83 49 ~miscible A series of three structurally isomeric molecules (based on C3H9N) becoming more compact in the sequence: primary ==> secondary ==> tertiary aliphatic amines - resulting decreasing boiling points (see 8.2.3).
N-ethylmethylamine CH3CH2NHCH3 -71 33 very high
trimethylamine (CH3)3N -117 4 very high
           

See also section 7.10 for physical properties of aromatic amines


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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 the biodegradation !!!

 

(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

solubility boiling points skeletal formula linear primary aliphatic amines methylamine ethylamine propylamine butylamine pentylamine hexylamine heptylamine octylamine nonylamine decylamine

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-Hδ+) 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.

 

diagram graph of boiling point of linear aliphatic alkanes versus electrons in the molecule comparison with linear alkanes

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-Hδ+) so the intermolecular forces are increased by permanent dipole - permanent dipole attractions including hydrogen bonding (N-Hδ+llllδ-N-H) between the amine molecules (diagram below).

hydrogen bonding in aliphatic amines stronger intermolecular force bonds bonding higher boiling points compared to alkanes with no hydrogen bonding

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

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

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

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

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) (c) doc b, (c) doc b propylamine (primary) bpt. 49oC

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

(ii) (c) doc b, (c) doc b  N-ethylmethylamine (secondary) bpt. 33oC

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

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

(iii) (c) doc b ,(c) doc b  trimethylamine (tertiary) bpt. 4oC

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 29oC).

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-Cδ+ 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).

polar aliphatic amines more soluble in water due to hydrogen bonding solvation not possible with non-polar alkanes

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.

solubility boiling points skeletal formula linear primary aliphatic amines methylamine ethylamine propylamine butylamine pentylamine hexylamine heptylamine octylamine nonylamine decylamine

-

 

See also section 7.10 for physical properties of aromatic amines


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Doc Brown's Advanced Level Chemistry Revision Notes

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INDEX of Organic Nitrogen Compound NOTES

 All Advanced Organic Chemistry Notes

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