Intermolecular forces
Read our general notes on Risk Assessment
What determines the strength of hydrogen bond, for example between primary and secondary amine?
Ulex replies …
This is a complex situation. Let us suppose we are comparing two isomeric amines, one primary and one secondary. The strength of the hydrogen bonding depends on the polarity of the nitrogen – hydrogen bond.
Alkyl groups tend to feed electrons onto the nitrogen atom and hence onto the hydrogen atom which reduces the polarity of the N – H bond. In the secondary amine there are two alkyl groups doing this to one hydrogen atom whereas in the primary amine there is only one alkyl group but two hydrogen atoms. The hydrogen bonding should therefore be weaker in the secondary than in the primary amine which should result in a lower boiling point for the secondary amine. This seems to be borne out in the case of ethylamine (bp 289.7 K) and the isomeric dimethylamine (bp 280.5 K) but one should also take into account differences in van der Waals forces due to the different overall shapes of the molecules.
Nice question – makes one think!
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
Rate this page or react
Share your views on this page, 6 ratings so far
, rated at 
updated: 25 August 2003
