Intermolecular forces
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I know that 1-bromobutane and water do not mix, however I am not sure why they don't. It is my understanding that bromine is quite electronegative, but not as electronegative as fluorine, so why can't it form hydrogen bonds with the hydrogen atom on the water molecule?
Igloo writes
A hydrogen-bonding system involves two electronegative atoms and a hydrogen atom:
X-H....X.
Here X is one of three elements: F, O or N. Bromine is not sufficiently electronegative to give rise to hydrogen bonding.
The C-Br bond in bromobutane is polar as is the the O-H bond in water. You are quite correct in predicting that the bromine atom of a 1-bromobutane molecule will be attracted to the hydrogen atom of a water molecule, but if the liquids are to mix, the 1-bromobutane molecules will need to work their way into the water layer, and break existing hydrogen bonds between the water molecules, which would be much stronger than any bonds that could be formed between the water and 1-bromobutane molecules.
To summarize, when considering whether or not liquids mix, you must take into account not only the “new” bonds which will be formed between molecules of the different liquids, but also the “old” bonds which will need to be broken in each of the individual liquids. If all these bonds are similar in strength, e.g. methanol and water, butane and pentane, 1-bromobutane and 1-chlorobutane, etc. then mixing will occur, otherwise not. Remember that when mixing does occur, the energy released on forming new bonds will be sufficient to break the “old” bonds, but in the case of water and 1-bromobutane, for example, there is a considerable energy deficit which makes the mixing impossible.
Risk assessment
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updated: 22 April 2004
