Catalysts used in dehydration of alcohols?

1.
If concentrated hydrochloric acid is used as a catalyst for the dehydration of secondary alcohols, chloro-alkene will be formed instead of alkene.

2.
if seconadary alcohol and concentrated sulphuric acid are mixed, the alkene formed will undergone polymerisation. Moreover, concentrated sulphuric acid should not be used as it may produces messy results. Not only is it an acid, but it is also a strong oxidising agent. It oxidises some of the alcohol to carbon dioxide and at the same time is reduced itself to sulphur dioxide. Both of these gases have to be removed from the alkene. It also reacts with the alcohol to produce a mass of carbon. There are other side reactions as well.

3.
To prevent loss of the product, alkene, by evaporation.

I hope this can help your understanding. =)

2007-09-25 10:55:03 補充：
1.Concentrated hydrochloric acid will never dehydration of secondary alcohols, it will only replace the functional group OH with Cl, or from alcohol to chloroalkane.

2007-09-25 10:56:28 補充：
2.When seconadary alcohol and concentrated sulphuric acid are mixed, first gives a ketone with the same number of carbon atoms as the original alcohol. Ketones are fairly difficult to oxidise,

2007-09-25 10:56:43 補充：
but prolonged action of the oxidising agents (with heating) produces a mixture of acids, each containing fewer carbon atoms than the original alcohol. Moreover, the reaction yields a mixture of two alkene products, and polymerisation of alkenes can be resulted.

alcohol^2 dehydration ?
I don't know, sorry