[smellor]

Hiya, i have been looking at this reaction and it seems to me two mechanisms are possible:
has to be said it is a synthesis from cyclohexanol with phosphoric acid (h3po4)

now, the first step is pretty obvious, the oh group is protonated and thus becomes the leaving group (h2o) leaving the cyclohexane cation.
This would leave h2po4- which i would think would be a pretty good candidate for deprotonating the ring, leaving cyclohexene, but from what i've been able to gather from books/internet, the deprotonating is done by an h2o molecule instead. I understand that this is an acidic environment, but would the basicity of the h2po4 not come before that of the h2o?

Edit: or does the fact that h3po4 readily deprotonates in aqaeus solution factor in, meaning that it is in fact unlikely to function as a nucleophile in this reaction?

any help much appreciated, thanks!

[smellor]

For the reaction in question, we have now been told that the deprotonation of the cation does indeed occur by the conjugate base of phosphoric acid. I am still slightly confused as to what factors are in effect, and how does one go about determining whether it is water or h2po4 that would be dominant in the reaction?? i would love a bit of clarification...

[Fry]

Well phosphoric acid isnt a strong acid so the first conjugate base may re protonate to form phosphoric acid.

It probably acts as the base instead of water because H3O+ is a stronger acid than phosphoric acid. (Someone please correct this if its wrong)

[RBF]

H₂PO_4^- is more basic than H₂O, so it will deprotonate the cation to give cyclohexene.  However, if the reaction is run in aqueous phosphoric acid, both water and the dihydrogen phosphate ion are likely to be involved in the deprotonation step. The important concept is the E1 nature of the mechanism (protonation, loss of water, deprotonation)

[smellor]

it was a 85% H3PO4 solution so i guess the most of the deprotonating should come from H2PO4-.
thanks for the replies!