In alcohols, the -OH can be "activated" wherein it is converted to a better leaving group. This results in substitution and elimination reactions.
For example bromination and chlorination with PBr3 and SOCl2 respectively occur by a substitution. Recall that these reagents convert the OH to a better leaving group which is substituted by the Cl- or Br- anions. The displacement is an SN2 reaction so there's an inversion of stereochemistry that's apparent if the alcohol is chiral.
Halogenation of Tertiary (3o) Alcohols
Tertiary alcohols can be converted to the corresponding chlorides and bromides by using HCl and HBr. Recall that this proceeds via and SN1 pathway.
Chlorination of 1o and 2o alcohols
Bromination of 1o and 2o alcohols
Tosylation of 1o and 2o alcohols
However, in tosylations, the reaction of an alcohol with tosyl chloride does not invert the stereochemistry. This is a result of the direct reaction of the hydroxyl group O atom with the tosyl chloride.
Dehydration reactions typically occur by way of elimination reactions. Treating acohols with strong acid in absence of water typically results in eliination
Typically utilizes concentrated H2SO4, H3PO4 or pTSA.
This occurs by an E1 elimination. The -OH is protonated and the water expelled to generate a carbonation. The carbocation is then deprotonated to form an alkene. This does not work well for substrates that are sensitive to acid, since the alcohol must be heated in the presences of a strong acid.
POCl3/pyridine is useful for acid sensitive substrates
Recall that the mechanism proceeds as follows. Notice the E2 elimination in the second step in which pyridine behaves as a base.