Review Rxns. of Alcohols

In alcohols, the -OH can be "activated" wherein it is converted to a better leaving group.  This results in substitution and elimination reactions.

Chlorination, bromination, and tosylation

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 S_N2 reaction so there's an inversion of stereochemistry that's apparent if the alcohol is chiral.

Halogenation of Tertiary (3^o) 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 1^o and 2^o alcohols 

Bromination of 1^o and 2^o alcohols 

Tosylation of 1^o and 2^o 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

Dehydration reactions typically occur by way of elimination reactions.  Treating acohols with strong acid in absence of water typically results in eliination

Acid Catalyzed

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

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.