Advices For Studying Organic Chemistry

R B R CH 3 C O B

Organoborane Alkane O

CH 3 CO 2

heat H +

H

11.8 REACTIONS OF ALCOHOLS

1. The oxygen atom of an alcohol polarizes both the C–O bond and the O–H bond:

Cδ+ Oδ− Hδ+

The functional group of an alcohol An electrostatic potential map for methanol

1) Polarization of the O–H bond makes the hydrogen partially positive ⇒ alcohols

are weak acids.

2) The OH– is a strong base ⇒ OH– is a very poor leaving group.

3) The electron pairs on the oxygen atom make it both basic and nucleophilic.

i) In the presence of strong acids, alcohols act as bases and accept protons:

C OH O

H

+ H A C + H + A−

Alcohol Strong acid Protonated alcohol

2. Protonation of the alcohol converts a poor leaving group (OH–) into a good one
   (H 2 O).

1. It also makes the carbon atom even more positive (because –OH 2 + is more
   electron withdrawing than –OH) ⇒ the carbon atom is more susceptible to
   nucleophilic attack ⇒ Substitution reactions become possible (SN2 or SN1,
   depending on the class of alcohol).