Organic Chemistry

49 Reactions

49.1 Conversion of alcohols to haloalkanes

Recall that haloalkanes can be converted to alcohols through nucleophilic substitution.

Conversion of a haloalkane to an alcoholR-X + OH-→R-OH + X-

This reaction proceeds because X (a halogen) is a good leaving group and OH-is a good
nucleophile. OH, however, is a poor leaving group. To make the reverse reaction proceed,
OH must become a good leaving group. This is done by protonating the OH, turning it
into H 2 O+, which is a good leaving group. H+must be present to do this. Therefore,
the compounds that can react with alcohols to form haloalkanes are HBr, HCl, and HI.
Just like the reverse reaction, this process can occur through SN2 (backside attack) or SN 1
(carbocation intermediate) mechanisms.

SN2 conversion of an alcohol to a haloalkaneR-O-H + H++ X-→R-O+-H 2 + X-→

R-X + H 2 O

SN1 conversion of an alcohol to a haloalkaneR-O-H + H++ X-→R-O+-H 2 + X-→R++ H 2 O +

X-→R-X + H 2 O

As stated in the haloalkane chapter, the two mechanisms look similar but the mechanism
affects the rate of reaction and the stereochemistry of the product.

49.2 Oxidation of alcohols.

Oxidation in organic chemistry always involves either the addition of oxygen atoms (or
other highly electronegative elements like sulfur or nitrogen) or the removal of hydrogen
atoms. Whenever a molecule is oxidized, another molecule must be reduced. Therefore,
these reactions require a compound that can be reduced. These compounds are usually
inorganic. They are referred to asoxidizing reagents.

With regards to alcohol, oxidizing reagents can be strong or weak. Weak reagants are able to
oxidize a primary alcohol group into a aldehyde group and a secondary alcohol into a ketone.
Thus, the R-OH (alcohol) functional group becomes R=O (carbonyl) after a hydrogen atom
is removed. Strong reagents will further oxidize the aldehyde intoa carboxylic acid (COOH).
Tertiary alcohols cannot be oxidized.