Chemistry Reference
In-Depth Information
This acid-catalyzed approach is limited because it does not apply to all alcohols.
Tertiary alcohols can easily give a reasonably stable carbocation. However, as
seen in Chapter 5, 1° and 2° alcohols give less stable carbocations. A more
general alternative uses the special reagents thionyl chloride (SOCl
2
) or phos-
phorus trihalides (PCl
3
and PBr
3
). The mechanism of these reactions does not
go through free carbocation intermediates.
FIGURE 7.21
Alkyl halides from alcohols.
7.6.4 Alcohol Oxidation
The oxidation chemistry of alcohols depends on whether the substitution at
alcohol carbon is 1°, 2°, or 3°. As
Figure 7.22
shows, alcohols can be com-
FIGURE 7.22
Combustion of alcohols.
busted in oxygen to give CO
2
and H
2
O. This is similar to the combustion of
alkanes in
Section 7.2.1
, but the heat given off is less because one carbon is
already in an oxidized state.
Many oxidation reagents are available. However, we can see the principles in
the dichromate system of Na
2
Cr
2
O
7
or K
2
Cr
2
O
7
in aqueous acid. As
Figure 7.23
shows, the number of hydrogen ligands on the alcohol carbon determines how
much oxidation can occur.
Oxidation simply means the removal of H
2
from the C-OH bond. Therefore, 1°
alcohols give aldehydes, and 2° alcohols give ketones. Because aldehydes still
have a hydrogen on the carbonyl carbon, they can be further oxidized to carbox-
ylic acids. With 3° alcohols, no simple oxidation can occur because they have no
hydrogen on the alcohol carbon.
7.6.5 Ether Cleavage
Ethers are relatively unreactive derivatives of alcohols. Because of this, several ethers
are widely used as polar solvents. As seen in
Figure 7.24
, strong acid is reacted with
the oxygen Lewis base in order to break the O-C bond in an ether. The proton-
ation of the oxygen gives a dialkyloxonium ion. This ionizes by S
N
1, or is subject
to S
N
2 attack by the conjugate base nucleophile to give cleavage products.
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