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Unformatted text preview: ium dichromate in
aqueous sulfuric acid!
C! ! ! ! +! H! !O!
K! ! ! ! ! ! !
2 Cr 2 O7
dichromate! H! !S! ! !
2 O4 H! !S! ! !
2 O4 H! !Cr! ! ! !
2 ! 2 O7 H! !Cr! ! !
2 2! ! !Cr! ! !
H2 ! O 4
Chromic acid! 10-13! Chromic Acid Oxidation of ROH!
• Step 1: formation of a chromate ester!
Cyc loh exanol fas t and
revers ible O
+ HO-Cr - OH O
O- Cr - OH
O + H2 O H
An alkyl chromate O • Step 2: reaction of the chromate ester with a base,
here shown as H2O!
chromiu m(V I) chromiu m(IV) O
H O H slow, rate
Cr -OH determining
O + O+ H +
Cyclohexan on e O - Cr -OH
O 10-14! Oxidation: 1° ROH to RCHO!
Oxidation of a primary alcohol gives an aldehyde or a
carboxylic acid, depending on the experimental
• to an aldehyde is a two-electron oxidation!
• to a carboxylic acid is a four-electron oxidation!
C H3 - C H
alcohol ! [O] O
C H3 - C - H
An aldehyde [O] O
C H3 - C - O H
acid Difﬁcult to stop at aldehyde!
10-15! Chromic Acid Oxidation of RCHO!
• chromic acid oxidizes a 1° alcohol ﬁrst to an aldehyde
and then to a carboxylic acid!
• in the second step, it is not the aldehyde per se that is
oxidized but rather the aldehyde hydrate!
R -C- H + H2 O
An ald ehyde fas t and
revers ible OH
R- C-OH H2 CrO4 H
An ald ehyde
hydrate H2 O O-Cr O3 H
R -C- OH
R -C- OH + HCr O3 - + H3 O+
A c arb oxylic
acid 10-16! Oxidation: 1° ROH to RCHO!
! Use pyridinium chlorochromate (PCC) to limit the
• A form of Cr(VI) prepared by dissolving CrO3 in
aqueous HCl and adding pyridine to precipitate PCC as
pyrid inium ion
chloroc hromate ion
Cr O3 + HCl + N Pyrid ine ClCrO3 - N
Pyrid inium ch lorochromate
(PCC) • PCC is selective for the oxidation of 1° alcohols to
aldehydes; it does not oxidize aldehydes further to
• PCC can also be used to oxidize 2° alcohols to
10-17! Oxidation: 1° ROH!
• PCC oxidizes a 1° alcohol to an aldehyde!
N H CrO3Cl
H CH2Cl2! Geraniol Geranial • PCC also oxidizes a 2° alcohol to a ketone! OH P CC O CH2Cl2! Menthol Menthone 10-18! Alkyl Sulfonates!
Sulfonic acids, like sulfuric acid, are strong acids!
• sulfonate anions are very weak bases (the conjugate base of a
strong acid) and are very good leaving groups for SN2 reactions
• sulfonyl chlorides are derived from sulfonic acids O
R S O–
R! ! ! ! ! !
A sulfonate anion!
A sulfonic acid!
(a very weak base and !
(a very strong acid)!
stable anion; a very!
good leaving group)! O!
R! ! ! ! !!
chloride! Conversion of an alcohol to a sulfonate ester converts
-OH, a very poor leaving group, into a sulfonic ester, a
very good leaving group
10-19! Alkyl Sulfonates!
A commonly used sulfonyl chloride is p-toluenesulfonyl
chloride (Ts-Cl) ! C H
OSO N p-toluenesulfonyl chloride
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