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Lecture outline
Ch 18 — Additions to the carbonyl group; chemistry of aldehydes and ketones
Before we begin studying reactions of aldehydes and ketones, it's worthwhile to revisit
some chemistry that can be used for their preparation. We've seen several reactions recently that
have been described as oxidations or reductions. Although these are familiar concepts, a brief
discussion of these principles as they apply to organic molecules will help us recognize oxidation
and reduction reactions of common organic compounds. This is covered in section 10.14 (p 380)
of the text.
Principles of oxidations and reductions as applied to organic cmpds
a. The general method —
Assign an oxidation level to each C involved in rxn by adding:
+1 for every bond to a more electronegative atom (O, N, halogen)
–1 for every bond to a less electronegative atom (H, metal, etc)
0 for every bond to another C
increase => oxidation
decrease => reduction
e.g.,
OH
–1 –1 +1 = –1
0
–1
CO2H
+1 +1 +1 = +3
–1 –2
∴ oxidation
∴ reduction
Important: the numbers above are oxidation levels, not charges
b. The easier method for alcohols and carbonyl cmpds —
again focusing only on the C(s) involved in the rxn ...
oxidation => gain of bonds to O and/or loss of bonds to H
reduction => loss of bonds to O and/or gain of bonds to H
How does this apply to the two examples above? Does this method lead to the same
conclusions? More generally, how does the first method demonstrate that counting bonds to O
and H is a valid shortcut for recognizing reductions and oxidations?
oxidation series —
oxidations that replace bonds to H with bonds to O are easy; once all the Hs are
gone, the carbon cannot be oxidized further (without cleavage of CC bonds — this
requires powerful oxidants and is normally of no use synthetically.)
O
R CH2 OH
R
CH O
R
C
OH
note that OH
R CH
OH
is the "hydrate" of the aldehyde
(i.e. RCH=O + H2 O) — hydrates
are generally less stable than the
carresponding carbonyl compound
— this has the same ox. state
as the aldehyde
R2CH
R3C
OH
R2C
O
OH
Write the series of cmpds that would be produced by oxidation of methanol.
Oxidation of alcohols to aldehydes, ketones, and carboxylic acids
Two useful Cr (VI) oxidants —
a. H2CrO4, H2O (aqueous chromic acid)
R2CH
OH
H2CrO4
R2C
O
RCH
O
H2O
RCH2
OH
H2CrO4
— made from CrO3, H2SO4, H2O
or from Na2CrO7, H2SO4, H2O
H2O
H2CrO4
H2O
RCO2H
chromic acid ox can't
be stopped at the aldehyde
partial mech:
great l.g.
O
OH
C
O
C
H
Cr OH
OH
H
a protonated
"chromate ester"
E2 rxn
O
C
OH2
(+ H2 CrO3 )
Cr (IV)
Notice that the carbon undergoes a 2-e– oxidation and the chromium a 2-e–
reduction. Cr (IV) is unstable. Three of these get together and disproportionate
— they swap electrons to make 1 Cr (VI) and 2 Cr (III).
This is a standard laboratory test for an oxidizable functional group — Cr (VI) is orange;
Cr (III) is green — the color change that accompanies reduction of the Cr is a clear
indication that something else (an alcohol, an aldehyde, etc) must have been oxidized.
b. Milder oxidant — CrO3 and pyridine (with no water)
— one popular variation of this is pyridinium chlorochromate (PCC)
CrO3 oxidizes alcohols to carbonyl compounds just like aq H2CrO4;
The only difference between CrO3 and aqueous chromic acid is that
CrO3 does not oxidize aldehydes.
e.g.,
R2CH
RCH2
OH
CrO3
pyr.
OH
CrO3
pyr.
Draw the products of the following rxns — assume excess oxidant in each case.
OH
OH
CrO3
pyr.
OH
OH
H2CrO4
H2O
Reactions of aldehydes and ketones —
— carbonyl additions
Nucleophiles
(RO– , R3N:,
H:–, "R:– ", etc)
attack C
C
Electrophiles
(H+, AlCl3, etc)
attack O
O
I don't understand the reason for the order in which the text covers the reactions in this
chapter, so I'm going to rearrange them. We'll start with the wimpy oxygen-based nucleophiles
(sections 18.3 and 18.9), then move to nitrogen-based nucleophiles (18.8), then the powerful
hydride and carbanionic nucleophiles.
1. H2O and ROH as nucleophiles —
a. Hydration (reversible)
OH
C
O
+
C
H2O
O
H
a geminal diol
or "carbonyl hydrate"
— base-catalyzed rxn
O
HO
H2O
HO
OH
mech:
— acid-catalyzed rxn
O
H
H2O
mech:
HO
OH