Section 18.10 The Wittig Reaction 76118.9 Addition of Sulfur Nucleophiles
Aldehydes and ketones react with thiols to form thioacetals and thioketals. The mech-
anism for addition of a thiol is the same as that for addition of an alcohol. Recall that
thiols are sulfur analogs of alcohols (Section 12.10).
Thioacetal (or thioketal) formation is a synthetically useful reaction because a
thioacetal (or thioketal) is desulfurized when it reacts with and Raney nickel.
Desulfurization replaces the bonds with bonds.
Thioketal formation followed by desulfurization provides us with a third method
that can be used to convert the carbonyl group of a ketone into a methylene group. We
have already seen the other two methods—the Clemmensen reduction and the
Wo l ff–Kishner reduction (Sections 15.15 and 18.6).
18.10 The Wittig Reaction
An aldehyde or a ketone reacts with a phosphonium ylide (pronounced “ILL-id”) to
form an alkene. An ylideis a compound that has opposite charges on adjacent cova-
lently bonded atoms with complete octets. The ylide can also be written in the double-
bonded form because phosphorus can have more than eight valence electrons.
The reaction of an aldehyde or a ketone with a phosphonium ylide to form an
alkene is called a Wittig reaction. The overall reaction amounts to interchanging the
double-bonded oxygen of the carbonyl compound and the double-bonded carbon
group of the phosphonium ylide.
a phosphonium ylide+ −
(C 6 H 5 ) 3 PCH 2 (C 6 H 5 ) 3 PCH 2H 2
Raney NiH 2
Raney NiS S H HCH 3 CH 2 CH 2 CH 2 CH 3SS
C
CH 3 CH 2 CH 2 CH 3C¬S C¬HH 2HClS SHSCH 2 CH 2 CH 2 SHa thioketal+ + H 2 OOHCl
CH 3 CH 2 CH 2 CH 3 2 CH 3 SHa thioketal+ CH 3 CH 2 CCH 2 CH 3 + H 2 OSCH 3SCH 31,3-propanedithiolmethanethiolCOGeorg Friedrich Karl Wittig
(1897–1987)was born in Germany.
He received a Ph.D. from the
University of Marburg in 1926.
He was a professor of chemistry at
the Universities of Braunschweig,
Freiberg, Tübingen, and Heidelberg,
where he studied phosphorus-
containing organic compounds.
He received the Nobel Prize in
chemistry in 1979, sharing it with
H.C. Brown (Section 4.9).