Chemistry Reference
In-Depth Information
secondary
vinyl cation
H
2
SO
4
H
2
O
Hg
2+
H
H
R
H
R
H
H
R
H
R
H
R
H
H
O
H
H
O
H
H
O
H
H
2
O
H
enol
more stable
keto tautomer
Protonation of the alkyne produces the more
favourable secondary vinyl cation, which is then
attacked by water, since water is the predominant
nucleophile available. Loss of a proton from this
produces an enol, which is transformed into a more
stable isomeric form, the ketone. This transforma-
tion is termed
tautomerism
, and we shall meet it
later (see Section 10.1) as an important consideration
in the reactivity of many organic compounds. This
reaction involves only one electrophilic addition, and,
although it does not occur readily using simply aque-
ous acid, it can be achieved by the use of mercuric
salts as catalyst. The mercuric ion may function as a
Lewis acid to facilitate formation of the vinyl cation.
Box 8.2
Electrophilic alkylation in steroid side-chain biosynthesis
Polyene macrolide drugs such as
amphotericin
and
nystatin
have useful antifungal activity but no antibacterial
action (see Box 7.14). Their activity is a result of binding to
sterols
in the eukaryotic cell membrane; they display
no antibacterial activity because bacterial cells do not contain sterol components. Binding to sterols modifies cell
wall permeability and leads to pores in the membrane and loss of essential cell components. Fungal cells are
also attacked in preference to mammalian cells, since the antibiotics bind much more strongly to
ergosterol
,the
major fungal sterol, than to
cholesterol
, the main animal sterol component. This selective action allows these
compounds to be used as drugs, though a limited amount of binding to cholesterol is responsible for side-effects
of the drugs.
H
H
H
H
H
HO
HO
cholesterol
(animals)
ergosterol
(fungi)
H
H
H
H
H
H
HO
HO
stigmasterol
(plants)
sitosterol
(plants)
