CHEM 203
Topics Discussed on Sept. 28
Technological importance of alkyl halides in the chemical industry
Even greater technological significance of alcohols in the contemporary world
Reminder: an alcohol is an organic compound in which a carbon atom bears an OH group:
Hydration reaction of alkenes: the addition of water across the π bond leading to alcohols:
H
OH
C
C
[?]
H OH
C
C
an alcohol
Principle: no reaction is possible between an intact alkene and H2O because both are Lewis bases
Possible reaction of Lewis basic H2O with a carbocation generated by protonation of an alkene
Inability of H2O (pKa ≈ 16) to protonate an alkene and consequent requirement for a strong
Bronsted acid in the hydration reaction
Reminder: the pKa of H2O as defined on the basis of the law of mass action is:
both = 10–7 M at equil. (25 °C)
this is an abstraction: there
is no free "H+" in solution
Keq = Ka =
[ H+ ] [
OH]
=
10–7 x 10–7
[ H2O ]
H–OH
H+
+
OH
= 1.8 x 10–16
55.55
no. of moles of H2O in one liter (≈ 1 kg)
of H2O ≈ 1000 g / 18 g/mol = 55.55 mol
therefore, pKaH2O = – log (1.8 x 10–16) ≈ 15.7 ≈ 16
Inadequacy of HCl, HBr, HI for the hydration of olefins (nucleophilic Cl–, Br–, I– are likely to
capture the carbocation intermediate)
Requirement for Bronsted acids that are strong enough to protonate the olefin (pKa << 0), but
that release a poorly nucleophilic conjugate base that is unlikely to compete effectively with H2O
for the intermediate carbocation
Sulfuric acid (pKa ≈ – 5), trifluoromethanesulfonic acid (pKa ≈ –10), tetrafluoroboric acid (pKa
≈ –10), as Bronsted acids that release poorly nucleophilic conjugate bases
(pKa ≈ –5) O (pKa ≈ +2)
H O S OH
sulfuric acid
O
dissoc.
pKa ≈ –5
O
O S OH
O
hydrogen sulfate ion:
a poor nucleophile
Lecture of Sept 28
p. 2
trifluoromethanesulfonic
(= "triflic ") acid
F O
F C S O–H
F O
F O
F C S O
F O
dissoc.
pKa ≈ –10
dissoc.
tetrafluoroboric acic
HBF4
pKa ≈ –10
F
F B F
F
trifluoromethanesulfonate
(triflate) ion: very poor
nucleophile
tetrafluoroborate ion:
very poor nucleophile
the conjugate bases of these acids are poor nucleophiles and are not overly inclined to react
with carbocation intermediates, permitting faster capture of the cations by other nucleophiles
Principle: in the absence of prior knowledge, it is generally not easy / possible to predict whether
the conjugate base of a Bronsted acid will be a good nucleophile, or a poor one: only experiment
can ascertain the nucleophilic character of such conjugate bases.
General mechanism of the hydration reaction of an olefin, e.g., of cyclohexene:
H–OSO3H
better nucleophile:
H
O H reacts with the
cation faster than HSO4–
protonation
unfavorable:
it occurs
reversibly
H
OSO3H
poorer nucleophile:
reacts more slowly
with the cation
B = generic base (e.g., OSO3H,
H
H2O, possibly another olefin ... ) a proton is
O H
protonated cyclohexanol:
returned to
pKa ≈ – 2
the medium
OH
cyclohexanol
Thermodynamically highly favorable capture of a carbocation by a molecule of H2O (completion
of a Lewis octet)
Principle: acid (=protons) are not consumed during the hydration of alkenes
Catalysts: species that promote chemical reactions but that are not consumed in the process
The addition of water to alkenes as a process that is catalytic in protons (acid)
The hydronium ion, H3O+, as a strong Bronsted acid with pKa ≈ –2
+
Reminder: the pKa of H3O as defined on the basis of the law of mass action is:
H3O+
H+
+
H2O
this is an abstraction: there
is no free "H+" in solution
Keq = Ka =
[ H+ ] [ H2O ]
[ H3O+ ]
same thing!!
= [ H2O ] = 55.55
Lecture of Sept 28
p. 3
therefore, pKaH3O+ = – log (55.55) ≈ – 1.7 ≈ – 2
"Markownikov" selectivity in the addition of water to unsymmetrical alkenes: formation of
alcohols derived from the more highly stabilized carbocation, e.g.:
OH
H2SO4
H2SO4
H2O
H2O
OH
2-propanol or
"isopropanol"
Possibility of rearrangement during addition of water to alkenes as a consequence of the fact that
the reaction involves carbocation intermediates. Examples:
1,2-hydride shift
H OSO3H
H
H
CH3
H
OH2
H
CH3
symbol for
proton exchange
H
CH3
CH3
H
H
H
O H
CH3
±H+
H
H
CH3
CH3
OH
CH3
CH3
CH3
and
1,2-alkyl shift
H OSO3H
H
CH3
CH3
CH3
CH3
symbol for
proton exchange
OH
H
CH3
CH3
CH2
H
CH3
H
O H
CH3
±H+
CH3
CH3
H
CH3 OH
CH3
CH3
CH3
Stereochemical aspects of proton-initiated hydration of alkenes: formation of racemic alcohols
(notes of Sept 25):
Example: the hydration of 1-butene leading to 2-butanol:
top-face attack:
pathway A
H–OSO3H
H2O
H
H2O
H
C
C CH2CH3
H
1-butene
CH3 C
OH
plane of the
cationic C
A
CH3
CH2-CH3
H
bottom-face attack:
pathway B
B
CH3
(S)-2-butanol
C CH CH
2
3
H
enantiomers
CH2CH3
C H
(R)-2-butanol
OH