Lecture Notes Oct 22nd

CHEM 203
Topics Discussed on Oct. 22
Successful radical bromination of unusually weak C–H bonds, such as allylic, benzylic,
propargylic, or tertiary ones (bond dissociation energy ≈ 85-90 kcal/mol)
Allylic, benzylic, propargylic hydrogens: those connected to a carbon atom, which in turn is
attached to an alkene, a benzene, or an alkyne framework:
H
H
H
H C–C
H C
H C–CH=CH2
H
H
H
red: allylic hydrogens
blue: allylic carbon
C–H
red: benzylic hydrogens
blue: benzylic carbon
red: propargylic hydrogens
blue: propargylic carbon
Weakening of allylic (and propargylic) C–H bonds through hyperconjugative delocalization of
electron density from the σC–H bond into the π* orbital (notes of Oct. 8), and also of electron
density from the neighboring π bond into the σ*C–H orbital:
H
H
H
C
C
lobes of the
H π*C=C orbital
C
(phases omitted
H for clarity)
allylic C–H bonds
dissociation energy
≈ 85-90 kcal / mol
H
electron donation from the σC–H bond into the π*C=C
orbital (notes of Oct. 8) weakens the allylic bond . . .
red: allylic
C–H bonds
H
H
C
H
C
C
H
H
H
as does electron donation from the πC=C
bond into the allylic σ*C–H orbital orbital
(identical considerations hold for propargylic C–H bonds):
Radical bromination of allylic positions as a favorable process that is widely employed in
contemporary organic chemistry
Principle: the success of an allylic radical bromination reaction depends on the presence of a very
small instant quantity of Br2 in the reaction medium (for reasons not covered in CHEM 203).
This avoids addition of Br• or of Br2 to the π bond, and other side reactions.
N-Bromosuccinimide ("NBS", structure below): a reagent that releases very small amounts of
Br2 over time through reaction with HBr (common contaminant in NBS), and that is especially
valuable for the radical bromination of allylic positions:
O
NBS
N
Br
O
H–Br
O
Br
N
Br
OH
Br2 + O
N
OH
± H+
O
N
H
O
succinimide
Lecture of Oct 22
p. 2
Especially successful radical bromination of allylic positions with NBS / light; e.g.:
NBS
CH2 CH–CH3
CH2 CH–CH2–Br
hν
via :
trace of
O
NBS
N
Br
O
H–Br
O
N
H
+
O
Br—Br
very small amounts
succinimide
reacts with NBS
to make more Br2
hν
Br—Br
2 Br
H–Br
propagates the
chain reaction
Br
Br—Br
H
CH2 CH–CH2
CH2 CH–C
Br
H
CH2 CH–CH2
H
an allylic radical
Weakening of benzylic C–H bonds through the same electronic effects outlined above:
red: benzylic
C–H bonds
H
C
H
C
C
C
C
H
lobes of a benzene
π*C=C orbital
benzylic C–H bonds
dissociation energy
(phases omitted
≈ 85-90 kcal / mol
for clarity)
H
electron donation from σC–H bonds into a π*C=C orbital
of benzene weakens the benzylic C–H bonds . . .
H
H
H
C
H
C
C
C
C
C
H
C H
H
H
as does electron donation from a πC=C bond of
benzene into the the benzylic σ*C–H orbital
Radical bromination of benzylic positions as a favorable process that is also widely employed in
contemporary organic chemistry, and that works particularly well with NBS:
H
CH2
Br
NBS
CH2
hν
via :
trace of
O
NBS
N
Br
O
H–Br
O
N
H
O
succinimide
+
Br—Br
very small amounts
Lecture of Oct 22
p. 3
reacts with NBS
to make more Br2
Br—Br
hν
2 Br
H–Br
propagates the
chain reaction
Br
Br—Br
H
CH2
C
Br
H
CH2
H
a benzylic radical
reminder:
the "olefinic" bonds in a benzene ring are unusually unreactive toward Br2,
HBr, BH3, OsO4, radicals, etc, due to "aromaticity" (a set of properties, to
be discussed in detail in CHEM 213, that cause the π bonds of benzene
to behave differently from those of ordinary alkenes).
High degree of stabilization of allylic and benzylic radicals through resonance interactions:
CH2
CH2
CH2=CH–CH2 •
• CH2-CH=CH2
etc.
notice the symbol employed to indicate resonance:
Elevated strength of sp2-C–H and of sp-C–H bonds: ΔHdiss ≈ 110-120 kcal/mol:
H
H
C H
C C
H
H
H
H
H
H
H
C
H
H
H
H
bold: C–H bonds connecting to an sp2 carbon: unusually strong (ΔHdiss ≈ 110 kcal/mol)
Weakening of tertiary C–H bonds through hyperconjugative delocalization of electron density
from neighboring C–H bonds into the σ*C–H orbital:
H3C
H
C
H
H
H
methane:
no hyperconj.
ΔHdiss C–H ≈
105 kcal/mol
H3C
C
H
H
H
ethane:
3 hyperconj.
(blue H's)
ΔHdiss C–H ≈
101 kcal/mol
H
H3C
H3C
C
H
propane:
6 hyperconj.
(blue H's)
ΔHdiss C–H ≈
98 kcal/mol
H3C C
H3C
H
2-Me-propane:
9 hyperconj.
(blue H's)
ΔHdiss C–H ≈
95 kcal/mol
blue: lobes of the σ*C–H orbital
Radical bromination of tertiary C–H bonds as a feasible, but not particularly useful, reaction

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