Lecture13-notes

Lecture13-notes - 9/28/09


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Unformatted text preview: 9/28/09
 The
π
bond
of
an
alkene
is
sufficiently
weak
to
undergo
reaction
with:
 Hydrogen
(H2)
 Halogen
(F2,
Cl2,
Br2)
 Hydrogen
halide
(HCl,
HBr,
HI)
 Water
 Each
of
the
above
reagents
undergoes
an
addition
reaction
with
an
alkene:
 
Reactions
 •  Alkenes
are
similar
in
structure
and
do
 similar
reactions.
 –  All
contain
a
double
bond
 –  All
contain
the
same
functional
group
 •  Reactions
are
categorized
through
different
 types
of
mechanisms.
 1
 9/28/09
 
Reactions
 Typical
for
unsaturated
systems

 is
the
addition
reaction:
 
A+B

C
 
Reactions
 2
 9/28/09
 
Reactions
 WHAT
IS
THE
NATURE

 OF
THIS
REAGENT?
 
Reactions
 electrophile
 Hydrogen
bromide
is
a
strong
acid
and
forms
 hydronium
ions,
H3O+,
and
bromide,
Br–,
when

 dissolved
in
water.
 H3O+
is
positively
charged,
 
 thus
it
is
electron
deficient
 
it
is
electrophilic
“electron
loving ”
 3
 9/28/09
 
Reactions
 In
the
presence
of
an
electron‐rich
species

 the
hydronium
ion
reacts:
 electrophile
 A
new
positively
charged
species
is
formed.
 
Reactions
 The
newly
formed
species,
a
carbocation,
is

 again
electron
deficient,
thus
it
is
electrophilic.
 electrophile
 4
 9/28/09
 
Reactions
 One
species
present
that
is
rich
in
electrons
is
Br–.
 Since
Br–
bears
a
negative
charge
it
seeks
for

 neutralization.
 It
is
nucleophilic
(nuclei
are
positively
charged).
 Reactions
 The
two
species,
electrophile
and
nucleophile,
combine
 and
form
a
new
compound.
 electrophile
 nucleophile
 5
 9/28/09
 
Reactions
  Electrophiles:
 
 molecules
that
contain
atoms
with
empty
 
 
 orbitals,
which
can
accommodate
electrons.

 Typically,
these
are
positively
charged.
  Examples:
 Boron
has
only
6
valence
electrons.

 BF3
is
a
Lewis
acid.
 
Reactions
  Nucleophiles:
 
 
 
 
 molecules
that
contain
atoms
with
lone
pairs,

 which
can
donate
electrons.
Often
these
are

 negatively
charged.
Almost
all
the
time
they

 contain
elements
from
groups
5–7
of
the

 periodic
table,
since
those
have
lone
pairs.
  Examples:
 6
 9/28/09
 
Reactions
 Nucleophiles:
 
 
 Organic
molecules
with
double
bonds
 (alkenes,
alkynes)
are
also
nucleophilic.
 Examples:
 
Mechanism
 Summarizing
our
reaction,
we
realize

 
 
it
is
a
2‐step
mechanism
 STEP
1
 STEP
2
 7
 9/28/09
 
Mechanism
 Step
1
reaches
a
carbocation
“intermediate.”

 One
new
bond
is
formed.
 Intermediates
are
species
with
a
very
short
lifetime.
 However,
their
stability
(energy)
often
determines

 the
outcome
of
a
reaction.
 Step
2
completes
the
reaction
by
forming
a
second
 bond.
Again,
it
is
the
interplay
between
positively
 charged
(electrophilic)
and
negatively
charged
 (nucleophilic)
species.
 1st step energy
maximum
 activation
 energy 8
 9/28/09
 2nd step energy
maximum
 activation
 energy Transition
state
 The
chemical
species
that
exists

 at
the
transition
state,
with
old
 bonds
in
the
process
of
breaking
 and
new
bonds
in
the
process
of
 forming:

 TS
2
 TS
1
 bond
breaking
 bond
 forming
 bond
 forming
 9
 9/28/09
 Overall
reaction
coordinate
 
Reactions
 10
 ...
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This note was uploaded on 07/02/2011 for the course CHEM R340 taught by Professor Tolbert during the Summer '11 term at Indiana.

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