Organometallics notes 2

1. 25
Introduction
to Organic
Reaction
Mechanismn
tetrahedral configuration.
as the
c a r b o n i u m
ion is formed.
The r e a s o n
for the relieving
in
strain is that the threc groups
are at an angle of 120° to each other
in
c a r b o n i u m
ion as
compared
to 109° in
t e t r a h e d r a l
parent mole
cule.
Hence in such cases the equilibrium
will lie far to the right.
This
strain will be
relieved
as sooD
3. Carbanions
A
carbanion may be defined as an
anion in
A
c a r b a n i o n
is
formed
I n t r o d u c t i o n .
which negative charge is carried by a
carbon.
if one of the atoms or groups
singly
bound to a
carbon is removed
without the bonding pair of electrons.
For example,
removal
ofta
hydrogen
of methyl part of
acetaldehyde
molecule as a
hydrogen
ion results in the
formation ofa
carbanion as
shown
beloW:
H
H
H:C-CHO ->
: C - C H O
+ H
H
H
Acetaldehyde
carbanion
N o m e n c l a t u r e .
Individual
carbanions are
named
after the
parent alkyl group
and adding the word
carbanion.
For example,
CH,-CH,: Ethylcarbanion
CH
CH-CH:Isopropylcarbanion
C a r b a n i o n as a
nucleophile.
Due to the presence
of the
electron pair,
carbanion can be
considered
as a Lewis base, i.e., an
electron pair donor. Thus, it is a nucleophile.
Methods of
f o r m a t i o n
of
c a r b a n i o n s .
The
c a r b a n i o n s
are formed in solution by the following
methods:
(a) It is formed by heterolytic
fission of a bond
attached to
the carbon atom.
There
The carbanions
occurs
the breaking ofa bond in sucha
way
that the carbon atom
retains the
electron pair, thus getting
negative charge. The group
attached to the
carbon is lost asa
cation which is most commonly a proton.
R-H
R:+ H*
- +
As the removal of a proton
requires a base, it means
that
Thus, the
which are
the
formation of
carbanion is an
acid-base
reaction.
a-hydrogen
atoms of carboxyl
and nitro
compounds
slightly
acidic can be
removed as a proto» by a base to forma
carbanion. O
+
NaOH+
H - C H , - C - H »
Na:CH-C-H + H,O
(Base)
(Acid)
(conjugate
base)
(conjugate acid)

2. Introduction
to
Organic
Reaction
Mechanism
11ere
it
is
assumed
that
if
R-H
is
a
stronger
acid,
then
it
will
nrcfcrably
exist
as
RLI
rather
than
RI.
The
carbanion
has
heen
1ahilised
by
any
factor
which
decreases
negative
charge
on
the
carben
atom.
The
net
charge
on
the
carbon
atom
can
be
decreased
if
it
gets
distributed
over
the
molecule.
The
following
are
the
factos
which
stabilise
the
car
banions:
(i,
Induc
tive
efect.
The
relative
order
of
stabilities
of
the
simple
carbanions
is
given
as
follows:
R
H,C:R»
CH,>
CH>
R-C:
R
R
It
is
important
to
note
that
this
order
is
reverse
of
that
of
carbonium
ions
in
which
tertiary
is
most
stable
while
methyl
car-
bonium
ion
is
least
stable,
i.e.,
tertiary
secondary>
primary
>
metbyl.
The
above
order
of
stability
of
carbanions
has
been
explained
on
the
basis
of
the
electron
repelling
effect
of
the
alkyl
groups
(shown
by
arrow
heads
above).
Due
to
this
inductive
effect,
there
occurs
an
increase
in
electron
density
on
carbon
atom
carrying
negative
charge
and
it
(carbanion)
gets
destabilised.
This
destabili-
sing
is
maximum
in
the
case
of
tertiary
carbanion
as
it
has
three
alkyl
groups
attached
to
it
and
least
in
the
case
of
the
metbys
carbanion
as
it
has
one
methyl
group.
If
the
electron
withdrawing
groups
are,
however,.
introduced
in
the
alkyl
groups,
then
the
stability
of
carbanion
is
increased.
The
negative
charge
can
be
shared
by
these
groups
by
-I
efect.
example,
the
introduction
of
fluorine
atoms
greatly
increases
the
stability
of
the
carbanions.
This
happens
so
because
the
duorine
atom
has
electron-withdrawing
inductive
effect
and
this
brings
about
dispersal
of
the-ve
charge
of
the
anion
with
consequent
increase
in
the
stability
of
the
carbanion.
However,
the
effect
is
less
with
chlorine
atoms
(less
electronegative).
(CF,),C
is
more
stable
thaD
FC
which
is
in
turn
more
stable
than
H,C
For
CF
F
H
FC-C:>
F-+C
H-C:
CF,
H
The
s-character
of
the
carbon
bearing
nega-
(ii)
s-character.
tive
charge
also
affects
the
stability
of
a
carbanion.
Greater
the
S-character
of
the
carbon
atom
having
negative
charge,
greater
will
be
its
stability.
The
s-character
of
hybrid
orbital
increases
in
the
order
sp"
<spë
<
sp.
As
the
electrons
in
s-orbitals
are
closely

3. Introduction
to
Organic
Reaction
Mechanism
29
held
by
the
nucleus
than
those
in
p-orbitals,
the
sp
orbital
will
have
more
s
character
and
hence
the
sp
carbon
is
more
electronegative
than
sp
or
sp*
carbons.
Hence
sp
carbon
atom
can
accommodate
electrons
better
than
sp"
or
sp3
carbons.
following
relative
stability
of
the
carbanions
This
factor
explains
the
HCC
sp-Carbon
HC=CH>
sp2.Carbon
H.C-CH
sp-Carbon
(iii)
Resonance.
Carbanions
can
also
be
stabilised
due
to
resonance
when
they
have
double
bond
or
an
atomic
ring
adjacent
to
the
charged
carbon
centre.
more
stable
than
ethyl
carbanion.
For
example,
benzyl
carbanion
is
-CH
CH-CH,
Ben2yl
carbanion
Ethyl
carbanion
The
stabilisation
by
resonance
is
due
to
the
delocalisation
of
the
negative
charge
which
is
then
distributed
over
other
carbon
atoms
in
the
hybrid
structure.
The
canonical
forms
of
benzyl
carbanion
in
its
resonance
bybrid
structure
responsible
for
its
stabilisation
are:
CH
CH
CH2
-
-
-
O
The
stability
of
the
carbanion
is
greatly
increased
if
the
negative
charge
gets
conjugated
wih
a
carbonyl,
nitro
or
cyano
group.
The
reason
for
this
is
that
the
electronegative
atoms
such
as
oxygen
and
nitrogen
can
accommodate
negative
churge
better
than
carbon
atom.
The
resonance
structures
of
such
carbanions
may
be
represented
as
follows
:0
:0
CH-C-
H,
*
CH,-C=CH
CH,-N
CH=N
:CH-C=N:
CH,=C=N:
(tv)
When
certain
cyclic
carbanions
satisfy
the
Huckel's
rule,
1.e.,
they
have
(4n-+2)
r
electrons
for
r
e
s
o
n
a
n
c
e
,
they
are
stabilised

4. Introduction to
Organic Reaction
Mechonism
by aromatisafion. F
example,the
cyclopentadienyl anion has six
clections for
resonanceln=). Hence it geta
ence it gets atomatic
stabilisation.
orbital havi g six
electrons.
the ive carbon atoms like the
delocalised aromatic system of
The p-orbitals overlap to form a
completely dele
elocalised r molecular
These six
electrons are
spread over
benzene ring and the
carbanion is
quite stable.
Canonical structures of the cyclopentadienyl
carbanion hybrid
Resonance hybrid of
cyclopentadienyl carbanion
When the
carbanionic carbon gets attached directly to
sulphur
or
phosphorus atoms (i.e., d-period elements), the unshared pair gets
conjugated with the vacant d orbital. Thus, the unshared pair can
overlap with the vacant d orbital. For
example, -SO,R group
Stabilise be vacant"a orbital,T,the unshO sulphur
stabilises the carbanion by involving this type of overlap.
:0 :0
R-S-CH, R-S=CH R-S=CH,
:0:
:0: :0:
Reactions of Carbanions.
As carbanions are
electron-rich
species, they behave as
potent nucleophiles, i.e., the carbanions take
for bond
formation. Carbanions are known to take part in a number of
addition and substitution reactions. However, these are also few
examples are
part in the reactions by giving its electron pair
rearrangement reactions. Some
representative
asfollows
(i) Addition reactions. Carbanions add on to the carboayl
group of aldehydes (aldol condensation) and ketones.
:0
:0
RG
R
A number of condensation reactions like aldol condensation,
Perkin and Claisen condensations are some examples of addition
reactions of carbanions.
(ii) Substitution reactions. Carbanions take part in nucleo-
philic substitution reaction at saturated carbon atoms, i.e., Sz
reaction.
R: +CH,-X R-CH, + X

5. Introduction to
Organic Reactlon
Mechanims 31
common examples of
substitution reac
of
malonic estert, B-keto esters, B
dicarbony com
reaction, actions are alkylation
Reimer- Tiemann reaction,
halogenation of ketones,
naloform reaction and
decarboxylation of a number of
carboxyc
acids.
iit) Rearrangements: An
Wittig reaction or
rcarrangement.example involving carbanion is
Configuration
carbanions, the
charge is sp3 hybridised.
form sigma bonds and the fourth sp orbital
accommodates the
unshared electron pair.
Hence the
negatively charge carbon in a
carbanion has a
pyramidal shape similar to
that of
ammonia. Actually, a
carbanion and
ammonia are
isoelectronic species.
The loss of
optical activity associated
with the
explained by the
pyramidal structure because
there occurs rapid inversion of
configuration during its life
time.
of
carbon carbanions. In
carrying negative
The three sp orbital
Cooniu
asymmetric carbanion has been
Fyomidc
R2
R
R
=
R3
R
Tetrahedral structuree
Spontaneous inversion)
It is believed that carbanions,
wbich are stabilised by resonance,
are assumed to have planar configu-
ration and 120" geometry to accom-
modate. the sp hybridisation for
their resonance hybrid structure.
To sum up it may be said that
the carbon atom of an unconjugaled
carbanion is in sp bybridised state
with a
pyramidal sbape whereas the
carbon atom of a conjugated carbanion is in sp* hybridi sed state
with a planar structure.
Planor ybrid structure
Classical
carbonium ions are those which get stabilised by the movementof
either a lone pair of electrons or w-clectrons, in conjugation tothe
oSitively charged carbon atom toform a new T-bond.
non-ciassical carbonium ions.
Bridged or