Organometallics notes 1

1. Tniroduction to Organic Reaction Mechanism 1
electrons due to the presence of unshared electrons pair, but they
areelectrically neutral. Important examples are as follows:
H-0-H, R-0-H,R-S-H,R-0-R, :NH,R-N-H
H
Neutral nucleophiles attack on the positively charged sub-
strates forming positively charged products.
-C + N C-N+
Substrate Nucleophile Product
The reactions which involve the attack of nucleophiles are
known as nucleophilic reactions, e.g., nucleophilic addition and
substitution reactions.
The various differences between nucleophiles and electrophiles
are given in the following Table.
Nucleophilic entities
(Nucleophiles)
Electrophilic entities
(Electrophiles)
(i) They are electron rich. (i) They areelectron deficient.
) They provide an electron (ii) They accept an electron pair.
pair.
(ii) They attack on electron-|(iii) They attack on electron-rich
deficient atoms.
(iv) They are Lewis bases.
(v) They have an unshared
pair of electrons not hold-
ing too strongly to the
atomic nucleus (generally
atoms of groups V and
VI of the periodic table).
(vi) They increase theircova- (vi) They form an extra or alter-
lency by one unit.
atoms.
(iv) They are Lewis acids.
(v) They have an empty orbital
which receives the electrons
pair from the nucleophile.
native bond with tbe nucleo-
phile.
(i) They are often anions.(vii) They are often cations.
1.6. Reaction Intermediates
Introduction. Before we try to understand the mechanism
of organic reacti1ons and work out a detailed
classification ot general
Organic
reactions we must try to know the nalure of the inter
mealates that are supposed to be formed in the intervening steps
between the reactants and the producis.
These transient inter-
mediates are known as reaction intermediates.
Their
The reaction
intermediates vary in their stabilities. Their
1alf-lives range from fraction of a second to several minutes. Al

2. 12 Introduction to Organic Reaction Mechanism
theintermediatescannotbeisolatedbuttheir existence is proved by
spectroscopic studies.
are molecules having carbon atoms in abnormal valence states.
They are called free radicals, carbonium ions, carbanions, enamines,
carbenes, nitrenes, benzynes. We shall discuss these one by one.
The most important of these intermediates
1. Free radicals. Any species which is having an odd or
.1umpaired electron is called a free radical.
Free radicals are electrically neutral. But they have unpaired
electrons. Therefore, they have tendency to pair up and hence
these are highly reactive species.
It is to be pointed out that the geometry of free radicals is not
known with certainty although it is readily believed that the carbon
ree radicals are either planar or having inverted pyramidal shape
depending upon the substituents attached to them.
the methyl free radical has planar structure in which carbon is in
sp hybridised state. The three sp* hybrid orbitals of carbon form
bonds with the three alkyl groups whereas the odd electron is lying
in the unused p-orbital,
For instance,
The relative order of stabilities of the common free radicals is
asfollows:
Benzyl>Akyi>Tertiary>Secondary> Primary> Methyl> Vinyl
Extra stability of the aromatic and alkyl radicals has been
attributed to their resonance. For their detailed study, please see
chapter on "Free Radicals".
2. Carbonium lons
Introduction. A carbonium ion is a cation in which the
positive charge is carried by a carbon having only six electrons in
itsvalence shell, i e., carbon atom lacks a pair of electrons in its
valence shell. A carbonium ion may be assumed to be a fragment
of a molecule in which an atom or group bonded to a cardon has
been removed alongwith the pair of bonding electrons. For
example, a methyl carbonium ion would be formed if a hydroxide
ion is removed from a molecule of methyl alcohol.
H
H:C:OH-» H :C*
H
OH
H
Carbonium ions have. been classified as primary, secondary or
tertiary according as the carbon having the positive charge is
primary, secondary or tertiary in nature.
carbonium ion having only one carbon is regarded as a special case.
In addition, methyl

3. Introduction to Organic Reaction Mechanism
H H H
CH,
CH-C CH-C CH-C+
H
H-C+
CH, CH,
tertButy
Methyl
carbonium ion Ethyl
carbonlum ion Isopropyl
carbonium jon carbonium ion
(Primary, I°) (Secondary, 2°) (Tertiary, )
The carbonium ions which are formed as reactionintermediates
are very reactive.
carbon atom has only six clectrons in the outer shell and has a
marked tendency to complete its octet. The octet can be completed
by combining with either of the following:
The reason for their reactivity is that the central
(i) An anionic species such as OH", CN", etc. For
example,
CH CH
OH
CH-C+
CH-C-OH
CH3 CH,
tertiary Butyl carbonium ion
-Butyl alcohol
(ii) An electron rich molecule such as H,0, NH,, etc. For
example,
-H+
(CH,),CH 4HO: -
(CH,), CHOH, (CH),CHOH
Isopropyl carbonium ion
fsopropyl alcobol
(iil) By losing an atom or group (usually bydrogen) without its
bonding electrons from the adjacent carbon atom, For example,
CH-H CH
CH-C CH-C + H
CH CH
2-Methyl propene
tert-Butyl carbonium ion
In some cases there occurs the conversion of less stable
carbonium ion into more stable carbonium ions
(rearrangement).
For example, a primary carbonium ion has tendency to form a
secondary or tertiary carbonium ion.
R R
R-C-C-H R-C-C-H
H
Primary carbonium ion
R RH
Secondary carbonium ion
the cases
Formation of carbonium ions. In most of
carbonium ions cannot be isolated but this instability does not
minimise their importance as reactive intermediates in a great
number of chemical reactions.

4. Introduction to Orgonic Reactior Mechanism
14
The carbonium ions may be formed in sofution, either in free
Some more stable ones have been obtained
state or as 1on pairs.
as solid states. However, the carbonium jons are formed inthe
following ways
(i) Direet ionisation. Many organic halides are known to
form carbonium ions in the presence ot a highly polar medium such
as SO
(CH),C-CI (CH,),C +C
r-Butyl chloride
CHCH.CH,CI- CH,=CH-CH, + CF
Allyl chloride
CH, CH-CI CH,.CH, +C
iquidSO
(CH),C-CI
Triphenyl
chloride (white solid)
(CgH)C+ + CI
Triphenylmetbyl
carbonium ion
(orange solution)
is very
Direct ionisation can occur only if the carbonium ion
stabie.
(ii) By protonation of unsaturated compounds. Carbonium ions
may be produced by dissolving olefins, carbonyl compounds and
nitriles in proton-donating solvents or treating them with Lewis
acids.
-CH=CH-+ H* -CH-CH,
c-o+ H* c-OH
C=0 + AICl c-0-AICI,
-C=N + AICi3 -C--AIC1,
-CEN+ H C=N-H
This method olfers satisfactory explanation for the values of
molar freezing point depression of most of aldehydes, ketones, acids,
esters and nitriles in 100 per cent sulphurie acid.
These values have been found to be twice as compared to that
produced by a non-clectrolyte.
=0+ H,s0, c-OH + HSO

5. Introduction to Organlic Reaction Mechanism 15
-C=N + H,SO, -C-NH + HSO
(ii) Br the protonation of atom having tlone pair of electrons.
Carbonium ions are formed when organic compounds having lone
pair ot elctrons on an atom especially oxygen are treated with
acid, e.g
H
R-0-H+H R-0-H R*+ H,0
H
R-0-R+H R-0-R Rt+ ROH
O
H
R-C-0-R + H* = R-C-0-R R-C+ ROH
O
H
O
R-C-O-C-R+H* R-C-0-C-R =
R-C+ HOOCR
This method of formation of carbonium ions offers explanation
for the fact that why the white crystalline alcohol triphenylmethano
on dissolvingin sulphuric acid forms an orange-coloured solution
and shows a molar freezing point depression four times as compare
to that produced for a solution of undissociated triphenylmethano
(CH),C.OH +HSO (CH)C+ SOH +H,0
Triphenyl methanol
(white solid)
Triphenyl methy
carbonium ion
(orange solution)
H,O+H,SO H,0*+ HSO
(CH),C.OH +2H,SO, (C,H,,C + H,0+2HSO,
iv) By the protonation of alkyl or acyl halides. Carbonium ions
are also formed when alkyl or acyl halides are treated with a proton-
donating solvent or Lewis acid. This is how the carbonium ions
R-X + H* R + H-X
R-C1 + AlCl, R+ AlCI
O
R-C-Cl+ AICI, R-C + AICI
are produced during Friedel-Craft's reaction using alkyl halides and
acyl halides.

6. Introduction to Organic Reaction mechanism
16
(v) By the decomposition of lest stable cations. Carbonium ions
are also produced by the decomposition of certain less stable
For example, the diazonium cations, which are obtained
cations.
by the action of a nitrite and a dilute acid on an aromatic amine at
low temperature, decompose to form alkyl carbonium ions.
Ar-NH,+O=N-OH+HCI Ar-N=NCI +2H,O
[Ar-N=NAr-N=N]- Ar+Na
Stability of carbonium ions. As the carbonium ions are
charged species, they are less stable than neutral molecules. Hence,
theyare formed only as transient intermediates. However, some
carbonium ions are known which are more stable than the others.
Thus, it becomes useful if we study the factors that affect the sta
bilities of carbonium ions. This will be useful in knowing which
path will be followed during a reaction.
Hence,
Stability of carbonium ions has been explained on the basis of
the principle that any situation in which positive charge on the
carbon atom is dispersed increases the stability of ion. If the charge
is localised, then it will be less stable. The factors which affect the
stability ofcarbonium ions are as follows
1. Relative stability of carbonium ions, The relative
stability ofsimple carbonium ions is as follows:
tertiary secondary>primary> methyl.
This order has beenexplained by two factors:
(a) Inductive effect. As the alkyl groups have electron donat-
ing effect (+), they increase the electron density on the carbon,
thus decaeasing the positive charge on it, The positive charge thus.
gets dispersed over all the alkyl groups and this dispersal of charge
incteases the stability of the wtole system, i.e., the carbonium ion.
The tertiary carbonium ions have maximum electron donating effect
due to three alkyl groups.
are more stable than secondary ones which in turn are stabler than.
primary carbonium ions.
Accordingly, tertiary carbonium ions
R R
RC> CH R-CH,> H,
CH,
R
Tertiary
R
Seconday Primary
(6) Hyperconjugation. The greater stability of alkyl substituted
carbonium ions has been partly attributed to the phenomenon of
hyperconjugation
G-electrons of a C-H bond into the unfilled p orbital of the positive
carbon atom. This spreads the charge all over the alkyl groups,.
thus increasing the stability of carbonium ion.
in which there occurs the delocalisation of

7. Introduction to Organic Reaction Mechanism 17
Molecular p Orbita
orbital
Further the tertiary carbonium ion has maximum number of
(hyperconjugation) structures compared to that of
Tesonance
secondary or primary.
stability than secondary carbonium ion which in turn bas more sta-
bility than primary carbonium ion.
Hence tertiary carbonium ion has more
The hyperconjugation energies for ethyl,isopropyland tertiary.
butyl carbonium ions have been found to be 151.2, 277.2 and 352.8
k/mole respectively, which refect their stabilities.
CH CH,H
CH-C CH-C. (Total 10 Structures)
CH
Tertiary
CH
CH-CH-CH, CH-CH=CH,H (Total 7 Structures)
Secondary
CH-CH CH,=CH
Primary
H(Total 4Structures)
2. Conjugation of +ve charge with a double bond. The
carbonium ions in which the positive charge gets conjugated with a
double bond have been found to be more stable. The increased
stability has been ascribed to the resonance which involves the elec-
trons of the double bond. A carbonium ion may delocalise its
positive charge through the canonical structures in its hybrid. Thus,
the positive charge on the central carbon of the carbonium ion gets
dispersed over other carbon atoms and makes the ion stable. As a
rule, more the resonance structures more the stability of the reso
nance hybrid.
more stable than propyl carbonium ion because of the marked
stabilisation of their resonance hybrids through the canonical
structures. Also, allyl cerbonium ion is more stable than propyl
carboniun ion because of resonance.
For example, benzyl çarbonium ion is considerably
CH-CH CH,CH-CH=CH, (equivalentresonance structures)
Allylic carbonium ion

8. 18 Introduction to
Organic Reaction
Mechanitm
CH CH, CH, CH
/4
/
Benzylic carbonium ion
For resonance to take place, the molecule should be essentially
planar.
The triphenylmethyl carbonium ion is extremely stable. Tbis
can be seen from the fact that if we place triphenylmetbyl bromide
in liquid sulphur dioxide (a solvent with which no reaction occurss
with carboninm ion), then one
can determine it quantitavively by
measuring electrical conductivity of the solution.
CH, CH e
liquid SO
CH-C-Br CH-C + Br
C.H
ECPh CPh
The extra atability of triphenylmethyl carbonium on has been
attributed to extensive resonance with the three benzene rings.
However, the benzene rings are slightiy out of plane and in fact they
have a propeller shape.
c
Total contribution structures for triphenylmethyl carbonium
ion will be 3x 3+1, i.e., 10 because 3 contributing structures result
for one ring.
In all these cases there occurs the delocalization due to the
Overlap of the vacant p-0rbital of carbon (carrying positive cbarge
With the 7 molecular orbitals. The delocalization has been foutd
to
be somewhat less than expected in triphenyl carbonium ion due
to
slightly non-planar rings.

9. Introductlon to
Organic Reaction Mechanism 19
H
C C
UTU
t Molecular Vacant
orbital orbital
Delocalization in allylic carbonium ion
Some of the carbonium ions are so stable that even their
3.
solid states are known.
yellow solid. In fact tropylium ion is about 1011 times more stable
than triphenylmethyl carbonium ion. Actually, tropylium ion is
the most stable carbonium ion. It is so slow that its reaction with
water, alcohol, etc. is very slow.
For example, tropylium bromide is a
The tropylium ion is planar and has six 7 electrons for
resonance which is in accordance with Huckel's rule (n=1). Due to
this it has extensive resonance resulting in sharing of the positive
charge by all the carbon atoms.
stabilised by aromatisation.
Thus, the tropylium ion is
--OOO-O-
Canonical structures of the tropylium ion
Further, substituted oyclopropenyl cation possesses even more
aromatic stabilization (n=0), e.8.
CyH CyH CH
C CHCH CH C4 cg
,2,3-Tripropyl cyclopropeny 2 ElectroD syste
In these cations all the carbon atoms are sp hybridized.
all the p orbitals overlap (including the
positively charged carbon) to form a delocalised n molecular orbital
which is common to all the atoms.
4. Substitution. The stability of carbonium ions is also
Thus
vacant p orbital of the
effected by substitution. For example, the presence of electron
donating (resonance) groups (e.g., -OH, -NH,-OR,etc.) in a

10. Introduction to Organic Reaction Mechanism
20
further.
carbonium ion increascs its resonance
delocalisation of electrons which urther stabilises the carbonium
ion. However, the presence of electron wjthdrawing groups (ez.
This increases
-NO, -CN, > C=0) in a carbonium ion decreases the electron
density on the carbon atom, thus making it less stable.
CH2
7
C:0H
5. Solvent. It is possible to stabilise the carbonium ioas
by solvation by a polar solvent if it has no reaction
solvent. However, if a more polar solvent is used, then there
occurs a greater stabilisation of carbonium ion. In order to stabilise
the carbonium ions by solvation, it is not necessary that they should
be planar.
It is also found that the stability of the carbonium ion is also
increased if it gets closely associated with a negative ion, ie., as an
ion pair.
with the
Classification of Carbonium ions. On the basis of
relative stabilities, it is possible to classify carbonium ions into two
types, namely (i) transient (short lived) carbonium ions
stable carboniumn ions.
and (ii)
() Translent carbonium ions. These ions do not have much
stabilisation by resonance.
transitory intermediates in certain organic reactions.
very reactive and can combine readily with any substance that can
donate an electron pair. Some
carbonium ions are alkyl carbonium ions, ie., primary, secondary
and tertiary. The relatively stability of the carbonium ions is as
follows
These are assumed to be formed as
These are
typical examples of transient
Tertiary > Secondary > Primary
(CH,,C>
(CH,,CH>CH,CH,
i) Stable carbonium ions. These ions ae so stable that can
be isolated and srudied. For example, benzyl aud allyl carbonium
ions are most stable than simple alkyl carbonium ions. However,
some carbonium ions have been found to be so stable that even their
solid salts are
exists as a red crystalline solid whereas tropylium
known. For example, tripbenymethyl perchlorate
bromide is a
yellow solid.

11. Introduction to Organic Reactlon Mechanism 21
C. CI0 Br
Triphenylmethyl perchlorate Tropylium bromide
It is to be remembered that tropylium ion is the most stable
carboniuum ion. The order of stability of some of the carbonium
ions is as follows
Tropylium cation (C,H,),C> CH,CH, > CH,=CH-CH,>
Benzy
carbonium ion carbonium ion
Allyl
carbooium ion
Tripbeoyl
CH
Methyl
carbonium ion carboaium ion carbonium ion carbonium ion
(CH,)C > (CH,),CH> CH,CH, > CH,
teri-Butyl Isopropy Ethyl
Reactions of carboniunm ions. A carbonium ion once
produced during a reaction may react further in any one of the
following ways:
() Elimination ofa hydrogen ion to form an alkene. Acarbo
Dium1on may eliminate a hydrogen from a carbon adjacent to that
carrying the positive charge.
ehe removal of hydrogen (as H* ion) now forms a T bond with the
tlectron deficient carbon so as to form an alkene.
propyl carbonium loses a proton to form propylene.
A carbo
The pair of electrons left behind after
For example, a
H
CH-CH-CH2
CH-CH =
C
Propylene
Propyl carbonium ion
(ii) Rearrangement to form imore stable carbonium ion. Car
bonium ions have a tendency to rearrange
themselves forming more
stable carbonium ions.
Car
In these rearrangements,
there occurs the
migration of a hydride jon from an adjacent carbon atom to the
electron-deficient carbon of the carbonium ion. Such a migration
is known as hydride shift.
C H
H

12. 22 Introductlon to Orgonie Reactlon Mechanim
For exmple
CH-CH CH CH,-CH,-CH-CH,
Botyl ca boatum ion
(Primary, en stable)
Me Botyl Carboniun ion
(Secondary, more nable)
In some rearrangements there may occur the migration of
an alkyl group with its pair of bonding electron from an adjacent
carbon to the carbon having positive charge. This type of migration
is known as alkyl shift.
R
For example.
CH CH
CHy- CH CH CH--CH-CH
CH CH
33-Dimethy! 2- butyl 2.3-Dimethyl -2- buty
car boni um ion car bonium on
(secondary,
less stable) Tertiary, more stabie)
As both bydride and alkyl migratioos involve adjacent carboa
atoms, they are collectively known as ), 2-shifts.
() Combination wlth nucleophile. A carboaium ioa may
accept a pair of clectrons from aucleophile, forming a boad.
6 mple ethyl bromide is formed by the reaction of a bighly reactive
For
carbonium ion with a bromide.
CH-CH 8r CH CH B
Ethyl orbonium ion Ethyl bromide
Ancthtraannpleis the reaction ofa ncutral ucleophile H,o
he 1 huvl
choniunn forming protonated iertbutyl

13. 13
Inttoduc ion to Organic Reaction Mechamism
CH
CH3
CH3 O-H
H CH H
CH
rtiary butl corbonium ion "ert butyl alcohol
The protonated alcohol further loses the proton to form butyi
alcchoi.
CH CH,
-H+
CH,-C-O -H- --CH,-C-0-H
CH,
Tert. butyl alcohol
CH, H
r} Addirion to an alkene. A carbonium ion may add on to
an alkne to form a bigger carbonium ion. For example,
CH CH CH,
CH
CH-C=CH, + C-CH, CH,-C--CH,-C-CH,
CH CH,
(v) Abstraction ofa hydride ion. A carbonium ion may
remoe a hydride ion to form an alkane. For example,
CH
CH
CH
-C C HC-CH, CH-C-CH-CH in
CH
CH CH
CH
Configuration of carbonium ions. In the carbonium ions,
the carbon carrying positive charge is 5p' bybridised. This carbon
uses the three bybrid orbitals for single bonding to thrce substitu
ents the remaining p-orbital is empty.
has a fat structure in which all the three bonds aro in one plane
with the bond angle of 120° betiween them Enough evidence is
available which shows that carbonium ions are planar structures as
shown in the following bgure.
Thus, the carbonium ioa
R
120
R
120

14. Introdhection to Organtfe Reaetlon Merhanisme
24
Such planar tructures having 120
hbridisation are considered to be neceeary conditinne for the
8bilisetion ot carbon ium fons through
resonanee or
hyper con in-
gation. If some how the strucrure ot a molerule preludes 10
geometry and sp' hybridsation in the corresponding carhonium jon.
the ion will not be formed. This ie the etplanation for the faet that
the triphenylchloromethane(in liquid SO,) and the ordinary t-alkvt
halides get ionised readily to form the tripheny Imethyl carboninm
ion and a -butyl carbonium ion (transient) respectively whereas
the bridged analogues 1-bromotriptycene and 1-chloroapocamphane
do not.
geometry and p
B
liquid
No ioniZation
SO
1-Bromotript
ycene
CH CH
AgNO-alcohol
-CI No reaction
heat 48 hours
1-Chloroapocamphane
The reason for the failure of such bridged structures to generate
carbonium ions is that it is geometrically impossible for tbe resuiting
cerbonium 1ons to have the planar structures required for its stabi-
lisation due to the rigid structural constraint of their rings.
Another cxperimental cvidence, which proves the plaaar
nature of the carbonium ions, is the reating of an opically acuve
ubstrate witb a nucleopbilic reagent whca a racemie mixiure is
formed.The formalion of tacemic miture can bc explaincd only
if the carbonium ion has a planar structuc Due to the planar
tructurc of carbonium 1on, the nucleopbilic reagent bas cqual-
tendency to attack the posilive car boa alom from cilher side of the
plane (i.t., from above or below), resultiag a the lormatioa of an
an equimolar mixture of d and torm (racemie mixture).
Another importänt stereocbem1cal aspect, which needs expla-
nation, is tbat a lertiary carboaium ion having three large graups
1s formed very readily.
Broups have a steric pressure on each other provided carbon has a
The explanation tor this is that the large

15. Introduiction to Organic Reaction Mechanism
tetrahedral configuration.
as the carbonium ion is formed.
strainis that the three groups are at an angle of 120° to each other
in carbonium ion as comparrd to 109° in tetrahedral parent mole-
cule. Hence in such cases the equilibrium will lie far to the right.
This strain will be relieved as soon
The reason for the relieving in
3. Carbanions
Lntroduction. A carbanion may be defined as an anion in
which negatire charge is carried by a carbon. A carbanion is formed
t one of the atoms or groups singly bound to a carbon is removed
without the bonding pair of electrons. For example, removal ota
hydrogen of methyl part of acetaldehyde molecule as a hydrogen
ion results in the formation of a cardanion as shown below :
H H
H:C-CHO- :C-CH0 H
H
Acetaldehyde
H
carbanioon
Nomenclature. Individual carbanions are named after the
parent alkyl group and adding the word carbanion. For example,
CH-CH,: Ethyl carbanion
CH
CH-CH : Isopropyl carbanion
Carbanion as a nucleophile. Due to the presence of the
electron pair, carbanion can be considered as a Levis base, i.e., an
electron pair donor. Thus, it is a nucleophile.
Methods of formation of carbanions. The carbanions
are formed in solution by the following methods:
(a) It is formed by heterolytic fission of a bond attached to
the carbon atom. There 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 as a
cation which is most commonly a proton.
R-H R:+ H*
As the removal of a proton requires a base, it means that
the formation of carbanion is an acid-base reaction. Thus, the
a-hydrogen atoms of carboxyl and nitro compounds
slightly acidic can be removed as a proton by a base to form a
carbanion.
which are
O
NaOH + H-CHC-H Na:CH,-C-H +
HO
(Base)
(conugate base) (conjugate acid)
(Acid)

16. Introductton to Organic Reaction Mechanísm
26
O
C.HON +H-CH,-C-CH,- Na:CH,-C-CH,+C,H.OH
KOH +H - CH,-NO,-^ K : CH,-NO, + H,O
In order to remove protons from hydrocarbons (very weak
acids), a stronger base such as sodamide in Tiquid ammonia is to be
used.
liquid
(C,H,),C-H+NaNH, -(C,H,),C: Na+NH,
NH
R-CEC-H+NaNH,-~R-C=C:Na+NH,
On the other hand, aromatic compounds of the type of tri-
phenylmethane can form carbanion very easily in the presence of
base.
(CH,CH +OH-(CsH,),C+ H,O
(b) Carbanions may be formed by breaking of carbon-metal
bonds of organo-metallic compounds.
-C:M -C 4 Mt (M=metal)
The nature of the carbon-metal bond has been found to
depend on the electronegativity of metal and nature of the organic
part. If the metal is less electro negative and the organIC part
possesses electron withdrawinggroups, then the bond will become
more polar. Hence it might ionise to form carbanion which exists
as ion pair.
CHNa :CH,+Na
R-MgX R:+ MgX
XZn-CHcOOC.H, XZn+:CH,COOC,H,
Less polar
(c) The negative ion can add on to a carbon-carbon double
bond, forming a carbanion. Such type of additions will occur only
when the double bond gets activated by the electron withdraw ing
groups such as nitro, cyano, carbonyl, eic. By this method, the car-
banion is formed in the Michael addition.
O O
- Y--C-C-

17. Introduction to Organic Reaction Mechanism
27
Carbanions are also formed by loss of carbon dioxide during
the decarboxylatiqn of carboxylic acids using their salts.
O
R-C--0-^R:+Co,
Stability of carbanions. As carbanion contains an electron
pair, it acts as a Lewis base and accepts a proton and then it is itself
converted into its conjugated acid
tions of carbanions may be represented by an acid-base reaction.
Thus, the formation and reac
RC+ HY R,CH+:Y
Base Acid Cunjugate
Acid
Coniugate
base
Carbanioo
Hence it is possible to relate the stability of a carbanion to the
strength of its conjugate acid. If the acid is stronger (smaller pk
value), then the carbanion has little or no tendency to accept the
proton (i.e., conjugate base is weaker) and hence it will
Thus, by knowing the pka values of the conjugate acids it is possible
to know the relative stabilities of the carbanions.
table, the carbanions have been arranged in the increasing order of
their stabilities (The carbanions will be formed by rcoioval of a
proton from the acid).
be stable.
In the following
Conjugate acid pka Codjugate acid pk
CH 43 CH, (COOC,H,), 13.3
CH,-CH, 37 CH, (CN), 12
CaH 37 HC (CF)
CH,CH, 37 CH,COCH,cooCH, 10.7
33 CH,NO, 10.2
CH,),CH
8.8
28 (CH,CO),CH,
CFH
25 (CHCO),CH
HC=CH
25 CH,(NO,
CH,CN
20 CH (NO,),
CH,COCH,
19 CH (CN),
CH,COCH,
However, it is dificult to ascertain the pka values for extremely
weak acids. in such cases it is possible to measure the relative
acidities of RH and R'H by measuring the equilibrium constant for
he reaction.
R-Li + R'-I R-1+ R-Li

18. Introduethon
to
Organic
Reaction
Mechonism
For
example,
the
cyclopentadienyl
anion
has
six
by
a
r
o
m
a
t
i
s
a
t
i
o
n
.
Telections
for
resonance(n
1),
Hence
it
gets
aromatic
stahi
SIX
The
p
orbitals
overlap
to
form
a
completely
delocalised
molecular
orbital
havig
six
electrons.
These
six
electrons
are
spread
over
all
the
tive
carbon
atoms
ike
the
delocalised
aromatic
syst
benzene
rng
and
the
carbanion
is
quite
stable.
Canonical
structures
of
the
cyelopentadieny
l
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
lises
the
carbinion
by
involving
this
type
of
overlap.
:0:
:0
:0
R-S-CH,R-SCH,R-S=CH,
:0:
:0:
:0:
Reactions
Carbanions.
As
carbanions
are
electron-ricb
species,
they
behave
as
potent
nucleophiles,
i.e.,
the
carbanio
part
in
the
reactions
by
giving
formation.
Carbanions
are
known
to
take
part
in
a
number
of
addition
and
substitution
reactions.
However,
these
are
also
few
rearrangement
reactions.
as
follows:
()
Addition
reactions.
group
of
aldebydes
(aldol
condensation)
and
ketones.
s
e
its
electron
pair
for
bond
Some
representative
examples
are
Carbanions
add
on
to
the
carbonyk
:0
R:
R-
A
number
of
condensation
reactions
like
aldol
condensation,
Perkin
and
Claisen
condensations
are
some
examples
of
addition
reactions
of
carbanions.
(i)
Substitution
reactions.
Carbanions
take
part
in
nucleo-
philie
substitution
reaction
at
saturated
carbon
atoms,
i.e.,
Sz
reaction.
R:+CH,-X
R-CH,
+
X

19. Introduction
to
Organic
Reaction
Mechanims
31
Common
examples
of
substitution
reactions
are
alkylation
of
malonic
ester,
B-keto
esters,
B-dicarbonyl
com
pounds,
Wurtz
reaction.
Reimer-Tiemann
reaction,
halogenation
of
ketones,
halofornm
reaction
and
decarboxylation
of
a
number
of
carboxylic
acids.
(iii)
Rearrangements:
An
example
involving
carbanion
is
Wittig
reaction
or
rearrangement.
Configuration
of
carbanions.
In
carbanions,
the
carbon
charge
is
sp3
hybridised.
The
three
sp3
orbital
form
sigma
bonds
and
the
fourth
sp3
orbital
accommodates
the
unsbared
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
asymmetric
carbanion
has
been
explained
by
the
pyramidal
structure
because
there
occurs
rapid
inversion
of
configuration
during
its
life
time.
carrying
negative
C
a
n
b
a
electron
pair.
R
Corbonium
yramidal)
R
R2
=
R3
Rg
Tetrahedral
structure
(spontaneous
inversion)
It
is
believed
that
carbanions,
wbich
are
stabilised
by
resonance,
are
assumed
to
have
planar
configu
ration
and
120°
geometry
to
accom-
9
modate
the
ap*
hybridisation
for
their
resonance
hybrid
structurc.
To
sum
up
it
may
be
said
that
the
carbon
atom
of
an
unconjugated
carbanion
is
in
sp3
hybridised
statec
with
a
pyramidal
shape
whereas
the
carbon
atom
of
a
conjugated
carbanion
is
in
sp*
hybridi
sed
state
with
a
planar
structure.
Bridged
or
carbonium
ions
are
those
which
get
stabilised
by
the
movement
of
either
a
lone
pair
of
electrons
or
r-clectrons,
in
conjugation
to
the
positively
charged
carbon
atom
to
form
a
new
T-bond.
R
R
Planor
hybrid
structure
Classical
non-classical
carbonium
ions.

20. Introduction
to
Organlc
Reactlon
Mechanisn
32
R
R
R
-
109
120
R
R
On
the
other
hand,
if
in
a
carbonium
ion
the
positive
charge
does
not
get
conjugated
with
the
double
bond,
then
the
r
e
s
o
n
a
n
c
e
s
r
u
c
t
u
r
0
8
cannot
be
written
in
a
normal
way.
cases
the
r
e
s
o
i
a
n
c
e
structures
can
be
written
by
the
participation
of
the
neighbouring
groups.
This
results
in
the
formation
of
bridged
carbonium
ions
which
are
also
ca!led
n
o
n
-
c
l
a
s
s
i
c
a
l
carbonium
ions.
However,
in
some
Such
type
of
r
e
s
o
n
a
n
c
e
has
been
exhibited
by
homoallylic
carbanions
ions,
in
which
there
is
a
carbon
atom
between
the
pOsl
tively
charged
carbon
and
the
unsaluration.
Hence
in
2-phenylethyl
carbonium
ion
the
r
e
s
o
n
a
n
c
e
structurers
may
be
written
by
partici
pation
of
the
phenyl
groups.
Such
ions
with
a
bridging
phenyl
group
are
known
as
phenonium
ions.
CH2
CH
CH
cH2s
Phenonium
jon
Non-classical
carbonium
ion
The
existence
of
non-classical
carbonium
ions
is
quite
doubtful.
However,
certain
cases
are
known
from
which
evidence
is
available
for
the
existence
of
bridged
carbonium
ions
as
reaction
intermediates,
e.g.,
()
The
remarkable
stereospecificity
seen
in
the
solvolysis
of
optically
active
threó-and
glacial
acetic
acid
medium
has
provided
a
strong
evidence
for
the
formation
of
non-classical
carbonium
ions.
If(-)-threo
isomer
gets
acetolysed,
the
product,
formed
is
a
racemic
mixture
of
threo
isomer
(main)
and
a
very
small
amount
of
erythro
isomer.
The
formation
of
bridged
head
or
non-classical
carbonium
ion
as
inter-
mediate
could
only
explain
the
exclusive
formaion
of
threo
isomer.
The
non-classical
carbonium
ion
may
be
attacked
by
the
solvent
(acetic
acid)
equally
at
either
of
the
carbon
atoms
forming
enantio-
meric
pair.
erythro-3-phenyl-2-butyl
tosylates
in

21. Introductlon
to
Organic
Reactlon
Mechanism
33
H
CHs
CH
0TS
H
Cors
CH
H
-
CH3
CHS
-)-
Threo
isomer
of
3-Phenyl-2
Buty!
Tsylote
Symmetric
phenonium
ion
OAcO
Attack
on
B-carbon
Attack
on
a-carbon
CgHs
cHs
HC
-
-
-
cH
OAC
CAC
CH3
Racemic
threo-acetate.
On
the
other
hand,
only
one
optically
active
product
of
the
erythro
isomer
is
formed
when
erythro
isomer
of
3-pheny1-2-
butyl
tosylate
is
treated
with
acetic
acid.
The
reason
for
this
is
that
the
non-classical
carbonium
ion
is
asymmetric
and
thus
attack
by
acetate
ion
at
either
carbon
atom
of
the
carbonium
ion
will
result
in
the
formation
of
the
same
product.
The
reason
for
this
is
that