Car Bani On 193m1t

Carbanion Mr. Prashant S. Munnolli M.Pharm-1st year Dept. of Ph.Chemistry

H.S.K College Of Pharmacy Bagalkot

• CARBANIONS • A carbanion defined as anion which are electron rich species and carry negative charge. • Geometry:..

R

C

-

R

R

• In carbanion, carbon is Sp3 hybridized, three Sp3 orbital form sigma bond and the fourth Sp3 orbital accommodates the unshared electron pair. Hence, the negatively charged 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 is associated with the pyramidal structure due to rapid inversion of configuration during its life time. • Carbanion is stabilized by resonance which have planner configuration to accommodate the Sp3 hybridization for their resonance hybrid structure.

R2

R2 R1

C R3

:

:

C

R1 R3

Methods of formation of carbanion : • Carbanions are formed by heterolytic fission of bond. Carbanion are formed by following method: • By acid – base reaction : O

O

+ NaOH + H - CH2 - C - OH (Base)

+ Na CH2 - C - OH + H2O

(Acid)

carbanion

• By removal of proton from hydrocarbons :

+ (C6H5)3 C - H + Na NH2 Tri phenyl methane

Liquid NH3

+ (C6H5)3 C : Na + NH3 carbanion

• By breaking carbon-metal bonds of organometallic compounds : R - Mg - X

- + R + MgX

• By adding negative ion to carbon – carbon double bond : O

O Y: + C = C - C -

Y-C-C-C-

• By decarboxylation of carboxylic acids : O R

C

O

R + CO2

Stability of Carbanions : • Inductive effects : The relative order of stabilities of the simple Carbanions is given as follows : H3C:

R - CH2

R R

CH

R R C R

• Due to inductive effect, electron density increase on carbon atom carrying negative charge and it gets destabilized. This destabilities is maximum in case of tertiary carbanion as it has three alkyl groups attached to it and least in case of the methyl carbanion as it has one methyl groups.

• If the electron withdrawing groups are introduced in the alkyl groups, then the stability of carbanion is increased. The negative charge are shared by these groups by – I effects.

F3C

CF3 C CF3

F F

C F

H

H C H

•

•

S character: The S character of the carbon bearing negative charge affects the stability of a carbanion. Greater the S character of the carbon atom having negative charge, greater will be its stability. The character of hybrid orbital increases in the following order

SP3 < SP2 < SP HC

C SP

H2C

CH SP2

H3C

CH2

SP3

•

•

•

Resonance: Carbanions can be stabilized due to resonance when they have double bond or an aromatic ring adjacent to the charged carbon centre. The stabilization by resonance is due to the delocalization of the negative charge which is then distributed over other carbon atom in the hybrid structure. e.g. resonance structure of benzyl carbanion. CH2

CH2

CH2

CH2

CH2 -

-

• The stability of the carbanion is greatly increased if the negative charge gets conjugated with carbonyl nitro or cyano group. The reason because electronegative atoms such as oxygen and nitrogen can accommodate negative charge better than carbon atoms. • The resonating structure of such carbanions as follow : .. :O: CH3 :CH2 :CH2

C + N

C

:O: CH2

CH3

O O N:

CH2

CH2

C

+ N

C

CH2 O O .. N:

•

•

Aromatisation : Cyclic carbanions follow the Huckel’s rule they have (4n + 2) electron for resonance. They are stabilised by aromatisation. e.g. the cyclopentadienyl anion has six  electron for resonance (n = 1). Hence it gets aromatic stabilisation. -

-

..-

Resonance hybrid structure

-

Reaction of carbanions : • •

Addition reactions : Carbanions add to the carbonyl group of aldehydes and ketones. e.g. aldol condensations, Perkin and Claisen condensations. .. :O:

O R + C

R

C

•

•

Substitution reactions : Carbanions take part in nucleophilic substitution reaction at saturated carbon atoms e.g. SN2 reaction. e.g. Reimer - Tiemann reaction, Halogenation of ketones, Wurtz reaction.

R + CH3

X

R

CH3 + X

• •

•

Aldol condensation : Aldehydes containing a α-hydrogen atom undergo a reversible self addition in presence of dil. Alkali to give condensation products βhydroxy Aldehydes. The hydroxyl aldehyde formed from acetaldehyde is originally called as aldol & the reaction is known as aldol condensation. O

O -

CH3 - C - H + CH3 - C - H Acetaldehyde

Acetaldehyde

OH

OH

O

CH3 - CH - CH2 - C - H 3-hydroxybutanol (aldol)

• Mechanism : • The enolate ion formation : O

O H

CH2

C

H

:CH2

+ OH

H + H2O

C

Enolate ion (nucleophile)

Acetaldehyde

• The enolate ion attacks the carbonyl carbon of another un-ionized aldehyde molecule. O

O

+

C CH 3

H

Acetaldehyde

-

H2C

Enolate ion

O H

-

O H

•

The negative oxygen in the product accepts a proton from water to give aldol.

HO

H O

CH3

CH

OH

O CH2 C

H

CH3

CH

O CH2 C

3-hydroxybutanol (aldol)

+ OH H

•

Benzoin condensation : The self condensation of aromatic aldehydes (with no αhydrogen ) in presence of cynide ions as a catalyst to α-hydroxy ketone (benzoin) is called benzoin condensation. This reaction is not successfully with aliphatic aldehydes under these condtions.

C6H5CHO + C6H5CHO Benzaldehyde

Benzaldehyde

KCN

C6H5

O

OH

C

C H

Benzoin

C6H5

•

Mechanism :

•

The cyanide ion react with benzaldehyde and form carbanion (A). O

O C6H5

C

H

-CN

C6H5

C CN

Benzaldehyde

OH H

C6H5

C CN (A)

• The carbanion (A) can then react with another benzaldehyde molecule subsequently proton-transfer and loss of the cyanide ion gives benzoin. OH C6H5

C

OH O-

O +

CN

C

C6H5

C6H5

H

(A)

C

C

C6H5

CN H

Benzaldehyde Proton shift

O

OH -

C6H5

C

C H Benzoin

C6H5

-CN

C6H5

O

OH

C

C

CN H

C6H5

•

Cannizzaro reaction :

•

Aromatic aldehydes ( that do not have α-hydrogen atom ) on treatment with conc. Alkali undergo self oxidation and reduction to give alcohol & the salt of the corresponding carboxylic acid. This disproportionation reaction or self oxidation & reduction reaction of aldehydes with lack of α-hydrogen is known as cannizzarose reaction. Ortho and para phenolic aldehydes do not give this reaction.

•

O C6H5CHO + C6H5CHO Benzaldehyde

Benzaldehyde

NaOH

C6H5CH2 - OH + C6H5 - C - ONa Benzyl alcohol

Sodium benzoate

• Mechanism : • Step 1 : Attack of OH- on the carbonyl carbon O

O C6H5

C

H

+ OH

C6H5

• Step 2 : Hydride – ion transfer. C6H5 C OH

O

O H + C6H5 C

H

OH (A)

Benzaldehyde

O

C

H

C6H5

Benzaldehyde

C H CH O C + 6 5 2 OH

• Step 3 : Acid – base reaction. O C6H5 C

O OH + C6H5CH2O + Na+

C6H5 C

ONa + C6H5CH2OH

Sodium benzoate

Benzyl alcohol

•

Perkin reaction :

•

Condensation of aromatic aldehydes with an acid anhydride to produce α-β unsaturated carboxylic acid is called perkin reaction. The reaction of an aliphatic acid anhydride to produce in presence of sodium or potassium salt of the acid corresponding to anhydride to give an α,βunsaturated acid is known as perkin reaction. The reaction is carried out by aldehyde and salt of acid with excess amount of aldehyde.

•

C6H5CHO + (CH3CO)2O Benzaldehyde

Acetic anhydride

CH3COONa

C6H5CH = CH - COOH + CH3COOH Cinnamic acid

Acetic acid

Mechanism : a) Formation of the anion of acetic anhydride (A). CH3COOH + :CH2 - CO - O - COCH3

CH3COO + CH2 - CO - O - COCH3 H

(A)

b) Reaction of (A) with benzaldehyde gives (B). O

O C6H5

C H

+

:CH2 - CO - O - COCH3

C6H5 - CH - CH2CO - O - COCH3 (B)

(A)

c) Protonation of (B) gives (C). O C6H5

+

H

CH CH2 - CO - O - COCH3

OH C6H5 - CH - CH2CO - O - COCH3 (C)

d) Internal proton transfer in (C) followed by elimination of water molecules gives (D).

OH H C6H5

CH CH2 - CO - O - COCH3

- H2O

C6H5 - CH = CH - CO - O - COCH3 (D)

e) Hydrolysis of (D) gives cinnamic acid.

O C6H5 - CH = CH - C - O - CO - CH3

O H2O

C6H5 - CH = CH - C - OH + CH3COOH Cinnamic acid

Acetic acid

Claisen ester condensation : The base catalysed condensation of an ester containing an α-hydrogen atom with a molecule of the same ester or a different ester to give β-keto ester is known as claisen condensation of ethyl-acetate in the presence of sodium ethoxide.

O

CH3 - C - O C2H5 + CH3 - C - O - C2H5 Ethylaceticacid

O

O C2 H5 ONa

+ H

O

CH3 - C - CH2 - C - O - C2H5 + C2H5OH Ethylacetoacetate

Mechanism :

a) Formation of ester anion by reaction with C2H5O-

:CH2COOC2H5 + C2H5OH

C2H5O + H - CH2 - COOC2H5

Ester anion

b) Attack of nucleophile with ester anion. O CH3

C

O

O +

:CH2

C

CH3

OC2H5

Ester anion

OC2H5

C

O CH2

C

OC2H5

OC2H5

c) Elimination of C2H5OH. O

O +

CH3

C

CH2

OC2H5

C

OC2H5

O

O

H

CH3

C

CH2

C

OC2H5 + C2H5OH

Ethyl acetoacetate

6) Favorskii rearrangement :- Transformation of α – haloketone into esters in the presence of alkoxide via base catalyzed rearrangement is known as Favorskii rearrangement.

OR

O

O

C Cl OR Alkoxide

2-Chlorocycloheptanone

Ester

Mechanism: (I) Abstraction of an α – proton in order to form carbanions. O

O H

Cl

Cl

+

OR

R - OH

2-Chlorocycloheptanone

a (II) Nucleophilic attack on carbanion and rearrangement take place in order to form esters O

O Cl

OR

O

OR

Cl

b

a O

OR

c OR

O

OR

C

C ROH

+ ester

d

REFERENCES -:

1) Organic chemistry by Morrison and Boyd. 2) Organic chemistry by Bahl and Bahl. 3) Organic Reaction and Mechanism by Grudeep Chatwal.

4) Reaction mechanism in organic chemistry by S.M.Mukherji and S.P.Singh. 5) organic reaction mechanisms by V.K. ahluwalia, Rakesh Kumar Prashar