This document discusses carbanions, which are negatively charged organic species where carbon carries three bond pairs and one lone pair. Carbanions are stabilized through conjugation, resonance effects, field effects, and aromaticity. They are generated through heterolytic bond cleavage or addition of a negative ion to a carbon-carbon multiple bond. As nucleophiles, carbanions undergo reactions such as alpha-halogenation of ketones, additions to carbonyls, nucleophilic acyl substitutions, substitutions with alkyl halides, and Michael additions.
An organic species which has a carbon atom bearing only six electrons in its outermost shell and has a positive charge is called carbocation.
The positively charged carbon of carbocation is sp2 hybridized.
The unhybridized p-orbital remains vacant.
They are highly reactive and act as reaction intermediate.
They are also called carbonium ion.
An organic species which has a carbon atom bearing only six electrons in its outermost shell and has a positive charge is called carbocation.
The positively charged carbon of carbocation is sp2 hybridized.
The unhybridized p-orbital remains vacant.
They are highly reactive and act as reaction intermediate.
They are also called carbonium ion.
Benzene has 6π electrons delocalized in 6p orbitals that overlap above and below the plane of the ring. Because benzene’s six pie electrons satisfy Huckel’s rule, benzene is especially stable. Reaction that keep the aromatic ring intact are therefore favoured
This is Power Point Presentation on Topic "Electrophilic Aromatic Substitution Reactions" as per syllabus of "University of Mumbai" for S.Y. B. Pharmacy (Sem.: IV) students.
Benzene has 6π electrons delocalized in 6p orbitals that overlap above and below the plane of the ring. Because benzene’s six pie electrons satisfy Huckel’s rule, benzene is especially stable. Reaction that keep the aromatic ring intact are therefore favoured
This is Power Point Presentation on Topic "Electrophilic Aromatic Substitution Reactions" as per syllabus of "University of Mumbai" for S.Y. B. Pharmacy (Sem.: IV) students.
A carbanion is an ion with a negatively charged carbon atom. The most stable carbanions have six electrons in the valence shell of the carbon atom. Carbanions are important in organic chemistry because they can act as nucleophiles, which means they can donate electrons to other molecules. Carbanions are also important in biochemistry because they can be used to transfer electrons between molecules
1. Introduction
2. History of benzene
3. Nomenclature
4. Orbital structure
5. Kekule structure
6. Resonance structure
7. Resonance energy and stability 8. Structural evidence
9. Synthetic evidence
10. Analytical evidence
11. Aromaticity and huckle rule
12. Method of preparation of benzene
13. Electrophilic substitution of benzene
14. Classification of substituent 15. Directive effect : Ortho and para director, meta director
16. Reaction of monosubstituted benzene
17. Effect of Substituents on reactivity and orientation of monosubstituted benzene towards electrophilic substitution 18. Structure and uses of BHC,DDT, Saccharine and chloramine
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1. CARBANION
DEPARTMENT OF PHARMACEUTICAL SCIENCES,
DR. H. S GOUR VISHWAVIDYALAYA, SAGAR, M.P
PRESENTED TO
Prof. Asmita Gajbhiye
PRESENTED BY
Mr. Sushanta Bhattacharya
M.Pharm (II Sem)
2. Contents
INTRODUCTION
DEVELOPMENT OF CARBANION CHEMISTRY
CHARACTERISTICS OF CARBANIONS
STABILIZATION OF CARBANIONS
GENERATION OF CARBANIONS
REACTIONS INVOLVING CARBANIONS AS NUCLEOPHILES
REFERENCES
5. Carbanion
Negatively charged Organic species in which carbon carries three
bond pairs and one lone pair, is known as cabanion.It is represented
as
6. Characteristics Of Carbanions
• Hybridisation and geometry : Alkyl carbanion has three bond pairs
and one lone pair. Thus hybridisation is sp3 and geometry is
pryamidal.
• There are eight electrons in the outermost orbit of carbanionic carbon hence
its octet is complete.
• It behaves as charged nucleophile.
• It is diamagnetic in character because all eight electrons are paired.
• It is formed by heterolytic bond fission.
• It reacts with electrophiles.
7. • A very important way of generating carbon nucleophiles
involves removal of a proton from a carbon by a base. The
anions produced are carbanions.
• The negative charge gives good nucleophilic properties and
it can be used in the formation of new carbon-carbon
bond.
8. • The C-C bonds of a molecule
under construction constitute its
scaffolding.
• Reactions which form new C-C
bonds are exceptionally useful
synthetically.
10. Conjugation Of Unshared Pair With An
Unsaturated Bond
• In cases where a double or triple bond is located α the
carbanionic C
• The ion is stabilized by resonance in which the unshared
pair overlaps with the ∏ electrons of the double bond
• This factor is responsible for the stability of the allylic and
benzylic types of carbanions
11. • Diphenylmethyl and triphenylmethyl anions are still more stable and can be
kept in solution indefinitely if water is rigidly excluded
• Carbanionic carbon is conjugated with a C–O or C–N multiple bond (Y=O or N),
the stability of the ion is greater than that of the triarylmethyl anions, since
these electronegative atoms are better capable of bearing a negative charge
than carbon.
12. Stabilization By >C=O Group Present On Carbanionic
Carbon
Carbanion Enolate ion
(I) (II)
Contribution of structure (II) will be more than (I) because in (II) negative
charge present on electronegative oxygen.
13. Carbanions Increase In Stability With An Increase In The
Amount Of S Character At The Carbanionic C
Stability directly proportional to electronegativity of carbanionic carbon
which is directly proportional to % s-character of carbanionic carbon
14. Stabilization By Sulphur Or Phosphorus
•Attachment of carbanionic carbon of a sulphur and
phosphorus atom causes an increase in carbanion stability.
•The cause of stability is due to the delocalization of
negative charge of carbanion by vacant d-orbital (ppi–dpi
bonding) of phosphorus and sulphur.
15. Field Effect
• Most of the groups that stabilize carbanions by resonance effects have electron-
withdrawing field effects and thereby stabilize the carbanion further by spreading
the negative charge.
• Nitrogen ylid, where a positive nitrogen is adjacent to the negatively charged
carbon, only the field effect operates.Ylids are more stable than the corresponding
simple carbanions.
• Carbanions are stabilized by a field effect if
there is any heteroatom (O, N, or S) connected to the carbanionic carbon,
provided that the heteroatom bears a positive charge in at least one important
canonical form.
16. Certain Carbanions Are Stable Because They
Are Aromatic
• Anions in which negative charge is present on carbon of aromatic system is
known as aromatic carbanions.
• Aromatic carbanions are most stable carbanions.
• Anions obeying Huckel rule are stable because they are aromatic and there is
complete delocalization of negative charge.
18. Generation Of Carbanions
1. A group attached to a carbon leaves without its electron pair
2. A negative ion adds to a carbon–carbon double or triple bond
19. Reactions Involving Carbanions As
Nucleophiles
1. Alpha-halogenation of ketones
2. Nucleophilic addition to aldehydes/ketones
a) Aldol and Crossed aldol
b) Aldol related reactions
c) Grignard synthesis of alcohols
d) Wittig synthesis of alkenes
3. Nucleophilic acyl substitution with esters and acid chlorides
a) Claisen and crossed Claisen
b) R2CuLi + RCOCl
20. Cont.
4. SN2 with alkyl halides
a) Corey-House
b) metal acetylide
c) Malonate synthesis
d) Acetoacetate synthesis
5. Michael Addition to α,β-unsaturated carbonyl
compounds
22. The enolate anion is a strong base and a good nucleophile
H3C
C
CH3
O
H3C
C
CH2
O
+ OH + H2O
R
C
CH2
O
R
C
CH2
O
"enolate" anion
Hydrogens that are alpha to a carbonyl are weakly
acidic due to resonance stabilization of the carbanion.
The enolate anion is a strong base and a good nucleophile
23. Mechanism for base promoted alpha-bromination
of acetone
H3C
C
CH3
O
H3C
C
CH2
O
+ OH + H2O
RDS
H3C
C
CH2
O
+ Br Br
H3C
C
CH2Br
O
+ Br
24. REFERENCES
• Smith M.B & March J,”MARCH’S ADVANCED ORGANIC CHEMISTRY”, Sixth edition,
John Wiley & Sons , New Delhi, 2007, 249-65.
• Morrison R.T, Boyd & Bhattacharjee S.K, “Organic Chemistry”, Seventh edition,
Dorling Kindersley(India) Pvt. Ltd., South Asia, 1992, 69-72.
• https://www.slideshare.net/shazi5250/carbanions-i-1
• https://slideplayer.com/slide/12672704/
