This document discusses organic reaction intermediates, specifically carbocations and carbanions. It defines them as positively or negatively charged carbon-containing ions that are formed during chemical reactions and then react further to form final products. The key features, methods of formation, factors affecting stability, and synthetic applications of carbocations and carbanions are described. Inductive effects, resonance effects, and hyperconjugation influence the stability of these intermediates. Carbocations and carbanions are involved in many common organic reaction types such as eliminations, substitutions, additions, and rearrangements.

1. Organic
Intermediates
Presented by : Panchal Ashitosh
M. Pharm Sem-I Roll No. 546
Guided by Ms. Tejashree Deokule
Department of Pharmaceutical Chemistry
Dr.D.Y. Patil College Of PharmacyAkurdi Pune

2. Contents
• Introduction
• Structure of Carbocations and Carbanions
• Features of Carbocations and Carbanions
• Method of Formation
• Stability of Carbocations and Carbanions
• Synthetic Applications
• References

3. Reaction Intermediates
• A reaction intermediate or an intermediate is a molecular entity (atom,
ions, molecule) with a lifetime appreciably longer than a molecular
vibration that is formed from the reactants and reacts with further to
give the directly observed products of a chemical reaction.
• Main carbon reactive intermediates:
• Carbocations
• Carbanions
• Carbenes
• Free radicals
• Nitrenes

4. Carbocations ( Carbonium ions ):
• Organic ions which contain a positively charged atom are called
Carbocations or Carbonium ions.

5. Salient features of Carbocations:
• It is a positively charged carbon atom having six electrons in its
outermost shell or Valence shell.
• Generally, it undergoes sp2 hybridization.

6. • It is an electron deficient ( electrophile).
• It is represented as
• The order of stability of Carbonium ions are as;

7. Method of Generation or Formation:
• Carbocations can be generated (formed) in a variety of ways:
1. Heterolysis of Alkyl halides:
2. Protonation of Alkenes:

8. 3. Protonation followed by dehydration of alcohols:

9. Stability of Carbocations:
• The stability of Carbonium ion depends upon:
• Inductive effect
• Resonance effect
• Hyperconjugation
1. Inductive effect
a. Positive inductive effect
b. Negative inductive effect

10. 1. Positive inductive effect
• The atom or group of atom which donates ( or release) or shared pair
of electrons from itself and thus acquires partial positive charge, called
positive inductive effect.
• Note: +I group: Electron donating or releasing group shows +I effect.

11. 2. Negative inductive effect:
• The atom or group of atom which withdraw or attract shared pair of
electron towards itself and thus acquires partial negative charge called
negative inductive effect ( -I effect ).
• Note: electron withdrawing or accepting group show –I effect.

12. • The stability of carbocations increase when +I groups like
alkyl groups are present adjacent to positively charged
carbon. The +I groups reduce the positive charge on the
carbon by donating negative charge density through positive
inductive effect. This results in greater stability of
carbocation.
• Whereas, the -I groups destabilize the carbocations as they
increase the positive charge by withdrawing electron density.
• Note that any factor that increases the charge (either negative
or positive) on an ion results in destabilization while any
factor that reduces the charge results in stabilization of that
ion.

13. • For example, the order of stability of a few carbocations
containing alkyl groups is as follows:
The tertiary carbocation containing three alkyl groups is more stable
than the secondary carbocation with two alkyl groups and which in turn
is more stable than the primary carbocation. Methyl carbocation is the
least stable among the given.

14. • Resonance Effect
• Definition – The resonance effect can be defined as a chemical
phenomenon which is observed in the characteristic compounds
having double bonds in their structures and usually have the
overlapping of the p-orbitals on the two adjacent sides of carbon
atoms.

15. • Types Of Resonance Effects
• There are two types of Resonance effects namely positive resonance
effect and negative resonance effect.
• Positive Resonance Effect- Positive resonance effect occurs when the
groups release electrons to the other molecules by the process of
delocalization. The groups are usually denoted by +R or +M. In this
process, the molecular electron density increases. For example- -OH, -
SH, -OR,-SR.
• Negative Resonance Effect- Negative resonance effect occurs when
the groups withdraw the electrons from other molecules by the process
of delocalization. The groups are usually denoted by –R or –M. In this
process, the molecular electron density is said to decrease. For
example- -NO2, C=O, -COOH, -C≡N.

16. • Carbocations involving resonance are more stable than those which
do not involve resonance.
The resonance delocalization of positive charge increases the
stability of Carbocation.

17. •Hyperconjugation:
• Hyperconjugation effect is a phenomenon in which localization of σ
electrons of C-H bond of an alkyl group directly attached to an atom
of the unsaturated system or to an atom with an unshared p orbital
takes place.

18. Hyperconjugation
• In general, more number of hyperconjugation structures; the greater is
the stability of Carbocations.
• Hence, the decreasing order of stability of alkyl Carbocations is,
Number of hyperconjugative
structures 9αH. 6αH. 3αH. 0

19. Carbanions
• Organic ions which contain a negatively charged carbon
atom are called as Carbanions.
• It is a negatively charged carbon atom having eight
electrons in its outermost shell or Valence shell.
• Generally, it undergoes sp3 hybridization.
• Trigonal Pyramidal
geometry
• Bond angle is 109.28°

20. The order of stability of Carbanions is as,

21. Method of Formation:
• Carbanions are generated (formed) by following reactions;
1. Heterolysis
2. Abstraction of proton by base:

22. Stability of Carbanions
• The stability of Carbanions depends upon;
1. Inductive Effect
2. Resonance Effect
3. S-character of orbital
4. Aromaticity

23. • InductiveEffect:
Stability of alkyl carbanions can be explained by inductive effect.
1. With increase in +I effect stability decreases
2. With increase in –I effect stability increases
Greater the number of alkyl group [+ I effect] attached to the carbon
atom bearing negative charge, lesser is the stability. Because there
inductive effect increase negative charge density on the carbon

24. • Effect of resonance:
• A carbanion is stabilized by resonance if a double bond is located α to
the anionic carbon. Due to delocalization negative charge diffuse over
whole molecule and thus stability increases.
• This explains the stability of the allylic and benzylic carbanions

25. • Electronegativity of carbanionic carbon:
• Stability directly proportional to electronegativity of
carbanionic carbon which is directly proportional to % s-character of
carbanionic carbon.

26. • Stability of Aromatic Carbanions:
• Anion in which negative charge is present on carbon of aromatic
system is known as aromatic carbanions.
• Aromatic carbanions are most stable carbanions because unshared
electron pair of carbanion itself take part in delocalisation.
• Anions obeying Huckel rule are stable because they are aromatic and
there is complete delocalisation of negative charge
Cyclopentadienylanion

27. Synthetic Applications of Carbocations
and Carbanions
• Carbocations are strong Lewis acids while Carbanions are strong bases. So
they shows more reactivity towards each other.
• Carbanions and Carbocations are part of most of the common reaction types
such as;
1. Elimination Reaction ( E1)
2. Substitution Reactions ( SN1)
3. Displacement Reactions
4. Addition Reactions
5. Rearrangement Reaction
6. Condensation Reactions etc.

28. References:
1. Advanced organic chemistry, Reaction, Mechanisms, and Structures, J March,
John Wiley and Sons, New York.
2. Reactive Intermediates in organic chemistry, Maya Shankar Singh, Wiley-
VCH.
3. https://www.researchgate.net/publication/335396136ADVANCED
ORGANIC CHEMISTRY –I (MPC 102T) UNIT-I: Basic Aspects of Organic
Chemistry
4. Principles Of Drug Action 1, Spring 2005, Resonance and Inductive Effect.
5. Blanksby, Stephen J. And Bowie, John H.: Carbanions: formation, structure
and thermochemistry 2005, 261-270. https://ro.uow.edu.au/scipapers/3237
6. http://www.wikipedia.com_organic_chemistry