Carbocations and carbanions

Organic Intermediates Carbocations and Carbanions.

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 Pharmacy Akurdi 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. ReactionIntermediates
•Areactionintermediateoranintermediateisamolecularentity(atom,
ions,molecule)withalifetimeappreciablylongerthanamolecular
vibrationthatisformedfromthereactantsandreactswithfurtherto
givethedirectlyobservedproductsofachemicalreaction.
•Maincarbonreactiveintermediates:
•Carbocations
•Carbanions
•Carbenes
•Freeradicals
•Nitrenes

4. Carbocations ( Carbonium ions ):
•Organicionswhichcontainapositivelychargedatomarecalled
CarbocationsorCarboniumions.

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: +Igroup: 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. •Thestabilityofcarbocationsincreasewhen+Igroupslike
alkylgroupsarepresentadjacenttopositivelycharged
carbon.The+Igroupsreducethepositivechargeonthe
carbonbydonatingnegativechargedensitythroughpositive
inductiveeffect.Thisresultsingreaterstabilityof
carbocation.
•Whereas,the-Igroupsdestabilizethecarbocationsasthey
increasethepositivechargebywithdrawingelectrondensity.
•Notethatanyfactorthatincreasesthecharge(eithernegative
orpositive)onanionresultsindestabilizationwhileany
factorthatreducesthechargeresultsinstabilizationofthat
ion.

13. •Forexample,theorderofstabilityofafewcarbocations
containingalkylgroupsisasfollows:
Thetertiarycarbocationcontainingthreealkylgroupsismorestable
thanthesecondarycarbocationwithtwoalkylgroupsandwhichinturn
ismorestablethantheprimarycarbocation.Methylcarbocationisthe
leaststableamongthegiven.

14. •ResonanceEffect
•Definition–Theresonanceeffectcanbedefinedasachemical
phenomenonwhichisobservedinthecharacteristiccompounds
havingdoublebondsintheirstructuresandusuallyhavethe
overlappingofthep-orbitalsonthetwoadjacentsidesofcarbon
atoms.

15. •TypesOfResonanceEffects
•TherearetwotypesofResonanceeffectsnamelypositiveresonance
effectandnegativeresonanceeffect.
•PositiveResonanceEffect-Positiveresonanceeffectoccurswhenthe
groupsreleaseelectronstotheothermoleculesbytheprocessof
delocalization.Thegroupsareusuallydenotedby+Ror+M.Inthis
process,themolecularelectrondensityincreases.Forexample--OH,-
SH,-OR,-SR.
•NegativeResonanceEffect-Negativeresonanceeffectoccurswhen
thegroupswithdrawtheelectronsfromothermoleculesbytheprocess
ofdelocalization.Thegroupsareusuallydenotedby–Ror–M.Inthis
process,themolecularelectrondensityissaidtodecrease.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:
Stabilityofalkylcarbanionscanbeexplainedbyinductiveeffect.
1.Withincreasein+Ieffectstabilitydecreases
2.Withincreasein–Ieffectstabilityincreases
Greaterthenumberofalkylgroup[+Ieffect]attachedtothecarbon
atombearingnegativecharge,lesseristhestability.Becausethere
inductiveeffectincreasenegativechargedensityonthecarbon

24. •Effect of resonance:
•A carbanion is stabilized by resonance if a double bond is located αto
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-characterof
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
Cyclopentadienyl anion

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