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Carbocations & ChargeStabilization

Carbocations

Structures with a positive formal charge oncarbon

What hybridization will carbocations have?

sp2

What geometry will carbocations have?

trigonal planarR

C

R R

Carbocation Stability Carbocations are inherently unstable due to

Incomplete octet for carbon

Positive charge

However, some are not as bad as others

Substituents that release electrons willstabilize carbocations by spreading thecharge over a larger volume

Substituents that withdraw electrons willdestabilize carbocations by packing agreater charge onto the carbon

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Electron Release/WithdrawalMechanisms

Induction

Induction involves the push and pull of electronsthrough sigma bonds

Example: bromine is more electronegative than

carbon and will pull at the electrons in theirsigma bond

Resonance => STRONGER EFFECT

Resonance involves the push and pull of

electrons through a pi system

Electron ReleasingSubstituents

Inductive only

Alkyl groups

Resonance

Atoms with lone pairs

Alkenyl groups

Aromatic rings

CH3

CCH3H3C

H

CCH3H3C

H

CHH3C

is morestable than

is morestable than

tertiary C+, 3 secondary C

+, 2 primary C

+, 1

OH

CCH3H3C

OH

C CH3H3C

Structure has 1/2 of positive chargeon each atom, much more stable thanhaving the entire charge on one atom

Problem 1

Draw resonance structures to demonstratewhich atoms in the structure below sharethe positive charge H

C

H

H

H

H

H

H

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H

C

H

H

H

H

H

H

H

C

H

H

H

H

H

H

H

C

H

H

H

H

H

H

H

C

H

H

H

H

H

H

Notes on resonance forms:

None of the Lewis structures tothe right are exactrepresentations of the electronand charge positions in themolecule (none actually existand the molecule does NOTresonate between them)

The behavior of the molecule isbest described as a HYBRID ofthe resonance forms.

H

C

H

H

H

H

H

H

+0.25+0.25

+0.25 +0.25

Notes on arrows:The double-headed arrows are used betweenresonance forms instead of reaction arrows, becauseNO reaction occurs

Curved arrows are used to show movement of

electrons (NOT atoms)

Four Resonance Forms

Resonance Hybrid

Problem 2

Rank the following carbocations from mostto least stable

C

H

H H

C

CH3

H H

C

H

H C

C

CH3

H CH3

C

H

H

H

C

H

H H

C

CH3

H H

C

H

H C

C

CH3

H CH3

C

H

H

H

Most stable as single electron-releasing substituent provides resonancestabilization by sharing the positive charge between two atoms

Least stable, no electron-releasing substituents

Two electron-releasing substituents, but provide charge stabilization by theweaker inductive mechanism through sigma-bond framework

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Electron WithdrawingSubstituents

Electronegative atoms

(at least those moreelectronegative than O):Br, Cl, I, F

Groups with positive

(or partial positivecharges)

HC

HH3Cis more

stable than

primary C+, 1

H

CHBr

H

CHH3C

is morestable than

primary C+, 1

H

CHHC

O

Problem 3

Which of the groups below is the MOSTelectron-withdrawing?

Hint: draw out each complete Lewisstructure with electrons, formal charges,and partial charges.

NO2 Br C C

OH O

OCH3

NO O

Full formal positive chargeon nitrogen makes this groupvery electron-withdrawing