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Carbocations and Charge Stabilization | CHEM 3311, Study notes of Organic Chemistry

University of Memphis (U of M)Organic Chemistry

Material Type: Notes; Class: Gen Organic Chemistry I; Subject: CHEM Chemistry; University: University of Memphis; Term: Spring 2003;

Typology: Study notes

Pre 2010

Uploaded on 08/19/2009

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Carbocations & Charge
Stabilization
Carbocations
•Structures with a positive formal charge on
carbon
•What hybridization will carbocations have?
–sp2
•What geometry will carbocations have?
–trigonal planar
R
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 will
stabilize carbocations by spreading the
charge over a larger volume
•Substituents that withdraw electrons will
destabilize carbocations by packing a
greater charge onto the carbon
pf3
pf4

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Download Carbocations and Charge Stabilization | CHEM 3311 and more Study notes Organic Chemistry in PDF only on Docsity!

Carbocations & Charge

Stabilization

Carbocations

• Structures with a positive formal charge on

carbon

• What hybridization will carbocations have?

– sp

• What geometry will carbocations have?

– trigonal planar

R

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 will

stabilize carbocations by spreading the

charge over a larger volume

• Substituents that withdraw electrons will

destabilize carbocations by packing a

greater charge onto the carbon

Electron Release/Withdrawal

Mechanisms

• Induction

– Induction involves the push and pull of electrons

through sigma bonds

– Example: bromine is more electronegative than

carbon and will pull at the electrons in their

sigma bond

• Resonance => STRONGER EFFECT

– Resonance involves the push and pull of

electrons through a pi system

Electron Releasing

Substituents

• Inductive only

– Alkyl groups

• Resonance

– Atoms with lone pairs

– Alkenyl groups

– Aromatic rings

CH 3

C

H 3 C CH 3

H

C

H 3 C CH 3

H

C

H 3 C H

is more stable than is more stable than

tertiary C+, 3° secondary C+, 2° primary C+, 1°

OH

C

H 3 C CH 3

OH

C

H 3 C CH 3

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

Problem 1

• Draw resonance structures to demonstrate

which atoms in the structure below share

the positive charge H

C

H

H

H

H

H

H

Electron Withdrawing

Substituents

• Electronegative atoms

(at least those more

electronegative than O):

Br, Cl, I, F

• Groups with positive

(or partial positive

charges)

H

C

H 3 C H

is more stable than

primary C+, 1°

H

C

Br H

H

C

H 3 C H

is more stable than

primary C+, 1°

H

C

HC H

O

Problem 3

• Which of the groups below is the MOST

electron-withdrawing?

• Hint: draw out each complete Lewis

structure with electrons, formal charges,

and partial charges.

NO 2 Br C C

O

H

O

OCH 3

N

O O

Full formal positive charge on nitrogen makes this group very electron-withdrawing