Lingo Electrophile – Electron Loving – Looks for a pair of electrons H+ CH3CH2+ BH3 Nucleophile...

Lingo

Electrophile – Electron Loving – Looks for a pair of electrons

H+ CH3CH2+ BH3

Nucleophile – Has a pair of electrons to donate

OH- Cl- CH3NCH3

Mechanism

• Practice HBr addition

What in the world do all those arrows mean anyway???

Why are Different Products Formed

Carbocations

• Carbocations are:

classified as 1°, 2°, or 3° depending on the number of carbons bonded to the carbon bearing the positive charge

Carbocation Structure

• Bond angles about the positively charged carbon are 120°

• Carbon uses sp2 hybrid orbitals to form sigma bonds to the three attached groups

• The unhybridized 2p orbital lies perpendicular to the sigma bond framework and contains no electrons

R CRR+

Carbocation Stability• A 3° carbocation is more stable than a 2°

carbocation – A 2° carbocation is, in turn, more stable than a 1° carbocation

• The higher the degree, the lower the activation energy needed for its formation

• The methyl cation is so unstable that it is never observed in solution

Methyl cation

(methyl)

Ethyl cation (1°)

Isopropyl cation (2°)

tert-Butyl cation (3°)

Increasing carbocation stability

+ + + +C

H

H

CH3 CCH3

CH3

H

C

CH3

CH3CH3CH

H

H

Carbocation Stability

• The alkyl groups attached to the positively charged carbon are electron-releasing and thereby delocalize the positive charge of the cation

• We account for this electron-releasing ability of alkyl groups by (1) the inductive effect, and (2) hyperconjugation

Carbocation Stability• The inductive effect

– the electron-deficient carbon bearing the positive charge polarizes electrons of the adjacent sigma bonds toward it

– the positive charge on the cation is not localized on the trivalent carbon, but delocalized over nearby atoms

– the larger the volume over which the positive charge is delocalized, the greater the stability of the cation

H3C

C CH3

H3C

+

+

++

Carbocation Stability

• Hyperconjugation– partial overlap of the bonding

orbital of an adjacent C-H bond with the vacant 2p orbital of the cationic carbon delocalizes the positive charge and also the electrons of the adjacent bond

– replacing a C-H bond with a C-C bond increases the possibility for hyperconjugation

Carbocations

•

Evidence to the ideas of induction and hyperconjugation

Markovnikov’s Rule

The hydrogen will add to the carbon with the most number of hydrogens

So, you mean that all that information can be summed up in this one easy rule to remember about the addition of things across a double bond???………….. So why didn’t you just say this?

Mechanism

• Practice H2O and OH addition

I think I’m starting to like this arrow thing!!!!

Regioselectivity

• Regioselective Reaction:

“A reaction in which two or more constitutional products could be obtained

as products but one of them

predominates.”

C+ Rearrangements• In electrophilic addition to alkenes, there is

the possibility for rearrangement

• Rearrangement: any change in connectivity of the atoms in a product compared with the connectivity of the same atoms in the starting material

Methyl Shift

• In addition of HCl to an alkene

+

+3-Methyl-1-butene

2-Chloro-3- methylbutane

(expected) (40%)

CH3

CH3 CH3

ClCl

CH3CHCH=CH2 HCl

CH3CHCHCH3 CH3CCH2CH3

2-Chloro-2- methylbutane

(rearrangement) (60%)

Methyl Shift

• In acid-catalyzed hydration of an alkene

+

3-Methyl-1-butene 2-Methyl-2-butanol

CH3 CH3

OH

CH3CHCH=CH2 H2O CH3CCH2CH3H2SO4

Hydride Shift

• Driving force is rearrangement of a less stable carbocation to a more stable one

+

A 2° carbocation intermediate

3-Methyl-1- butene

+

+

-

CH3

H

ClH

CH3

H

CH3CCH=CH2

ClCH3C-CHCH3

slow, rate- limiting step

Hydride Shift

• The less stable 2° carbocation rearranges to a more stable 3° one by shift of a hydrogen atom with its bonding electrons

++

A 3° carbocation intermediate

CH3

H

CH3

H

CH3C-CHCH3 CH3C-CHCH3fast

.

Hydride Shift

• Reaction of the more stable carbocation (a Lewis acid) with chloride ion (a Lewis base) completes the reaction

2-Chloro-2-methylbutane

++

-

Cl

CH3 CH3

ClCH3C-CH2CH3 CH3C-CH2CH3fast

Rearrangements

• 1,2-hydride shifts

Rearrangements

• 1,2-methyl shift

Rearrangements

• 1,3-hydride shifts usually do NOT occur

Rearrangements

• Ring expansion through rearrangement

Mechanism

• Practice hydride shift, methyl shift, ring opening