Lingo Electrophile – Electron Loving – Looks for a pair of electrons H+ CH3CH2+ BH3 Nucleophile – Has a pair of electrons to donate OH- Cl- CH 3 NCH 3

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 sp 2 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

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

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

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 H 2 O 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

Methyl Shift In acid-catalyzed hydration of an alkene

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

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

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

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