Organic Chemistry Review of Markovnikov and Anti-Markovnikov Additions MARKOVNIKOV ADDITION ELECTROPHILIC ADDITION Electrophilic addition reactions occur between an alkene and an electrophile. General Reaction Mechanism

The pi electrons of the alkene bond are used to form a new bond between one of the sp2 carbons of the alkene

and the electrophile (E+). The pi electrons are pulled away from the other sp2 carbon of the alkene leaving a formal charge of 1+ on this carbon. This positively charged carbon species is called a carbocation.

All electrophilic addition reactions of alkenes involve a carbocation intermediate and occur through the same

mechanism. Different electrophiles give rise to different types of products. ADDITION OF H2O

Addition of water to an alkene results in formation of a Markovnikov alcohol. A catalyst is necessary for water to react with the alkene. There are two common catalysts that are used for this reaction.

a. H+ (acid such as sulfuric, phosphoric, acetic etc...) b. HgX2 followed by treatment with sodium borohydride (NaBH4)

The reaction of an alkene with water and acid occurs through the mechanism below:

The pi e- of the alkene attack the proton to form a new bond between the C2 of the alkene and the hydrogen atom.. The most stable carbocation is generated as in the previous examples. Water behaves as Nu and attacks the carbocation to form an oxonium ion. Loss of a proton from the oxonium ion gives the alcohol product. The rate determining step (rds) of the reaction is formation of the carbocation. An additional reaction intermediate (oxonium ion) forms in this reaction. The reaction energy diagram for this process has three transition states (TS) and two reaction intermediates (RI).

C C C C EE+

Carbocation Intermediate

Nu:

C C ENu

HH

CH3

H

HH

CH3

HH

HH

CH3

HH

OH

H

HH

CH3

HH

OHH-OH2

1

2

H2O

Carbocation Intermediate Oxonium ion Intermediate

Organic Chemistry Review of Markovnikov and Anti-Markovnikov Additions

Hydration of alkenes can also be accomplished using mercuric catalysts (Hg(Ac)2 or HgX2) The pi electrons of the alkene react with the mercury atom of the catalyst to generate the carbocation.in the first, rds of the reaction. Water reacts with the carbocation (as in the previous example to give an oxonium ion. After loss of the proton, an alcohol forms but the mercury must be remove. This is achieved with NaBH4. H- reacts with the carbon atom bonded to the mercury and displaces it to give the final product.

The reaction energy diagram for this process is similar but has additional steps to describe the NaBH4 part of the reaction. The rds is still formation of the carbocation.

Energy

Reaction Progress

Reactant Product

TS2TS1

C+

(Start) (End)

Activation energy DG=

TS3

O+

HH

CH3

H

HH

CH3

HHgX H

H

CH3

HHgX

O

H

H

HH

CH3

HHgX

O

H

HgX21

2

H2O

NaBH4

H

Oxonium ion Intermediate

HH

CH3

HH

OH

Carbocation Intermediate

Organic Chemistry Review of Markovnikov and Anti-Markovnikov Additions

ANTI-MARKOVNIKOV ADDITION ADDITION OF BH3 followed by reaction with H2O2, NaOH

Addition of BH3 to an alkene results in formation of an alkyl borane.Addition of BH3 to an alkene occurs in ANTI-MARKOVNIKOV fashion, i.e., B bonds to least substituted carbon.

Addition of BH3 to an alkene occurs via SYN addition, i.e., B and H atoms are added on the same side of the double bond. In cycloalkenes, this results in the B atom and the H atom having a CIS relationship

Energy

Reaction Progress

Reactant Product

TS2TS1

C+

(Start) (End)

TS3

O+

TS3

Hg-OH

HH

H3C HC C

HH

H3C H

H-BH2

C CHH

H3C HH BH2

δ+δ−

δ+ δ−

Alkyl borane

CH3 CH3HH2B H2B H

CH3H

Organic Chemistry Review of Markovnikov and Anti-Markovnikov Additions Reaction of alkylboranes with H2O2, NaOH gives an alcohol. Reaction of the alkyl borane with hydroxide, and peroxide causes an oxidation. It occurs with “retention of configuration”, meaning the stereochemistry of the OH group in the alcohol product is the same as the BH2 group in the alkyl borane. Since the BH2 was bonded to the least substituted carbon of the starting alkene, the OH of the alcohol ends up bonded to the same carbon. This results in an ANTI-MARKOVNIKOV alcohol.

CH3 CH3HH2B H2B H

CH3H

NaOH, H2O2

OOH

H2B H

CH3H

OHO

H

CH3H

OBH2H2OHHO

CH3H