Chapter 9: Addition Reactions of Alkenes and Alkynes

Electrophilic addition

2 step reaction

1. Electrophile adds to pi bond making a carbocation

2. Nucleophile adds to carbocation

Regiochemistry

Orientation of reagent attachment to pi bond

Markovnikov’s Rule

H attaches to less substituted carbon

Hydration of alkene

Markovnikov addition to alkene

1. H from acid attaches to pi bond

2. H2O attaches to other carbon but becomes OH

Addition of Br2 and Cl2 to alkene

1. Partial positive atom attaches to both carbons in alkene forming a halonium bridge

2. Partial negative atom attached to other carbon

   Halogens are trans

Addition of Br2 or Cl2 in H2O to alkene

1. Halogen attached to both carbons in alkene forming halonium intermediate

2. H2O attaches to more highly substituted carbon, then becomes OH

OH and halogen are trans

If a new chiral center is produced from an achiral reactant

There are 2 products, R and S

Enantiospecific

Produces one enantiomer

Enantioselective

Produces more of one enantiomer

Regioselective

Produces some anti-Markovnikov product

Stereospecific

One stereoisomer is produced

Oxymercuration

Markovnikov addition of OH and HgOAc to alkene

1. HgOAc forms bridged intermediate

2. H2O adds to more substituted carbon, becomes OH

OH and Hg are trans

Demercuration

Using NaBH4 to replace HgOAc with H in alkene

Addition of HBr to alkyne

1. H adds to less substituted carbon, making vinyl carbocation

2. Br adds to carbocation

Addition can continue again to get to single bond if reagent is in excess

Hydroboration of alkenes

BH3 reacts with cyckohexene in a concerted process to form an organoborane (ANTI MARKOVNIKOV)

1. Four ring transition state BOX is made when BH2 and H attach to each carbon in alkene

BH2 attaches to less substituted carbon

BH2 and H are cis

Oxidation after hydroboration

BH2 gets replaced by R or OH and configuration is retained

Anti Markovnikov addition of water to double bond

Hydration of alkyne and tautomerism

1. Metal ion adds to pi bond in bridge intermediate

2. H2O adds to more highly substituted carbon and becomes OH

3. Metal ion gets replaced by H

4. Tautomerism: OH becomes ketone, another H is added to other side

Adding carbocation to alkene

1. H adds to alkene, making it a carbocation

2. New alkene comes and attaches to positive charged carbon (losing its double bond) and becomes a carbocation

3. Carbocation is deprotonated so double bond comes back

Carbene

Carbon with only 6 electrons

Cyclopropanation

Concerted process where carbene adds to alkene to form cyclopropane (3 membered ring)

Simmons Smith reaction (zinc and iodide reagent)

Ch2 attach to pi bond both carbons forming 3 membered ring

Simmmons smith is chunkyy

Anti addition configuration

cis reactant makes racemic products

trans reactant makes meso products