Addition to Alkenes- Markovnikov’s rule-

Nanoplasmonic Research Group

Organic Chem-istry

Chapter 3Part II

Reactions of Alkenes: Addition

• Electrophile– Literally ‘ELECTRON’ lovers: electron-POOR reagents– Positive ions or electron-deficient species

• Nucleophile– Literally ‘NUCLEUS’ lovers: electron-RICH reagents– Donate electrons to an electrophile

E+ Nu-+ E Nu

Addition of Unsymmetric Reagents to Unsymmetric

Alkenes: Markovnikov’s Rule

Markovnikov’s Rule

The RULE says “The addition of a protic acid such as H-X to an alkene, the acid hydrogen (H) be-comes attached to the carbon atom with the greatest number of hydrogens, and the halide (X) group becomes attached to the carbon with the fewest hydrogens”

Why ???The stability of Carbocation(1) Hyperconjugation & (2) Induction

Mechanism of Electrophilic Addition to Alkenes

The Point is which carbocation is more stable !!

Hyperconjuga-tion: Alkyl group stabi-lize carbocation

Induction:The transmission of charge through a chain of atoms in a molecule by electrostatic in-duction

Types of Addition Rxns• Addition of Halogens

• Addition of Water (hydration)

• Addition of Acids

Hydroboration• Addition of a hydrogen-boron bond to an

alkene• The Point is that ‘hydrogen’ is more elec-

tronegative than ‘boron’

Quick Reminder

• Alkene Reaction goes through the pathway of the most stable intermediate

• The more substituents on an electron deficient carbon (carbocation or radical), the greater its stability

Additions to Conjugated Sys-tems

1,2-addition

1,4-addition

1. Electrophile adds to alkene according to Markovnikov’s rule

2. Draw possible resonance structures and see how many structures come out

3. Selectivity may differ depending on the stability of each resonance structure

How to Predict ?

Cycloaddition to Conjugated Di-enes

- The Diels-Alder Reaction -

1. Concerted as with hydroboration2. Easy to synthesize a cyclic compound3. Converts three pi bonds to two sigma

bonds & one new pi bond4. HOMO of diene and LUMO of dienophile

Features

Free-Radical Additions

1. Initiation by a radical initiator: A radical is created from a non-radical precursor

2. Chain propagation: A radical reacts with a non-radical to produce a new radical species

3. Chain termination: Two radicals react with each other to create a non-radical species

4. Chain branching: see page 102 middle

Features

Other reactions with alkenes• Ozonolysis

– The oxidation of alkenes with ozone to give carbonyl com-pounds

– One can deduce the structure of an unknown alkene