Organic Chemistry Chapter 3 Part II
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Transcript of Organic Chemistry Chapter 3 Part II
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