organic - carey 10e ch. 10 - introduction to free...
TRANSCRIPT
CONCEPT: RADICAL INITIATORS
□ Chemical bonds can be cleaved in two ways: Heterolytically (ionic cleavage) and homolytically (radical cleavage).
● We use full curved arrows to indicate the movement of ______ electrons and a half arrow to indicate _____.
● Homolytic dissociation energy is much ___________ than a corresponding heterolytic dissociation energy.
□ There are a few molecules that are known as radical initiators due to the fact that they contain relatively weak bonds that
can be easily cleaved by homolysis.
1. Diatomic Halogen:
2. Peroxides:
3. N-bromosuccinimide (NBS):
ORGANIC - CAREY 10E
CH. 10 - INTRODUCTION TO FREE RADICALS
Page 2
CONCEPT: RADICAL STABILITY
□ Radicals are electron deficient, therefore they also benefit from the stabilizing effects of __________________________.
● The relative stability of radicals is slightly different than that of carbocations:
● Unlike carbocations, allylic / benzylic radicals are ALWAYS most stable
EXAMPLE: Determine which of the following radicals is most stable.
ORGANIC - CAREY 10E
CH. 10 - INTRODUCTION TO FREE RADICALS
Page 3
CONCEPT: RADICAL CHAIN REACTION
□ Alkanes are the backbone of organic molecules, yet they are almost completely unreactive.
● The only reaction they undergo is radical halogenation, the gateway to the rest of organic synthesis.
The Radical Chain Reaction
● Alkanes will react with diatomic halogens in the presence of heat, light or any other radical initiator.
Initiation:
Propagation:
Termination:
ORGANIC - CAREY 10E
CH. 10 - INTRODUCTION TO FREE RADICALS
Page 4
EXAMPLE: Show the entire chain reaction mechanism for the following radical halogenation reactions, predicting which
would be structure of the major alkyl halide produced.
a.
ORGANIC - CAREY 10E
CH. 10 - INTRODUCTION TO FREE RADICALS
Page 5
CONCEPT: RADICAL SELECTIVITY – QUALITATIVE
□ Selectivity is defined as the ability to only halogenate the carbons with most stable radical intermediates.
Fluorination: Chlorination:
● Overall ΔHo = -432 ● Overall ΔHo = -101
● No useful radical fluorination reactions. Call 911! ● The only useful radical chlorinations are reactions with a
single type of - H
Bromination: Iodination:
● Overall ΔHo = -26 ● Overall ΔHo = +53
● The only useful method for selectively halogenating alkanes ● Not a spontaneous, don’t even try it.
Chiral Products are ALWAYS racemized
ORGANIC - CAREY 10E
CH. 10 - INTRODUCTION TO FREE RADICALS
Page 6
EXAMPLE: Draw the product of the following radical chlorinations. Would the following radical chlorinations be synthetically
useful? (Yielding only one product).
1)
2)
ORGANIC - CAREY 10E
CH. 10 - INTRODUCTION TO FREE RADICALS
Page 7
EXAMPLE: Predict the following monobrominated products of the following radical brominations:
a.
b.
ORGANIC - CAREY 10E
CH. 10 - INTRODUCTION TO FREE RADICALS
Page 8
CONCEPT: RADICAL SELECTIVITY -- QUANTITATIVE
□ Selectivity is defined as the ability to only halogenate the carbons with most stable radical intermediates.
● The Hammond Postulate explains why halogen radicals have differing selectivities.
Radical Chlorination
Radical Bromination
ORGANIC - CAREY 10E
CH. 10 - INTRODUCTION TO FREE RADICALS
Page 9
CONCEPT: CALCULATING PERCENTAGE YIELDS
□ We need equations to make quantitative predictions about the exact ratios of these products formed.
● Chlorination is non-selective, meaning that the difference between relative rates of halogenation is ___________
● Bromination is highly selective, meaning that the difference between relative rates of halogenation is ___________
● These ratios are only valid at room temperature. At higher temperature the ratios get _________________
EXAMPLE: Draw all of the monochlorination products of butane. Calculate the percentage yields of each product.
ORGANIC - CAREY 10E
CH. 10 - INTRODUCTION TO FREE RADICALS
Page 10
CONCEPT: CALCULATING PERCENTAGE YIELDS
PRACTICE: Calculate the percentage yield of all monobromination products at room temperature.
PRACTICE: Calculate the percentage yield of all monobromination products.
ORGANIC - CAREY 10E
CH. 10 - INTRODUCTION TO FREE RADICALS
Page 11