ORGANIC - KLEIN 3E CH.10 - RADICAL...

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ORGANIC - KLEIN 3E

CH.10 - RADICAL REACTIONS

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):

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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.

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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:

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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.

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CONCEPT: ENTHALPY

□ Bond dissociation energies describe the strength of chemical bonds. They can be determined experimentally.

● Enthalpy is the sum of bond dissociation energies for the reaction.

_____ = ___________ bonds = Exothermic _____ = ___________ bonds = Endothermic

EXAMPLE: Predict the sign and magnitude of ΔH⁰ for the following reaction. Give your answer in units of kilojoules per

mole, and identify whether the reaction is expected to be endothermic or exothermic.

PRACTICE: Predict the sign and magnitude of ΔH⁰ for the following reaction. Give your answer in units of kilojoules per

mole, and identify whether the reaction is expected to be endothermic or exothermic.

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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

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EXAMPLE: Draw the product of the following radical chlorinations. Would the following radical chlorinations be synthetically

useful? (Yielding only one product).

1)

2)

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EXAMPLE: Predict the following monobrominated products of the following radical brominations:

a.

b.

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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

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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.

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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.

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CONCEPT: ALLYLIC HALOGENATION

□ Unlike carbocations, allylic / benzylic radicals are ALWAYS most stable

Resonance plays a central role in the mechanisms of conjugated reactions (radical and carbocation intermediated).

General Mechanism:

Initiation:

Propagation:

Termination:

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CONCEPT: ALLYLIC HALOGENATION

Allylic Chlorination:

Allylic Bromination:

EXAMPLE: Predict the product(s) of the following reaction.

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CONCEPT: RADICAL HYDROHALOGENATION

□ Recall the hydrohalogenation mechanism:

● ____________________ intermediate

● _____________________ addition of bromine

Compare to the radical mechanism that predominates in the presence of peroxide:

● ____________________ intermediate

● _____________________ addition of bromine

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EXAMPLE: Provide the complete mechanism for the following radical hydrohalogenation.

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CONCEPT: RADICAL POLYMERIZATION

□ Radical polymerization reactions use ________________ in excess to extend the propagation step almost indefinitely.

● Petroleum is converted into various plastics through this mechanism

General Mechanism:

Initiation:

Propagation:

Termination:

EXAMPLE: Use the polymerization mechanism to draw the polymer formula for the following compound

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EXAMPLE: Provide the MAJOR product for the following multi-step syntheses

a.

b.

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ORGANIC - KLEIN 3E CH.10 - RADICAL...

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