organic - mcmurry 9e ch.10 -...

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ORGANIC - MCMURRY 9E

CH.10 - ORGANOHALIDES

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):



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.



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:



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.



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



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

useful? (Yielding only one product).

1)

2)



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

a.

b.



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



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.



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.



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:



CONCEPT: ALLYLIC HALOGENATION

Allylic Chlorination:

Allylic Bromination:

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



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

a.

b.



CONCEPT: CONVERSION OF ALCOHOLS TO GOOD LEAVING GROUPS

□ Alcohols are terrible leaving groups. If we want to react with them, they need to be transformed.

●They can be converted into ____________ _________________ or ______________ _______________

2o and 3o Alcohols with HX: SN1 mechanism

1o alcohols with HX: SN2 mechanism




□ SOCl2 and PBr3 are two of the most commonly used reagents to transform alcohols into alkyl halides

●They always proceed with __________________________ of configuration.

1o and 2o Alcohols to RX: SN2 mechanism

EXAMPLE: Determine the mechanism for the following reaction. Draw the final product.




□ Alcohols can attack sulfonyl chlorides to create sulfonate esters.

●They always proceed with __________________________ of configuration.

Alcohols to Sulfonate Esters:



Alcohol Conversion Summary:



CONCEPT: NUCLEOPHILIC ADDITION

□ One of the most important ways that carbonyl compounds react is through nucleophilic addition.

● The carbonyl carbon is _____________________________

Nucleophilic Addition General Mechanism:

Nucleophilic Addition General Reactions:



CONCEPT: REACTIONS OF ORGANOMETALLICS

1. Substitution/Elimination on Alkyl Halides

2. Nucleophilic Addition on Ketones and Aldehydes

3. Nucleophilic Acyl Substitution on Esters

4. Base-Catalyzed Epoxide Ring Opening



CONCEPT: PREPARATION OF ORGANOMETALLICS

□ Organometallics are alkylating agents that have mostly group IA or group IIA metals bonded to a carbon structure

● They can also react as strong ____________. Beware of cross-reactions with acidic hydrogens.

1. Sodium Alkynides

2. Grignard Reagents

3. Organolithium Reagents

4. Gilman Reagents



CONCEPT: INTRO TO REDOX

● Oxidation reactions involve an increase in the ______________ content of a molecule

● Reduction reactions involve an increase in the ______________ content of a molecule

EXAMPLE: Label the following transformations as oxidation or reduction.

a.

b.

c.



organic - mcmurry 9e ch.10 -...

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