E2 reactions

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E2 Reactions

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E2 = elimination, bimolecular Reaction is concerted (one step) Rate = k[substrate][base] (a second order process) E2 can occur with S N 2.

Occurs by abstraction of

H + from a C adjacent

to the C with the LG.

Products follow Zaitsev’s Rule.

If the base is strong, E2 will occur instead of E1 for 3° alkyl halides.

E2 Reaction Mechanism

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One-step and requires a strong base.

Best transition state is anti-coplanar.

Example: t-butyl bromide + methoxide Why don’t S N 2, S N 1, or E1 occur?

E2 Reaction Mechanism

Example: t-butyl bromide + methoxide

E2 Reaction Mechanism

Anti-periplanar alignment allows pi bond to form from sp 3 orbitals.

E2 Reaction Profile

rate = ?

k = ?

Factors Affecting E2 Reactions

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Structure of the substrate Strength of the base Nature of the leaving group The solvent in which the reaction is run.

Factors Affecting E2 Reactions Structure of the Substrate

Structure of the alkyl halide: 3° > 2° >1°

This is due to the stability of the more highly substituted alkene (Zaitsev’s Rule).

Factors Affecting E2 Reactions Structure of the Substrate Decide whether the following substrates could react by E1 or E2 (and by S N 1 or S N 2).

Br Cl I Br

Factors Affecting E2 Reactions Strength of the Base

The base must be strong. Which of the following bases favor E2 and which favor E1 reactions?

CH 2 OH H 2 O N

Factors Affecting E2 Reactions the Leaving Group

The LG should be good. Which of the following substrates have good LGs?

Cl NH 2 OCH 3 H O O S O CH 3

Factors Affecting E2 Reactions Solvent Effects

Polarity is not so important because negative charge is spread over the transition state.

Zaitsev’s Rule

When two or more elimination products are possible, the more stable alkene will predominate. This is usually the product with the more substituted double bond.

Zaitsev’s Rule

Show the E2 elimination products that could come from:

Cl  Br

Which E2 reaction would occur more quickly?

E2 Reactions - Summary

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The structure of the substrate affects the rate.

Relative rates for E2: 3°>2°>1°.

The base must be strong.

The LG should be good.

The solvent should be polar.

Coplanar (usually anti) transition state is required.

Products will follow Zaitsev’s Rule.

Can occur with S N 2 for 2° alkyl halides.

E2 Reactions - Summary

Predict the products

Cl RO Cl CH 3 CH 2 OH Br RO -

E2 Competes with S N 2 for 2° Alkyl Halides

E1 and E2 Reactions

Base Substrate Leaving group Solvent E1 weak ones work E2 strong 3°>2° 3°>2°>1° good good polar, ionizing polar, aprotic is better

E1 and E2 Reactions

E1 Rate =k[RX] E2 =k[RX][Base] Carbocation intermediate?

Stereochemistry Y Zaitsev’s rule N Zaitsev’s rule Rearrangement ~H, ~ CH 3 possible No rearrangements

How Do You Decide Which Reaction(s) Happen?

The strength and structure of the nucleophile/base is paramount.

Strong ones always give a second order reaction (S E2). N 2 or

If the base is bulky, E2 will occur instead of S N 2.

How Do You Decide Which Reaction(s) Happen?

Now, look at the structure of the substrate.

1° alkyl halides will undergo S N 2 and possible.

3° alkyl halides will NOT undergo S N 2.

2° alkyl halides are the toughest to predict.

How Do You Decide Which Reaction(s) Happen?

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Evaluate, in order,

Nucleophile, strength and structure

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-or- Base, strength and structure Structure of the alkyl halide/carbocation Leaving group Solvent YOU MUST BE ABLE TO DRAW THE MECHANISM!

Elimination Reactions

Predict the products

Br Na + OCH 2 CH 3 ethanol CH 3 OH OCH 3

Did this product come from the reaction of 1-chlorobutane with a) sodium acetate or b) sodium t-butoxide?

This product came from the reaction of 1-chlorobutane with sodium acetate. The C=O and C-O peaks show ester.

O O C CH 3

Did this product come from the reaction of 1-chloropentane with a) sodium acetate or b) sodium t-butoxide?

This product came from the reaction of 1-chloropentane with sodium t-butoxide. The sp 2 C-H and C=C peaks show alkene.

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