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CHAPTER 8: ELIMINATION REACTIONS

ALKENES

CIS-TRANS ISOMERS

RESTRICTED ROTATION

CIS-TRANS ISOMERS

C C

H3C

H H

CH3C C

H3C

H CH3

H

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IDENTIFY CIS-TRANS ISOMERISM

Which of these compounds can exist as cis and trans isomers? When this type of isomerism is possible, draw the other isomer (if trans is shown, draw cis).

ALKENE STABILITY

CIS VS. TRANS ISOMERS

Complete Reaction DH˚ (kcal/mol)1

trans-2-butene + H2 (+ catalyst) à butane -27.6

cis-2-butene + H2 (+ catalyst) à butane -28.6

1 Jones, M., Fleming, S.A., Organic Chemistry, 4th ed., Norton, 2010, pp. 441

O3S

N N

SO3SO3

N

SO3

N SO3

O3S

ClBr

Br

FD&C Blue No. 1

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NIGHT VISION

beta-carotene Vitamin A

In the back of the eye, beta-carotene is converted to Vitamin A (then retinal and isomerized) and attached to the protein opsin (with molecule bound is now rhodopsin). With the absorption of dim light, the pi bond is temporarily broken and reforms in a new configuration. This large molecular change causes a nerve impulse to be sent to the brain: light was received.

DEGREE OF SUBSTITUTION

dim light

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1.   How would you classify the following alkenes?

a)   Monosubstituted b)   Disubstituted c)   Trisubstituted d)   Tetrasubstituted

a)   Monosubstituted b)   Disubstituted c)   Trisubstituted d)   Tetrasubstituted

2.   Which compound should release more heat upon full hydrogenation (reaction with H2)?

ELIMINATION REACTIONS

GENERAL PATTERN

PRODUCT MIXTURES

Product composition2

2 Jones, M., Fleming, S.A., Organic Chemistry, 4th ed., Norton, 2010, pp. 322

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Draw all possible products (including stereoisomers) of this elimination and identify the expected major product.

E2 REACTION

MECHANISM

ENERGY DIAGRAM + TRANSITION STATE OF RDS

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Give the major organic product of this E2 reaction, and draw the transition state leading to the major product.

E2 REACTIVITY TRENDS

Trend Specifics Explanation

Type of base Neutral vs. charged

Leaving group I vs. Br vs. Cl

Solvent Protic vs. aprotic

Alkyl halide 3˚ vs. 2˚ vs. 1˚

BrNaOH

E2

Br NaOH

E2

NaOH

E2

Br

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REACTION OF BULKY BASE WITH 3˚ RX

Product composition3

PROBLEM

For each set of reactions, choose which would undergo a faster E2 reaction. Then draw the expected major product for the faster reaction.

3 Wade, L.G., Organic Chemistry, 8th ed., Pearson, 2013, pp. 301

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ANTI GEOMETRY OF LEAVING GROUP AND BETA H

RELATIONSHIP

Anti-coplanar configuration

EXAMPLES

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Br

CH2CH3

CH3O–

Br

CH2CH3

CH3O–

NH2–

Cl CH3

CH3

NH2–

Cl CH3

CH3

I

CH3

NH2–

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E1 REACTION

MECHANISM

ENERGY DIAGRAM + TRANSITION STATE OF RDS

E1 REACTIVITY TRENDS

Trend Specifics Explanation

Type of base Neutral vs. charged

Leaving group I vs. Br vs. Cl

Solvent Protic vs. aprotic

Alkyl halide 3˚ vs. 2˚ vs. 1˚

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PROBLEM

For each set of reactions, choose which would undergo a faster E1 reaction. Then draw the expected major product for the faster reaction.

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SUBSTITUTION VERSUS ELIMINATION

EFFECT OF HEAT

TREND

Substitution Products Elimination Products

At 25 ˚C

At 65 ˚C

EXPLANATION

Substitution

Elimination

BrSN2CN+

BrE2OH+

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WHICH MECHANISM DOMINATES WHEN

MOST DECISIVE FACTORS

Alkyl Halide Structure: 3˚ 2˚ 1˚ methyl

Nucleophile/Base: Neutral Charged

THE SUMMARY GRID

CH3Br

1˚ RX

2˚ RX

3˚ RX

Weak base Poor nucleophile

Weak Base Good nucleophile

Strong base Good nucleophile

Strong base Poor nucleophile

Methyl RX

Br

Br

CH3

Br

CH3

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PROBLEMS

ClNaSH

ClKOH