Substitution and Elimination Reactions

Question 1

What is a furan? A tetrahydrofuran (THF)?

Answer 1

Question 2

What is a diethyl ether?

An ethyl methyl sulfide?

Answer 2

Question 3

What is a vinyl alcohol?

An allyl alcohol?

Answer 3

Question 4

What is chloroform?

Vinyl or allyl chloride?

Answer 4

Question 5

What are primary, secondary, and tertiary alcohols?

Answer 5

Question 6

What does SN2 mean?

Answer 6

Question 7

What are the charactestics (and examples) of good leaving groups?

Answer 7

Good leaving groups are weak bases

(meaning, they make strong acids)

Question 8

What are the characteristics (and examples) of poor leaving groups?

Answer 8

Poor leaving groups are strong bases (weak acids)

Question 9

What are the characterstics (and some examples) of good nucleophiles?

Answer 9

• high energy donor
• (-) negative formal charge
• electropositive

Question 10

What makes for a rapid S_N2 reaction?

Answer 10

Steric Effects (most important)

• Good if the halide is methyl or 1^o
• Fair if the halide is 2^o
• Poor if the halide is 3^o, β-branch. Or if the nucleophile is very bulky.

Leaving-Group ability

• Good leaving groups are weak bases.

Nucleophilicity

• Good: High energy lone pairs (C, P, S, I)
• Fair: OH^-, Br^-, N, Cl^-
• Poor: F^-, ROH, H₂O

Question 11

Each of the following carbocations is significantly more stable than the methyl cation. Explain why each of these species is especially stable. You should use both molecular orbital and resonance arguments in your explanations.

Answer 11

Question 12

What are the primary requirements for…

• S_N2 reactions
• E2 reactions
• S_N1 and E1 reactions

Answer 12

• S_N2 reactions: need good steric interactions
  
  • (Me, 1° is good, 2° less good, 3° bad)
• E2 reactions: need strong attacking base
  
  • at least as strong as OH^–
• S_N1 and E1 reactions: need stable carbocation
  
  • 3° is good, 2° is OK, 1° is bad, protic conditions

Question 13

Predict the major product, and indicate the type of mechanism.

Answer 13

Question 14

Predict the major product, and indicate the type of mechanism.

Answer 14

Question 15

Answer 15

Question 16

Saytzeff’s rule

Answer 16

Saytzeff’s rule: more substituted alkene is more thermodynamically stable (that is, the one with more carbon-carbon bonds).

Question 17

What are the alkene isomers formed by this reaction?

Answer 17

Note: 20% are S_N2

Question 18

S_N2 vs E2: The “GFP” Reaction Hierarchy

Answer 18

Question 19

What is the rate law of an S_N1 or E1 reaction?

What would the reaction energy profile look like?

Answer 19

Rate = K[R-X]

first order reaction (unimolecular)

K depends on the stability of carbocation

Question 20

Answer 20

Question 21

Here are two other examples of “carbocations.” Can you make any statement about their stability, relative to each other?

Answer 21

Question 22

In an S_N1 reaction, the racemic product is not exactly 50/50.

Why not?

Answer 22

Question 23

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Question 24

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Question 25

S_N1, E1 and S_N2, E2 The “GFP” Hierarchy Revisted

Answer 25

* S_N2 reactions: **Require minimal steric interactions (and a good nucleophile)** * **Good** if the halide is methyl or 1^o * **Fair** if the halide is 2^o * **Poor** if the halide is 3^o or if the nucleophile is bulky (e.g. tBuO^– ) or poor * E2 reactions: **Require a strong base** * **Good** if the base is at least as strong as OH * **Fair** if the base is between H₂O and OH^– in strength * **Poor** if the base is weaker than H₂O * S_N1 and E1: **Require a stable carbocation** * **Good** if the halide is 3^o * **Fair** if the halide is 2^o * **Poor** if the halide is methyl or 1^o (Note that S N 1 and E1 reactions almost always occur together, although you can ensure elimination by treating an alcohol with concentrated mineral acid.)

Question 26

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Question 27

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Question 28

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Question 29

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Question 30

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Question 31

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Question 32

Each of the following products can be synthesized in one step from an alkyl halide or alkyl sulfonate. Be sure to consider stereochemistry where relevant.

Answer 32

Question 33

Each of the following products can be synthesized in one step from an alkyl halide or alkyl sulfonate. Be sure to consider stereochemistry where relevant.

Answer 33

Question 34

Each of the following products can be synthesized in one step from an alkyl halide or alkyl sulfonate. Be sure to consider stereochemistry where relevant.

Answer 34