Substitution Reactions

Question 1

What is a S_N1 Reaction?

Answer 1

A Unimolecular Substitution Reaction, with the rate determined by the formation of a carbocation intermediate.

Question 2

What is a S_N2 Reaction?

Answer 2

A Bimolecular Substitution Reaction, where a lone pair from a nucleophile attacks the back side of an electrophile, expelling the Leaving Group

Question 3

S_N1 OR S_N2 ?

Answer 3

S_N2

Question 4

S_N1 OR S_N2 ?

Answer 4

S_N1

Question 5

Rank the stability of carbocations in or from least stable to most stable.

Answer 5

Question 6

What makes a good electron source?

Answer 6

π-bonds, Lone Pair, Polar Bond (ς+- - - ς-)

Question 7

What makes a good electron sink?

Answer 7

Cations, Empty P-Orbitals, ς+

Question 8

What is Solvolysis?

Answer 8

A nucleophilic substitution in which the solvent is also the nucleophile.

Question 9

What is the steriochemisrty seen in S_N1 reactions?

Answer 9

S_N1 reactions form both ( S ) and ( R ) enantiomers forming a racemic mixture

Question 10

What is the steriochemistry seen in S_N2 reactions?

Answer 10

The product formed is the inverted form of the original.

(S) to (R) or (R) to (S)

Question 11

How does the structure of the alkyl portion of a haloalkane affect S_N1 and S_N2 reactions?

Answer 11

S_N1 reactions are infulenced by electronics; the alkyl portion stabilizes the carbocation formed in the rate determining step. The more alkyl groups the more stable the carbocation

S_N2 reactions are influenced by sterics; the larger the alkyl group, the less likely the Nucleophile is to reach the carbon.

Question 12

What affects does ß-Branching have on substitution reactions?

Answer 12

In S_N2 reactions, branching on the ß-carbon can block the Nucleophile from reaching the A-carbon.

The less ß-branches the faster the reaction

Question 13

With a methyle halide (CH₃-X), is S_N2 or S_N1 favored?

Answer 13

S_N2 is Favored

S_N1 does not occur. The methyl cation is so unstable that it is never observed in solution.

Question 14

With a primary alkyl halide (RCH₂-X), is S_N2 or S_N1 favored?

Answer 14

S_N2 is favored

S_N1 rarely occurs. Primary cations are so unstable that they are rarely formed in solution (allylic and benzylic cations can be exceptions).

Question 15

Whith a secondary alkyl halide (R₂CH-X), is S_N2 or S_N1 favored?

Answer 15

S_N1 is favored in protic solvents with poor nucleophiles. Carbocation rearrangements may occur

S_N2 is favored in aprotic solvents with good nucleophiles.

Question 16

In a tertiary alkyl halide (R₃C-X), is SN2 or SN1 favored?

Answer 16

S_N1 is favored because of the ease of formation of tertiary carbocations.

S_N2 does not occur because of steric hindrance around the reaction center.

This is also true for 2° and 3° allylic and benzylic carbocations

Question 17

What makes a good leaving group?

Answer 17

• Leaving group builds up negative charcter
• The best leaving group can stabilize the negative charge
• Weaker C-LG bonds make better leaving groups.

Question 18

Whats the difference between protic and aprotic solvents?

Answer 18

Protic Solvents

• Can donate a hydrogen bond (look for OH groups)
• Help to stabilize the T.S. in S_N1 reactions
• The more polar the protic solvent the more it will stabilize the T.S. in S_N1 reactions
• Slows reation rate in S_N2, because it reacts with the nucleophile

Aprotic Solvent

• Can not donate a hydrogen bond
• Provide a “naked” nucleophile in S_N2 reactions

Question 19

What nucleophiles have “very good” reactivity?

Answer 19

I^-, HS^-, RS^-

Question 20

What nucleophiles have “good” reactivity?

Answer 20

Br^-, OH^-, RO^-, CN^-, N₃^-, R-C≣C^-

Question 21

What nucleophiles have “fair” reactivity?

Answer 21

NH₃, Cl^-, F^- RCO₂^-,R₂S

Question 22

What nucleophiles have “weak” reactivity?

Answer 22

H₂O, ROH

Question 23

What nucleophiles have “very weak” reacticity?

Answer 23

Carboxylic Acids

Question 24

List Common Protic Solvents

Answer 24

Question 25

List Common Aprotic Solvents

Answer 25