TOPIC: 5. Nucleophilic Substitution
SUBTOPIC: 50. SN2 Mechanism
5042
The reaction of n-butyl chloride with sodium hydroxide to give n-butyl alcohol is catalyzed by sodium iodide.
(a)
Write amechanism to account for the iodide ion catalysis of this reaction.
(b)
Determine the stereochemistry that would be expected for both the catalyzed and uncatalyzed reactions using R-1-chlorobutane-1-d as the starting material.
5055
There are eight possible isomeric structures which satisfy the molecular formula for pentyl bromide, C5H11Br. When they are reacted separately with sodium ethoxide in refluxing ethanol, they exhibit very different behavior, and can be divided according to their behavior into four classes:
(a)
three give mainly ethers, the reaction of one being more sluggish than the others.
(b)
two give ethers, plus a large amount of alkenes, C5H10.
(c)
one is inert.
(d)
two give alkenes with little or no ether being formed.
Correlate these facts with the structure of the eight possible pentyl bromides, and write an illustrative equation for each type of reaction, using structural formulas.
5057
The 1-propyl halides undergo the SN2 reaction only about 40 percent as fast as the ethyl halides. Draw three-dimensional molecular structures of the substitution process that will account for this difference.
5149
Explain the fact that I– is both a good leaving group and a good nucleophile. Be complete by giving proper definitions to terms such as nucleophile and polarizability.
5151
Discuss how a change from non-polar to polar solvent would affect the rate of each of the following SN2 reactions.
(a)
(b)
(c)
5152
Arrange the following species in decreasing order as regards quality of the leaving group in nucleophilic substitution reactions. Explain your answer using appropriate structures to support your argument.
5156
Optically active phenylbromoethane reacts with methanol to give a product with 63% inversion, 37% retention. However, the same reaction with added methoxide in the methanol produces completely inverted product. Write mechanisms to account for these experimental facts and include a brief written explanation to support your structures. Also include an explanation as to why complete racemization is not observed during the reaction with methanol.
X / % / %1. CH3OH / 37% / 63%
2. CH3O–/CH3OH / 0% / 100%
5157
Compound A reacts via an SN2 reaction when treated with methanol, whereas compound B reacts via SN1. Provide an explanation for the above fact.
5176
Arrange the following compounds in order of decreasing effectiveness as leaving groups in nucleophilic substitution reactions. Explain your rationale in terms of factors which affect the quality of the leaving group.
5224
Show by appropriate curved arrows, the electron movement implied by each of the following transformations:
(a)
(b)
(c)
(d)
5240
The reaction of benzyl bromide (-bromotoluene) with the nucleophilic azide ion (N3–) follows the SN2 pathway.
(a) Write the expected rate expression for the reaction.
(b) How would you expect the rate of the reaction to change if the concentration of azide were doubled?
(c) How would you expect the rate of the reaction to change if the concentration of both azide and benzyl bromide were doubled?
5241
Arrange the following set of ions in decreasing order as leaving groups in nucleophilic substitution.
H–, Cl–, Br–, CH3CO2–, HO–
5368
Show how 3-decyne can be synthesized from an alkylating agent plus an appropriate nucleophile.