10 • Additional Problems
10 • Additional Problems
The following alkyl bromide can be prepared by reaction of the alcohol (S)-2-pentanol with PBr3. Name the compound, assign (R) or (S) stereochemistry, and tell whether the reaction of the alcohol results in the same stereochemistry or a change in stereochemistry (reddish brown = Br).
In light of the fact that tertiary alkyl halides undergo spontaneous dissociation to yield a carbocation plus halide ion (see Problem 10-41), propose a mechanism for the following reaction.
Naming Alkyl Halides
Synthesizing Alkyl Halides
A chemist requires a large amount of 1-bromo-2-pentene as starting material for a synthesis and decides to carry out an NBS allylic bromination reaction. What is wrong with the following synthesis plan? What side products would form in addition to the desired product?
What product(s) would you expect from the reaction of 1-methylcyclohexene with NBS? Would you use this reaction as part of a synthesis?
What product would you expect from the reaction of 1-phenyl-2-butene with NBS? Explain.
Oxidation and Reduction
Which of the following compounds have the same oxidation level, and which have different levels?
Alkylbenzenes such as toluene (methylbenzene) react with NBS to give products in which bromine substitution has occurred at the position next to the aromatic ring (the benzylic position). Explain, based on the bond dissociation energies in Table 6.3.
How would you carry out the following syntheses?
Why do you suppose it’s not possible to prepare a Grignard reagent from a bromo alcohol such as 4-bromo-1-pentanol? Give another example of a molecule that is unlikely to form a Grignard reagent.
Addition of HBr to a double bond with an ether (−OR) substituent occurs regiospecifically to give a product in which the −Br and −OR are bonded to the same carbon. Draw the two possible carbocation intermediates in this electrophilic addition reaction, and explain using resonance why the observed product is formed.
Identify the reagents a–c in the following scheme:
Tertiary alkyl halides, R3CX, undergo spontaneous dissociation to yield a carbocation, R3C+, plus halide ion. Which do you think reacts faster, (CH3)3CBr or H2CCHC(CH3)2Br? Explain.
Carboxylic acids (RCO2H; pKa ≈ 5) are approximately 1011 times more acidic than alcohols (ROH; pKa ≈ 16). In other words, a carboxylate ion (RCO2−) is more stable than an alkoxide ion (RO−). Explain, using resonance.