7. Substitution Reactions

Show all the products, including their configurations, that are obtained from the above reaction.

You were told in [SECTION 7.11] that is best to use a methyl halide or a primary alkyl halide for the reaction of an acetylide ion with an alkyl halide. Explain why this is so.

Would you expect acetate ion (CH₃CO₂⁻) to be a better nucleophile in an S_N2 reaction with an alkyl halide carried out in methanol or in dimethyl sulfoxide?

Draw a perspective structure or a Fischer projection for the products of the following S_N2 reactions.

(e)

Draw a perspective structure or a Fischer projection for the products of the following S_N2 reactions.

(f)

Predict the compound in each pair that will undergo the S_N2 reaction faster.

(c)

(d)

For each pair of compounds, state which compound is the better S_N2 substrate.

c. 2-bromobutane or isopropyl bromide

d. 1-chloro-2,2-dimethylbutane or 2-chlorobutane

e. 1-iodobutane or 2-iodopropane

Triethyloxonium tetrafluoroborate, (CH₃CH₂)₃O⁺ BF₄^–, is a solid with melting point 91–92°C. Show how this reagent can transfer an ethyl group to a nucleophile (Nuc:⁻) in an S_N2 reaction. What is the leaving group? Why might this reagent be preferred to using an ethyl halide? (Consult Table 6-2)

Explain why a much better yield of primary amine is obtained from the reaction of an alkyl halide with azide ion (^-N₃), followed by catalytic hydrogenation. (Hint: An alkyl azide is not nucleophilic.)

Consider the reaction of 1-bromobutane with a large excess of ammonia (NH₃). Draw the reactants, the transition state, and the products. Note that the initial product is the salt of an amine (RNH₃⁺Br⁻), which is deprotonated by the excess ammonia to give the amine.

Draw a perspective structure or a Fischer projection for the products of the following S_N2 reactions.

(a) trans-1-bromo-3-methylcyclopentane + KOH

(b) (R)-2-bromopentane + KCN

Draw a perspective structure or a Fischer projection for the products of the following S_N2 reactions.

(c)

(d)

Predict the major products of the following reactions.

(d) the tosylate of cyclohexylmethanol + excess NH₃

(e) n-butyl tosylate + sodium acetylide, H–C≡C:^{– +}Na

Rank the following compounds in decreasing order of their reactivity toward the S_N2 reaction with sodium ethoxide (Na^{+ –}OCH₂CH₃) in ethanol.

methyl chloride

tert-butyl iodide 

neopentyl bromide

isopropyl bromide

methyl iodide

ethyl chloride

When (±)−2,3−dibromobutane reacts with potassium hydroxide, some of the products are (2S,3R)-3-bromobutan-2-ol and its enantiomer and trans-2-bromobut-2-ene. Why is no cis-2-bromobut-2-ene formed?