7. Substitution Reactions

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?

Which reacts faster in an S_N2 reaction?

Explain why the alkyl halide shown here reacts much more rapidly with guanine than does a primary alkyl halide (such as pentyl chloride).

Under appropriate conditions, (S)-1-bromo-1-fluoroethane reacts with sodium methoxide to give pure (S)-1-fluoro-1-methoxyethane.

a. Why is bromide rather than fluoride replaced?

b. Draw perspective structures (as shown on the previous page for 2-bromobutane) for the starting material, the transition state, and the product.

c. Does the product show retention or inversion of configuration? d. Is this result consistent with reaction by the SN2 mechanism?