8. Elimination Reactions

What halides would undergo E2 dehydrohalogenation to give the following pure alkenes?

a. hex-1-ene

b. isobutylene

c. pent-2-ene

When the following compound is treated with sodium methoxide in methanol, two elimination products are possible. Explain why the deuterated product predominates by about a 7:1 ratio (refer to Problem 7-75).

Protonation converts the hydroxy group of an alcohol to a good leaving group. Suggest a mechanism for each reaction.

(b)

Draw the products of the following intramolecular reactions:

a.

b.

After a proton is removed from the OH group, which compound in each pair forms a cyclic ether more rapidly?

a.

b.

cis-4-Bromocyclohexanol and trans-4-bromocyclohexanol form the same elimination product but a different substitution product when they react with HO⁻.

c. How many stereoisomers does each of the elimination and substitution reactions form?

Explain how each of the following changes affect the rate of the reaction of 1-bromobutane with ethoxide ion in DMF.

a. The concentration of both the alkyl halide and the nucleophile are tripled.

b. The solvent is changed to ethanol.

Explain how the following changes affect the rate of the reaction of 2-bromo-2-methylbutane with methanol:

a. The alkyl halide is changed to 2-chloro-2-methylbutane.

b. The alkyl halide is changed to 2-chloro-3-methylbutane.

The following substitution reaction, between a strong base and a 1° haloalkane, occurs in a single step via backside displacement. Yet it is not technically an S_N2 reaction. Why?

Propose a mechanism for each of the following reactions:

a.

Which of the following two compounds eliminates HBr more rapidly in a basic solution?

Propose a mechanism for the following reaction. (Hint: The rate of the reaction is much slower if the nitrogen atom is replaced by CH.)

Show a mechanism for the following elimination reactions. Label the mechanism as E1 or E2.

(d)

Paying close attention to the stereochemical outcome, predict the product of these elimination reactions.

(d)

Predict the product(s) that would result when molecules (a)–(p) are allowed to react under the following conditions: (i) SOCl₂ ; (ii) PBr₃ ; (iii) SOCl₂ , NEt₃ (iv) 1. TsCl, Et₃N 2. NaCN; (v) 1. TsCl, Et₃N 2. NaOt-Bu (vi) H₂SO₄ (vii) HCl; (viii) HBr; (ix) PCC; (x) H₂CrO₄ , H₂O (xi) HOCl, H₂O (xii) HIO₄ If no reaction occurs, write 'no reaction.'

(c)