8. Elimination Reactions

Predict the mechanism for the following reactions. Provide the full mechanism and draw the final product.

Predict the mechanism for the following reactions. Provide the full mechanism and draw the final product.

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

(d) <IMAGE>

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

(d) <IMAGE>

(••) 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) <IMAGE>

(••) Predict the product(s) that would result when molecules (a)–(p) are allowed to react under the following conditions: (v) 1. TsCl, Et₃N 2. NaOt-Bu (vi) H₂SO₄ If no reaction occurs, write 'no reaction.'

(o) <IMAGE>

(••) Predict the product(s) that would result when molecules (a)–(p) are allowed to react under the following conditions: (v) 1. TsCl, Et₃N 2. NaOt-Bu (vi) H₂SO₄ If no reaction occurs, write 'no reaction.'

(k) <IMAGE>

(••) Predict the product(s) that would result when molecules (a)–(p) are allowed to react under the following conditions: (vi) H₂SO₄. If no reaction occurs, write 'no reaction.'

(c) <IMAGE>

(••) Predict the product(s) that would result when molecules (a)–(p) are allowed to react under the following conditions: (i) SOCl₂ ; (iii) SOCl₂, NEt₃. If no reaction occurs, write 'no reaction.'

(c) <IMAGE>

(••) Predict the product(s) that would result when molecules (a)–(p) are allowed to react under the following conditions: (ii) PBr₃ If no reaction occurs, write 'no reaction.'

(c) <IMAGE>

(••) Predict the product(s) that would result when molecules (a)–(p) are allowed to react under the following conditions: (vii) HCl; (viii) HBr; If no reaction occurs, write 'no reaction.'

(c) <IMAGE>

(••) Predict the product(s) that would result when molecules (a)–(p) are allowed to react under the following conditions: (v) 1. TsCl, Et₃N 2. NaOt-Bu (vi) H₂SO₄ If no reaction occurs, write 'no reaction.'

(f) <IMAGE>

(•••) Predict the product(s) of the following substitution or elimination reactions, paying close attention to the stereochemical outcome of the reactions.

(a) <IMAGE>

(•••) Predict the product(s) of the following substitution or elimination reactions, paying close attention to the stereochemical outcome of the reactions.

(d) <IMAGE>

(•••) Predict the product(s) of the following substitution or elimination reactions, paying close attention to the stereochemical outcome of the reactions.

(e) <IMAGE>

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

(c) <IMAGE>

Would you expect the following bases to favor E1 or E2 elimination?

(d) <IMAGE>

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

(c) <IMAGE>

Indicate the type of catalysis that is occurring in the slow step in each of the following reaction sequences:

a. <IMAGE>

Indicate the type of catalysis that is occurring in the slow step in each of the following reaction sequences:

b. <IMAGE>

Why is melphalan a good cancer drug?

Indicate which species in each pair gives a higher substitution-product-to-elimination-product ratio when it reacts with isopropyl bromide:

c. Cl⁻ or Br⁻

The following three nitrogen mustards were studied for possible clinical use. One is now used clinically, one was found to be too unreactive, and one was found to be too insoluble in water to be injected intravenously. Which is which? (Hint: Draw resonance contributors.)

<IMAGE>

Finish [SOLVED Problem 7-3] <IMAGE> by showing how the rearranged carbocations give the four products shown in the problem.

Be careful when using curved arrows to show ­deprotonation and/or nucleophilic attack by the solvent.

The curved arrows always show ­movement of electrons, not movement of protons or other species.

Give the substitution and elimination products you would expect from the following reactions.

c. 1-bromo-2-methylcyclohexane + silver nitrate in water (AgNO₃ forces ionization)

Make models of the following compounds, and predict the products formed when they react with the strong bases shown.

HINT: Anti-coplanar E2 eliminations are common.

Syn-coplanar E2 eliminations are rare, usually occurring when free rotation is not possible.

a. <IMAGE> + KOH ——> (substitution and elimination)

Deuterium (D) is the isotope of hydrogen of mass number 2, with a proton and a neutron in its nucleus. The chemistry of deuterium is nearly identical to the chemistry of hydrogen, except that the C―D bond is slightly (5.0 kJ/mol, or 1.2 kcal/mol) stronger than the C―H bond. Reaction rates tend to be slower if a C―D bond (as opposed to a C―H bond) is broken in a rate-limiting step. This effect on the rate is called a kinetic isotope effect. (Review PROBLEM 4-57 <IMAGE>)

b. When the following deuterated compound reacts under the same conditions, the rate of formation of the substitution product is unchanged, while the rate of formation of the elimination product is slowed by a factor of 7.

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Explain why the elimination rate is slower, but the substitution rate is unchanged.

Show how you would prepare cyclopentene from each compound.

a. cyclopentanol

b. cyclopentyl bromide

Show how this 1° alcohol can be made from the following:

<IMAGE>

(a) a 1° alkyl bromide

SN1 substitution and E1 elimination frequently compete in the same reaction.

Propose a mechanism and predict the products for the solvolysis of 2-bromo-2,3,3-trimethylbutane in methanol.

Why is the S_N 1 reaction shown an inefficient way of synthesizing ethers?

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