8. Elimination Reactions

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

c. <IMAGE>

(•••) Suggest a bromoalkane and the conditions necessary to produce the alkenes shown.

(b) <IMAGE>

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

(d)

Suggest an appropriate base to synthesize the alkene as the major product from the starting haloalkane.

(b)

Given the reactants shown, what type of elimination would you expect to occur?

(c) <IMAGE>

For which of the following reactions would you expect elimination to be more favored than substitution?

(c) <IMAGE>

vs.

<IMAGE>

For which of the following reactions would you expect elimination to be more favored than substitution?

(c) <IMAGE>

vs.

<IMAGE>

(•••) Give a mechanism for the following substitution and elimination reactions.

(c) <IMAGE>

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

(g) <IMAGE>

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

(h) <IMAGE>

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

(f) <IMAGE>

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

(c) <IMAGE>

Predict the product of the following reactions.

(b) <IMAGE>

Predict the product of the following reactions.

(d) <IMAGE>

Predict the product of the following reactions.

(f) <IMAGE>

Which of the following SN2 and E2 reactions, respectively, is faster? Justify your choice.

a. <IMAGE

Which of the following SN2 and E2 reactions, respectively, is faster? Justify your choice.

b. <IMAGE

Predict the product of the following reactions.

(e) <IMAGE>

For which of the following reactions would you expect elimination to be more favored than substitution?

(e) <IMAGE>

vs.

<IMAGE>

(••) For each pair, choose the haloalkane that would react most quickly in an Sₙ1 or E1 reaction.

(a) <IMAGE> vs. <IMAGE>

(••) Predict the product(s) that would result when the following molecules are allowed to react under the following conditions: (i) 1. BH₃ 2. NaOH, H₂O₂ (ii) 1. Hg(OAc)₂ 2. NaBH₄ (iii) H₂SO₄ , H₂O (iv) 1. OsO₄ 2. NaHSO₃ (v) H₂O (vi) NaOH, H₂O . If there is no reaction, write 'no reaction.'

(f) <IMAGE>

(••) Identify whether each of the following reactions proceed by an S_N1 ,S_N2 , E1, or E2 mechanism.

(c) <IMAGE>

(••) Identify whether each of the following reactions proceed by an SN1 ,SN2 , E1, or E2 mechanism.

(b) <IMAGE>

For which of the following reactions would you expect elimination to be more favored than substitution?

(a) <IMAGE>

vs.

<IMAGE>

For which of the following reactions would you expect elimination to be more favored than substitution?

(d) <IMAGE>

vs.

<IMAGE>

When the following compound undergoes solvolysis in ethanol, three products are obtained. Propose a mechanism to account for the formation of these products.

<IMAGE>

Draw the substitution and elimination products for the following reactions, showing the configuration of each product:

c. 1-chloro-1-methylcyclohexane + CH3O−

d. 1-chloro-1-methylcyclohexane + CH3OH

For each of the following reactions, draw the major elimination product; if the product can exist as stereoisomers, indicate which stereoisomer is obtained in greater yield.

a. (R)-2-bromohexane+high concentration of CH3O−

b. (R)-3-bromo-3-methylhexane+CH3OH

What are the products of the following reactions?

a. <IMAGE>

b. <IMAGE>

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

c. The alkyl halide is changed to 1-chlorobutane.

d. The alkyl halide is changed to 2-bromobutane.

What are the products of the following reactions?

g. <IMAGE>

h. <IMAGE>

Draw the products of each of the following S_N2/E2 reactions. If the products can exist as stereoisomers, show which stereoisomers are formed.

c. (3S,4R)-3-bromo-4-methylhexane + CH₃O⁻

What products (including stereoisomers, if applicable) are formed from the reaction of 3-bromo-3-methylpentane:

b. with H₂O?

Which of these reactions are likely to produce both elimination and substitution products?

a. 2−bromopentane + NaOCH3

b. 3−bromo−3−methylpentane + NaOMe. (Me=methyl,CH3)

c. 2-bromo-3-ethylpentane + NaOH

For each reaction, decide whether substitution or elimination (or both) is possible, and predict the products you expect. Label the major products.

c. chlorocyclohexane+NaOCH₃ in CH₃OH

d. chlorocyclohexane + NaOC(CH₃)₃ in (CH₃)₃COH

Silver-assisted solvolysis of bromomethylcyclopentane in methanol gives a complex product mixture of the following five compounds. 

Propose mechanisms to account for these products.

<IMAGE>  AgNo3, CH3OH, heat—> <IMAGE>

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>)

a. Propose a mechanism to explain each product in the following reaction.

CH3CH(Br)CH3 KOH, alcohol—> CH2=CH-CH3 elimination product + CH3CH(OH)CH3 substitution product 

a. Two stereoisomers of a bromodecalin are shown. Although the difference between these stereoisomers may seem trivial, one isomer undergoes elimination with KOH much faster than the other.

Predict the products of these eliminations, and explain the large difference in the ease of elimination.

Fast elimination <IMAGE> slow elimination <IMAGE>

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.

b. meso-1,2-dibromo-1,2-diphenylethane + (CH3CH2)3N:

Predict the products and mechanisms of the following reactions. When more than one product or mechanism is possible, explain which are most likely.

e. isobutyliodide+KOH in ethanol/water

f. isobutylchloride+AgNO3 in ethanol/water

The solvolysis of 2-bromo-3-methylbutane potentially can give several products, ­including both E1 and products from both the unrearranged carbocation and the rearranged ­carbocation.

Mechanisms 6-6 (page 282) <IMAGE> and 7-2 (previous page) <IMAGE> show the products from the rearranged carbocation.

Summarize all the possible products, showing which carbocation they come from and whether they are the products of E1 or reactions.

For each reaction, decide whether substitution or elimination (or both) is possible, and predict the products you expect. Label the major products.

b. 1-bromo-1-methylcyclohexane + triethylamine(Et3N:)

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

b. 1-iodo-1-phenylcyclopentane heated in ethanol

Predict the major and minor elimination products of the following proposed reactions (ignoring any possible substitutions for now).

In each case, explain whether you expect the mechanism of the elimination to be E1 or E2.

b. <IMAGE> AgNO3, CH3OH heat——>

Predict the products and mechanisms of the following reactions. When more than one product or mechanism is possible, explain which are most likely.

a. 1−bromohexane+sodium ethoxide in ethanol

Predict the products and mechanisms of the following reactions. When more than one product or mechanism is possible, explain which are most likely.

b. 2−chlorohexane+NaOCH3 in methanol

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.

Give mechanisms to account for these products.

3 bromobutan-2-ol <IMAGE> <IMAGE>

trans-2-bromobut-2-ene <IMAGE>

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.

c. (d,l)-1,2-dibromo-1,2-diphenylethane + (CH3CH2)3N:

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.

d. <IMAGE> + NaOH in acetone

Propose mechanisms for the following reactions. Additional products may be formed, but your mechanism only needs to explain the products shown.

(c) <IMAGE> BrOH, heat—> <IMAGE> + <IMAGE> + <IMAGE>

Draw the products of each of the following SN2/E2 reactions. If the products can exist as stereoisomers, show which stereoisomers are formed.

b. (3R,4R)-3-bromo-4-methylhexane+CH3O−

Which of the following compounds would react faster in an

<IMAGE>

<IMAGE>

a. E1 reaction?

b. E2 reaction?