Alkyl Halides and Elimination Reactions

Label the $\mathbf{\text{α}}$ and $\mathbf{\text{β}}$ carbons in each alkyl halide. Draw all possible elimination products formed when each alkyl halide is treated with ${\mathbf{\text{K}}}^{\mathbf{+}\mathbf{-}}{\mathbf{\text{OC(CH}}}_{\mathbf{3}}{\mathbf{\text{)}}}_3$.

a.

b.

c.

Classify each alkene in the following vitamins by the number of carbon substituents bonded to the double bond.

a.

b.

For which double bonds are stereoisomers possible?

a.

b.

c.

(a) Which double bonds in (E)-ocimene, a major component of the odor of lilac flowers, can exhibit stereoisomerism? (b) Draw a diastereomer of (E)-ocimene

Label each pair of alkenes as constitutional isomers, stereoisomers, or identical.

a.

b.

c.

d.

Which alkene in each pair is more stable?

a.

b.

c.

Several factors can affect alkene stability. Explain why alkene A is more stable than alkene B even though both contain disubstituted carbon-carbon double bonds.

Use curved arrows to show the movement of electrons in the following E2 mechanism. Draw the structure of the transition state.

Consider an E2 reaction between ${\mathbf{\text{CH}}}_{\mathbf{3}}{\mathbf{\text{CH}}}_{\mathbf{2}}\mathbf{\text{Br}}$ and ${\mathbf{\text{KOC(CH}}}_{\mathbf{3}}{\mathbf{\text{)}}}_{\mathbf{3}}$. What effect does each of the following changes have on the rate of elimination? (a) The base is changed to KOH. (b) The alkyl halide is changed to ${\mathbf{\text{CH}}}_{\mathbf{3}}{\mathbf{\text{CH}}}_{\mathbf{2}}\mathbf{\text{Cl}}$.

Rank the alkyl halides in each group in order of increasing reactivity in an E2 reaction.

a.

b.

How does each of the following changes affect the rate of an E2 reaction?

a. tripling $\mathbf{\text{[RX]}}$

b. halving $\mathbf{\text{[B:]}}$

c. changing the solvent from ${\mathbf{\text{CH}}}_{\mathbf{3}}\mathbf{\text{OH}}$ to DMSO 

d. changing the leaving group from $\mathbf{\text{I–}}$to  $\mathbf{\text{Br–}}$

e. changing the base from ${}^{\mathbf{-}}\mathbf{\text{OH}}$ to ${\mathbf{\text{H}}}_{\mathbf{2}}\mathbf{\text{O}}$

f. changing the alkyl halide from ${\mathbf{\text{CH}}}_{\mathbf{\text{3}}}{\mathbf{\text{CH}}}_{\mathbf{\text{2}}}\mathbf{\text{Br}}$ to ${\mathbf{\text{(CH}}}_{\mathbf{\text{3}}}{\mathbf{\text{)}}}_{\mathbf{\text{2}}}\mathbf{\text{CHBr}}$

What alkenes are formed from each alkyl halide by an E2 reaction? Use the Zaitsev rule to predict the major product.

a.

b.

c.

d.

Draw an E1 mechanism for the following reaction. Draw the structure of the transition state for each step.

What alkenes are formed from each alkyl halide by an E1 reaction? Use the Zaitsev rule to predict the major product.

a.

b.

How does each of the following changes affect the rate of an E1 reaction?

a. doubling $\mathbf{\text{[RX]}}$ 

b. doubling $\mathbf{\text{[B:]}}$ 

c. changing the halide from ${\mathbf{\text{(CH}}}_{\mathbf{\text{3}}}{\mathbf{\text{)}}}_{\mathbf{\text{3}}}\mathbf{\text{CBr}}$ to ${\mathbf{\text{CH}}}_{\mathbf{\text{3}}}{\mathbf{\text{CH}}}_{\mathbf{\text{2}}}{\mathbf{\text{CH}}}_{\mathbf{\text{2}}}\mathbf{\text{Br}}$

d. changing the leaving group from ${\mathbf{\text{Cl}}}^{\mathbf{\text{– }}}{\mathbf{\text{to Br}}}^{\mathbf{\text{–}}}$

e. changing the solvent from DMSO to ${\mathbf{\text{CH}}}_{\mathbf{\text{3}}}\mathbf{\text{OH}}$

Draw both the ${\mathbf{\text{S}}}_{\mathbf{\text{N}}}\mathbf{\text{1}}$ and E1 products of each reaction.

a.

b.

Given that an E2 reaction proceeds with anti periplanar stereochemistry, draw the products of each elimination. The alkyl halides in (a) and (b) are diastereomers of each other. How are the products of these two reactions related? Recall from Section 3.2A that ${\mathbf{\text{-C}}}_{\mathbf{6}}{\mathbf{\text{H}}}_{\mathbf{5}}$ is a phenyl group, a benzene ring bonded to another group.

a.

b.

Draw the major E2 elimination products from each of the following alkyl halides.

a.

b.

Explain why cis-1-chloro-2-methylcyclohexane undergoes E2 elimination much faster than its trans isomer.

Which mechanism, E1 or E2, will occur in each reaction?

a.

b.

c.

d.

Draw the alkynes formed when each dihalide is treated with excess base.

a.

b.

c.

d.

Draw the products in each reaction.

a.

b.

c.

d.

Draw a stepwise mechanism for the following reaction.

Rank the alkenes shown in the ball-and-stick models (A–C) in order of increasing stability.

Name each compound and decide which stereoisomer will react faster in an E2 elimination reaction. Explain your choice.

What is the major E2 elimination product formed from each alkyl halide?

a.

b.

Draw all possible constitutional isomers formed by dehydrohalogenation of each alkyl halide.

a.

b.

c.

d.

What alkyl halide forms each of the following alkenes as the only product in an elimination reaction?

a.

b.

c.

d.

Which double bonds in the following natural products can exhibit stereoisomerism? Nerolidol is isolated from the angel’s trumpet plant, caryophyllene is present in hemp, and humulene comes from hops.

a.

b.

c.

Label each pair of alkenes as constitutional isomers, stereoisomers, or identical.

a.

b.

c.

d.

Rank the following alkenes in order of increasing stability:

$\mathbf{\text{∆H°}}$values obtained for a series of similar reactions are one set of experimental data used to determine the relative stability of alkenes. Explain how the following data suggest that cis-but-2-ene is more stable than but-1-ene (Section 12.3A).

Draw all constitutional isomers formed in each E2 reaction and predict the major product using the Zaitsev rule.

a.

b.

c.

d.

For each of the following alkenes, draw the structure of two different alkyl halides that yield the given alkene as the only product of dehydrohalogenation.

a.

b.

c.

Consider the following E2 reaction.

a. Draw the by-products of the reaction and use curved arrows to show the movement of

electrons.

b. What happens to the reaction rate with each of the following changes? [1] The solvent is changed to DMF. [2] The concentration of ${\mathbf{\text{-OC(CH}}}_{\mathbf{\text{3}}}{\mathbf{\text{)}}}_{\mathbf{\text{3}}}$ is decreased. [3] The base is changed to ${}^{\mathbf{-}}\mathbf{\text{OH}}$. [4] The halide is changed to ${\mathbf{\text{CH}}}_{\mathbf{\text{3}}}{\mathbf{\text{CH}}}_{\mathbf{\text{2}}}{\mathbf{\text{CH}}}_{\mathbf{\text{2}}}{\mathbf{\text{CH}}}_{\mathbf{\text{2}}}{\mathbf{\text{CH(Br)CH}}}_{\mathbf{\text{3}}}$ [5] The leaving group is changed to ${\mathbf{\text{I}}}^{\mathbf{\text{–}}}$.

Dehydrohalogenation of 1-chloro-1-methyl cyclopropane affords two alkenes (A and B) as products. Explain why A is the major product despite the fact that it contains the less substituted double bond.

What is the major stereoisomer formed when each alkyl halide is treated with ${\mathbf{\text{KOC(CH}}}_{\mathbf{\text{3}}}{\mathbf{\text{)}}}_{\mathbf{\text{3}}}$?

a.

b.

What alkene is the major product formed from each alkyl halide in an E1 reaction?

a.

b.

c.

Draw all constitutional isomers formed in each elimination reaction. Label the mechanism as E2 or E1.

a.

b.

c.

d.

e.

f.

Rank the alkyl halides in order of increasing E2 reactivity. Then do the same for E1 reactivity

Pick the reactant or solvent in each part that gives the faster elimination reaction.

a. reaction of ${}^{\mathbf{-}}\mathbf{\text{OH}}$ with 1-chloro-1-methylcyclohexane or 1-chloro-3-methylcyclohexane

b. reaction of ${\mathbf{\text{H}}}_{\mathbf{2}}\mathbf{\text{O}}$ with ${\mathbf{\text{CH}}}_{\mathbf{\text{3}}}{\mathbf{\text{CH(Cl)CH}}}_{\mathbf{\text{2}}}{\mathbf{\text{CH}}}_{\mathbf{\text{3}}}$or ${\mathbf{\text{(CH}}}_{\mathbf{\text{3}}}{\mathbf{\text{)}}}_{\mathbf{\text{2}}}{\mathbf{\text{C(Cl)CH}}}_{\mathbf{\text{2}}}{\mathbf{\text{CH}}}_{\mathbf{\text{3}}}$

c. reaction of ${\mathbf{\text{(CH}}}_{\mathbf{\text{3}}}{\mathbf{\text{)}}}_{\mathbf{\text{3}}}\mathbf{\text{CCl}}$ with ${}^{\mathbf{-}}\mathbf{\text{OH}}$ in ${\mathbf{\text{H}}}_{\mathbf{2}}\mathbf{\text{O}}$ or DMSO

In the dehydrohalogenation of bromocyclodecane, the major product is cis-cyclodecene rather than trans-cyclodecene. Offer an explanation.

Elimination of HBr from 2-bromobutane affords a mixture of but-1-ene and but-2-ene. With sodium ethoxide as base, but-2-ene constitutes 81% of the alkene products, but with potassium tert-butoxide, but-2-ene constitutes only 67% of the alkene products. Offer an explanation for this difference.

What is the major E2 elimination product formed from each halide?

a.

b.

c.

Taking into account anti periplanar geometry, predict the major E2 product formed from each starting material.

a.

b.

Does cis- or trans-1-bromo-4-tert-butylcylohexane react faster in an E2 reaction?

a. Draw three-dimensional representations for all stereoisomers of 2-chloro-3-methylpentane, and label pairs of enantiomers.

b. Considering dehydrohalogenation across C2 and C3 only, draw the E2 product that results from each of these alkyl halides. How many different products have you drawn?

c. How are these products related to each other?

Explain why compound A does not undergo an E2 elimination with a strong base.

What alkyl chloride affords the following alkene exclusively under E2 reaction conditions?

Draw the products formed when each dihalide is treated with excess ${\mathbf{\text{NaNH}}}_{\mathbf{2}}$.

a.

b.

c.

d.