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Organic Chemistry

Janice Gorzynski Smith

Chapter 8

Alkyl Halides and Elimination Reactions

Educators


Problem 1

Label the $\alpha$ and $\beta$ carbons in each alkyl halide. Draw all possible elimination products formed when each alkyl halide is treated with $K^{+ -}OC(CH_3)_3$,

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Problem 2

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

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Problem 3

For which alkenes are stereoisomers possible?

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Problem 4

(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 .

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Problem 5

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

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Problem 6

Which alkene in each pair is more stable?

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Problem 7

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.

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Problem 8

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

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Problem 9

Consider an $E2$ reaction between $CH_3CH_2Br$ and $KOC(CH_3)_3$, 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 $CH_3CH_2CI$.

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Problem 10

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

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Problem 11

How does each of the following changes affect the rate of an $E2$ reaction?
a. tripling [RX]
b.halving [B:]
c. changing the solvent from $CH_3OH$ to $DMSO$
d. changing the leaving group from $I^-$ to $Br^-$
e. changing the base from $^-OH$ to $H_2O$
f. changing the alkyl halide from $CH_3CH_2Br$ to $(CH_3)_2CHBr$

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Problem 12

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

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Problem 13

Draw an $E1$ mechanism for the following reaction. Draw the structure of the transition state for each
step. $$(CH_3)_2C(CI)CH_2CH_3+CH_3OH \longrightarrow(CH_3)_2C=CHCH_3 + CH_3OH_2 + CI^-$$

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Problem 14

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

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Problem 15

How does each of the following changes affect the rate of an $E1$ reaction?
a. doubling [RX]
b.doubling [8:]
c. changing the halide from $(CH_3)_3CBr$ to $CH_3CH_2CH_2Br$
d. changing the leaving group from $CI^-$ to $Br^-$
e. changing the solvent from $DMSO$ to $CH_3OH$

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Problem 16

Draw both the S$_N$1 and E1 products of each reaction.

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Problem 17

Draw the anti periplanar geometry for the $E2$ reaction of $(CH_3)_2CHCH_2Br$ with base. Then draw the product that results after elimination of $HBr$.

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Problem 18

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 $C_6H_5 -$ is a phenyl group, a benzene ring bonded to another group.

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Problem 19

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

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Problem 20

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

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Problem 21

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

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Problem 22

Draw the alkynes formed in each reaction. Two equivalents of each base are used.

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Problem 23

Draw the products in each reaction.

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Problem 24

Draw a stepwise mechanism for the following reaction.

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Problem 25

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

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Problem 26

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

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Problem 27

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

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Problem 28

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

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Problem 29

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

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Problem 30

Which double bonds in the following natural products can exhibit stereoisomerism? Nerolidol is isolated from the Angel's trumpet plant, and caryophyllene is present in hemp.

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Problem 31

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

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Problem 32

$PGF_{2\alpha}$ is a prostaglandin, a group of compounds that are responsible for inflammation (Section 19.6). (a) How many tetrahedral stereogenic centers does $PGF_{2\alpha}$ contain? (b) How many double bonds can exist as cis and trans isomers? (c) Considering both double bonds and tetrahedral stereogenic centers, what is the maximum number of stereoisomers that can exist for $PGF_{2\alpha}$?

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Problem 33

Rank the following alkenes in order of increasing stability: $CH_2=C(CH_3)CH_2CH_3, CH_2=CHCH(CH_3)_2$, and $(CH_3)_2C=CHCH_3$.

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Problem 34

$\Delta H^\circ$ 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 $cis$-2-butene is more stable than 1-butene (Section 12.3A).

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Problem 35

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

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Problem 36

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.

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Problem 37

Explain why $(CH_3)_2CHCH(Br)CH_2CH_3$ reacts faster than $(CH_3)_2CHCH_2CH(Br)CH_3$ in an E2 reaction, even though both alkyl halides are 2$^\circ$.

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Problem 38

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 $^-OC(CH_3)_3$ is decreased. [3] The base is changed to $^-OH$. [4] The halide is changed to $CH_3CH_2CH_2CH_2CH(Br)CH_3$. [5] The leaving group is changed to $I^-$.

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Problem 39

Dehydrohalogenation of 1-chloro-1-methylcyclopropane 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.

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Problem 40

What is the major stereoisomer formed in each reaction?

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Problem 41

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

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Problem 42

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

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Problem 43

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

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Problem 44

Which elimination reaction in each pair is faster?

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Problem 45

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

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Problem 46

Explain the following observation. Treatment of alkyl chloride $A$ with $NaOCH_2CH_3$ yields only one product $B$, whereas treatment of $A$ with very dilute base in $CH_3CH_2OH$ yields a mixture of alkenes $B$ and $C$, with $C$ predominating.

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Problem 47

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

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Problem 48

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

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Problem 49

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

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Problem 50

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?

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Problem 51

Draw the products of each reaction.

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Problem 52

Draw the structure of a dihalide that could be used to prepare each alkyne. There may be more than one possible dihalide.

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Problem 53

Under certain reaction conditions, 2,3-dibromobutane reacts with two equivalents of base to give three products, each of which contains two new $\pi$ bonds. Product $A$ has two $sp$ hybridized carbon atoms, product $B$ has one $sp$ hybridized carbon atom, and product $C$ has none. What are the structures of $A$, $B$, and $C$?

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Problem 54

For which reaction mechanisms$-S_N1, S_N2, E1$, or $E2-$are each of the following statements true? A statement may be true for one or more mechanisms.
a. The mechanism involves carbocation intermediates.
b. The mechanism has two steps.
c. The reaction rate increases with better leaving groups.
d. The reaction rate increases when the solvent is changed from $CH_3OH$ to $(CH_3)_2SO$.
e. The reaction rate depends on the concentration of the alkyl halide only.
f. The mechanism is concerted.
g. The reaction of $CH_3CH_2Br$ with NaOH occurs by this mechanism.
h. Racemization at a stereogenic center occurs.
i. Tertiary (3$^\circ$) alkyl halides react faster than 2$^\circ$ or 1 $^\circ$ alkyl halides.
j. The reaction follows a second-order rate equation.

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Problem 55

Draw the organic products formed in each reaction.

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Problem 56

What is the major product formed when each alkyl halide is treated with each of the following reagents: [1] NaOCOCH$_3$; [2] NaOCH$_3$; [3] KOC(CH$_3$)$_3$? If it is not possible to predict the major product, identify the products in the mixture and the mechanism by which each is formed.

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Problem 57

a. The cis and trans isomers of 1-bromo-4-$tert$-butylcyclohexane react at different rates with KOC(CH$_3$)$_3$ to afford the same mixture of enantiomers $A$ and $B$. Draw the structures of $A$ and $B$.
b. Which isomer reacts faster with KOC(CH$_3$)$_3$? Offer an explanation for this difference in reactivity.
c. Reaction of $cis$-1-bromo-4-$tert$-butylcyclohexane with NaOCH$_3$ affords $C$ as the major product, whereas reaction of $trans$-1-bromo-4-$tert$-butylcyclohexane affords $D$ as the major product. Draw the structures for $C$ and $D$.
d. The cis and trans isomers react at different rates with NaOCH$_3$. Which isomer reacts faster? Offer an explanation for the difference in reactivity.
e. Why are different products formed from these alkyl halides when two different alkoxides are used as reagents?

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Problem 58

Draw all products, including stereoisomers, in each reaction.

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Problem 59

Draw all of the substitution and elimination products formed from the following alkyl halide with each reagent. Indicate the stereochemistry around the stereogenic centers present in the products, as well as the mechanism by which each product is formed.

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Problem 60

The following reactions do not afford the major product that is given. Explain why this is so, and draw the structure of the major product actually formed

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Problem 61

Draw a stepwise, detailed mechanism for each reaction.

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Problem 62

Draw the major product formed when ($3R$)-1-chloro-3-methylpentane is treated with each reagent: (a) NaOCH$_2$CH$_3$; (b) KCN; (c) DBU.

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Problem 63

Draw a stepwise, detailed mechanism for the following reaction.

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Problem 64

Explain why the reaction of 2-bromopropane with NaOCOCH$_3$ gives (CH$_3$)$_2$CHOCOCH$_3$ exclusively as product, but the reaction of 2-bromopropane with NaOCH$_2$CH$_3$ gives a mixture of (CH$_3$)$_2$CHOCH$_2$CH$_3$ (20%) and CH$_3$CH = CH$_2$ (80%).

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Problem 65

Draw a stepwise detailed mechanism that illustrates how four organic products are formed in the following reaction.

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Problem 66

Although there are nine stereoisomers of 1,2,3,4,5,6-hexachlorocyclohexane, one stereoisomer reacts 7000 times more slowly than any of the others in an E2 elimination. Draw the structure of this isomer and explain why this is so.

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Problem 67

Explain the selectivity observed in the following reactions.

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Problem 68

Draw a stepwise mechanism for the following reaction. The four-membered ring in the starting material and product is called a $\beta$-lactam. This functional group confers biological activity on penicillin and many related antibiotics, as is discussed in Chapter 22. (Hint: The mechanism begins with $\beta$ elimination and involves only two steps.)

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Problem 69

Although dehydrohalogenation occurs with anti periplanar geometry, some eliminations have syn periplanar geometry. Examine the starting material and product of each elimination, and state whether the elimination occurs with syn or anti periplanar geometry.

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Problem 70

(a) Draw all products formed by treatment of each dibromide ($A$ and $B$) with one equivalent of NaNH$_2$. (b) Label pairs of diastereomers and constitutional isomers.

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