Organic Chemistry

John McMurry

Chapter 7

Alkenes: Structure and Reactivity - all with Video Answers

Educators

Chapter Questions

Problem 1

Calculate the degree of unsaturation in each of the following formulas, and then draw as many structures as you can for each:
(a) $\mathrm{C}_{4} \mathrm{H}_{8}$
(b) $\mathrm{C}_{4} \mathrm{H}_{6}$
(c) $\mathrm{C}_{3} \mathrm{H}_{4}$

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Problem 2

Calculate the degree of unsaturation in each of the following formulas:
(a) $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{N}$
(b) $\mathrm{C}_{6 \mathrm{H}_{5} \mathrm{NO}_{2}}$
(c) $\mathrm{C}_{8} \mathrm{H}_{9} \mathrm{Cl}_{3}$
(d) $\mathrm{C}_{9} \mathrm{H}_{16} \mathrm{Br}_{2}$
$\mathrm{C}_{10} \mathrm{H}_{12} \mathrm{N}_{2} \mathrm{O}_{3}$
(f) $\mathrm{C}_{20} \mathrm{H}_{32} \mathrm{CIN}$

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Problem 3

Diazepam, marketed as an antianxiety medication under the name Valium, has three rings, eight double bonds, and the formula $\mathrm{C}_{16} \mathrm{H}_{7} \mathrm{CiN}_{2} \mathrm{O}$. How many hydrogens does diazepam have? (Calculate the answer, don't count hydrogens in the structure.)

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Problem 4

Give IUPAC names for the following compounds:

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Problem 5

Draw structures corresponding to the following IUPAC names:
(a) 2 -Methyl-1,5-hexadiene
(b) $3-$ Ethyl- 2,2 -dimethyl-3-heptene
(c) 2,3,3 -Trimethyl-1,4,6-octatriene
(d) $3,4-$ Diisopropy $1-2,5$ -dimethyl-3-hexene

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Problem 6

Name the following cycloalkenes:
A.
B.
C.

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Problem 7

Change the following old names to new, post- 1993 names, and draw the structure of each compound:
(a) 2,5,5 -Trimethyl-2-hexene
(b) 2,3 -Dimethyl-1,3-cyclohexadiene

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Problem 8

The sex attractant of the common housefly is an alkene named cis-9-tricosene. Draw its structure. (Tricosane is the straight-chain alkane $\mathrm{C}_{23} \mathrm{H}_{48}$ )

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Problem 9

Which of the following compounds can exist as pairs of cis-trans isomers? Draw each cis-trans pair, and indicate the geometry of each isomer.
(a) $\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}$
(b) $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CHCH}_{3}$
(c) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CHCH}_{3}$
(d) $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{C}\left(\mathrm{CH}_{3}\right) \mathrm{CH}_{2} \mathrm{CH}_{3}$
(e) CICH=CHCI
(f) $\mathrm{BrCH}=\mathrm{CHCl}$

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Problem 10

Name the following alkenes, including the cis or trans designation:

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Problem 11

Which member in each of the following sets ranks higher?
(a) $-\mathrm{H}$ or $-\mathrm{CH}_{3}$
(b) $-\mathrm{Cl}$ or $-\mathrm{CH}_{2} \mathrm{Cl}$
(c) $-\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}$ or $-\mathrm{CH}=\mathrm{CH}_{2}$
(d) $-\mathrm{NHCH}_{3}$ or $-\mathrm{OCH}_{3}$
(e) $-\mathrm{CH}_{2} \mathrm{OH}$ or $-\mathrm{CH}=\mathrm{O}$
(f) $-\mathrm{CH}_{2} \mathrm{OCH}_{3}$ or $-\mathrm{CH}=\mathrm{O}$

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Problem 12

Rank the substituents in each of the following sets according to the sequence rules:
(a) $-\mathrm{CH}_{3},-\mathrm{OH}_{4},-\mathrm{H},-\mathrm{Cl}$
(b) $-\mathrm{CH}_{3},-\mathrm{CH}_{2} \mathrm{CH}_{3},-\mathrm{CH}=\mathrm{CH}_{2},-\mathrm{CH}_{2} \mathrm{OH}$
(c) $-\mathrm{CO}_{2} \mathrm{H},-\mathrm{CH}_{2} \mathrm{OH},-\mathrm{C}=\mathrm{N},-\mathrm{CH}_{2} \mathrm{NH}_{2}$
(d) $-\mathrm{CH}_{2} \mathrm{CH}_{3},-\mathrm{C}=\mathrm{CH},-\mathrm{C}=\mathrm{N},-\mathrm{CH}_{2} \mathrm{OCH}_{3}$

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Problem 13

Assign E or $Z$ configuration to the following alkenes:

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Problem 14

Assign stereochemistry (E or $Z$ ) to the double bond in the following compound, and convert the drawing into a skeletal structure (red = O):

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Problem 15

Name the following alkenes, and tell which compound in each pair is more stable:

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Problem 16

Predict the products of the following reactions:

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Problem 17

What alkenes would you start with to prepare the following products?

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Problem 18

Show the structures of the carbocation intermediates you would expect in the following reactions:

Ronald Prasad

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Problem 19

Draw a skeletal structure of the following carbocation. Identify it as primary, secondary, or tertiary, and identify the hydrogen atoms that have the proper orientation for hyperconjugation in the conformation shown.

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Problem 20

What about the second step in the electrophillic addition of HCI to an alkene- the reaction of chloride ion with the carbocation intermediate? Is this step exergonic or endergonic? Does the transition state for this second step resemble the reactant (carbocation) or product (alkyl chloride)? Make a rough drawing of what the transition-state structure might look like.

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Problem 21

On treatment with HBr, vinylcyclohexane undergoes addition and rearrangement to yield 1-bromo-l-ethylcyclohexane. Using curved arrows, propose a mechanism to account for this result.

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Problem 22

Name the following alkenes, and convert each drawing into a skeletal structure:

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Problem 23

Assign $E$ or $Z$ stereochemistry to the double bonds in each of the following alkenes, and convert each drawing into a skeletal structure (red $=\mathbf{O},$ green $=\mathbf{C} \mathbf{l}$ ):

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Problem 24

The following carbocation is an intermediate in the electrophilic addition reaction of HCl with two different alkenes. Identify both, and tell which C-H bonds in the carbocation are aligned for hyperconjugation with the vacant $p$ orbital on the positively charged carbon.

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Problem 25

The following alkyl bromide can be made by HBr addition to three different alkenes. Show their structures.

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Problem 26

Calculate the degree of unsaturation in the following formulas, and draw five possible structures for each:
(a) $\mathrm{C}_{10} \mathrm{H}_{16}$
(b) $\mathrm{CgH}_{8} \mathrm{O}$
(c) $\mathrm{C}_{7} \mathrm{H}_{10} \mathrm{Cl}_{2}$
(d) $\mathrm{C}_{10} \mathrm{H}_{16} \mathrm{O}_{2}$
(e) $\mathrm{C}_{5} \mathrm{H}_{9} \mathrm{NO}_{2}$
(f) $\mathrm{C}_{8} \mathrm{H}_{10} \mathrm{CINO}$

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Problem 27

How many hydrogens does each of the following compounds have?
(a) $\mathrm{C}_{8} \mathrm{H}_{7} \mathrm{O}_{2},$ has two rings and one double bond
(b) $\mathrm{C}_{7} \mathrm{H}_{7} \mathrm{N},$ has two double bonds
(c) $\mathrm{C}_{9} \mathrm{H}_{7} \mathrm{NO},$ has one ring and three double bonds

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Problem 28

Loratadine, marketed as an antiallergy medication under the name Claritin, has four rings, eight double bonds, and the formula $\mathrm{C}_{22} \mathrm{H}_{2} \mathrm{ClN}_{2} \mathrm{O}_{2}$. How many hydrogens does loratadine have? (Calculate your answer; don't count hydrogens in the structure.)

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Problem 29

Name the following alkenes:

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Problem 30

Draw structures corresponding to the following systematic names:
(a) $(4 E)-2,4-$ Dimethyl-1, 4 -hexadiene
(b) cis-3,3-Dimethyl-4-propyl-1,5-octadiene
(c) 4-Methyl-1,2-pentadiene
(d) $(3 E, 5 Z)-2,6$ -Dimethyl- 1,3,5,7 -octatetraene
(e) 3-Butyl-2-heptene
(f) trans-2,2,5,5 -Tetramethyl-3-hexene

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Problem 31

Name the following cycloalkenes:

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Problem 32

Ocimene is a triene found in the essential oils of many plants. What is its IUPAC name, including stereochemistry?

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Problem 33

$\alpha$ -Farnesene is a constituent of the natural wax found on apples. What is its IUPAC name, including stereochemistry?

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Problem 34

Menthene, a hydrocarbon found in mint plants, has the systematic name 1-isopropyl-4-methylcyclohexene. Draw its structure.

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Problem 35

Draw and name the six alkene isomers, $\mathrm{C}_{5} \mathrm{H}_{10},$ including $E, Z$ isomers.

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Problem 36

Draw and name the 17 alkene isomers, $C_{6} H_{12}$, including $E, Z$ isomers.

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Problem 37

Rank the following sets of substituents according to the Cahn-Ingold-Prelog sequence rules:
(a) $-\mathrm{CH}_{3},-\mathrm{Br},-\mathrm{H},-\mathrm{I}$
(b) $-\mathrm{OH},-\mathrm{OCH}_{3},-\mathrm{H},-\mathrm{CO}_{2} \mathrm{H}$
(c) $-\mathrm{CO}_{2} \mathrm{H},-\mathrm{CO}_{2} \mathrm{CH}_{3},-\mathrm{CH}_{2} \mathrm{OH},-\mathrm{CH}_{3}$
(d) $-\mathrm{CH}_{3},-\mathrm{CH}_{2} \mathrm{CH}_{3},-\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH},-\mathrm{CCH}_{3}$
(e) $-\mathrm{CH}=\mathrm{CH}_{2},-\mathrm{CN},-\mathrm{CH}_{2} \mathrm{NH}_{2},-\mathrm{CH}_{2} \mathrm{Br}$
(f) $-\mathrm{CH}=\mathrm{CH}_{2},-\mathrm{CH}_{2} \mathrm{CH}_{3},-\mathrm{CH}_{2} \mathrm{OCH}_{3},-\mathrm{CH}_{2} \mathrm{OH}$

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Problem 38

Assign $E$ or $Z$ configuration to each of the following compounds:

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Problem 39

Which of the following $E, Z$ designations are correct, and which are incorrect?

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Problem 40

trans-2-Butene is more stable than cis-2-butene by only 4 kJ/mol, but trans-2,2,5,5-tetramethyl-3-hexene is more stable than its cis isomer by 39 kJ/ mol. Explain.

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Problem 41

Cyclodecene can exist in both cis and trans forms, but cyclohexene cannot. Explain. (Making molecular models is helpful.)

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Problem 42

Normally, a trans alkene is more stable than its cis isomer. trans-Cyclooctene, however, is less stable than cis-cyclooctene by $38.5 \mathrm{kJ} / \mathrm{mol}$. Explain.

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Problem 43

trans-Cyclooctene is less stable than cis-cyclooctene by $38.5 \mathrm{kJ} / \mathrm{mol}$, but transcyclononene is less stable than cis-cyclononene by only $12.2 \mathrm{kJ} / \mathrm{mol}$. Explain.

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Problem 44

Tamoxifen, a drug used in the treatment of breast cancer, and clomiphene, a drug used as a fertility treatment, have similar structures but very different effects. Assign $E$ or $Z$ configuration to the double bonds in both compounds.

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Problem 45

Predict the major product in each of the following reactions:

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Problem 46

Predict the major product from addition of HBr to each of the following alkenes:

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Problem 47

Alkenes can be converted into alcohols by acid-catalyzed addition of water. Assuming that Markovnikov's rule is valid, predict the major alcohol product from each of the following alkenes.

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Problem 48

Each of the following carbocations can rearrange to a more stable ion. Propose structures for the likely rearrangement products.
(a) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2}^{+} \quad$ (b) $\mathrm{CH}_{3} \mathrm{CH}^{+} \mathrm{CHCH}_{3}$
$$
\mathrm{CH}_{3}
$$

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Problem 49

Addition of HCl to 1-isopropylcyclohexene yields a rearranged product. Propose a mechanism, showing the structures of the intermediates and using curved arrows to indicate electron flow in each step.

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Problem 50

Allene $(1,2-\text { propadiene }), \quad H_{2} C=C=C H_{2}$ has two adjacent double bonds. What kind of hybridization must the central carbon have? Sketch the bonding $\pi$ orbitals in allene. What shape do you predict for allene?

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Problem 51

The heat of hydrogenation for allene (Problem 7.50 ) to yield propane is $-295 \mathrm{kJ} / \mathrm{mol},$ and the heat of hydrogenation for a typical monosubstituted alkene such as propene is $-126 \mathrm{kJ} / \mathrm{mol}$. Is allene more stable or less stable than you might expect for a diene? Explain.

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Problem 52

Retin $\mathrm{A},$ or retinoic acid, is a medication commonly used to reduce wrinkles and treat severe acne. How many different isomers arising from double-bond isomerizations are possible?

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Problem 53

Fucoserratene and ectocarpene are sex pheromones produced by marine brown algae. What are their systematic names? (Ectocarpene is a bit difficult; make your best guess, and then check your answer in the Study Guide and Solutions Mamual.)

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Problem 54

tert-Butyl esters $\left[\mathrm{RCO}_{2} \mathrm{C}\left(\mathrm{CH}_{3}\right)_{3}\right]$ are converted into carboxylic acids $\left(\mathrm{RCO}_{2} \mathrm{H}\right)$ by reaction with trifluoroacetic acid, a reaction useful in protein synthesis (Section $26.7)$. Assign $E, Z$ designation to the double bonds of both reactant and product in the following scheme, and explain why there is an apparent change of double-bond stereochemistry:

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Problem 55

Addition of HC to 1-isopropenyl-1-methylcyclopentane yields 1-chloro-1,2,2trimethylcyclohexane. Propose a mechanism, showing the structures of the intermediates and using curved arrows to indicate electron flow in each step.

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Problem 56

Vinylcyclopropane reacts with HBr to yield a rearranged alkyl bromide. Follow the flow of electrons as represented by the curved arrows, show the structure of the carbocation intermediate in brackets, and show the structure of the final product.

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Problem 57

Calculate the degree of unsaturation in each of the following formulas:
(a) Cholesterol, $\mathrm{C}_{27} \mathrm{H}_{46} \mathrm{O}$
(b) $\mathrm{DDT}, \mathrm{C}_{14} \mathrm{H}_{9} \mathrm{Cl}_{5}$
(c) Prostaglandin $\mathrm{E}_{1}, \mathrm{C}_{20} \mathrm{H}_{34} \mathrm{O}_{5}$
(d) Caffeine, $\mathrm{C}_{8} \mathrm{H}_{10} \mathrm{N}_{4} \mathrm{O}_{2}$
(e) Cortisone, $\mathrm{C}_{21} \mathrm{H}_{28} \mathrm{O}_{5}$
(f) Atropine, $C_{17} \mathrm{H}_{23} \mathrm{NO}_{3}$

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Problem 58

The isobutyl cation spontaneously rearranges to the tert-butyl cation by a hydride shift. Is the rearrangement exergonic or endergonic? Draw what you think the transition state for the hydride shift might look like according to the Hammond postulate.

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Problem 59

Draw an energy diagram for the addition of HBr to 1-pentene. Let one curve on your diagram show the formation of 1-bromopentane product and another curve on the same diagram show the formation of 2-bromopentane product. Label the positions for all reactants, intermediates, and products. Which curve has the higher-energy carbocation intermediate? Which curve has the higherenergy first transition state?

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Problem 60

Make sketches of the transition-state structures involved in the reaction of HBr with 1-pentene (Problem 7.59). Tell whether each structure resembles reactant or product.

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Problem 61

Limonene, a fragrant hydrocarbon found in lemons and oranges, is biosynthesized from geranyl diphosphate by the following pathway. Add curved arrows to show the mechanism of each step. Which step involves an alkene

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Problem 62

cpi-Aristolochene, a hydrocarbon found in both pepper and tobacco, is biosynthesized by the following pathway. Add curved arrows to show the mechanism of each step. Which steps involve alkene electrophilic addition(s), and which involve carbocation rearrangement(s)? (The abbreviation H-A stands for an unspecified acid, and "Base" is an unspecified base in the enzyme.)

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Problem 63

Aromatic compounds such as benzene react with alkyl chlorides in the presence of AlCla catalyst to yield alkylbenzenes. The reaction occurs through a carbocation intermediate, formed by reaction of the alkyl chloride with $\mathrm{AlCl}_{3}(\mathrm{R}-\mathrm{Cl}+$

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Problem 64

Reaction of 2,3 -dimethyl-1-butene with HBr leads to an alkyl bromide, $\mathrm{C}_{6} \mathrm{H}_{13} \mathrm{Br}$. On treatment of this alkyl bromide with KOH in methanol, elimination of HBr occurs and a hydrocarbon that is isomeric with the starting alkene is formed. What is the structure of this hydrocarbon, and how do you think it is formed from the alkyl bromide?

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