Content check — Organic Chemistry, Chapter 7: Alkenes: Structure and Reactivity

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Chapter 7: Alkenes: Structure and Reactivity

21 problems · 39 figures

Calculate the degree of unsaturation in each of the following formulas, and then draw as many structures as you can for each:

(a)

C₄H₈

(b)

C₄H₆

(c)

C₃H₄

Answer (3 parts)

(a)

(b)

(c)

7-2 §7.2 source ↗

Calculate the degree of unsaturation in each of the following formulas:

(a)

C₆H₅N

(b)

C₆H₅NO₂

(c)

C₈H₉Cl₃

(d)

C₉H₁₆Br₂

(e)

C₁₀H₁₂N₂O₃

(f)

C₂₀H₃₂ClN

Answer (6 parts)

(a)

(b)

(c)

(d)

(e)

(f)

7-3 §7.2 source ↗

Diazepam, marketed as an antianxiety medication under the name Valium, has three rings, eight double bonds, and the formula C₁₆H_?ClN₂O. How many hydrogens does diazepam have? (Calculate the answer; don’t count hydrogens in the structure.)

Answer

C₁₆H₁₃ClN₂O

7-4 §7.3 source ↗

Give IUPAC names for the following compounds:

(a)

(b)

(c)

(d)

Answer (4 parts)

(a)

3,4,4-Trimethyl-1-pentene

(b)

3-Methyl-3-hexene

(c)

4,7-Dimethyl-2,5-octadiene

(d)

6-Ethyl-7-methyl-4-nonene

7-5 §7.3 source ↗

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-Diisopropyl-2,5-dimethyl-3-hexene

Answer (4 parts)

(a)

(b)

(c)

(d)

7-6 §7.3 source ↗

Name the following cycloalkenes:

(a)

(b)

(c)

Answer (3 parts)

(a)

1,2-Dimethylcyclohexene

(b)

4,4-Dimethylcycloheptene

(c)

3-Isopropylcyclopentene

7-7 §7.3 source ↗

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

Answer (2 parts)

(a)

2,5,5-Trimethylhex-2-ene

(b)

2,3-Dimethylcyclohexa-1,3-diene

7-8 §7.4 source ↗

The sex attractant of the common housefly is an alkene named cis-9-tricosene. Draw its structure. (Tricosane is the straight-chain alkane C₂₃H₄₈.)

Answer

7-9 §7.4 source ↗

Which of the following compounds can exist as pairs of cis–trans isomers? Draw each pair, and indicate the geometry of each isomer.

(a) CH₃CH = CH₂ (b) (CH₃)₂C = CHCH₃ (c) CH₃CH₂CH = CHCH₃ (d) (CH₃)₂C = C(CH₃)CH₂CH₃
(e) ClCH = CHCl (f) BrCH = CHCl

Answer

Compounds (c), (e), and (f) have cis–trans isomers.

7-10 §7.4 source ↗

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

(a)

(b)

Answer (2 parts)

(a)

cis-4,5-Dimethyl-2-hexene

(b)

trans-6-Methyl-3-heptene

7-11 §7.5 source ↗

Which member in each of the following sets ranks higher?

(a)

−H or −CH₃

(b)

−Cl or −CH₂Cl

(c)

–CH₂CH₂Br or –CH = CH₂

(d)

−NHCH₃ or −OCH₃

(e)

−CH₂OH or −CH = O

(f)

−CH₂OCH₃ or −CH = O

Answer (6 parts)

(a)

−CH₃

(b)

−Cl

(c)

−CH = CH₂

(d)

−OCH₃

(e)

−CH = O

(f)

−CH = O

7-12 §7.5 source ↗

Rank the substituents in each of the following sets according to the sequence rules:

(a)

−CH₃, −OH, −H, −Cl

(b)

−CH₃, −CH₂CH₃, −CH = CH₂, −CH₂OH

(c)

−CO₂H, −CH₂OH, −C ≡ N, −CH₂NH₂

(d)

−CH₂CH₃, −C ≡ CH, −C ≡ N, −CH₂OCH₃

Answer (4 parts)

(a)

−Cl, −OH, −CH₃, −H

(b)

−CH₂OH, −CH = CH₂, −CH₂CH₃, −CH₃

(c)

−CO₂H, −CH₂OH, −C ≡ N, −CH₂NH₂

(d)

−CH₂OCH₃, −C ≡ N, −C ≡ CH, −CH₂CH₃

7-13 §7.5 source ↗

Assign E or Z configuration to the following alkenes:

(a)

(b)

(c)

(d)

Answer (4 parts)

(a)

(b)

(c)

(d)

7-14 §7.5 source ↗

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

Answer

7-15 §7.6 source ↗

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

(a)

(b)

(c)

Answer (3 parts)

(a)

2-Methylpropene is more stable than 1-butene.

(b)

trans-2-Hexene is more stable than cis-2-hexene.

(c)

1-Methylcyclohexene is more stable than 3-methylcyclohexene.

7-16 §7.8 source ↗

Predict the products of the following reactions:

(a)

(b)

(c)

(d)

Answer (4 parts)

(a)

Chlorocyclohexane

(b)

2-Bromo-2-methylpentane

(c)

4-Methyl-2-pentanol

(d)

1-Bromo-1-methylcyclohexane

7-17 §7.8 source ↗

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

(a)

(b)

(c)

(d)

Answer (4 parts)

(a)

Cyclopentene

(b)

1-Ethylcyclohexene or ethylidenecyclohexane

(c)

3-Hexene

(d)

Vinylcyclohexane (cyclohexylethylene)

7-18 §7.9 source ↗

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

(a)

(b)

Answer (2 parts)

(a)

(b)

7-19 §7.9 source ↗

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.

Answer

In the conformation shown, only the methyl- group C−H that is parallel to the carbocation p orbital can show hyperconjugation.

7-20 §7.10 source ↗

What about the second step in the electrophilic addition of HCl 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.

Answer

The second step is exergonic; the transition state resembles the carbocation.

7-21 §7.11 source ↗

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

Answer

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Problems and answers are adapted from Organic Chemistry: A Tenth Edition by John McMurry, OpenStax, and are used under the CC BY-NC-SA 4.0 licence. Access for free at https://openstax.org/books/organic-chemistry/pages/1-why-this-chapter. Illustrations: Copyright Rice University, OpenStax, under CC BY-NC-SA 4.0 license. This adaptation is released under the same licence, is non-commercial, and is neither affiliated with nor endorsed by OpenStax or Rice University.