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Chapter 4: Organic Compounds: Cycloalkanes and Their Stereochemistry
21 problems · 29 figures4-1 §4.1 source ↗
Give IUPAC names for the following cycloalkanes:
(a)
(b)
(c)
(d)
(e)
(f)
Answer (6 parts)
(a)
1,4-Dimethylcyclohexane
(b)
1-Methyl-3-propylcyclopentane
(c)
3-Cyclobutylpentane
(d)
1-Bromo-4-ethylcyclodecane
(e)
1-Isopropyl-2-methylcyclohexane
(f)
4-Bromo-1-tert-butyl-2-methylcycloheptane
4-2 §4.1 source ↗
Draw structures corresponding to the following IUPAC names:
(a)
1,1-Dimethylcyclooctane
(b)
3-Cyclobutylhexane
(c)
1,2-Dichlorocyclopentane
(d)
1,3-Dibromo-5-methylcyclohexane
Answer (4 parts)
(a)
(b)
(c)
(d)
4-3 §4.1 source ↗
Name the following cycloalkane:
Answer
3-Ethyl-1,1-dimethylcyclopentane
4-4 §4.2 source ↗
Name the following substances, including the cis- or trans- prefix:
(a)
(b)
Answer (2 parts)
(a)
trans-1-Chloro-4-methylcyclohexane
(b)
cis-1-Ethyl-3-methylcycloheptane
4-5 §4.2 source ↗
Draw the structures of the following molecules:
(a)
trans-1-Bromo-3-methylcyclohexane
(b)
cis-1,2-Dimethylcyclobutane
(c)
trans-1-tert-Butyl-2-ethylcyclohexane
Answer (3 parts)
(a)
(b)
(c)
4-6 §4.2 source ↗
Prostaglandin F2α, a hormone that causes uterine contraction during childbirth, has the following structure. Are the two hydroxyl groups (–OH) on the cyclopentane ring cis or trans to each other? What about the two carbon chains attached to the ring?
Answer
The two hydroxyl groups are cis. The two side chains are trans.
4-7 §4.2 source ↗
Name the following substances, including the cis- or trans- prefix (red-brown = Br):
(a)
(b)
Answer (2 parts)
(a)
cis-1,2-Dimethylcyclopentane
(b)
cis-1-Bromo-3-methylcyclobutane
4-8 §4.3 source ↗
Each H ⟷ H eclipsing interaction in ethane costs about 4.0 kJ/mol. How many such interactions are present in cyclopropane? What fraction of the overall 115 kJ/mol (27.5 kcal/mol) strain energy of cyclopropane is due to torsional strain?
Answer
Six interactions; 21% of strain
4-9 §4.3 source ↗
cis-1,2-Dimethylcyclopropane has more strain than trans-1,2-dimethylcyclopropane. How can you account for this difference? Which of the two compounds is more stable?
Answer
The cis isomer is less stable because the methyl groups nearly eclipse each other.
4-10 §4.4 source ↗
How many H ⟷ H eclipsing interactions would be present if cyclopentane were planar? Assuming an energy cost of 4.0 kJ/mol for each eclipsing interaction, how much torsional strain would planar cyclopentane have? Since the measured total strain of cyclopentane is 26 kJ/mol, how much of the torsional strain is relieved by puckering?
Answer
Ten eclipsing interactions; 40 kJ/mol; 35% is relieved.
4-11 §4.4 source ↗
Two conformations of cis-1,3-dimethylcyclobutane are shown. What is the difference between them, and which do you think is likely to be more stable?
(a)
(b)
Answer
Conformation (a) is more stable because the methyl groups are farther apart.
4-12 §4.6 source ↗
Draw two different chair conformations of cyclohexanol (hydroxycyclohexane), showing all hydrogen atoms. Identify each position as axial or equatorial.
Answer
4-13 §4.6 source ↗
Draw two different chair conformations of trans-1,4-dimethylcyclohexane, and label all positions as axial or equatorial.
Answer
4-14 §4.6 source ↗
Identify each of the colored positions—red, blue, and green—as axial or equatorial. Then carry out a ring-flip, and show the new positions occupied by each color.
Answer
Before the ring-flip, red and blue are equatorial and green is axial. After the ring-flip, red and blue are axial and green is equatorial.
4-15 §4.7 source ↗
What is the energy difference between the axial and equatorial conformations of cyclohexanol (hydroxycyclohexane)?
Answer
4.2 kJ/mol
4-16 §4.7 source ↗
Why do you suppose an axial cyano (–CN) substituent causes practically no 1,3-diaxial steric strain (0.4 kJ/mol)?
Answer
Cyano group points straight up.
4-17 §4.7 source ↗
Look back at Figure 4.13 and estimate the percentages of axial and equatorial conformations present at equilibrium in bromocyclohexane.
A plot of the percentages of two isomers at equilibrium versus the energy difference between them. The curves are calculated using the equation ΔE = −RT ln K.
Answer
Equatorial = 70%; axial = 30%
4-18 §4.8 source ↗
Draw the more stable chair conformation of the following molecules, and estimate the amount of strain in each:
(a)
trans-1-Chloro-3-methylcyclohexane
(b)
cis-1-Ethyl-2-methylcyclohexane
(c)
cis-1-Bromo-4-ethylcyclohexane
(d)
cis-1-tert-Butyl-4-ethylcyclohexane
Answer (4 parts)
(a)
2.0 kJ/mol (axial Cl)
(b)
11.4 kJ/mol (axial CH3)
(c)
2.0 kJ/mol (axial Br)
(d)
8.0 kJ/mol (axial CH2CH3)
4-19 §4.8 source ↗
Identify each substituent in the following compound as axial or equatorial, and tell whether the conformation shown is the more stable or less stable chair form (green = Cl):
Answer
4-20 §4.9 source ↗
Which isomer is more stable, cis-decalin or trans-decalin (Figure 4.18)? Explain.
Representations of cis- and trans-decalin. Hydrogen atoms at the bridgehead carbons are on the same face of the rings in the cis isomer but on opposite faces in the trans isomer.
Answer
trans-Decalin is more stable because it has no 1,3-diaxial interactions.
4-21 §4.9 source ↗
Look at the following structure of estrone, the primary sex hormone in females, and tell whether each of the two indicated (red) ring-fusions is cis or trans.
Answer
Both ring-fusions are trans.
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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.