Organic Compounds: Cycloalkanes and Their stereochemistry Chapter 4

What is this Cycloalkane Molecule?

What is this Cycloalkane Molecule?

This is a Cyclopropane

What is this Cycloalkane Molecule?

This is a Cyclopropane

What is this Cycloalkane Molecule?

Cyclobutane

What is this Cycloalkane Molecule?

Cyclobutane

What is this Cycloalkane Molecule?

Cyclopentane

What is this Cycloalkane Molecule?

Cyclopentane

What is this Cycloalkane Molecule?

Cyclohexane

What is this Cycloalkane Molecule?

Cyclohexane

(4-1) Give IUPAC names for the following cycloalkanes

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

Name the following cycloalkane

Name the following substances, including the cis- or trans- prefix:

a) trans-1,3-Dimethylcyclopentane (b) cis-1,2-Dichlorocyclohexane

Problem 4-4

Name the following substances, including the cis- or trans- prefix:

Problem 4-5 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

Problem 4-6

Prostaglandin F2a, a hormone that causes uterine contraction during childbirths, has the following structure. Are the two hydroxyl groups (O OH) on the cyclopentane ring cis or trans to each other? What about the two carbon chains attached to the ring?

Problem 4-7

Name the following substances, including the cis- or trans- prefix (red-brown 5 Br):

Angle strain

the strain due to expansion or compression of bond angles

Torsional strain

the strain due to eclipsing of bonds between neighboring atoms

Steric strain

the strain due to repulsive interactions when atoms approach each other too closely.

Problem 4-8

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?

Problem 4-9

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?

What is Cyclopropanes Angle?

60 degrees

What is Cyclobutane’s Angle?

90 degrees

What is cyclopentanes angle?

108 degrees

What is Cyclohexane’s angle?

120 degrees

What is a cyclopropane?

Cyclopropane is the most strained of all rings, primarily because of the angle strain caused by its 60° C - C- C bond angles. In addition, cyclopropane has considerable torsional strain because the C -H bonds on neighboring carbon atoms are eclipsed.

What is a Cyclobutane?

Cyclobutene has less angle strain than cyclopropane but has more torsional strain because of its larger number of ring hydrogens. As a result, the total strain for the two compounds is nearly the same—110 kJ/mol (26.4 kcal/mol) for cyclobutene versus 115 kJ/mol (27.5 kcal/mol) for cyclopropane. Cyclobutene is not quite flat but is slightly bent so that one carbon atom lies about 25° above the plane of the other three.

What is a Cyclopentane?

nonplanar conformation that strikes a balance between increased angle strain and decreased torsional strain. Four of the cyclopentane carbon atoms are in approximately the same plane, with the fifth carbon atom bent out of the plane. Most of the hydrogens are nearly staggered with respect to their neighbors

Problem 4-10

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?

Problem 4-11

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?

Problem 4-12

Draw two different chair conformations of cyclohexanol (hydroxycyclohexane), showing

Problem 4-13

Draw two different chair conformations of trans-1,4-dimethylcyclohexane, and label all positions as axial or equatorial.

Problem 4-14

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.

Problem4-15

What is the energy difference between the axial and equatorial conformations of cyclo hexanol (hydroxycyclohexane)?

Problem 4-16

Why do you suppose an axial cyano (–CN) substituent causes practically no 1,3-diaxial steric strain (0.4 kJ/mol)? Use molecular models to help with your answer

Problem 4-17

Look at Figure 4-12 on page 105, and estimate the percentages of axial and equatorial conformations present at equilibrium in bromocyclohexane

Problem 4-18

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

Problem 4-19

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 5 Cl):