Chapter 4: Cycloalkanes, Strain, and Chairs

Organic Chemistry

How does one ring affect the hydrogen count of a saturated hydrogen?

It has two fewer hydrogens than the corresponding open-chain alkane C_nH_2n for a single saturated ring.

When is a ring normally chosen as the parent rather than an attached chain?

When the ring has at least as many carbons as the attached chain. Then use a cycloalkane parent name.

What does cis mean for substituents on a cycloalkane?

Same face of the ring

What does trans mean for substituents on a cycloalkane?

Opposite faces of the ring.

What two main kinds of strain are especially important in small rings?

Angle strain and torsional strain.

What is angle strain?

Distorted bond angles — when the bond angles in a molecule deviate from their ideal values, resulting in increased energy and destabilization.

What is torsional strain?

Eclipsing bonds — strain that arises from eclipsing interactions between adjacent atoms or groups in a molecule, causing increased energy and destabilization.

What is steric strain?

Repulsion between non-bonded atoms or bulky groups that are forced too close together in physical space, resulting in increased energy and destabilization.

Why is cyclopropane relatively strained?

Its bond angles depart greatly from the preferred tetrahedral angle, and its bonds are substantially eclipsed.

Why is a cyclohexane chair especially stable?

It minimizes both torsional and steric strain by allowing all bonds to be staggered and placing bulky groups in equatorial positions, resulting in a more stable structure.

What are axial bonds on a cyclohexane chair?

Bonds pointing approximately vertically, alternating up and down around the ring.

What are equatorial bonds on a cyclohexane chair?

Bonds pointing roughly outward around the ring. At each carbon, an equatorial bond points opposite the axial bond’s up/down direction.

What changes during a chair flip?

Each substituent’s up/down direction, the compound’s cis/trans relationship. and its stom connectivity.

Why is an equatorial substituent usually favored over an axial one?

An axial substituent has unfavorable 1,3-diaxial interactions with groups on the same side of the ring.

How do you choose the more stable chair of a substituted cyclohexane?

Preserve the correct up/down stereochemistry in both chairs, then compare axial substituents. The chair placing larger groups equatorial is generally favored.

Which stereochemical patterns can have two equatorial substituents in one chair: 1,2-, 1,3-, and 1,4- distributed cyclohexanes?

1,2: trans

1,3: cis

1,4: trans

The chair flip makes both axial.

For cis-1,3-dimethylcyclohexane, which chair is more stable?

The chair with both methyl groups equatorial, its flip has both methyl groups axial.

For cis-1,4-dimethylcyclohexane, how many methyl groups are axial in each chair?

One axial and one equatorial in each chair. Because the substituents are identical, the two chairs have equal energy.

List the cyclic constitutional isomers of C₅H₁₀

Cyclopentane, methylcyclobutane, ethylcyclopropane, 1,1-dimethylcyclopropane, and 1,2-dimethylcyclopropane.

Cis and trans forms of the last structure are stereoisomers, not additional constitutional isomers.