Conformational Analysis of Alkanes and Cycloalkanes in Organic Chemistry

What is the bond angle in a tetrahedral sp3 hybrid carbon?

109.5°

What is a conformation?

Different arrangements of atoms caused by rotation about a single bond.

What projection is used to judge the stability of different conformations?

Newman projection

How is the stability of alkane conformers primarily determined?

By steric interactions between atoms or groups.

What happens to potential energy as steric repulsion increases?

Potential energy increases, leading to lower stability.

What is torsional strain?

Changes in potential energy resulting from rotation around a C-C bond.

Which conformation of ethane is the most stable?

The staggered conformation.

What is the energy barrier between eclipsed and staggered conformations of ethane?

Small and easily overcome at room temperature.

What is the torsional strain of propane compared to ethane?

Propane's torsional strain is 0.3 kcal/mol higher than for ethane.

What are the two staggered conformations of butane?

Gauche (60°) and anti (180°).

What is steric strain?

Interference between two bulky groups causing strong repulsion.

What is the formula for cycloalkanes?

CnH2n

What is angle strain?

Increase in potential energy due to departure from the ideal bond angle.

What is the bond angle in cyclopropane?

60°

What is the most stable conformation of cyclohexane?

Chair conformation.

What is the difference between axial and equatorial positions in cyclohexane?

Axial bonds are vertical; equatorial bonds are directed outward.

What is a 1,3-diaxial interaction?

Interference of an axial substituent with axial hydrogens on C3 and C5.

Which conformation of cis-1,3-dimethylcyclohexane is more stable?

Both conformations have the same energy.

Which conformation of trans-1,3-dimethylcyclohexane is more stable?

The diequatorial conformation.

What is the most stable conformation of cis-1,4-di-tert-butylcyclohexane?

The twist boat conformation.

What is the nomenclature for bicyclic systems?

Bicyclo [#.#.#]alkane, where the #s are the numbers of carbons on the bridges.

What is the difference between cis-decalin and trans-decalin?

Cis-decalin has two cis bonds; trans-decalin has two trans bonds.

Why is trans-decalin more stable than cis-decalin?

Because the alkyl groups are equatorial in trans-decalin.

What type of strain is present in cyclopropane?

Both angle strain and torsional strain.

What conformation does cyclobutane adopt to relieve strain?

The folded conformation (envelope).

What is the significance of the chair conformation in cyclohexane?

It has no eclipsing interactions and optimal bond angles.

What are the two conformations of cyclopentane?

Slightly folded, like an envelope, to reduce eclipsing.