Conformational Isomers

What is a conformation?

A particular 3D arrangement of a molecule that can change through rotation around single bonds without breaking bonds.

What stays the same when a molecule changes conformation?

Its molecular formula and the way its atoms are connected.

What is a Newman projection?

A drawing of a molecule viewed straight down a bond, usually a C—C bond.

In a Newman projection, which carbon is the dot? Which is the circle?

Dot = front carbon

Circle = back carbon

What does staggered mean?

The bonds on the back carbon appear between the bonds on the front carbon. They do not line up.

What does eclipsed mean?

Bonds on the front and back carbons line up when viewed down the bond.

Which is generally lower in energy: staggered or eclipsed? Why?

Staggered, because it has less torsional strain.

What is torsional strain?

The increase in energy when atoms or groups crowd each other in space.

As you rotate one carbon by 60º in a Newman projection, what happens?

It switches between staggered and eclipsed arrangements.

On an energy versus bond rotation graph, what do valleys represent?

Lower energy staggered conformations.

What does anti mean in a staggered conformation of butane?

The two CH₃ groups are 180º apart. This conformation minimizes steric strain.

What does gauche mean in a staggered conformation of butane?

The two CH₃ groups are 60º apart, leading to increased steric strain.

Which is more stable in butane: anti or gauche? Why?

Anti, because its CH₃ groups are father apart.

What is the stability order of butane’s main conformations, from most to least stable?

Anti → gauche → CH₃/H eclipsed → CH₃/CH₃ eclipsed

Which butane conformation has the highest energy?

The fully eclipsed conformation in which the two CH₃ groups line up.

How do you rank Newman projections by stability?

1st: check staggered vs. eclipsed

2nd: locate the largest groups: farther apart generally means more stable; large groups eclipsing is especially unstable

Why does cyclohexane form a chair shape?

The chair reduces angle and torsional strain compared with a flat ring

What happens to axial and equatorial positions during a cyclohexane ring flip?

Axial becomes equatorial; equatorial becomes axial.

What happens to a substituent’s up or down direction during a ring flip?

It stays the same. For example, axial up becomes equatorial up.

Where does a bulky substituent usually prefer to sit on a cyclohexane chair? Why?

Equatorial, because that avoids unfavorable 1,3-diaxial interactions that increase steric strain.

For substituents on a ring, what does cis mean?

Same side (both up or both down)

For substituents on a ring, what does trans mean?

Opposite sides (one up, one down)

Can a ring flip change cis into trans?

No. A ring flip preserves each substituent’s up or down direction.