Cycloalkanes & Chair Conformations

Cycloalkanes

are saturated hydrocarbons whose carbon atoms form a ring (different from aromatic rings who have double bonds).

Cycloalkane formula

C_nH_2n

Ring Strain

A carbon atom prefers:

• Bond angle = 109.5°

• Staggered arrangement of bonds

When a ring forces carbons away from these ideal values, strain develops.

Chair Conformation

the most stable, three-dimensional arrangement that a six-carbon ring, such as cyclohexane, can adopt. Instead of being a flat hexagon, cyclohexane bends so that some carbons are above the plane and some are below it. This reduces strain and makes the molecule much more stable. (Basically Cyclohexane in 3D).

Axial Positions

• Axial bond → points straight up or straight down

• Equatorial bond → points outward, around the edge of the ring

(see image)

Axial

• points straight up or straight down

• More crowded

• Higher energy

• Less stable

Equatorial

• points outward, around the edge of the ring

• Less crowded

• Lower energy

• More stable

That's why larger groups usually prefer the equatorial position.

What happens during a ring flip?

The group stays attached to the same carbon, but its orientation changes. So if it was axial, it becomes equatorial.

Diaxal and Diequatorial

Two substituents facing up or two facing down, if you have one up one down it is trans. (remember to number them based on the carbon it is placed in, and count where substituents get the lowest possible number).

How to tell which Chair Conformation is more stable

Has a larger group on equatorial, more stable.

Lower energy =

Lower energy = more stable

Higher energy = less stable