Chapter 4- Cycloalkanes

Chem 203

What are the THREE different kinds of strains affecting cycloalkanes?

Torsional, steric, and angle

What is angle strain?

When bond angles are NOT 109.5 degrees and there is an INCREASE in energy as a result (expansion/compression)

Why do cycloalkanes distort their shape?

To alleviate angle and torsional strain

What is torsional strain?

Neighboring atoms experience ECLIPSING interactions that cause an INCREASE in energy

What is steric strain?

Atoms that are TOO close will result in strain caused by REPULSIVE interactions

When cycloPROPANE is involved, what are the bond angles? What is the shape?

Triangle shape; 60 degree bond angle

What kind of strains does cyclopropane endure?

Torsional and angle; due to reactive and weak bonds. Reactions that OPEN the ring are favored

When cycloBUTANE is involved, what is the SHAPE and BOND ANGLE?

Square shape; 90 degree angle

What types of strain does cyclobutane endure?

Torsional and angle strain

When cyclopentane is involved, what is the shape and bond angle?

Pentagonal shape; bond angle of 108 degrees

What kind of strain does cyclopentane endure?

Torsional strain and very SLIGHT angle strain. This is the most stable

With cyclohexane, what is the shape and bond angle?

A hexagon shape; 120 degrees

What kind of strain does cyclohexane undergo?

Torsional strain but GREATER angle strain

What characteristic of a cycloalkane influences the heat of combustion?

The size of the ring

What is the general formula when an alkane is burned in the presence of oxygen?

Carbon dioxide + water+ heat

When the heat of combustion per CH2 is involved, what is the formula used to calculate this?

Difference between CYCLOALKANE heat of combustion per CH2 and ACYCLIC alkane heat of combustion per CH2, all multiplied by number of CH2 units in a ring

When heat of combustion per CH2 is involved, how is the size of ring playing a role?

Small and medium rings will be strained, but CYCLOHEXANE rings and VERY LARGE rings are strain free

What bond angle is found in CYCLOhexane CHAIR conformation?

109.5 degrees

What is the PLACEMENT of hydrogens in cyclohexane chair conformations? What is the stability of this conformer?

Hydrogens are in a STAGGERED position; this is the MOST STABLE conformer

All the carbons within a chair conformer have TWO different kinds of hydrogens. What are they?

Axial and equatorial hydrogens

What are axial hydrogens?

They are hydrogens that are located ABOVE and BELOW a ring along the perpendicular axis

What are equatorial hydrogens?

They are located IN the ring plane

What is the specific name when cyclohexanes undergo a conformation change? How do the positions differ?

Cyclohexanes will undergo “ring flipping”; this is when the headrest becomes the foot rest and vice versa

In between a cyclohexane undergoing ring flipping, it will experience an intermediate phase known as what?

A boat conformation; the boat conformation is LEAST stable. Both steric and torsional strain are present

When larger AXIAL substituents are involved, at what specific areas do they occur?

The substituents will take place a 1 and 3, leading to UNFAVORABLE diaxial interactions

How can larger substituents become favored?

If they are in the EQUATORIAL position; the equatorial position provider more space

How can we differentiate cis isomer and trans isomer?

Cis isomers will have two groups on the SAME side of the ring that either BOTH face up OR down. Trans isomers have two groups on the OPPOSITE side of the ring with one facing up/down and the other OPPOSITE