Question

Why are cyclopropane and cyclobutene so reactive?

Short answer

Cyclopropane and cyclobutene are reactive due to a large ring strain. Both the compounds react easily to obtain the preferred 109.5° bond angles.

Step-by-step solution

Ring strain

Angle strain, conformational strain or Pitzer strain, and transannular strain, also known as Van der Waals strain or Prelog strain, combine to generate a ring strain.

Small cycloalkanes like cyclopropane and cyclobutene are the simplest instances of an angle strain.

Step 2: Angle and torsional strain

The high ring strain in cyclopropane and cyclobutane causes their reactivity. Angle strain and torsional strain are two elements that contribute to enormous ring strain.

• Angle strain is the increase in a molecule's potential energy due to bond angles that differ from ideal values.
• Torsional strain is the increase in a molecule's potential energy caused by electron repulsion in bonds that do not share an atom.

Step 3: Reasons for reactivity

Furthermore, all carbon-carbon bonds are eclipsed, resulting in torsional strain, which contributes to the total ring strain.

Reprsentation of ring strain in cyclobutene

Cyclobutene has nearly the same ring strain as cyclopropane, but it is dispersed over four carbon atoms. Cyclobutene is in a somewhat folded shape with bond angles of 88° to reduce torsional strain.

Decrease in torsional strain