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Ring strain results from a combination of angle strain, conformational strain or Pitzer strain (torsional eclipsing interactions), and transannular strain, also known as van der Waals strain or Prelog strain. The simplest examples of angle strain are small cycloalkanes such as cyclopropane and cyclobutane.
Cyclopropane is the cycloalkane with the molecular formula (CH 2) 3, consisting of three methylene groups (CH 2) linked to each other to form a triangular ring. The small size of the ring creates substantial ring strain in the structure.
Because of the large ring strain energy of cyclopropanes (29.0 kcal per mole), they are often used as substrates for C-C activation through oxidative addition of a transition metal into one of the three C-C bonds leading to a metallacyclobutane intermediate. Substituents on the cyclopropane affect the course of its activation. [3]
The strain energy of cyclopropane and cyclobutane are 27.5 and 26.3 kcal mol −1, respectively. [1] Cyclopentane experiences much less strain, mainly due to torsional strain from eclipsed hydrogens: its preferred conformations interconvert by a process called pseudorotation. [4]: 14 Ring strain can be considerably higher in bicyclic systems.
In organic chemistry, transannular strain (also called Prelog strain after chemist Vladimir Prelog) is the unfavorable interactions of ring substituents on non-adjacent carbons. These interactions, called transannular interactions, arise from a lack of space in the interior of the ring , which forces substituents into conflict with one another.
The small cycloalkanes – in particular, cyclopropane – have a lower stability due to Baeyer strain and ring strain. They react similarly to alkenes, though they do not react in electrophilic addition, but in nucleophilic aliphatic substitution. These reactions are ring-opening reactions or ring-cleavage reactions of alkyl cycloalkanes.
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Cyclopropane derivatives are numerous. [4] Many biomolecules and pharmaceutical drugs feature the cyclopropane ring. Famous example is aminocyclopropane carboxylic acid, which is the precursor to ethylene, a plant hormone. [5] The pyrethroids are the basis of many insecticides. [6] Several cyclopropane fatty acids are known.