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Reactions of Cyclopropane and Cyclobutane


Reactions of Cyclopropane

Some small cycloalkanes, including cyclopropane, behave differently than alkanes. A UV-induced substitution reaction occurs with cyclopropane as it does with non-cyclic alkanes. UV light is not necessary for the reaction to occur. It is possible to break the cyclopropane ring without UV light as added reactions take place. When bromine is added to cyclopropane, for instance, tribromopropane is formed.



If light is present, this can still happen - but a substitution reaction will also occur. It is very difficult to burn cyclopropane because the ring gets strained, resulting in rupture. When the carbon makes four single bonds, the bond angles are 60° instead of the normal value of 109.5°. In carbon-carbon bonds, there is less overlap between the orbitals than normally occurs, and the pair of bonding electrons are repelled by each other. If the ring is broken, the system will be more stable.

Corey-Chaykovsky Reaction


In the presence of carbonyl compounds, such as ketones, sulfuryl compounds react to form aziridines and carboxylic acids.

epoxidation

aziridination

Cyclopropanes are formed when sulfur ylides react with enones.

Cyclopropanation

Wurtz Reaction




It produces a simple dimer as a result of two equivalent alkyl halide molecules. Wurtz coupling is an organic reaction that has been around for thousands of years. Molecular versions of the reaction have also been successfully used to prepare strained ring compounds:



There will be an approximate statistical mix of products when using two different alkyl halides. Starting materials with different reactivity rates can undergo a more selective unsymmetric modification.

Hydrogenation (ring opening) - When hydrogenation of cycloalkanes occurs with catalysts such as Ni or Pt, saturated hydrocarbons are formed. The hydrogenation process becomes more difficult with increasing ring size. High-carbon cycloalkanes usually survive hydrogenation without reorganizing. These conclusions are evident from the conditions of the following reactions.



Halogenation

Addition reaction of halogen (leading to ring opening) - Dark reactions of cyclopropane with bromine and chlorine produce addition products. The solvent is CCl2.



Substitution reaction with halogens - By exposing cycloalkanes to UV light, the chlorine and bromine react to form substitution products.



Reactions of Cyclobutane

Hydrogenation (ring opening) - Using catalysts such as Ni or Pt, Cycloalkanes can be hydrogenated to produce saturated hydrocarbons. With increasing ring size, hydrogenation becomes more difficult. It is generally true that higher cycloalkanes with six and more carbon atoms are resistant to hydrogenation. The following reactions illustrate this.



Halogenation

Addition reaction of halogen (leading to ring opening) - Chromate and bromine react with cyclopropane in the dark to form addition products. The solvent used is CCl2.

Later the stability of the cyclo-alkane is done by following two tests:

Baeyer’s strain theory

Baeyer's strain theory holds the following postulates:
  • Molecular planarity characterizes the cycloalkanes.
  • Angle strain occurs when the bond angle variance departs from the normal tetrahedral value (1090 28').
  • As the angle strain increases, the ring becomes more unstable.
  • It is easier to form rings that are more stable
C-C bonds have a positive sign meaning they are bent inward, and a negative sign means they are bulged outward from the normal tetrahedral angle. It is only the magnitude of the angle strain that determines how much strain is present within the ring, regardless of the angle. Therefore, Baeyer's strain theory suggests that cyclopropane should be highly strained, and consequently most unstable, because it has the largest angle strain.

Sachse-Mohr theory

It was Sachse and Mohr (1918) who proposed that cyclohexane rings and higher members become stable if they are not confined to one plane as Baeyer proposed, because the carbon atoms are not confined to one plane. It is assumed that, in the event that the ring assumed a fold or puckered condition, the normal tetrahedral angles of 1090 28' would be retained and, therefore, the stretch within the ring would be relieved.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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