Reactions of Cyclic aliphatic compounds and Baeyer’s strain theory

Reactions of Cyclic aliphatic compounds

With fewexception, alicyclic compounds show a similartype of reactions as open chain aliphatic compounds. For example,

1. Cycloalkanes undergo chiefly free radical substitution reactions.

2. Cycloalkenes undergo addition reactions, both electrophilic and free radical, like open chain alkenes. These can also undergo cleavage and allylic substitution.
3. Reaction with halogens: Cycloalkenes undergo the additionreaction with halogens.

1. Reaction with hydrogen halides: Cycloalkanes are almost inert towards hydrogen halides whereas cycloalkenes undergo electrophilic addition across thedoublebond.

• Hydrogenation: Cycloalkenes undergo catalytic hydrogenation to give the corresponding cycloalkanes.

1. Allylic halogenation: Cycloalkenes undergo free radical substitution in allylicpositions when treated with halogens at high temperature. Allylic bromination can also be brought about by reaction with N-bromosuccinimide (NBS).

1. Ozonolysis: Like alkenes, cycloalkenes on ozonolysisfromcarbonylcompounds.

Similarly,alicyclichalides,alcohols, and ketones undergo the similartype of reactions as open chain aliphatic compounds.

Reactions of small ring compounds: (Cyclopropane and Cyclobutane)

It undergoes an exceptional reaction when treated with halogens, halogen acids or hydrogen except for free radical substitution small ring compounds like cyclopropane and cyclobutane in the presence of a catalyst. These compounds undergo addition reactions with the breaking of carbon-carbon bond and the two atoms of the reagent appear at the ends of the chain.

Baeyer’sstrain theory

Adolf von Baeyer in 1885 proposed a theory to account for the relative stabilities of various alicyclic rings known as Baeyer’s strain theory. This theorystandsfor the classical hypothesis ofVan’tHoff and Le-Bell. According to this, when a carbon atom is bonded to four other atoms, its four valencies are directed towards the four corners of a regular tetrahedron and thereby any two of its bonds form an angle of 109’, the ideal tetrahedral angle at the center of the tetrahedron.

The main assumptions in which its theory is based are given below:

1. All the carbon atoms constituting an alicyclic ring lie in the same plane and therefore, one pair of bonds to each carbon cannot assume the tetrahedralangle, i.e., 109^o 28’.
2. Any deviation of –C-C-C- bond angles of the ring carbons from the ideal tetrahedral angle develops a strain in the ring. This strain is known as angle strain and brings instability in the ring.
3. The greater the angle strain in the ring, the greater would be the instability of the molecule.
4. The more stable a ring system is, more easily it can be synthesized.

The amount of angle strain can be expressed in terms of the valence angle deviation of which can be calculated from the following relation:

d = ½(10928’ -α)

where, α ‘’ is the bond angle in various cycloalkanes. The factor ‘1/2’ implies that the deviation of the bond angle has been assumed to be equally shared between the two bonds.

For example:

The three carbon atoms are placed at the corner of the equilateral triangle of cyclopropane, therefore, the –C-C-C- bond angle of 109^o 28’ between any two valence bond is compressed to 60 during the formation of cyclopropane. Therefore, either of the two bonds involved is bent in by ½(109^o 28’) = +24^o44^,. Therefore the angle strain in Cyclopropane is +24^o44^,.

Similarly, the angle strain in cyclobutane is ½(109⁰ 28’- 90) = + 9^o 44’.

The angle strain in different cycloalkanes are calculated on the basis of Bayer Strain theory is listed below.

The negative and positive value of valance angle deviation d indicate the whether the bond angle α is less or greater than the ideal tetrahedral bond angle, i.e., whether the strain is inward or outward.

From above table we can confirm that cyclopropane has the maximum angle strain, therefore, it should be highly strained molecule and most unstable. This prediction is supported by experimental observations as cyclopropane is really found to be quite unstable, it easily opens up it's ring in reactions with H₂,Br₂, HBr etc. and thus releases the strain. Moreover, it is quite difficult to prepare cyclopropane, which further supports the above facts.

The angle strain for cyclopropane is minimum, therefore, it should have the least strain and it should become most stable. The cyclohexane contains slightly higher angle strain as compared it to the cyclopentane. Both, cyclopentane and cyclohexane are found to be quite stable as these do not undergo ring cleavage and can be easily synthesized.

Cyclopentane and higher cycloalkanes possess larger values of angle strain and hence, they should be highly unstable. Baeyer proposed that, because of this instability great difficulty had been encountered in the synthesis of larger rings. This was in agreement with the facts known at that time, as the rings containing 7 or more carbon atoms were unknown. According to Baeyer’s strain theory, rings smaller or larger than cyclopentane or cyclohexane are unstable.