Cycloalkanes – Method of preparation, Physical properties and stabilities of cycloalkanes, with best notes(1)

Cycloalkanes

Cycloalkanes are saturated hydrocarbons in which the carbon atoms are joined by a single covalent bond to form a ring. They are also called alicyclic compounds. These are Sp3 hybridized.

Method of preparation of cycloalkanes

• From Dihalides: Terminal Dihalides when treated with sodium or zinc form cycloalkane.

• Dieckmann reaction: When the ester of dicarboxylic acid is treated with sodium, B-keto-ester is formed. When further on hydrolysis give cyclic ketones which a clemmensen reduction gives cycloalkane.

• From alkene: When alkene is treated with methylene iodine (CH2I2) in the presence of a zinc-copper (Zn-cu) couple, then cyclopropane derivatives are formed.

• From aromatic hydrocarbons: six membrane cycloalkanes can be prepared by the catalytic reduction of benzene and its derivatives.

Physical properties of cycloalkanes

1. Cycloalkanes are lighter than water and non-polar in nature.
2. Cyclopropane and cyclobutane are gases at room temperature; the remaining are liquids.
3. Cycloalkanes are insoluble in water but dissolve in ethanol and ether.
4.  The melting and boiling points of cycloalkanes show a gradual increase with increases in molecular weight.

Stabilities of cycloalkanes

In the structure of an alkane, the hybridization is sp3 and the bond angle is 109°28′, and the shape is tetrahedral but in the case of a cycloalkane, the hybridization is the same sp3 but due to internal strain, their structure and stability are changed.

• Bayer’s strain theory: This theory is based on the fact that the normal angle between a pair of carbon atoms is 109° 28′. Now it is assumed that all the cycloalkanes are planned. Stability of cycloalkanes depend upon angle strain in cycloalkanes, the more angle strain then the more unstable. Angle strain, it is angle difference between desired angle 109°28 and actual angle.

Limitation of Bayer’s strain theory:

1. This theory only applies to the lower cycloalkanes; Bayer’s strain theory was not able to explain the effect of angle strain in a larger ring system.
2. According to Bayer’s strain theory cyclopentane should be much stable than cyclohexane but partially it is reversed.
3. Higher cycloalkanes do not follow this rule.

• Coulson Moffit’s modification: “This strain theory is only applicable for cyclopropane.” In the structure of cyclopropane, the internal bond angle of 60° produces strain and the cyclopropane structure is modified into a new structure. In this new structure, the bond between C and C contains the sigma and pi characters, so this is also known as the “banana bond.” In C-C bond pi(π) is present but C-H bond only sigma character is present.

• Sache Mohr theory:- according to this theory, the strain in this structure is seen in only up to cyclopentane ring but in higher structure like cyclohexane and cycloheptane no strain is present so this is called stainless ring. For example, cyclohexane is a multiplanar structure that exists in two forms: chair form and boat form. They can change their shape and don’t show the strain.