Introduction
Alicyclic compounds are simply aliphatic compounds that form rings. When carbon atoms join together in a circular manner, they form cyclic structures. Even though they are rings, they behave like normal aliphatic compounds; hence the name alicyclic compounds.
These compounds can be:
- Saturated – called cycloalkanes
- Unsaturated – like cycloalkenes
They can also be:
- Homocyclic – ring contains only carbon atoms
- Heterocyclic – ring contains at least one non-carbon atom (like N or O)
Examples: cyclopropane, cyclobutane, cyclopentane, cyclohexane, cyclohexene etc.
What Are Cycloalkanes?
Cycloalkanes are cyclic alkanes—that means:
- They have only single bonds
- They form a ring
- They contain two less hydrogen atoms than normal alkanes
The general formula for cycloalkanes is:
CnH2n
where n = number of carbon atoms.
First member: Cyclopropane (C3H6)
Preparation of Cycloalkanes
Cycloalkanes can be prepared by many methods. Below are the commonly used reactions explained in simple language.
1) Addition of Carbene to Alkenes
A carbene is a carbon species that has two bonds and a pair of electrons. When a carbene reacts with an alkene, a three-membered ring (cyclopropane) is formed.
This method mainly gives substituted cyclopropanes.
2) Action of Metallic Sodium on Dihaloalkanes
When a molecule contains two halogens at the two ends (like Cl-…-Cl), and it reacts with sodium metal, the ends join to form a ring.
This is similar to the Wurtz reaction.
Useful for preparing 3–6 membered rings.
3) From Calcium Salts of Saturated Dicarboxylic Acids (Wislicenus Method)
When calcium salts of dicarboxylic acids are heated, they form cyclic ketones. These ketones can then be reduced to give cycloalkanes.
Used for preparing: cyclopentane, cyclohexane, cycloheptane.
4) From Diethyl Malonate (Perkin’s Method)
Diethyl malonate reacts with dihalo compounds (like 1,3- or 1,4-dihaloalkanes) and forms a cyclic structure after several steps.
Useful for making 5-, 6- and 7-membered rings.
5) From Esters of Dicarboxylic Acids
Esters of long dicarboxylic acids can undergo intramolecular condensation (reaction within the same molecule) to form cyclic β-ketoesters. These further get converted into cycloalkanes.
Example: Cyclopentane can be prepared from diethyl adipate.
6) Diels–Alder Reaction
This reaction involves a diene and a dienophile. When both react under heat or light, a six-membered ring (cyclohexane derivative) is formed.
This is one of the classical methods to prepare cyclohexane rings.
7) Reduction of Aromatic Compounds
Benzene can be reduced (hydrogen added) in the presence of a catalyst like nickel under pressure to form cyclohexane.
8) From Dihalogen Compounds
Terminal dihalides (halogens at the ends of a molecule) when treated with sodium or zinc metal undergo a reaction similar to the Wurtz reaction and form cycloalkanes.
Detailed Notes:
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