Cycloalkanes

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Organic Compounds And Cycloalkanes

Earlier it was believed that organic compounds are those compounds that are obtained from nature and living organisms and cannot be prepared in the laboratories. This definition was disapproved when urea was formed in the lab by Wohler from an inorganic substance ammonium cyanate. Later organic compounds were defined as those compounds which have carbon elements in their constitution. Organic compounds that are compounds of carbon are studied with the help of a branch of science which is referred to as organic chemistry.


Carbon Compounds are Broadly Divided Into Two Categories -

  • Hydrocarbons

  • Derivatives of Hydrocarbons

Hydrocarbons are those organic compounds that are made of carbon and hydrogen. Cycloalkanes are also hydrocarbons.


Classification of Hydrocarbons

Hydrocarbons are classified into two broad categories based on the carbon skeleton in their structure. These are -

  • Open chain/Acyclic hydrocarbons

  • Closed chain/cyclic hydrocarbons

Open chain hydrocarbons have a straight chain of carbon compounds with or without branching. No ring structure is present.

Open chain hydrocarbons are again of two types-

  • Straight chain hydrocarbons

  • Branched-chain hydrocarbons

Straight chain hydrocarbons are further divided into two categories-

  • Saturated hydrocarbons

  • Unsaturated hydrocarbons

Saturated hydrocarbons are those compounds which have carbon-carbon single bonds. These are also known as alkanes.

Unsaturated hydrocarbons are those carbon compounds which have carbon-carbon double bonds (alkanes) or triple bonds(alkynes).

Closed chain hydrocarbons are those hydrocarbons whose carbon skeleton takes the form of a ring.

These hydrocarbons are further divided into two types-

  • Alicyclic Hydrocarbon

  • Aromatic Hydrocarbon


Structure of Cycloalkanes

Alicyclic hydrocarbons are those hydrocarbons which have a closed ring carbon structure. When a straight chain of carbon forms a ring two hydrogen atoms are removed. These are also known as non-benzenoid hydrocarbons.

Cycloalkanes are Alicyclic hydrocarbons. They are not aromatic. They have simply ringed hydrocarbons.

The structure of cycloalkane contains a single ring made of carbon atoms and all of the carbon-carbon bonds are single. The free valencies of carbon are satisfied with hydrogen atoms.

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Cycloalkanes Examples

Cycloalkanes can have only one ring in their structure like in Cyclopropane, cyclobutane, cyclohexane, etc. Cycloalkanes can have two rings in their structure like in Bicyclo[4,4,0] decane which has two rings with 10 carbon atoms.

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Nomenclature of Cycloalkanes

Cyclohexane which has only one ring is named by adding a prefix cyclo to the original name of the alkane that represents the number of carbon atoms in the chain.

For example, a carbon ring with 3 carbon atoms and 6 hydrogen atoms is named cyclopropane.


No. of Carbon Atoms in the Ring

Alkane Name

Prefix

IUPAC Name of Cycloalkane

4

butane

cyclo

cyclobutane

6

hexane

cyclo

cyclohexane


The above nomenclature is done when only carbon and hydrogen are present in the compound.

When anyone substituent is attached to the carbon ring by replacing any one hydrogen then suffix and prefix are added according to the priority series of the functional group. Also, the position of the group is not mentioned.


No of Carbon Atoms in the Ring

Substituent Group

Prefix

Suffix

IUPAC Name of the Cycloalkane

6

-OH

-

ol

cyclohexanol

6

-Cl

Chloro

-

Chlorocyclohexane


When more than one substituents are present by replacing 2 hydrogen atoms than the position of the substituent group then the group of highest priority gets the lowest number. Also, alphabetical order is followed in case of the same priority group. For example, if two groups i.e ethyl and methyl group are present then the ethyl group gets the lower number.


Cycloalkane Properties

Their physical properties of cycloalkanes are similar to the properties of open chain alkanes. But cycloalkanes have a higher boiling point, melting point, and density. This is due to the high amount of London forces or dispersion forces. These high London forces are due to the close vicinity of carbon atoms in the ring structure.

The cycloalkanes are highly reactive as compared to the alkanes. This is because of the angle strain present in them due to the ring structure. 

FAQ (Frequently Asked Questions)

1. What are Aromatic Hydrocarbons?

Aromatic hydrocarbons are named aromatic because of the specific fragrance associated with them. There are cyclic hydrocarbons with few properties that make the aromatic and also exceptionally stable despite the presence of the ring structure. 


The Conditions for Aromaticity are-

  • Presence of ring

  • Presence of alternate double and single bonds and presence of resonance in the ring(delocalization of pi bonds)

  • Presence of 4n+2pi electrons

  • Presence of planarity

These are Huckle's rules of aromaticity. Any cyclic hydrocarbon having all the above conditions satisfied then is said to be aromatic. If anyone out of 4 conditions is lacking then the compound will be non-aromatic. Benzene is an aromatic compound.

2.Explain the Conformers of Cyclohexane.

Cyclohexane is an example of cycloalkane with 6 carbons in its ring and 12 hydrogen atoms attached to the 6 carbons. Due to the rotation of carbon-carbon bonds cyclohexane can form many shapes and these are known as conformers. Every shape is different from one another and has different stability.


The conformers are-

  • Chair form - This is the most stable conformer. In this, the carbon 1 and carbon 4 are at trans position thus minimizing the strain providing stability.

  • Boat form- THis form is less stable and in this carbon 1 and carbon, 4 are on the same side. This increases the steric hindrance.

  • Half chair form - This form is of stability between chair form and boat form.