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.
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.
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.
FAQs on Cycloalkanes
1. What are cycloalkanes and what is their general formula?
Cycloalkanes are alicyclic saturated hydrocarbons in which carbon atoms are joined by single bonds to form a ring. Since they are saturated and have a ring structure, their general formula is CnH2n (where n ≥ 3). For example, a cycloalkane with three carbon atoms is cyclopropane (C3H6), and one with six carbon atoms is cyclohexane (C6H12).
2. How are cycloalkanes named according to IUPAC nomenclature rules?
The IUPAC nomenclature for cycloalkanes follows a simple set of rules:
- The prefix 'cyclo-' is added to the name of the corresponding straight-chain alkane with the same number of carbon atoms in the ring.
- If only one substituent is attached, no number is needed to indicate its position. For example, a methyl group on a cyclohexane ring is simply called methylcyclohexane.
- If multiple substituents are present, the carbon atoms are numbered to give the substituents the lowest possible numbers. The substituents are then listed in alphabetical order. For example, 1-ethyl-2-methylcyclopentane.
3. Why are smaller cycloalkanes like cyclopropane and cyclobutane highly reactive?
Smaller cycloalkanes are highly reactive due to significant angle strain, as per Baeyer's Strain Theory. Carbon atoms in alkanes prefer a tetrahedral bond angle of 109.5°. In cyclopropane, the C-C-C bond angle is forced to be 60°, and in cyclobutane, it is 90°. This large deviation from the ideal angle creates immense strain in the ring, making the C-C bonds weak and prone to breaking. This is why they readily undergo ring-opening reactions, unlike larger, more stable cycloalkanes like cyclohexane.
4. Why do cycloalkanes generally have higher boiling points and densities than their open-chain alkane counterparts?
Cycloalkanes exhibit higher boiling points, melting points, and densities than their corresponding acyclic alkanes. This is because the rigid, cyclic structure allows the molecules to pack more closely together. This close packing increases the surface area of contact between molecules, leading to stronger intermolecular van der Waals forces (specifically, London dispersion forces). Stronger forces require more energy to overcome, resulting in higher boiling and melting points.
5. What are the different conformations of cyclohexane, and why is the chair form the most stable?
Due to the free rotation around its C-C single bonds, cyclohexane can exist in several shapes called conformations. The main ones are:
- Chair form: The most stable conformation.
- Boat form: A less stable, more energetic conformation.
- Twist-boat form: Intermediate in stability between chair and boat.
- Half-chair form: The least stable conformation.
The chair form is the most stable because it minimises two types of strain: angle strain (all C-C-C bond angles are approximately 109.5°, the ideal tetrahedral angle) and torsional strain (all hydrogen atoms on adjacent carbons are in a staggered arrangement, reducing repulsion).
6. How do cycloalkanes differ from cycloalkenes and aromatic compounds?
The key difference lies in their bonding and structure. Cycloalkanes are saturated cyclic hydrocarbons containing only carbon-carbon single bonds. Cycloalkenes are unsaturated, containing at least one carbon-carbon double bond within the ring. Aromatic compounds, like benzene, are a special class of cyclic compounds that have a planar ring with a delocalised system of pi electrons, making them exceptionally stable and chemically distinct from both cycloalkanes and cycloalkenes.
7. Is a molecule with the formula C5H10 always a cycloalkane?
No, not necessarily. While cyclopentane is a cycloalkane with the formula C5H10, this formula corresponds to a degree of unsaturation of one. This means the molecule could also be an alkene, such as pent-1-ene or 2-methylbut-2-ene. Therefore, C5H10 represents constitutional isomers, which can be either a cycloalkane or an alkene. Without knowing the structure, the formula alone is insufficient to identify it as a cycloalkane.
