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# Cycloalkane

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 Sub Topics We know that the organic compounds which are composed of carbon and hydrogen are called as hydrocarbons. Hydrocarbons are most common compounds which we can found in our surroundings. For Example: crude oil is a mixture of different hydrocarbons. We know that crude oil is used to produce many useful materials. This mixture of hydrocarbon can separate with the help of fractional distillation. Many of the fractions of crude oil are also used as fuel in automobiles and machines. On the basis of presence of covalent bonds and structures of hydrocarbons, they can be classified as alkanes, alkene and alkynes. In this article we will discuss about alkanes and Cycloalkanes. Alkanes are one of the simplest hydrocarbons with all single covalent bonds between carbon atoms. The general formula of alkanes is $C_{n}H_{2n+2}$. In other words, we can say that the number of hydrogen atoms is more than double compare to carbon atoms in alkanes. Another type of hydrocarbons is alkenes with general formula $C_{n}H_{2n}$. They contain at least one double bond between carbon atoms. Therefore the smallest alkene contains two carbon atoms and named as ethene.  Alkynes are also example of hydrocarbons which contain at least one triple covalent bond between two carbon atoms. Ethyne or acetylene (CH$\equiv$CH) is the simplest alkyne.  Alkene and alkyne have double and triple covalent bonds which make them unsaturated. Therefore they show additional reactions quite easily compare to alkanes. Alkanes are saturated hydrocarbons with all the single covalent bonds between carbon atoms. Since sigma covalent bonds are stronger compare to pi-bonds, hence alkanes are least reactive hydrocarbons.

## Naming Cycloalkanes

Alkanes are usually named with according to IUPAC nomenclature. International Union of Pure and Applied Chemistry provided some rules for the naming of alkanes and other hydrocarbons.  The naming of hydrocarbon starts from the naming of parent chain which is the longest branch of carbon atoms in the alkane.

After naming of parent chain with the help of root word, we have to write the name of substituent with their position as prefix.  If one substituent is bonded at more than one position, they indicate with the help of di, tri, tetra- prefixes.

The suffix of alkanes is â€“ane which is added after root word. Hence a complete name of alkane can be written as:

Prefix + Root word + Suffix = Alkane

The IUPAC rules for naming cycloalkanes are as follows.

Rule-1:

The name of an unsubstituted cycloalkane is obtained by attaching the prefix cyclo to the name of the corresponding normal alkane having the same number of carbon atoms as in the ring. For convenience and simplicity cycloalkanes are often represented by simple geometric figures.

It is understood that each corner represents one carbon and two hydrogens.

Rule-2:

The ring is numbered so that the carbons bearing the substituents will have the lowest numbers.

Rule-3:

Naming alkyl substituted cycloalkanes. Count the number of carbon atoms in the rig and also in the largest alkyl substituent. If the number of carbon atoms in the ring is less than the number of carbon atoms in the substituent, it is named as a cycloalkyl-substituted alkane.

## Cycloalkanes Formula

To form a ring two hydrogen on the end carbons of a carbon chain are lost when the new C-C bond is formed closing the ring. Thus the molecular formula of a cycloalkane differs from that of an acyclic alkane by two hydrogens for each ring.

General formula:
1) Alkane - CnH2n+2
2) Cycloalkane, one ring - CnH2n
3) Cycloalkane, two rings - CnH2n-2

The parent chain or longest chain of alkanes can be named with the help of root words which indicate the number of carbon atoms in the molecule. Some root words of alkanes are as given below,
 Number of carbon atoms Root word 1 Meth- 2 Eth- 3 Prop- 4 But- 5 Pent- 6 Hex- 7 Hept- 8 Oct- 9 Non- 10 Dec-

If there are two parent chains with the same number of carbon atoms, we have to consider the parent chain with more substituent.  During the number of the parent chain, the substituent must get lowest number.

If there is more than one substituent in the parent chain then the sum of the positions of the substituent must be least. In case of branched alkanes, the branched substituent must get lowest number in the parent chain. The substituent is named as an alkyl group. The name of alkyl group is again dependent on root word or the number of the carbon atoms in the alkyl group. Some common alkyl groups with the number their formulas are as given below,

 Name of alkyl groups Structural Formula Methyl- $CH_{3}$- Ethyl- $C_{2}H_{5}$- Propyl- $C_{3}H_{7}$- Butyl- $C_{4}H_{9}$- Pentyl- $C_{5}H_{11}$- Hexyl- $C_{6}H_{13}$- Heptyl- $C_{7}H_{15}$- Octyl- $C_{8}H_{17}$-

After writing the root word of the branched alkane, start the numbering of the substituent chain bonded on the parent chain. The name of branched substituent must be placed in parentheses with the number of its position. In case of more than one substituent, they must be written in alphabetical order by ignoring numerical and prefixes.

## Stability of Cycloalkanes

Every cycloalkane does not have a similar degree of stability. It is determined based on their enthalpies of combustion values. Higher the enthalpy per CH2, lower is its stability. The stability of cycloalkanes has been explained in terms of Baeyer strain theory. The standard valence angle 109o28' in carbon compound can be altered by bending of valence bonds. As a result of bending a strain is set up in molecule. Greater is the deviation from the standard value, greater is the strain and greater is the unstability in organic compounds.

The strain on each carbon = $\frac{Normal\ valence\ angle}{2}$

e.g strain in C2H4 = $\frac{109^{o}28' - 0}{2}$ = 54o44'
Ethylene or double bond system may be regarded as a limiting case of a ring system with zero bond angle between its valence bonds:
1) Cyclopropane or C3H6 strain = $\frac{109^{o}28' - 60^{o}}{2}$ = 24o44'

2) Cyclobutane or C4H8 strain = $\frac{109^{o}28' - 90^{o}}{2}$ = 9o44'

3) Cyclopentane or C5H10 strain = $\frac{109^{o}28' - 108^{o}}{2}$ = 0.44'

4) Cyclohexane or C6H17 strain = $\frac{109^{o}28' - 120^{o}}{2}$ = 5o44'

5) Cycloheptane or C7H14 strain = $\frac{109^{o}28' - 128^{o}34'}{2}$ = 9o44'

## Alkanes and Cycloalkanes

 Name Number of carbon atoms Root word Structural formula Methane 1 Methyl- $CH_{4}$ Ethane 2 Ethyl- $CH_{3}CH_{3}$ Propane 3 Propyl- $CH_{3}CH_{2}CH_{3}$ Butane 4 Butyl- $CH_{3}(CH_{2})_{2}CH_{3}$ Pentane 5 Pentyl- $CH_{3}(CH_{2})_{3}CH_{3}$ Hexane 6 Hexyl- $CH_{3}(CH_{2})_{4}CH_{3}$ Heptane 7 Heptyl- $CH_{3}(CH_{2})_{5}CH_{3}$ Octane 8 Octyl- $CH_{3}(CH_{2})_{6}CH_{3}$ Nonane 9 Non- $CH_{3}(CH_{2})_{7}CH_{3}$ Decane 10 Dec- $CH_{3}(CH_{2})_{8}CH_{3}$ Undecane 11 Undec- $CH_{3}(CH_{2})_{9}CH_{3}$ Dodecane 12 Dodec- $CH_{3}(CH_{2})_{10}CH_{3}$ Tridecane 13 Tridec- $CH_{3}(CH_{2})_{11}CH_{3}$ Tetradecane 14 Tetrade- $CH_{3}(CH_{2})_{12}CH_{3}$ Pentadecane 15 Pentadec- $CH_{3}(CH_{2})_{13}CH_{3}$ Hexadecane 16 Hexadec- $CH_{3}(CH_{2})_{14}CH_{3}$ Heptadecane 17 Heptadec- $CH_{3}(CH_{2})_{15}CH_{3}$ Cyclopropane 3 Cyclo $C_{3}H_{6}$ Cyclobutane 4 Cyclo $C_{4}H_{8}$ Cyclopentane 5 Cyclo $C_{5}H_{10}$ Cyclohexane 6 Cyclo $C_{6}H_{12}$