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Alkane Formula

All known chemical compounds can be classify as organic and inorganic compounds. Hydrocarbons are organic compound which are mainly composed of carbon and hydrogen. On the basis of structure, hydrocarbons can be classified as open chain compounds and cyclic hydrocarbons. Open chain compounds are also known as aliphatic compounds.  Aliphatic or open chain compounds have straight or branch carbon chain as parent chain in the molecule. On the basis of presence of single or multiple covalent bonds, aliphatic compounds can further classify as alkanes, alkene and alkynes.  Alkanes are saturated hydrocarbons in which all the C-C is single covalent bond in the molecule.  For example; the formula of ethane molecule is $CH_{3}-CH_{3}$. In this molecule two Carbon atoms are bonded with single covalent bond. Another type of aliphatic hydrocarbons is unsaturated hydrocarbons which have multiple covalent bonds in parent carbon chain. On the basis of multiple bonds, unsaturated hydrocarbons can be classified as alkenes and alkynes. Alkenes are unsaturated hydrocarbons have double covalent bond in parent carbon chain whereas alkyne molecules have triple covalent bond between carbon atoms. Because of presence of multiple bonds, alkenes and alkynes are more reactive compare to alkanes. 

Hierarchy of Hydrocarbons

Another type of hydrocarbons is cyclic compounds which have carbon ring like aromatic and alicyclic compounds.   Aromatic compounds are very stable cyclic compounds with benzene ring whereas alicyclic compounds are cyclic compounds with single or multiple covalent bonds. Cycloalkanes are saturated alicyclic compounds whereas cycloalkenes and cycloalkens are unsaturated alicyclic compounds. In this article we will discuss about structure of alkanes. 

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General Alkane Formula

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Hydrocarbons are organic compounds which are composed of C and H atoms. The general formula of alkanes is $C_{n}H_{2n+2}$. Similarly for alkenes, general formula is $C_{n}H_{2n}$ and for alkynes it is $C_{n}H_{2n-2}$. Alkanes with different carbon atoms and same general formula are listed below.

 Number of C atoms 
 2  $C_{2}H_{6}$   Ethane 
 3  $C_{3}H_{8}$  Propane 
 4  $C_{4}H_{10}$ 
 5  $C_{5}H_{12}$  Pentane
 6  $C_{6}H_{14}$  Hexane
 7  $C_{7}H_{16}$  Heptane
 8  $C_{8}H_{18}$  Octane 

Alkane Formula and Names

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IUPAC provides certain rules for writing the names and formula for organic compounds. IUPAC name of organic compounds are composed of three parts;

Prefix   + $\frac{Base\ word}{Root\ word}$   +   Suffix

Here base word or root word indicates the number of carbon atoms in the longest parent chain of alkene. The root words for different numbers of C atoms are listed below;

 Number of C-atoms 
  Root word 
 1  Meth-
 2  Eth-
 3  Prop-
 4  But-
 5  Pent-
 6 Hex-
 7  Hept-
 8  Oct-
 9  Non-
 10  Dec-

Prefix indicates the side chain at parent chain and also position of it. For example; 2-methyl indicates that there is a methyl group at position 2-.
Suffix is added at the end of root word. There are two types of suffix; primary and secondary suffix. Primary suffix indicates the presence of single or multiple bonds in the parent chain. For Alkane, we use suffix –ane, for alkene the primary suffix is –ene and for alkynes it is –yne.  Secondary suffix indicates the presence of any functional group in the molecule like carboxy group (-COOH), hydroxyl group (-OH) etc.

Alkane Formula Examples

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Common examples of alkanes with their structure and formula.

(i)  Examples of Alkanes 
In the given structure, the longest chain contains 7 carbon atoms. So root word must be hept-. There are four side chains on the parent chain. At C-4, the side chain contains three carbon atoms so name of this side chain would be propyl. Apart from this side chain there are three methyl groups at C-5, C-3 and C-2. The name of side chain must be written alphabetically. There is no double bond, so primary suffix should be –ane which must be added at the end of root word. Hence IUPAC name of this alkane should be;

(ii)  Examples of Alkanes
In the above structural formula of alkane, the longest chain contains 9 carbon atoms so root word must be Non-. Since there is no other functional group so primary suffix will be used only, no secondary suffix will be part of name. The primary suffix for single covalent bond is –ane. Now we have to check the side chains which are bonded on the parent chain. There are three side chains on parent carbon chain. One side chain with 4 carbon atoms is sec-butyl group. It is located at 5 –position. There are two methyl groups at 2 and 7 positions respectively.

Alkane Formula List

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The structural formula for alkane can be written as condense form or line form. Structural formula must provide a correct number of all C and H atoms in alkane molecule with their bonding with each other. Condense formula involves all atoms including H. Like condense structural formula for ethane is $CH_{3}CH_{3}$. In line structural formula, atoms are represented as dots and bonds with lines. Few example of condense and line formula of alkanes are listed below.

 Sum Formula   Structural Formula   Half structural Formula  
 $CH_{4}$    $CH_{4}$
 $C_{2}H_{6}$    $H_{3}C - CH_{3}$
 $C_{3}H_{8}$    $H_{3}C - CH_{2} - CH_{3}$ 
 $C_{4}H_{10}$    $H_{3}C - CH_{2} - CH_{2} - CH_{3}$ 

Structural Formula of Alkane

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Alkanes are saturated hydrocarbons with all single bonded carbon atoms in the parent chain. The condense structural formulae for alkanes are listed below.  

Structural Formula for Alkanes Table

Since all carbon atoms are bonded with single covalent bonds hence no bond must be shown between carbon atoms.  The line structural formula for alkanes is shown by zig-zag lines. Each line indicates a single covalent bond between two carbon atoms.  Like the line –angle formula for butane is given below. 

Structural Formula for Butane

Each point indicates one carbon atom and no H atom is shown in the formula. Since all C-atoms are bonded with single covalent bond so remaining valency of carbon atoms will be satisfied with H atoms. Some other examples of alkanes with their structural formulae are

Ethylcyclopentane, Heptane, 3 - Ethylpentane and 3 - Chloropentane

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