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Isomers of Butane

Alkanes are simplest hydrocarbons with all C-C bonds.  Therefore they are called as saturated hydrocarbons. General formula for alkanes is CnH2n+2. All carbon atoms tend to complete their tetravalency by bonding with same or different atoms. In alkanes, all carbon atoms form single covalent bonds with other carbon atoms.  

The parent chain can be branched or un-branched and on the basis of that there chemical and physical properties get change. 

Alkanes are less reactive compare to other hydrocarbons like alkenes, alkynes etc. This is because in alkanes, all carbon atoms are bonded with single covalent bonds which are strong and less reactive compare to double or triple covalent bonds of alkenes and alkynes respectively. 

The members of homologues series of alkanes are differ from each other by –CH2 unit. As the number of carbon atoms increases in the series, the molecular mass also increases. The straight chain alkanes are listed below.

 Name of Alkane   Carbon atoms   Chemical Formula   Simple Structure 
 Methane  1  $C H_{4}$  Methane
 Ethane  2  $C_{2}H_{6}$  Ethane
 Propane  3  $C_{3}H_{8}$  Propane
 Butane  4  $C_{4}H_{10}$  Butane
 Pentane  5  $C_{5}H_{12}$  Pentane
 Hexane  6  $C_{6}H_{14}$  Hexane
 Heptane  7  $C_{7}H_{16}$  Heptane
 Octane  8  $C_{8}H_{18}$  Octane
 Nonane  9  $C_{9}H_{20}$  Nonane
 Decane  10  $C_{10}H_{22}$  Decane

 

Isomers of Butane C4H10

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 Isomers are molecules with same molecular formula but different structural formula. This phenomenon is called as isomerism.  Due to different structure, isomers may have different chemical and physical properties also. Let’s discuss the isomers of molecular formula C4H10.  There are four carbon atoms in given molecular formula. So these four carbon atoms can arrange in two different manners. They can either arrange in straight chain of four carbon atoms or they can form a chain of 3 carbon atoms with one side chain. 

 Structural formula   Name of isomer   Line –angle formula 
 $CH_{3} - CH_{2} - CH_{2} - CH_{3}$   n-butane  Line Angle Formula (n-butane)
 $CH_{3} - CH(CH_{3})CH_{3}$  2 - methylpropane or isobutane   Line Angle Formula (2 - methylpropane or isobutane)

How Many Isomers Does Butane have?

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Butane is an alkane with four carbon atoms so molecular formula is C4H10. It has two isomeres; n-butane and isobutane. 

Butane & Isobutane
 
Here n-butane is a straight chain compound with four carbon atoms bonded with single covalent bonds. 

n-butane

Another isomer is isobutane or 2-methylpropane in which three carbon atoms form the parent chain and one carbon atom is placed as the side chain at C-2 of parent chain. All carbon atoms have 4 valencies which are satisfied either by carbon atoms or hydrogen atoms. 

Isobutane or 2 - Methylpropane

Isomers of Butane Structural Formula

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The molecular formula for butane is $C_{4}H_{10}$. There are two possible isomers with this molecular formula. First one is n-butane which has all four carbon atoms in parent chain with structural formula as;

$CH_{3}-CH_{2}-CH_{2}-CH_{3}$

Another isomer is isobutane with three carbon atoms in parent chain and one in side chain as methyl group. 

Molecular Formula for Butane

Constitutional Isomers of Butane

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Constitutional isomers can be defined as the compounds that have the same molecular formula and different structural formula. In other words, constitutional isomers have different connectivity of atoms in molecules.  To determine that two molecules are constitutional isomers of each other or not, we have to count the number of each atom in both molecules.

The molecular formula must be same for both the molecules but the arrangement of atoms should be different.  For example; look at the structure of n-butane and isobutane. 
Constitutional Isomers of Butane
 
Both of these molecules have same molecular formula that is C4H10. Now look at the connectivity of atoms (carbon atoms). In case of n-butane, all carbon atoms are in straight chain whereas in case of isobutane, there is a side chain in the molecule. So they have different connectivity of atoms and are constitutional isomers of each other. 

Conformational Isomers of Butane

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Stereoisomers which can be converted into one another by rotation around a single bond are called as conformational isomers. Alkanes usually show conformation isomerism because of presence of C-C bonds. For example when we rotate the molecule of butane at the axis of C-C bond, we get eclipsed, gauche, and anti butane conformational isomers.  Here eclipsed conformation has identical groups directly in-line with one another that makes it unstable. Gauche conformation stands for the presence of identical groups at 60 degrees from one another. It is more stable compare to eclipse conformation due to less steric hindrance between same molecules. In anti conformation, identical groups are 180 degrees from one another which makes it most stable form. Each conformer is interconvert by rotating around the central carbon single bond like eclipsed conformer can convert into gauche conform by rotating 60 degrees. Rotation of eclipsed by 180 degree forms anti- conformer. Rotation of anti butane by 120 degrees forms gauche conformer. 

Conformational Isomers of Butane

Staggered Conformation of Butane

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The study of conformers mainly involves the arrangement of atoms or groups with respect to central atom. There are different ways to represents conformers like Fisher projection, Saw-horse or Newman projection. Newman projection can be defined as the representation of a molecule in which the atoms and bonds are viewed along the axis of rotation. In a Newman projection, the front carbon is indicated as dot and back carbon atom as circle. The substituent on carbon atoms can be viewed both in front of and behind the carbon-carbon bond. For example in ethane molecule, there are two possible conformations. One is with the dihedral angle 0° and another in which dihedral angle is 60 degree. First case is called as eclipsed conformation in which the H atoms of C-1 line up with the H-atoms of C-2. So we can say that in this conformation, the H-atoms are line up perfectly close to each other.  In another conformer with dihedral 60 degree, the H-atoms of C-1 are far enough to H-atoms of C-2. This is called the staggered conformation. 

Staggered Conformation of Butane

Since H-atoms are far enough in staggered conformer, it is more stable than eclipsed conformer. Staggered conformer of butane can exist in two forms; anti and gauche conformers. If we fix the position of one carbon atom of butane and rotate other, it gives three types of conformers; eclipsed, anti and gauche. There are two kinds of staggered conformations. In anti form, both methyl groups are at anti position. It is one of most stable form of butane. The other one is known as gauche or skew conformation. 
Staggered Conformation of Butane

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