[Chemistry Class Notes] Isomers of Butane Pdf for Exam

Introduction

Alkanes are described as the simplest carbons’>hydrocarbons having all the C-C bonds. Thus, they are referred to as saturated hydrocarbons. The general formula for these alkanes is given as CnH2n+2. All the carbon atoms tend to complete their respective tetra valency by bonding either with the different or same atoms. In the case of alkanes, all the carbon atoms produce single covalent bonds with the other carbon atoms.

About Alkanes

The parent chain is either branched or unbranched and based on the change in chemical and physical properties. Alkanes are concerned to be less reactive compared to other hydrocarbons such as alkynes, alkenes, and so on. In alkanes, all the carbon atoms are bonded with the single covalent bonds that are less reactive and strong compared to either triple or double covalent bonds of alkynes and alkenes, respectively.

The homologous series of alkane members are varied from each other by the unit of -CH2. As the carbon atom number in the series increases, the molecular mass also gets increases.

What are Isomers?

Isomers are described as the molecules having the same molecular formula but with different structural formulas. This phenomenon is referred to as isomerism. Because of the different structure, isomers can have different physical and chemical properties as well.

There exists 4 carbon atoms in the given molecular formula. Hence, these 4 carbon atoms can arrange in 2 different ways. They can either form a chain of 3 carbon atoms with one side chain or arrange a straight chain of 4 carbon atoms.

Structure of Butane

The structures of butane or the structural isomers of butane can be given as follows:

Constitutional Isomers of Butane

The constitutional isomers can be described as compounds having a similar molecular formula but with different structural formulas. To explain, in other words, constitutional isomers hold different connectivity of atoms in the molecules. To determine that 2 molecules are constitutional isomers of either each other or not, we have to count each atom in both the molecules.

The molecular formula should remain similar for both the molecules, whereas atoms’ arrangement should result differently. As an example, refer to the structure of isobutane and n-butane.

Both these molecules hold similar molecular formula, which is given as C4H10. The connectivity of carbon atoms. In the n-butane case, all the carbon atoms fall in a straight-chain, whereas, in the isobutane case, there exists a side chain in the molecule. Hence, they hold different connectivity of atoms, and they are constitutional isomers of each other.

conformational-isomers’>Conformational Isomers of Butane

• Stereoisomers that are converted to one another by the rotation around a single bond are known as conformational isomers.

• Usually, alkanes represent conformational isomerism due to the presence of C-C bonds.

• For instance, when we rotate the butane molecule at the C-C bond axis, we get gauche, eclipsed, and the conformational isomers of anti butane.

• Here, the eclipsed conformation holds identical groups directly in-line with one another, which makes it unstable.

• Moreover, gauche conformation stands for the identical group presence at an angle of 60 degrees from one another.

• It is also a more stable one compared to the eclipse confirmation because of the less steric hindrance between the same molecules.

• In the case of anti conformation, the identical groups exist at an angle of 180 degrees from one another, making it the most stable form.

• Here, every conformer can be interconverted by rotating around the central carbon single bond, such as eclipsed conformer can convert into gauche conform by rotating with an angle of 60 degrees.

• Rotation of eclipsed by an angle of 180-degree results in anti- conformer. The rotation of anti butane by an angle of 120 degrees produces a gauche conformer.

Staggered Conformation of Butane

• The conformers study primarily involves the arrangement of either groups or atoms concerning the central atom.

• There exists several ways to represent conformers, such as Saw-horse, Newman projection, or Fisher projection.

• Newman’s projection is defined as the molecule representation in which the bonds and atoms are viewed along the axis of rotation.

• In this Newman projection, the front carbon can be indicated as a dot, but the back carbon atom is indicated as a circle. The substituent on the carbon atoms can be viewed both behind and in front of the carbon-carbon bond.

• To explain this with an example, in the molecule of ethane, there exist 2 possible conformations. One is given with the dihedral angle of 0°, whereas the other in which the dihedral angle is given as 60 degrees.

• Here, the first case is referred to as eclipsed conformation, where the C-1H atoms line up with the C-2 H-atoms.

• Hence, we can say that in this form, the H-atoms can be lined up perfectly close to one another.

• In the other conformer with a dihedral angle of 60 degrees, the C-1 H-atoms is far enough from the C-2 H-atoms. This is known as the staggered conformation.

Since the H-atoms are far enough in the staggered conformer, it is more stable than that of the eclipsed conformer. Butane staggered conformer can exist in 2 forms; gauche and anti conformers. When we fix the position of 1 carbon atom of the butane and rotate another, it results in 3 types of conformers, named anti, gauche, and eclipsed.