Name: Structure of Benzene
Uploaded: 2019-07-19
Duration: 3 min 53 s
Description: Benzene is the most commonly known aromatic compound and is a significant part of gasoline. In this video, we will study the structure and properties of benz...

Question

Explain the differences between structural isomers and stereo isomers.

Accepted Answer

A structural isomer is one in which two or more or organic compounds have the same molecular formulas but different structures. The two pentane molecules below differ only in the location of the methyl group.

Example of alkane structural isomers

Alkenes can also demonstrate structural isomerism. In alkenes, there are multiple structural isomers based on where in the chain the double bond occurs. The condensed structural formulas of 1-butene and 2-butene show this.

$\underset{}{\underset{1 - Butene}{{CH}_{2} = CH - {CH}_{2} - {CH}_{3}} \underset{2 - Butene}{{CH}_{3} - CH = CH - {CH}_{3}}}$

Example of alkene structural isomers

The number in the name of the alkene refers to the lowest numbered carbon in the chain that is part of the double bond.

Geometric Isomers
With a molecule such as 2-butene, a different type of isomerism called geometric isomerism can be observed. Geometric isomers are isomers in which the order of atom bonding is the same but the arrangement of atoms in space is different. Geometric isomers are also referred to as stereoisomers. The double bond in an alkene is not free to rotate because of the nature of the pi bond. Therefore, there are two different ways to construct the 2-butene molecule. The image below shows the two geometric isomers, called cis-2-butene and trans-2-butene.

The cis isomer has the two single hydrogen atoms on the same side of the molecule, while the trans isomer has them on opposite sides of the molecule. In both molecules, the bonding order of the atoms is the same. In order for geometric isomers to exist, there must be a rigid structure in the molecule to prevent free rotation around a bond. If the double bond in an alkene was capable of rotating, the two geometric isomers above would not exist. In addition, the two carbon atoms must each have two different groups attached in order for there to be geometric isomers.

Explain the differences between structural isomers and stereo isomers.

Important Questions on Hydrocarbons

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