Frequently Asked Question

Here are three examples each of two types of isomers, followed by instructions on how to identify an isomer:
1. Structural Isomers (Constitutional Isomers)
Structural isomers have the same molecular formula but different connectivity of atoms. In other words, the atoms are bonded together in different orders.

Example 1:

Butane (C4H10): A straight-chain alkane with all four carbon atoms connected in a row.

Isobutane (C4H10): A branched-chain alkane with three carbon atoms in a row and one carbon atom branching off the middle carbon.

Example 2:

Pentane (C5H12): A straight-chain alkane with five carbon atoms connected in a row.

Isopentane (C5H12): A branched-chain alkane with four carbon atoms in a row and one carbon atom branching off the second carbon.

Neopentane (C5H12): A branched-chain alkane with three carbon atoms in a row and two carbon atoms branching off the middle carbon.

Example 3:

Propanal (C3H6O): An aldehyde with the carbonyl group at the end of the carbon chain.

Acetone (C3H6O): A ketone with the carbonyl group in the middle of the carbon chain.
2. Stereoisomers
Stereoisomers have the same molecular formula and the same connectivity of atoms, but they differ in the spatial arrangement of their atoms.

Example 1:

(R)-2-Butanol

(S)-2-Butanol

These are enantiomers (non-superimposable mirror images) due to the presence of a chiral center (carbon bonded to four different groups).

Example 2:

Cis-2-butene

Trans-2-butene

These are diastereomers (stereoisomers that are not enantiomers) because the two methyl groups are on the same side of the double bond in cis-2-butene and on opposite sides in trans-2-butene.

Example 3:

Cis-1,2-dichlorocyclohexane

Trans-1,2-dichlorocyclohexane

These are diastereomers because the two chlorine atoms are on the same side of the ring in cis-1,2-dichlorocyclohexane and on opposite sides in trans-1,2-dichlorocyclohexane.
How to Identify Isomers:
1. Compare the molecular formulas. If the molecular formulas are the same, the compounds could be isomers.
2. Draw out the structural formulas. This will help you visualize the connectivity of the atoms.
3. Look for differences in connectivity. If the atoms are bonded together in different orders, the compounds are structural isomers.
4. Look for differences in spatial arrangement. If the atoms are arranged in different ways in space, the compounds are stereoisomers.
5. Determine if the stereoisomers are enantiomers or diastereomers. Enantiomers are non-superimposable mirror images, while diastereomers are stereoisomers that are not enantiomers.
I hope this helps! If you have any more questions, please feel free to ask.