9701 · 13.4
Isomerism: structural isomerism and stereoisomerism
Isomers are compounds with the same molecular formula but different structures. This difference can be in how the atoms are connected (structural) or how they are arranged in 3D space (stereo).
Need to know
What you need to know
- **Chain Isomerism:** The carbon chain is arranged differently. For example, pentane (a straight chain of 5 carbons) and 2-methylbutane (a branched chain) are chain isomers with the formula C₅H₁₂.
- **Positional Isomerism:** The carbon skeleton is the same, but a functional group or substituent is in a different position. For example, butan-1-ol and butan-2-ol are positional isomers.
- **Functional Group Isomerism:** The isomers contain different functional groups. For example, ethanol (an alcohol, CH₃CH₂OH) and methoxymethane (an ether, CH₃OCH₃) are functional group isomers with the formula C₂H₆O.
Explanation
Molecular Mix-Ups
- Structural isomers have the same molecular formula but a different structural formula (connectivity). Use the simulator to build butan-1-ol and 2-methylpropan-2-ol from C₄H₁₀O.
- Functional group isomers are structural isomers with different functional groups. For example, an alcohol (R-OH) and an ether (R-O-R') can both have the formula C₂H₆O.
- E/Z isomerism occurs in alkenes due to restricted rotation around the C=C double bond. For this to happen, each carbon in the double bond must be attached to two different groups.
- Optical isomerism occurs when a molecule has a chiral centre: a carbon atom bonded to four different groups. This results in two non-superimposable mirror image molecules called enantiomers.