=What are Isomers?= There are several different types of isomers, and for some courses you may need to know only some types (not all). ==Structural (or constitutional) Isomers== This is the most common type of isomer. It is when there are compounds which share the same molecular formula, but they have a different arrangement of the atoms bonded together (different structural formula) The easiest way to see if you have isomers is to make a tally of the atoms in each molecule (they should be the same), and name each molecule (the names should be different). e.g. C6H14 could be drawn as hexane or 2-methylpentane, or a number of other ways! ==Positional Isomers== These isomers come under "structural isomers" but are a bit special because it involves changing the position of the functional group. Butan-1-ol and butan-2-ol are positional isomers. ==Geometric Isomers== These isomers only form in alkenes, or compounds which cannot freely rotate. The criteria for geometric isomers are: - A double C=C bond. - a group which is the same on each carbon of the double bond - NO group which is found twice on the same carbon. If the identical groups on either carbon are facing the same way, then the isomer is named as cis. See the first image for cis-but-2-ene. If the identical groups on either carbon are facing opposite ways, then the isomer is named as trans.See the second image for trans-but-2-ene. [image:https://upload.wikimedia.org/wikipedia/commons/thumb/8/84/Cis-2-butene.svg/220px-Cis-2-butene.svg.png] [image:https://upload.wikimedia.org/wikipedia/commons/thumb/1/1e/Trans-2-butene.svg/2000px-Trans-2-butene.svg.png] ==Optical Isomers== Not assessed in NCEA Level 2 These are isomers which exist as mirror images of each other. The criteria for an optical isomer to exist is that a carbon has 4 different groups attached to it. This carbon is known as a "chiral" carbon. [image:https://upload.wikimedia.org/wikipedia/commons/thumb/e/e8/Chirality_with_hands.svg/300px-Chirality_with_hands.svg.png] Optical isomers are labelled as R or S, depending on how the groups are arranged. See the video for more information if your course covers optical isomers.
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