Chemistry (university) · Part 09
Organic structure and stereochemistry
Functional groups, naming, isomers, 3D shapes of molecules, chirality and R/S labels.
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- Functional groupA functional group is a small group of atoms in an organic molecule that decides how it reacts. Molecules with the same group react in similar ways, whatever the rest of the molecule looks like.
- IUPAC naming of alkanesFind the longest carbon chain for the parent name (meth, eth, prop, but, pent, hex...). Number from the end that gives the lowest numbers to branches. Name each branch (methyl, ethyl) with its number and list them alphabetically.
- Constitutional isomersConstitutional (structural) isomers have the same molecular formula but different atom connections. They have different physical and often chemical properties. The number of isomers grows rapidly with chain length.
- Conformations and torsional strainConformations are the different shapes a molecule makes by rotating around single bonds. In ethane, the staggered shape is lowest in energy and the eclipsed shape is about 12 kJ/mol higher. In butane, the anti shape beats the gauche shape.
- Cyclohexane chairCyclohexane is not flat. It bends into a chair shape, which has no angle strain and no eclipsing. Each carbon has an axial position (straight up or down) and an equatorial position (pointing outward). Large groups prefer equatorial.
- E/Z isomerismRotation about a C=C double bond is blocked, so groups on each side stay fixed. Cis-trans (E/Z) isomers differ in which side groups sit. Rank the two groups on each carbon by priority: if the higher ones are on the same side it is Z, opposite sides is E.
- Chirality and stereocentresA molecule is chiral if it cannot be placed on top of its mirror image. This usually happens at a carbon with four different groups, called a stereocentre (chiral centre). A molecule with n stereocentres has up to 2ⁿ stereoisomers.
- R/S configurationThe Cahn-Ingold-Prelog (CIP) rules label a stereocentre R or S. Rank the four groups by atomic number at the first point of difference. Point the lowest priority away from you, then trace 1 to 2 to 3: clockwise is R, anticlockwise is S.
- Enantiomers, diastereomers and meso compoundsEnantiomers are non-superimposable mirror images. Diastereomers are stereoisomers that are not mirror images. A meso compound has stereocentres but is achiral because of an internal mirror plane, so it has fewer isomers than 2ⁿ.