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Isomerism
Uni of Notts, fundamentals of inorganic & organic chemistry, first year
| Term | Definition |
|---|---|
| Constitutional isomers | Same molecular formula but different atomic organisation |
| Conformational isomers | Different C-C bond rotation of the same constitutional isomer, they have the ability to interconvert by rotation around a single bond |
| Stereoisomers | Same atomic connectivity but different bond rotations, usually C=C bonds, these cannot be interconverted |
| Diastereoisomers | Non-superimposable, non-mirror image stereoisomers (e.g., trans/cis-isomers but not enantiomers) contain |
| Rule when synthesising chiral molecules using achiral reagents | The molecules will always form a racemic mixture |
| CIP rules: Step 1 | Order constituent groups in terms of priority, higher atomic numbers have higher priority |
| CIP rules: Step 2 | The group with the lowest priority goes at the back of the plane with the highest priority going upwards out of it |
| CIP rules: Step 3 | Draw a circle from 2-3-4. If it's clockwise then it is R, if anticlockwise then S |
| How enantiomers with multiple stereogenic (chiral) centres are drawn | Isomers written with 2 directional markers e.g., (S, R). Isomers with the same of each directional marker are enantiomers while those with different of each marker are diastereoisomers |
| Mesocompounds | Molecules with multiple chiral centres which are overall achiral with no optical activity due to possessing an internal plane of symmetry |
| Relative stereochemistry | Position of substituents on different stereogenic centres relative to each other. If they're both in the same direction in the plane then they have the prefix -"syn", vice versa for -"anti" |
| Newman projections | Technique to determine the confirmation of an organic molecule by looking lengthways down the bond of interest |
| Process of Newman projections | Front C drawn as black dot, back C drawn as circle around it, groups drawn as lines coming from them. For multiple Cs, they're drawn as groups coming off each C in the bond, similar to cis/trans-isomers |
| Eclipsed projections | Groups on front & back Cs align with 0° between bonds. These are energetically unfavourable due to repulsion of filled orbitals causing maximum torsional strain |
| Staggered projections | Non-overlapping groups on adjacent Cs with 60° dihedral bonds. These are energetically favourable due to attraction between filled & empty orbitals which stabilises the molecule |
| Chair conformations of cyclohexane | 3D diagrams of cyclohexane accurately displaying steric & torsional strain with alternating carbons above & below each other. Chair conformation is energetically favourable with 2 mirror image conformational isomers |
| Axial substituents | Groups coming off each C alternating directly up & down in the plane |
| Equatorial substituents | Slightly angular groups coming out of plane off the side of each C, more energetically favourable by minimising 1,3-diaxial interactions |
| 1,3-diaxial interactions | Steric repulsions occurring between axial substituents & those on the same side 3Cs away. This increases the energy of the molecule & overall make it less stable |
| How conformational isomers of cyclohexane are favoured | Larger substituents in the axial position cause more 1,3-diaxial interaction which means the other conformer with it as equatorial will be more stable & therefore more favoured |
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Beech47
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