C7 Stereochemistry 1 Flashcards
what is stereochemistry?
- a subdiscipline of chemistry which studies the spatial arrangement of individual atoms in a molecule
- these arrangements give molecules their shape
describe shape and bonding at 4-valent carbon
- tetrahedral
- sp3 hybridisation
- 109.5 degrees
describe shape and bonding at 3-valent carbon
- trigonal
- planar structure with sp2 hybridisation around carbons
- 120 degrees
- overlap of p orbitals generates pi bond
describe shape and bonding at 2-valent carbon
- linear
- sp hybridisation
- 180 degrees
- overlap of p orbits generates 2 pi bonds
what are isomers and state the 2 general types
- compounds that have the same molecular formula
- structural / constitutional isomers
- stereoisomers
what are structural / constitutional isomers?
- compounds with the same molecular formula but different structural formulae
what are stereoisomers?
- compounds that are exactly the same (same molecular and structural formula)
- differ in the way that the various groups are arranged in space
what are the 2 classes of stereoisomers?
- conformational
- configurational
what are conformational isomers?
- different conformations of the same molecule
- interconvert easily through rotation around a single bond or ring flip (for cyclic structures)
- represent different conformations of the exact same molecule through bond rotation
what are configurational isomers?
- identical groups differ in how they are arranged around the same central C atom
- do not easily interconvert
- need to break a bond to interconvert
what is a Newman projection?
looking at a molecule (eg. ethane) from the end instead of the front
what are the 2 conformers of ethane in Newman projection?
- staggered (you can see all 6 hydrogen through gaps)
- eclipsed (the back hydrogens are hidden behind the ones at the front)
what are the 3 conformers of a butane molecule in Newman projection?
- staggered conformer anti
- staggered conformer gauche
- eclipsed conformer
starting with the front methyl at the bottom and the back methyl at the top and rotating the back group anti-clockwise by 1/6 of a turn, list the conformers of butane in the Newman projection
- staggered conformer anti (lowest energy)
- eclipsed conformer
- staggered conformer gauche
- eclipsed conformer (highest energy)
- staggered conformer gauche
- eclipsed conformer
describe the structure of a staggered conformer anti of butane in Newman projection
- lowest energy
- carbons are linear and in plane
- methyls are as far apart as possible
describe the structure of an eclipsed conformer of butane in Newman projection
- slightly less energetically favourable than the staggered conformer anti due to repulsion
describe the structure of an eclipsed conformer after half a rotation of the back methyl group in butane in Newman projection (highest energy conformer)
- highest energy
- methyls are very close together and show lots of repulsion
- difficult to form this due to high energy required
describe the chair conformation of cyclohexane
- 109.5 degree angles
- no ring strain (can’t force to be planar, would put too much strain on the bonds)
- no eclipsing in chair conformation
which hydrogens in cyclohexane in chair conformation are axial and which are equatorial?
- axial: go straight up or down when you lay the molecule down
- equatorial: the hydrogens that stick out
describe cyclohexane ring flip
- interconversion of conformers via ring flip changes the axial/equatorial relationship
- axial and equatorial groups swap
describe cyclohexane boats
- no ring strain
- eclipsing in boat conformation
- flag-pole interaction in boat
- eclipsing and flag-pole interaction reduced in twist-boat form
- higher energy than chair
