Organic Structures and Mechanisms 1 Flashcards
Define isomer
Compounds with the same molecular formula but a different constitution, configuration and confirmation
Define structural/constitutional isomerism
Compounds different in nature/sequence of atoms in a molecule (connectivity)
e.g. C4H8O can be in lots of arrangements but as long as all are C4H8O they are constitutional isomers
Define stereoisomerism
Two or more compounds whose structure differ only in the 3D arrangement of their constituents (same constitution)
Define conformational isomerism (conformers)
Different spatial arrangement of a molecule due to rotation about a single bond (as opposed to a double bond)
Generally what is the more stable configuration of a conformer and why?
Staggered, they are 12KJMol^-1 more stable (lower torsional strain) than eclipsed in ethane. Therefore, at any given moment most molecules are staggered
What is torsional strain?
The resistance of a bond twisting in a chemical compound, highest (generally) in eclipsed, so staggered is favoured
What difference in eclipsed to staggered torsional strain will cause a slow spin?
Anything greater than 80KJMol^-1 will cause a slow spin of the C atoms around the single bond
What causes torsional strain?
Electrostatic repulsion between electron clouds
Define antiperiplanar
Torsion angle about C-C bond is 180 degrees (staggered)
Define synclinal
Torsion angle about C-C bond is 60 degrees (staggered)
Define anticlinal
Torsion angle about C-C bond is 120 degrees (eclipsed)
Define synperiplanar
Torsion about C-C bond is 0 degrees (eclipsed)
Define steric hinderance
The increase in energy as atoms are crowded together
What may cause a molecule to favour synclinal over antiperiplanar?
Internal hydrogen bonding will pull the groups towards eachother
What is an sp2 carbon atom?
A carbon atom that is bonded to three atoms (2 single bonds and one double bond)
Why can’t C=C double bonds rotate?
Rotation will break the overlap of the pi-orbital (overlap of the two p-orbitals, allowed by the planar nature of the C=C)
What is E/Z isomerism?
Isomerism found around a C=C bond due to each C atom having 2 different groups attached.
E - Groups are on opposite sides of the C=C double bond to each other
Z - Group are on the same side of the C=C double bond to each other
What are the CIP priority sequencing rules?
- Higher atomic number = higher priority
- Higher oxidation = higher priority
What is an sp3 carbon atom?
An sp3 carbon atom is a saturated carbon atom (bonded to 4 atoms)
What is a chiral carbon?
An sp3 carbon with 4 different groups attached
Define enantiomer/optical isomer
An isomer around a chiral carbon that cannot be superimposed onto its other isomers
What is a configurational isomer?
The spatial array of atoms that distinguishes a stereo-isomer other than distinctions due to differences in conformation
R vs S stereo-isomers
R - clockwise
S - anticlockwise
How do you determine enantiomeric purity?
1) Put sample in polarimeter and irradiate with Na-“D-line”
2) Measure specific rotation @ 25 degrees
Specific rotation = observed angle of rotation (degrees) / path length (dm) x density (gcm-3)
