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)
What specific rotation value would you get for a racemic mixture?
0
If a chiral compound is formed from an achiral starting mixture, what ratio of enantiomers would we get?
50:50
How many stereoisomers would you get for a molecule with 2 chiral centres?
4
What is a diastereomer?
Stereoisomers that are not entantiomers
If you attempted to reflect a molecule with two identical chiral carbons to form its other entantiomer what would you actually get?
A meso-isomer (no optical rotation)
Are meso-isomers also diastereoisomers?
Yes
Define Tautomers
Tautomers are isomers of a molecule that exist in solution or in a cell. They are interchangeable forms because chemical bonds are rearranged many times spontaneously
They are constitutional isomers
Define Atropisomer
stereochemistry arising from restricted bond rotation that creates a chiral axis
Features of cyclopropane
- Smallest cyclic compound
- Planar
- Can’t adopt non-planar conformations
- Bond angle: 60 degrees
Features of cyclobutane
- Can adopt a non-planar conformation
- bond angle: 90 degrees
- Favours non-planar conformation due to decreased eclipsing
Features of cyclopentane
- Can adopt a non-planar conformation
- Bond angle of 108 degrees
- No angle strain in planar confirmation but high torsional strain
Features of cyclohexane
- Can be planar
- 2 major non-planar conformations (Boat and chair)
Features of Boat confirmation of cyclohexane
- non-planar
- extensive eclipsing
- high torsional strain
Features of chair conformation of cyclohexane
- non-planar
- Low torsional strain
- Low eclipsing
What interactions occur in a monosubstituted cyclohexane molecule
- 1,3 diaxil interactions
- steric hindrance
Define racemate
a mixture of equal quantities of two enantiomers
Define axial
a bond which is parallel to the axis of the ring, or a group attached by such a bond
Define equatorial
a bond which is perpendicular to the axis of the ring (i.e., the bond lies along the equator of the chair), or a group attached by such a bond
