stereochemistry Flashcards
isomers
chemical compounds with the same molecular formula but different structural formula
stereoisomers
have the same bonding sequence , but differ in the orientation of their atoms in space - stereochemistry is the study of the three dimensional shape of compounds
diastereoisomers
stereoisomers whose molecules are not mirroring images of each other : geometric and conformers
enantiomers
stereoisomers whose molecules are non superimposable mirror images of each other
diastereoisomers : conformations
Generally when we consider alkanes, the carbon atoms are all sp3 hybridised and the carbon-hydrogen, carbon-carbon bonds have the same general electron distribution (sigma bonds).
In ethane the bond angles have been determined to be 109.5º; C-H length 1.10 Å; C-C length 1.53 Å
newman projections
we are not limited to a single arrangement of atoms for the ethane molecule , since the relationship between the hydrogens of one carbon and the hydrogen of the other carbon is not specified .
we could have arrangements like that seen in an eclipsed confirmation or a staggered conformation or anywhere in between which is called a skew conformation
unforuntatelyt certain physical properties show that rotation is not quiet free : energy barrier of -3kcal/mol
most ethane molecules occur naturally in the more stable staggered conformation , however an energy barrier of 3kcal is not very high therefore even at room temp there is sufficient energy to cause collisions with enough energy to cause rapid interconversion
torsional energy
the energy required to rotate the ethane about the c-c bond
torsional strain
Eclipsed conformation is relatively instable when compared to the staggered conformation due to torsional strain
Conformations of n-butane
We have established that the staggered conformation has the highest relative stability due to the minimum torsional strain
What about molecules with more than one staggered conformation?
-anti conformation and gauche conformation
anti is more stable than gauche due to van Der Waals repulsion
both are free from torsional strain
Cycloalkanes: cyclopropane
The carbon atoms in alkanes are all sp3 hybridized. The normal tetrahedral bond angle of an sp3 hybridized atom is 109.5 º.
In Cyclopropane the internal angles must be 60 °.
This compression is called angle strain
The C-C bonds are weaker in cyclopropane and as a result the molecule has greater potential energy.
As the molecule is planar, the Hydrogen atoms are all eclipsed, this leads to considerable torsional strain
Cycloalkanes: cyclobutane, cyclopentane
Cyclobutane also has considerable angle strain. The internal angles are 88 °.
However, the cyclobutane ring is not planar – but slightly bent or folded.
If the ring where planar then angle strain would be slightly less as the internal angle would be 90 °, however, torsional strain would be considerably larger as all 8 hydrogen atoms would be eclipsed.
Cyclopentane, if planar would have internal angles of 108 °, a value very close to the tetrahedral bond angle of 109.5. However, considerable torsional strain would be introduced if all 10 hydrogen atoms were eclipsed.
Cyclopentane assumes a slightly bent conformation, twisting of the C-C bonds can occur with little change in energy, therefore the molecule is flexible
Cycloalkanes: cyclohexane
Cylcohexane can be considered the most stable of the cycloalkane series
The reason for this stability to the stable conformation that cyclohexane can adopt.
The chair is the most stable conformation with 99% of all cyclohexane molecules in this configuration at any given time
Chair conformation is a non planar structure, all internal C-C bonds are 109.5 °, and therefore free of angle strain.
Free from torsional strain as all hydrogen atoms are staggered.
The boat conformation is also free of angle strain.
It is not free of torsional strain, the hydrogen atoms adjacent are eclipsed.
Additionally, two of the hydrogen atoms on C1 and C4 are close enough to each other to cause Van der Waals repulsion, this effect as been called the flagpole effect, or transannular strain
Torsional strain and flagpole interactions cause the structure to have significantly higher energy than the chair conformation.
The boat conformation is quite flexible, and can easily trans form to the twist form.
Substituted Cyclohexanes
If we consider the hydrogen atoms in the chair conformation
One hydrogen atom from each of the 6 carbon atom generally lies in the plane of the ring, by analogy with the equator of the earth, these are referred to as equatorial bonds.
The 6 remaining Hydrogen atoms are perpendicular to the plane of the ring, axial hydrogen atoms (red).
Cyclohexane rapidly flips back and forth between the two equivalent chair conformations.
When this happens the bonds switch with all the equatorial bonds becoming axial and vice versa
Disubstituted cyclohexanes
The presence of two substituents on a ring allows for the possibility of cis trans isomerism.
In cyclopentane this is easy to determine as the ring is relatively planar
Cis trans isomerism is a form of diastereoisomerism: Geometric Isomers
They differ only in their arrangement of atoms in space
The two forms cannot be interconverted without breaking bonds
Cis trans isomers have different physical properties, such as boiling point, melting point etc
They can be easily separated and stored
CIS/TRANS isomerism where there is more than two elements
to figure these out you must know the molecular weights of all the chemicals
the two heaviest will decide the isomerism
when there is more than 2 they have different names
E=OPP
Z=BESIDE
