Stereochemistry Flashcards
What are isomers?
Molecules with the same molecular formula but different structural formula.
Structural isomers occur when?
Atoms are bonded together in a different order in each isomer.
Stereoisomers occur when?
The order of binding in the atoms is the same but the spatial arrangement of the atoms is different in each isomer.
What are the 2 types of stereoisomers?
Optical and geometric.
Geometric isomers can occur when there is?
Restricted rotation around a carbon-carbon double bond or a carbon-carbon single bond in a cyclic compound.
Geometric isomers must have?
2 differnt groups attached to each of the carbon atoms that make up the bond with restricted rotation.
Geometric isomers can be labelled cis or trans according to?
Whether the substitutes groups are on the same side (cis) or on different sides (trans) of the bond with restricted rotation.
Cis and trans isomers have —— physical properties such as mp and bp
Differences in
What makes a cus isomer polar?
Both EN substitutes are on the same side of the double bond, making the molecule polar.
What makes the trans isomer non polar?
The EN substitutes are on opposite sides of the double bond. Due to the symmetry of the molecule, the polar groups cancel eachother out and the molecule is non polar.
Why do cis isomers have higher bps than trans isomers?
Between molecules of the cis isomer there will be pd-pd interactions. These are stronger than LDFs between the molecules of the trans isomer so the bp is higher.
Why do trans isomers have higher mp?
Trans isomers are more straight so pack better than cis isomers. As a result of poorer packing on the cis isomer, the IMFs are not as effective so less energy is needed to melt the solid, giving it a lower mp.
Optical isomers occur in compounds in which…
4 different groups are arranged tetrahedral around a central carbon atom (chiral carbon or chiral centre)
Optical isomers are —— and ————— —— —— — ——-
Asymmetric and non superimposable mirror images of eachother.
Optical isomers can be described as ———
Enantiomers
Optical isomers have identical physical properties EXCEPT…
That they rotate plane-polarised light in opposite directions.
Optical isomers have identical chemical properties EXCEPT…
When in a chiral environment such as that found in biological systems (only 1 optical isomer is usually present)
Optical isomers are optically active because
They rotate plane polarised light by the same amount but in opposite directions.
What is a racemic mixture?
When mixed in equal amounts, optical isomers are optically inactive because of the rotational effect of the plane polarised light cancels out- called a racemic mixture.
