Organic Chemistry 3.2 - Stereo Chemistry - Geometric Isomers Flashcards
What defines a geometric isomer?(2)
restricted rotation around a c=c bond or c-c bond in a cyclic compound, AND two unique groups (IE anything that’s not hydrogen), one on each carbon involved in the c=c bond.
Single bonds allow for ___ ___.
free rotation
Two molecules of 1,2-dichloroethane could have both chlorines on the ___ side, or on ___ sides, and they would still be ___.
same, different, isomers/the same
However, if two molecules of 1,2-dichloroethene had both chlorines on the same side, it would be called…
and if the chlorines were on different sides, it would be called…
cis 1,2-dichloroethene
trans 1,2-dichloroethene
What is a cis isomer?
What is a trans isomer?
an isomer with two notable groups on the same side of a double bond (IE above or below)
an isomer with two notable groups, one on either side of the double bond
In a cycloalkane, each of the carbon atoms has a ___ structure.
tetrahedral
If two hydrogen atoms (that were attached to two different carbons) on a cycloalkane were replaced with something else, eg chlorine, how many forms of that molecule would there be?
two:
cis 1,2-dichloropentane, where the chlorine atoms are on the same side,
and trans 1,2-dichloropentane, where the chlorine atoms are on different sides
in the exam draw c=c containing compounds using full structural formula
Geometric isomers must have one different ___ on both of the ___ involved in the ___ bond.
group, carbons, c=c bond (or anywhere on a cyclic compound).
Is C2H3Cl capable of geometric isomerism?
What about C2H2Cl2?
no. (only one carbon in the c=c bond has a different group/atom.
yes, (provided the chlorines are not on the same carbon)
Trans isomers have higher ___ points, due to…
melting, due to more effective LDFs, which is due to the molecules being able to stack together more closely
Cis isomers have higher ___ points, due to…
boiling, due to (sometimes) polarity across the molecule, due to both polar bonds (eg C-Cl) being on the same side of the molecule. This results in PD-PD attractions as well as LDFs
___ isomers have higher boiling points, due to __-__ attractions being present.
Cis, PD-PD.
___ isomers have higher melting points, due to stronger ___, which are present due to…
trans, LDFs, more dense stacking of molecules
Chemical properties may vary between geometric isomers - for example cis dibutandioic acid can be ___, but trans butanedioic acid cant
dehydrated (because both -OHs are on the same side
Why do trans isomers tend to have higher melting points?
Why do cis isomers tend to have higher boiling points?
because their LDFs are more efficient/stronger because the molecules are able to pack closer together
because the molecules will have PD-PD attractions (due to both of the polar groups are on one side)
