Unit 4 Flashcards
Constitutional isomers
molecules with the same chemical formula, but different connectivity
Conformational isomers
molecules with the same formula and connectivity, but occupy a different 3D space due to rotation around a sigma bond
Gauche
60º
Eclipsed
directly in back of
no matter which atoms/molecules are where, this is always a higher energy level than gauche or anti
Anti
180º
the most stable when the largest constituents are 180º away from each other
Enantiomers
non-superimposible mirror images of eachother
What did the example with 2-bromobutane show?
that depending on the possible configurations, sometimes a conformational change can make a superimposible mirror image but sometimes it can not
Do enantiomers have to have the same configuration?
No
For example, 2-bromobutane had two different configurations that made an enantiomer
Chiral
a molecule with a nonsuperimposable mirror image
Criteria for chiral
- does the molecule have a single asymmetric carbon? If yes, its chiral
- does the molecule have internal mirror plane/symmetry? If yes, its achiral
How to assign R and S configuration
- orient the molecule so lowest priority group goes into the plane (hydrogen normally)
- assign the priorities of the 3 remaining groups based on atomic mass
- look at counter clockwise vs. clockwise
S-configuration
goes counter clockwise
R-configuration
goes clockwise
Think: r = “right”
Does a conformational change result in a configurational change?
No.
You can rotate about the sigma bonds, but R and S configuration will still remain the same
What to check for after you have determined that two molecules are the same molecules with the same connectivity?
Check for if they actually have the same configuration just rotated in space
Configurational isomers with a non-superimposable mirror image
enantiomers
Configurational isomers with a superimposable mirror image
disastereomers
Stereoisomers
isomers with the same constitution but that differ in the respect to the arrangement of their atoms in space
Disastereomers
stereoisomers that have different configurations at one or more (but not all) of the equivalent asymmetric carbon centers and are NOT mirror images
Geometric
also called cis-trans isomerism
stereoisomers that involve a ring or double bond
identical substituents can either be on the same side or opposite sides of the bond or ring
As bond length increases what happens to dipole moment?
decrease
inverse relationship between bond length and dipole moment
Why are intramolecular covalent bonds stronger than intermolecular dipole-dipole bonds?
electrons are shared in a bond between two nuclei. this creates a strong force of attraction.
a dipole-dipole bond is only attracted to a partially positive/partially negative species. also, occur at further distances.
What is the strongest bond?
ionic
Hydrogen bonding donor
polar species that contains a H that is covalently bonded to an O, F, or N
Hydrogen bonding acceptor
any polar species with a lone electron pair on O, F, or N
*does not need a H atom, since this will be donated to it
Adhesive forces
forces between UNLIKE molecules
Cohesive forces
forces between LIKE molecules
Forces in chain molecules vs. branched molecules
in branched molecules, electrons can get closer together and potential energy increases.
therefore, chained molecules have stronger forces and higher boiling points because they have less PE and greater Force of Attraction to overcome
What do dispersion forces occur due to?
polarized e- clouds
therefore, larger molecules have greater intermolecular attractions
Dipole-dipole interactions
occur between two polar molecules
What is hydrogen a form of?
dipole-dipole interactions
Ex: if both ICl and Br2 have a dipole moment how can you tell which is stronger?
stronger = higher boiling point
Br2 only has a temporary dipole because there is no different in electronegativity
therefore, ICl will be stronger since it has dispersion forces and a permanent dipole
What causes temporary dipoles? (dispersion forces)
random fluctuations in e- clouds distributions
Polarizable
larger molecules have more polarizable e- clouds which makes them form stronger dispersion forces
What is another name for a stereoisomer?
constitutional isomer
Is a conformational isomer a stereoisomer?
no
Is a conformational isomer the same molecule?
yes.
it is just rotated around a sigma bond differently
Is a configurational isomer the same molecule?
no
have to physically move something on the same carbon
Is a constitutional isomer the same molecule?
no
there is different connectivity
Criteria for an enantiomer
- determine that it is a configurational isomer
- look at asymmetric carbons
- enantiomers have to have exact opposite configuration
ex: R S // S R
Are enantiomers chiral?
yes
Meso compound
referring to mirror image in ONE compound
two asymmetric carbons
one with R and one with S
this makes an internal plane of symmetry and ACHIRAL
An entaniomer with a single asymmetric center has a R configuration. What does the other isomer have to be?
has to have an S configuration
