Stereochemistry Flashcards
Who discovered ppl?
1800’s Louis Posteur
What do molecules do?
Molecules rotate the plane of polarized light
Equation for optical activity?
α
[α] = ————-
ℓ x C
[α] = optical activity specific rotation α = observation rotation ℓ = path/length 1dm C = g/mL
Chiral?
Handedness. Mirror image is non-superimposable. Rotates the plane of polarized light
Chiral carbon?
Has 4 different sides (environments)
Recemic Mixture?
1:1 mixture of enantiomers
Mesomers?
Mirror image, chiral center, internal plane of reflection. They are the same except one is R one is S. R reflects as S through a mirror plane
Optical rotation?
Rotation of plane of polarized light. Can have (+) or (-) rotations
Enantiomers (+-) or (d/ ℓ) or (D/L)
Who made fisher diagrams?
Emil Fisher
What are the rules for D/L?
1) Put enantiomer in a fisher with, highest oxidized carbon at the top.
(R-COO-H 3/4 bond to O) (CH3 not oxidized)
2) Put the lowest oxidized carbon (usually methyl) at bottom of fisher
3) Start at the bottom and look up until there is a atom with unpaired electrons
4) If atom with unpaired electrons is on the right is D, if on the left it’s L
What is the flaw with D and L?
Doesn’t take in to account the rest of the chiral centres. Cannot distinguish between enantiomers
Can you flip a fisher?
You cannot flip molecule because the horizontal arms will be going backwards instead of giving a hug
What do R and S mean?
R—> Rectus (clockwise +)
S—> Sinister (counter clockwise -)
Label chiral centre 1 to 4 in order from highest to lowest atomic number
Can R and S change?
The cannot change unless bonds are broken.
Is there a trick to R and S?
Yes! If the H is in your face. To the right of a fisher or the top of a staggered. Don’t flip the molecule do the opposite of what you originally got. S will be R and R will be S
Diostereomers?
Change of one chiral centre. Not the same. One is R one is S
Can you make any molecule a diostereomer?
Yes you can change any amount of R’s to S as long as it is not all of the chiral centres. You can change one, two, three, four, five
Wedged vs hashed
Wedged is coming out of page. If it is H it is coming out of page so switch R to S or S to R. Hashed is going behind
What do you do when converting Newman projection to fisher.
Flip the one side 180 degrees
What is 2^n
Indicated the total amount of stereoisomers. N is the amount of chiral centres in a molecule. If a mesomer is possible subtract 1
Optical activity?
Rotation the day of polarized light
When solving for the specific rotation what do you do?
Look at the ratio. Add together the whole ratio then use the final number as a fraction 1/x.
One enant has +10 the other must have -10. The ratio is 1:1
1/2(10) + 1/2(-10) = [α]
What is %ee?
Percent enantiomeric excess
How to solve for %ee?
Add together the whole ratio. Subtract each ratio number by the total. Subtract the bigger ratio number by the smaller. Put the express ratio number over the total for the %ee
3:1 —> 4
3-1 —>2 2/4 —> %50
Alkane?
Comes from crude oil. Aliphatic chemistry (greasy). Sp3, Sp2 must be all hydrocarbons (CH). The longer the chain the greasier the molecule.
What is catalytic cracking?
Breaks down long molecules of hydrocarbons into little pieces
Who discovered ring sizes?
Von Baeyer and Saches in the early 1800
What ring size is the most stable and least stable?
Most stable. 6 carbon ring. It is predominate in nature.
Least stable is the three carbon ring. There is ring strain, it is unstable and very reactive
What happens when rings pucker?
It forms a chair conformation. Forms from a 6 sided compound
Where are axial bonds?
The point upwards from the ring
Where are equatorial bonds?
They always point away from the ring. Count two bonds away from the axial bond to find the angle of the equatorial bonds.
What is more stable for larger molecules? Axial or equatorial?
Always put larger groups on equatorial bonds because of on an axial the molecules butt heads. Further, the bigger the molecule the lower the energy!
How does chair flipping work?
Makes every down peak up and up peak down. Changes every axial to equatorial and every equatorial to axial.
How do you lock a chair from flipping?
Add a big group onto the equatorial bond.
What is 1/2 a chair flip?
A boat conformation. They are more unstable.
How do you add substituents to a chair diagram?
Start with a blank chair conformation and number the carbons in your cyclohexane and your chair. Make sure your number the same direction for both molecules (clockwise or counterclockwise). Add the atoms to the chair. If wedged. Add it to the up position, of dashed add it to the down position.
What is the cahn- ingold- prelog system?
The absolute configurations. R and S
How do you rank the atoms for R and S?
By atomic number
Constitutional isomer?
Compounds with the same molecular formula but different structural formulas
Stereoisomers?
Isomers with the same composition but different orientations
Achiral?
A molecule that is superimposeable with its mirror image
What is a stereogenic centre?
A chiral centre
