2. Isomers Flashcards
constitutional isomers/structural isomers vs regioisomers/positional isomers vs stereoisomers vs conformational isomers vs configurational/geometric isomers. Give examples of each
same composition (aka same formula) but diff IUPAC name (ex: TAUTOMERS, hexane vs 2-methylpentane) vs constitutional isomers w/ same functional groups but those groups = at diff positions vs same constitution but diff stereochemistry (breaks into conformational and configurational isomers) vs rotate around C, same constitution, same configuration, diff conformation (ex: Newman projections) vs actually need to break bonds, same constitution (aka same IUPAC name) but diff configuration (aka diff prefix) (ex: R/S enantiomers, E/Z, cis/trans alkenes, or diastereomers)
Gauche vs anti vs eclipse vs totally eclipsed of Newman projections
biggest group = 60 degrees apart, more stable than eclipsed vs biggest group = 180 degrees apart, most stable, lowest energy state vs biggest group = 120 degrees apart vs biggest group = 0 degrees apart, least stable, highest energy state
Chiral vs achiral. Examples of optical isomers?
object with nonsuperimposable mirror images (hands) vs object with superimposable mirror images (forks). Both enantiomers and diastereomers
What’s a chiral carbon?
Tetrahedral carbon w/ 4 diff substituents (that means if a C has a double bond –> not a chiral center)
What’s a racemic mixture?
when both enantiomers are in equal conc –> cancel out –> no optical activity; RACEMIC MIXTURES MEANS A PAIR OF ENANTIOMERS
What does a Fischer projection tell you?
vertical lines away from you, horizontal lines coming at you
Know specific rotation and the units. Know % enant excess
[alpha] = alpha/(conc in g/mL * length in dm)
(Observed alpha*100)/[alpha]
Dextrorotatory vs levorotatory
pos alpha, clockwise vs neg alpha, counterclockwise
R vs S. What are they suppose to tell you?
- Assign priority at chiral C (highest atomic #’s have highest priority)
- Lowest priority away from you
- clockwise => R, counterclockwise => S
R/S determines chirality
ONLY CHIRAL MOLEC ARE OPTICALLY ACTIVE —> can rotate plane of polarized light
Yep. OPTICAL ACTIVITY IS ONLY DETERMINED EXPERIMENTALLY, ITS NOT RELATED TO ABSOLUTE CONFIG OF MOLEC (ie. Not related to R/S or D/L)
What are meso cmpds and are they optically active?
Sterosisomers that are symmetric and contain 2 opposite chiral centers, this not optically active. THEY MIGHT HAVE CHIRAL CENTERS BUT IS NOT OVERALL CHIRAL
Basically have both R/S within itself (ie. They would be named R,S or S,R)
Enantiomers vs diastereomers regarding physical and chemical properties
Same and usually same, enant = act similarly w/ achiral reagent but act diff w/ chiral reagent vs different and different
FYI, enant = have opposite absolute config at EVERY CHIRAL CENTER! (Ie. R,R and S,S = enant; not R,S and S,R (I just described meso cmpds here)). Diastereomers include epimers, anomers (which = type of epimers but w/ diff config fyi), meso cmpds, conformational isomers and geometric/configurational isomers
What causes ring strain? What are the differences?
Angle strain - bond angles deviate from ideal angle b/c bending and stretching, torsional strain - when cyclic molec = forced to assume conformation with gauche or eclipsed forms, non bonded strain/van der Waals repulsion - when no adjacent atoms or groups compete for same space
For substituted rings (usually cyclohexane), will bulky groups prefer axial or equatorial? Why?
Equatorial, to reduce non bonded strain
Axial and equatorial positions alternate on chair conformation. If you flip chair conformation, what happens to those positions?
They flip too
