Organic Chem I Stereochemistry Quiz

Answer: Four

That one was, I hope, elementary. (See what I did there?) No carbon can have more than four bonds, and an uncharged carbon will never have fewer than two distinct groups (i.e. a triple bond and a single bond). For a carbon to be defined as chiral, it must have four distinct groups (e.g., a methyl, an ethyl, a fluorine and a hydroxyl).

Answer: Entgegen, Zusammen

From the German. Entgegen is a trans isomer, zusammen a cis. Remember "Z Zame Zide!"

A zwitterion is a neutral molecule with a positively charged region and a negatively charged region that cancel each other out.

Zymurgic means being related to the process of fermentation.

Elastomeric means having the qualities of an elastomer, which is a polymer that is both viscous and elastic.

Zeolitic means related to zeolites, which are microporous, aluminosilicate minerals.

Answer: False

A monosubstituted alkene cannot show isomerism, since two of the groups (hydrogens) on one end are identical. Remember that for a double bond to show stereochemistry, each end of the bond must have two different groups (the groups can be symmetric on each end).

Answer: Racemization

Sn1 reactions go through a carbocation mechanism, which means the original stereochemistry of the molecule disappears (as the carbocation is trigonal planar, it is no longer a chiral carbon). Thus the nucleophile can attack from either side equally, leading to racemization (a 50/50 mixture of stereoisomers).

Answer: Inversion of Configuration

Sn2 reactions are bimolecular, so they go through inversion of configuration. The nucleophile attacks from behind, so the stereochemistry of the addition is the reverse of that of the original halide.

Think umbrellas! (Inversion of configuration can be compared to an umbrella flipping inside out.)

Answer: True

The number of possible stereoisomers is calculated using simple probability. There are two possible choices for each stereocenter, and three stereocenters total, so the overall answer is 2x2x2 or 8.

Answer: Meso, achiral.

A meso compound is a compound that has two stereocenters, but is achiral (i.e. the enantiomer of the meso compound is identical to the compound itself). Each meso compound has two diastereomers (S,S and R,R).

The easiest way to tell if a compound with two stereocenters is meso is by checking if each stereocenter has identical groups attached. If the groups are identical, assign R and S. A meso compound must have one S and one R stereocenter.

Answer: It has two diastereomers and no enatiomers.

Every meso compound has two diastereomers, which are enantiomers of each other. A meso compound itself does not have an enantiomer (because it is its own enantiomer).

Answer: They have the same connectivity and opposite stereoisomerism, but different properties.

Enantiomers are mirror images of a chiral molecule. As a result, they have the same connectivity (otherwise they would be constitutional isomers) and opposite stereoisomerism (e.g., R/R goes to S/S) but their properties are often completely different. In some cases this has caused major problems, such as with thalidomide. One thalidomide enantiomer is completely harmless and assuages nausea associated with pregnancy, but the other is very dangerous and causes birth defects. The compound was tragically produced and sold as a racemic mixture, as the critical differences in activities between the two isomers were not known.

In other cases, the results of racemization are less problematic; for instance, a drug could be active in one form but not in the other, though the inactive form may be completely safe. Thus, rather than attempt to separate the two, it is easier to simply make up doses with twice the amount of compound, to make up for half of it being inactive.

Answer: Cahn-Ingold-Prelog System

The Cahn-Ingold-Prelog System provides a blueprint for determining the R and S configuration of a chiral carbon. It lays down the rules for how priority is assigned and how ties in priority are resolved.

I call the rules by which ties are resolved the "Play War System" because the comparison of the "cards," or atoms, each carbon "plays" can be compared to the game of War (i.e., keep playing cards in turn until one person wins).

I guess they are priority assignment rules, but that is not the official name!

I simply made up "Cohen-Martin-Prevault Rules."