diastereomers and Nucleophilic Substition and Elimination

[BoneMental]

A few questions that I really cannot seem to understand. However, I hope these can be answered quickly. Let me know if I need to be more detailed!

1) Will a diastereomer (say RS) rotate plane polarized light with the same magnitude as its RR? (ie, one is +120 and the other is -120?)

2) If alkenes/alkynes are not on the MCAT (aside from elimination reactions), how come they are so pervasively discussed throughout all prep books? (ie, my books all talk about hydroboration with alkynes, etc...)

3) Are meso compounds, essentially, two enantiomers put together?

Thanks everyone! This is such a fun forum to help others out on and get help on. :]

EDIT: I know I misspelled Substitution, I was typing fast >_>

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[gmcguitar4]

1) don't worry about that..you won't be asked something like that on mcat and I assume that question hasn't popped up in any review material(has not in mine at least). in my opinion that is beyond scope of mcat, they just want to know what happens to a chiral molecule if it undergoes sn1 or sn2..they generally only refer to one chiral carbon. When asked about 2 chiral carbons they usually are referring to chirality of the whole molecule. Is it chiral or a meso compound?(look for symmetry).

2) as for these types of compounds, I guess they just want you to understand the types of reactions that form/destroy double bonds Instead of specific reactions. But I learned all of the reactions anyway because it can't hurt!

3) meso compounds are achiral compounds that contain more then 1 chiral carbon. When a compound has a plane of symmetry, it's achiral. Look for symmetry when looking at molecules with more then 1 chiral carbon. Mcat will give a molecule with MAX 3 chiral carbons..but I doubt you will see that, I havnt!

Hope that helps! If not, then someone else help me out so I can get a better understanding of this as well.

 

[Rabolisk]

No, diastereomers will not rotate plane-polarized light at equal magnitude. If RR rotates +120, then SS will rotate -120, but nothing can be said about RS and SR (except that they, as a pair of enantiomers, will rotate equal and opposite)

 

[BerkReviewTeach]

1) Will a diastereomer (say RS) rotate plane polarized light with the same magnitude as its RR? (ie, one is +120 and the other is -120?)

This is an important concept. Only enantiomers have opposite rotations, so if RS has a +120 degree rotation, then SR has a -120 degree rotation. RR will not be +120 or -120. SS will also not be -120 or +120, but it will be the opposite sign of RR.

A good question might be:

Compound X has a specific rotation of +42 degrees. A stereoisomer with which of the following rotations will exhibit the same boiling point as Compound X?
A) +21 degrees
B) +84 degrees
C) -42 degrees
D) -21 degrees

2) If alkenes/alkynes are not on the MCAT (aside from elimination reactions), how come they are so pervasively discussed throughout all prep books? (ie, my books all talk about hydroboration with alkynes, etc...)

They aren't discussed through ALL prep books. In BR you'll find a terminal alkyne's pKa, but that is in terms of acidity and not alkyne chemistry. Hydroboration of an alkyne is not discussed in BR. Hydration of an alkene is discussed, because of its relevance in biochemistry.

3) Are meso compounds, essentially, two enantiomers put together?

I might be more apt to say it's a molecule with two enantiomeric halves. Helps me visualize it better.