help !!! S and R assignment !!

[FutureDental88]

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Ive been struggling with this one for hours ! I just cant see why B is SSR... Help please.... my test is coming up on monday... Dont want to do any silly mistakes..





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[Clancyp]

I think b is the mirror image of it's enatiomer? Im confused too.

 

[yappy]

Cah-Ingold-Prelog

Compare the atomic number (Z) of the atoms directly attached to the stereocenter; the group having the atom of higher atomic number receives higher priority.

Therefore on B I got:

SSR.

NAMING: keep in mind that this molecule is named as 3(S)-bromo-4(S)-chloro-5(R)-floro-heptane. Because there was a tie to the first substituent from either side of the molecule we gave carbon number priority to the substituent that was closest to the letter "A" - in this case it was bromo. Therefore, the chirality from right to left or from carbon 1 to 7 is SSR.

BTW: I think the answer to the original question is A. My understanding is that if you keep all the molecules in the same spot but flip wedges for dashes you get the enantimer. In choice B you're flipping wedges for dashes AND flipping the molecule - isn't that the same exact compound just looking at it from the other side? ya/nah fellow SDN'ers?

C is flat out wrong.


Well... what does the answer booklet say OP?

 

[FutureDental88]

oh man,.. i was wasting so much time on it,, that i ended up mixing up the rules.. so now im stressing out about that !!..

There are other R and S problems in this book,, that are just plain WEIRD and hard.

 

[FutureDental88]

it answer in the back doesnt really explain it ,,, it just gave the answer that A is correct,,, and wrote out what the others are.

 

[radmazindds]

A is correct.

Your original molecule is SSR -- you have it assigned correctly. The molecule that looks exactly like that one structurally, with all the wedges reversed (choice A!) is your enantiomer. This one is RRS.

A good way to do it is to just switch all your Rs and Ss to get your enantiomer.
For example, if you have a molecule with assignments RSRRRSSS, your enantiomer would be SRSSSRRR (just switch them all).

EDIT: if ALL the Rs and Ss are not reversed, then you probably have a diastereomer.

Hope it helped. GOOD LUCK MONDAY!

Ive been struggling with this one for hours ! I just cant see why B is SSR... Help please.... my test is coming up on monday... Dont want to do any silly mistakes..





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[FutureDental88]

Yea, I got A correctly... But how they got B was what was bothering me...

 

[FutureDental88]

thx !!

 

[toothhornet88]

ANSWER B SHOULD BE RSS! they reversed the letters

 

[FutureDental88]

Thats what I thought,,, but nope,,,,, also D which i didnt include in the picture also had a WEIRD configuration, which I didnt know how they got....

 

[LetsGo2DSchool]

There is an easier way to approach this. Visualize the mirror image of the molecule (which is what an enantiomer is in the first place) and it would look like B but with wedges in place of those dashes. Then if you rotate this enantiomer 180 degrees, those wedges will now become dashes and you will wind up with A, the correct answer.