R/S Configuration problem

[NuckingFutz]

---

Members don't see this ad.

I can never tell when to flip the lowest priority group for these type of questions since there isn't any wedges or dashes to give you a clue.

The answer is A... but I don't understand why... Br should be the first priority group. COOH should be the second b/c of the attached O to the C, CH3 is the third group, which then leaves H as the lowest. But in my mind it comes out to being a S configuration.... am I missing something?

 

[klutzy1987]

You are forgetting that the grouo with the lowest priority has to be in the vertical position. therefore make two changes and switch it to the vertical position and see what you get.

 

[klutzy1987]

Also these projections are just basically fisher projections and follow basic fisher projection rules.

 

[harrygt]

I can never tell when to flip the lowest priority group for these type of questions since there isn't any wedges or dashes to give you a clue.

The answer is A... but I don't understand why... Br should be the first priority group. COOH should be the second b/c of the attached O to the C, CH3 is the third group, which then leaves H as the lowest. But in my mind it comes out to being a S configuration.... am I missing something?

ok, let me give you some tips for approaching this. As you know these are Fischer projections. In Fischer proj, the horizontal lines are protruding on top of the paper, and vertical lines are getting away from the paper.
The first thing you should do is assigning priorities. THen, see how you can make a circle from 1---> 2 ---> 3. A clockwise circle will give you an R config only if the last priority is away from you [on the vertical line]. Otherwise it would be an S. an anti clockwise circle would give you S, if the last priority is away from you, otherwise it is R.
At the first moment, this seems confusing, and hard to memorize. But it is not. Try to practice this method on lots of problems and it will become like a piece of cake for ya.
I hope it will help

 

[osimsDDS]

the largest MW will be group number 1 with highest priority then go as follows...

If you have group #4 on the horizontal axis then change the configuration...

For the first image A, the Br has 1st priority, then the COOH group, then the methyl CH3 group...this is S configuration BUT the Hydrogen group is on the horizontal axis so you need to change the configuration to R...

 

[TarheelKid24]

a little Kaplan tip that was actually useful....the Fischer projections are like bow ties...therefore, the horizontal groups are "wedges" (sticking out of the page) and the vertical groups aren't.

 

[Mrbubbles]

someone once told me that when assigning priority,
the most electronegative elemetn gets to be number 1..
or is that only when it's a tie? like CHO vs. COOH

 

[acupofchai]

basically if the first element is same CH3, COOH, you look at the next one - O. If the lowest priority is on the HORIZONTAL axis, still determine the direction of highest -> lowest priority and take the inverse (R is S, and S is R). but, if the lowest priority is on the VERTICAL axis, the direction of highest -> lowest priority is the one you take.

 

[NuckingFutz]

Thanks everyone 🙂

 

[eros215]

F = Priority one
Br = Priority two
CHO = Priority three
this is clockwise so R
and...
H is the last and it's vertical so no reversing happens

so shouldn't it be A and B...???? someone please explain why

 

[ThePandaFactor]

Br is priority 1, F is priority 2.

 

[eros215]

ohh thank you! i thought priority was off of electronegativity for some reason...thanks!

 

[atlanta213]

Why C is not R configuration ?

F>NH2>COH>CH3

I think answer should be A and C.

hmm am i wrong?

 

[Mamona]

Why C is not R configuration ?

F>NH2>COH>CH3

I think answer should be A and C.

hmm am i wrong?

CH3 is not in the vertical position you need to make two changes and then you get S configuration