Need Help for Org -->stereochemistry question

[predent steve]

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1 Br
F--c--CHO
H

I thought it was R- configuration..

(1- F (1) Br (2) CHO (3) -turn right)

but Answers is S- ..

Would you please explain why I should count Br 1st???

 

[rejuvenatedsoul]

Br has a higher atomic # than F.

 

[theragu]

It's actually by largest atomic mass. Not that that's much of a difference haha. and if the atoms are similar you move outward and again look at mass.

 

[predent steve]

I was confused.. THank You so much!!

 

[rejuvenatedsoul]

i was told you could go by either atomic # or mass...but according to wikipedia's article about cahn-ingold-prelog priority system, it's atomic number. w/e either works! link:http://en.wikipedia.org/wiki/Cahn–Ingold–Prelog_priority_rules#Stereocenters:_R.2FS

i know, wikipedia is not the best source 😛

 

[theragu]

http://www.cliffsnotes.com/study_gu...tem.topicArticleId-22667,articleId-22649.html

hahaha well cliffsnotes isnt either, but there you go ppl you cant trust everything wiki says 😛

doesnt matter, itll be 99.999% the same thing every time lol

 

[predent steve]

is it only for the Halogen group? right?

 

[rejuvenatedsoul]

http://www.cliffsnotes.com/study_gu...tem.topicArticleId-22667,articleId-22649.html

hahaha well cliffsnotes isnt either, but there you go ppl you cant trust everything wiki says 😛

doesnt matter, itll be 99.999% the same thing every time lol

haha okay thanks, good to know!

 

[UCB05]

http://www.cliffsnotes.com/study_gu...tem.topicArticleId-22667,articleId-22649.html

hahaha well cliffsnotes isnt either, but there you go ppl you cant trust everything wiki says 😛

doesnt matter, itll be 99.999% the same thing every time lol

I was taught to go by atomic number. neutrons vary, but protons always stay the same. Not much of a difference since I can't think of a case where the priority of an isotope changes due to number/mass difference, but meh. more convenient to think by atomic number since the periodic table is arranged that way.

 

[theragu]

Ya it's the same thing.

And now that we've had this endless debate over something so simple, no one will ever forget how to order them 👍

 

[UCB05]

You'd think it were a totally simple concept, right? I've posted in threads where people thought priority was based on the combined atomic mass of an entire substituent (e.g. -COOH > -Cl)

 

[predent steve]

Ok..

Last Question!

Br
H--C--COOH
OH Is this S??

 

[UCB05]

Ok..

Last Question!

Br
H--C--COOH
OH Is this S??

Br -> O -> (COOH), whichever direction that is. I can't tell if -- means toward or away.

 

[KappaSig]

Br -> O -> (COOH), whichever direction that is. I can't tell if -- means toward or away.

if the equation is :


Br
H-C-COOH Than it should be an R..
OH

Because you have to rearrange the atoms.. so in reality it would be

H
COOH-C-Br
OH

Is R the right answer?

 

[predent steve]

I didn't know the answer.. but I thought it was R too!!

 

[KappaSig]

where did you get this question from?

 

[TexasDDS]

which is the correct answer?

 

[rnbaxter]

Ok..

Last Question!

Br
H--C--COOH
OH Is this S??

If your showing H behind the plane it is S.