M

MSc44

if you have this Molecule


CHO
l
H3C - C- Br
l
OH

To find absolute config. obviously Br is 1. Whats lect is C from methyl, C from CHO and O from OH. Now since the O has a greater atomic weight then both C's shouldent this be priortized as 2 then go to the 2 carbons and since there is an O in the CHO assign that 3 and the methyl group 4, or should CHO be 2 B/C it would be as if 2 O's are connected to it.
 

EBI831

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i've learned that you only look at what's directly connected first. so it's (1) Br (2) OH then you look and see that there are 2 Carbons so you look at the most electronegative one and that would be CHO(plus it also weighs more and ch3 is almost always the bottom rung unless H is on the molecule) . so that's how i'd do it...you can jump straight to the O and bypass the C.
 
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M

MSc44

EBI831 said:
i've learned that you only look at what's directly connected first. so it's (1) Br (2) OH then you look and see that there are 2 Carbons so you look at the most electronegative one and that would be CHO(plus it also weighs more and ch3 is almost always the bottom rung unless H is on the molecule) . so that's how i'd do it...you can jump straight to the O and bypass the C.

thats exactly what i thought but EK say's otherwise, that the COH is given priority as 2, 3 the OH then the methyl group as 4.....confused now :scared:
 
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MSc44

UofT_475 said:
don't you look for the config with lowest total formal charge?


no it goes according to molecular weight

largest MW gets highest priority
 

UMP

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MSc44 said:
no it goes according to molecular weight

largest MW gets highest priority
(I think you mean atomic number...)


you prioritize what is immediately connected.... so first step one: Br, then OH, based on atomic number, and then you are left with the methyl and aldehyde who are both carbons... so this is the second step, and you take in to account the double bonded oxygen at this point, and the oxygen takes priority now...


1. Br
2. OH
3. CHO
4. CH3
 

Kikaku21

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MSc44 said:
thats exactly what i thought but EK say's otherwise, that the COH is given priority as 2, 3 the OH then the methyl group as 4.....confused now :scared:
Don't use stupid test prep books as a guide. Go look in an orgo book!! Its atom number of the first connected atom, and then if there is a tie, the second atom, etc. In his example, Br = 1, OH = 2, CHO = 3, and the CH3 = 4. But don't take my word for it. Look it up in a REAL textbook. I have seen mistakes in Kaplan and TPR.
 
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