Please help! A question about o chem.

[grettlin2]

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Hi, I am going to take DAT next week. I have a question about o chem. What is "absolute configuration"??? What is the difference from the one applying general rule (depends on the electronegativity of each atom which directly connects to the chiral carbon)?

Thanks!

 

[ecdoesit]

Abs configuration:
the detailed stereochemical picture of a molecule, including how the atoms are arranged in space. alternatively, the R or S configuration at each chiral carbon atom.

I am not sure what you mean by difference.

Basically for R or S, just push the H to the back, and start ranking from high to low electronegativity, then size. If cw, R(kinda like a right turn). If cc, then it is a S. Another convienent way is reverse the configuration if H is in the front side.

Just remember R/S are not correlated to +/- rotation.

 

[critterbug]

I assume you are referring to the general rule concerning prioritization: Just remember, prioritize by the SIZE (atomic mass) of the FIRST atom attached to the chiral carbon. If there are two identical atoms connected to the chiral carbon, look at the subsequent bonds of those atoms. And remember, a double bond is equal to TWO bonds. For example. If a chiral carbon has two substituents (sp?) : CH2 OH(alchohol) versus CHO (aldehyde) which will have the highest priority?

ANSWER: the carbonyl (aldehyde) because the C is double
bonded to oxygen.

If the image above is not showing up, right click it and select "show picture". PM me if you have any problems, I will be more than happy to help.

Substituents are ranked in the following priority:
1.) OH - because O has an atomic mass of 16
2.) CHO - because C has atomic mass of 12 and is double bonded to O
3.) CHOH- because C has atomic mass of 12 and is single bonded to O
4.) H- because H has the lowest atomic mass of all the substituents

Stick the lowest priority group (which is H in this example) in the back (into the page) and simply count 1-2-3. If the numbers go in a clockwise direction then it is R. If numbers go in a counterclockwise direction, then it is S.