R S configuration Br, Cl, F, I

[wantVCUdental]

When it comes to orgo and determining R and S
how do you rank these? is the priority ranked by the highest atomic number?

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[suhshi]

yes

 

[mikexima]

atomic number is a good starting point for bonds to halogens or single elements. more importantly, you should consider overall molecular weight, specially if similar molecules are attached...

for example NH2> CH2I > CH3

not trying to be harsh on you but it sounds like you have lots of studying to do to prepare better.

 

[trentsquawkbox]

atomic number is a good starting point for bonds to halogens or single elements. more importantly, you should consider overall molecular weight, specially if similar molecules are attached...

for example NH2> CH2I > CH3

not trying to be harsh on you but it sounds like you have lots of studying to do to prepare better.

Overall molecular weight is NOT a factor. Atomic number is the determining factor according to the Cahn-Ingold-Prelog (sp?) rules for assigning priority.

 

[superkc2004]

Technically, it's atomic number of all substituents attached to the bond. And you'll usually get it right if you go by molecular weight, since it's closely correlated to atomic number, but atomic number is the correct method.

 

[trentsquawkbox]

consider assigning chirality to the carbons of D-glucose. For example, the last chiral carbon is (R). The oxygen gets priority 1 because it has the highest atomic number, despite the fact that groups of far higher molecular weight are attached to that carbon.

 

[UCB05]

atomic number make the world go round >_<

When dealing with configuration, it's always always always atomic number of the first point of difference. -F trumps -CI3 every time.

 

[mikexima]

consider assigning chirality to the carbons of D-glucose. For example, the last chiral carbon is (R). The oxygen gets priority 1 because it has the highest atomic number, despite the fact that groups of far higher molecular weight are attached to that carbon.

O has higher molecular weight than C to begin with so you would stop there.... higher atomic weight = higher atomic number, samething 😛

but yes the correct method is the higher atmoic number of the immediate bonded element, regardless of all other bonds to that element

 

[americanpierg]

Technically, it's atomic number of all substituents attached to the bond. And you'll usually get it right if you go by molecular weight, since it's closely correlated to atomic number, but atomic number is the correct method.

It has nothing to do with molecular weight whatsoever, and "usually" doesn't cut it when you're shooting for a perfect score. Which has higher priority? CF3 or NH2? Te or I? Co or Ni? Yea...

 

[americanpierg]

atomic number is a good starting point for bonds to halogens or single elements. more importantly, you should consider overall molecular weight, specially if similar molecules are attached...

for example NH2> CH2I > CH3

not trying to be harsh on you but it sounds like you have lots of studying to do to prepare better.

not only is this post incorrect in the first place, but it even contradicts itself. CH2I weighs more than NH2, so why do you have NH2 first?

not trying to be harsh on you but it sounds like you have lots of studying to do to prepare better.

 

[gentile1225]

When considering R/S configuration you have to take several things into account:

1. electronegativity - the most electronegative element will always be considered #1 when numbering the groups, i.e. F > Cl > Br > I

2. size of the side chain - take this into consideratio when dealing mainly with hydrocarbons, i.e. Ethyl > Methyl

3. Stereochemistry - the last group, or what I number as group #4 should always be vertical in a Fischer projection or a bugle in stereochemical terms. If the group #4 is not vertical, then that means the last group lies in the same place as the chiral carbon and therefore you switch your thoughts, if it looks like a S then its an R and vice versa.

Check out the problems in Destroyer if you have it, there are a bunch of good examples. For the DAT I would also concentrate on R/S Configuration for molecules with more than one chiral center, and understanding the difference between enantiomers & diastereomers. Don't forget about meso compounds either.

 

[trentsquawkbox]

When considering R/S configuration you have to take several things into account:

1. electronegativity - the most electronegative element will always be considered #1 when numbering the groups, i.e. F > Cl > Br > I

2. size of the side chain - take this into consideratio when dealing mainly with hydrocarbons, i.e. Ethyl > Methyl

3. Stereochemistry - the last group, or what I number as group #4 should always be vertical in a Fischer projection or a bugle in stereochemical terms. If the group #4 is not vertical, then that means the last group lies in the same place as the chiral carbon and therefore you switch your thoughts, if it looks like a S then its an R and vice versa.

Check out the problems in Destroyer if you have it, there are a bunch of good examples. For the DAT I would also concentrate on R/S Configuration for molecules with more than one chiral center, and understanding the difference between enantiomers & diastereomers. Don't forget about meso compounds either.

I just hope this was sarcasm. Please I would advise many of you to bust out your organic books or even take a look at wikipedia it actually does a fair job at this. http://en.wikipedia.org/wiki/Cahn–Ingold–Prelog_priority_rules

 

[mikexima]

not only is this post incorrect in the first place, but it even contradicts itself. CH2I weighs more than NH2, so why do you have NH2 first?

not trying to be harsh on you but it sounds like you have lots of studying to do to prepare better.

LOL i took the test already and scored 19 on o chem....

N>C....... no need to look further