Priority for assigning R/S

[Shinpe]

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So I guess I don't remember this right. Which is the higher priority between these two groups if u were assigning R/S?

(-C(Cl)(H)(CH3)) or (-COOH)????

So the question comes down to whether you add the atomic number for the groups attached to the C (-COOH higher) or do you look at the highest one (first one higher due to Cl)???

 

[UCB05]

So I guess I don't remember this right. Which is the higher priority between these two groups if u were assigning R/S?

(-C(Cl)(H)(CH3)) or (-COOH)????

So the question comes down to whether you add the atomic number for the groups attached to the C (-COOH higher) or do you look at the highest one (first one higher due to Cl)???

No adding of atomic numbers. Just take them at face value. In this case, -CClH(CH3) has higher priority. -C-O < -C-OO < -C-OOO < -C-Cl

 

[AndyK]

So I guess I don't remember this right. Which is the higher priority between these two groups if u were assigning R/S?

(-C(Cl)(H)(CH3)) or (-COOH)????

So the question comes down to whether you add the atomic number for the groups attached to the C (-COOH higher) or do you look at the highest one (first one higher due to Cl)???

I believe it's by weight of what's attached. Because -COOH has a double bond, you have to separate it

so -C=O becomes -CO-OC
    |            |
    OH           OH

So the C next to the chiral carbon is bonded 3 oxygens... total MW 48

The C in the 1-chloroethyl group is bonded to a C, an H and Cl, total MW = 48.45

So the 1-chloroethyl wins over the acid... ever so slightly

someone please confirm I did this right.

 

[UCB05]

Your reasoning is incorrect. When assigning priority, you look at atomic numbers and find the first point of difference. Since a carbon has 3 bonds excluding the main-chain bond, you compare the three bonds in order of decreasing substituent priority.

In the case of a terminal alcohol vs an aldehyde, vs carboxyl groups,
an alcohol carbon has one O bond, 2 H bonds.
an aldehyde carbon has two O bonds, 2 H bonds.
One O on the alcohol and aldehyde are the same. However, the next bond comparison is another O on the aldehyde vs an H on the alcohol. The aldehyde group has higher priority.

The same goes for an aldehyde vs carboxyl (three O bonds). The aldehyde and carboxyl both have two O bonds, but the third is an O on the carboxyl vs H on the aldehyde. The carboxyl has higher priority.

In the case above, the difference is a carboxyl vs a chloromethyl group. The first point of difference is a Cl on the chloromethyl vs a O on the carboxyl. Cl > O, the chloromethyl group has higher priority. It is inconsequential that the chloromethyl has hydrogens remaining, or that there are two more effective O bonds on the carboxyl carbon. The first point of difference determines priority.

 

[Shinpe]

I think UCB is right.

Grazia ....

 

[dentrilla]

Your reasoning is incorrect. When assigning priority, you look at atomic numbers and find the first point of difference. Since a carbon has 3 bonds excluding the main-chain bond, you compare the three bonds in order of decreasing substituent priority.

In the case of a terminal alcohol vs an aldehyde, vs carboxyl groups,
an alcohol carbon has one O bond, 2 H bonds.
an aldehyde carbon has two O bonds, 2 H bonds.
One O on the alcohol and aldehyde are the same. However, the next bond comparison is another O on the aldehyde vs an H on the alcohol. The aldehyde group has higher priority.

The same goes for an aldehyde vs carboxyl (three O bonds). The aldehyde and carboxyl both have two O bonds, but the third is an O on the carboxyl vs H on the aldehyde. The carboxyl has higher priority.

In the case above, the difference is a carboxyl vs a chloromethyl group. The first point of difference is a Cl on the chloromethyl vs a O on the carboxyl. Cl > O, the chloromethyl group has higher priority. It is inconsequential that the chloromethyl has hydrogens remaining, or that there are two more effective O bonds on the carboxyl carbon. The first point of difference determines priority.

egggjactly👍👍👍