O Chem Q's from Destroyer

[tawaqul]

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I feel like I've been posting too many threads, so here are a few questions in one!

1)Do epoxides always open with the OH's anti to each other? I googled it and it seemed like they did, no matter if they were base or acid-catalyzed...is this correct? (#94 from Destroyer)

2) When dealing with rotation, I know a meso compound has a rotation of zero, but what about R and S? Is R always (-) and S always (+) or does it just depend on what the question gives you? (#133)

3) On #151, it says that NaHCO3 will extract acids, so it extracts the carboxylic acid group. But why wouldn't this base also extract the phenol? Why can a phenol only be extracted with a base like NaOH?

And that's it for now.

Thanks!

 

[Tina324]

I feel like I've been posting too many threads, so here are a few questions in one!

1)Do epoxides always open with the OH's anti to each other? I googled it and it seemed like they did, no matter if they were base or acid-catalyzed...is this correct? (#94 from Destroyer)

2) When dealing with rotation, I know a meso compound has a rotation of zero, but what about R and S? Is R always (-) and S always (+) or does it just depend on what the question gives you? (#133)

3) On #151, it says that NaHCO3 will extract acids, so it extracts the carboxylic acid group. But why wouldn't this base also extract the phenol? Why can a phenol only be extracted with a base like NaOH?

And that's it for now.

Thanks!

1. yup always...because the nuc that comes in to open the ring comes in anti (from behind) the C-O bind.
2. R and S have nothing to do with - and +. R and S are just notations describing where the atoms in a chiral C are relative to each other. - and + refer to which direction plane polarized light is turned. Remember L(-) and R (+) always.
3. i dunno...i didn't study lab stuff yet...can someone explain this one from the beginning?

 

[tawaqul]

ANyone have any idea about #3?

 

[tranv117]

3) On #151, it says that NaHCO3 will extract acids, so it extracts the carboxylic acid group. But why wouldn't this base also extract the phenol? Why can a phenol only be extracted with a base like NaOH?

And that's it for now.

Thanks!

It has to do with relative strengths of the acid and base you are extracting.

first of all, we are extracting acids...
To extract a strong acid, we'd need a weak base and to extract a weak acid, we'd need a strong base.

To extract phenol which is a weak acid, we'd need a very strong base like NaOH. Carboxylic acid is mildly acidic (pKa ~4-5?), so we'd need a weaker base. If you'd had only used NaOH, you'd extract both the phenol and carboxylic acid which would then defeat the purpose of the separation.

In separation experiments, you have to use different base/acid strengths to separate different organic molecules.

 

[tawaqul]

again thanks, i appreciate it

 

[dcl222]

this helped me also, thanks!

 

[doc3232]

Since when is phenol an acid? Terrible acid.