A Few Organic Chem Questions

[thechenster]

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1) Having trouble w/ oxidizing agents. What is the difference between KMnO4 and Na2Cr2O7?? Is it a matter of strength as in 1 degree vs. 2 degree?

2) What is a good way to tell if a benzene substituent is donating or withdrawing?
For example Benzene-O-COOH seems like it's a withdrawing group to me b/c the first O would just donate into the C=O bond, but it's actually donating.

3) What does the pKa of a leaving group mean?
In my notes, I have the pKa of Cl- as -7 which is really acidic...but it has no Hydrogen to donate................

All help is appreciated. Thanks

 

[PiBond]

1. both are strong oxidizing agents, capable of taking a primary alcohol to a carboxylic acid or a secondary alcohol to a ketone.

2. if the atom attached the ring has a lone pair of electrons on it or if it's an R (alkyl) group, then it's electron donating. if there's not electrons on the atom, it's electron withdrawing. Halogens are ortho/para because of their lone pairs but they are weakly deactivating because of their electronegativity. The lone pair stabilizes the carbocation intermediate and thus directs ortho/para
--in your example, the oxygen attached to the ring has lone pairs and is thus donating. if the carbonyl was attached the ring, the substituent would not be donating.

3. pka value is a constant denoting the acidity of functional group. the lower the pka the stronger the acid. Pka's are given for the conjugate acid of a species while the Pkb is given for the conjugate base to denote its basicity. The conjugate acid of Cl- is (-)7 which corresponds to HCl.

the Pka of a leaving group is generally used to denote the strength of the leaving group. the best leaving groups are those that are the conjugate bases of strong acids (for example, I-, Cl-, Br- are all good leaving groups--their pka's are all negative as well which signifies that they are also crappy bases).

 

[thechenster]

Thank you!