[lutesium]

Say that we're reducing Adrenaline to Dopamine so why can HI replace the benzyllic -OH groups easily while HCl or HBr can't???

[Mitch]

What mechanism are you considering?

[Rabn]

I would suggest you consider the nucleophilicity.

[lutesium]

This is the point that I cannot understand. Why is The iodide may be a better leaving group, however, it is a much better nucleophile than the Chloride???

[Mitch]

Nucleophilicity increases with the principle quantum number.

[ultrashogun]

This is the point that I cannot understand. Why is The iodide may be a better leaving group, however, it is a much better nucleophile than the Chloride???

In the nucleophilic aromatic substitution F is actually the best "leaving group". I write it that way because its not actually the best leaving group, but the best group to conduct the reaction on because being so electronegative helps stabilize the negative charge on the adduct the most. It is in fact the worst leaving group, but that doesnt matter because the elimination step is the fact step no matter what, the rate determining step is the addition in which the molecule looses its aromaticity.

In this kind of reaction I is the better nucleophile because its HOMO is large and spread out and thats what matters when the electrophile doesnt carry any charge itsself. Orbital overlap is what dominates this reaction.

In Sn2 reactions I is a great leaving group because it has the most stabil anion, which is also reflected in the fact that HI is the strongest acid of all the HX. But like I mentioned earlier, thats not what counts here.

[lutesium]

Say that we want to reduce Adrenaline ---> Dopamine may esterifying the -OH group work to reduce the molecule w/Na+Alcohol???  Because as far as I know benzylic -OH groups can't be reduced by Na+Alcohol (is this right)


Lutesium...

[Kryolith]

Is Na/ROH able to reduce an alcohol to a hydrocarbon?

[lutesium]

I'm asking the same question. Or is Na/ROH able to reduce an ester to a hydrocarbon!!!

[Kryolith]

I am sorry. I thought that was a proposal. 

Short answer: No.

[lutesium]

So what does this mean HI is a more powerful reducing reagent than Na/R-OH??? This is unbelieveable!!! I've done Prydine -> Piperidine w/Na+MeOH under 4 atm Hydrogen pressure!!! Is the same thing possible for reducing adrenaline ---> dopamine???