neutral nuc: acidic conditions

[nancy9494]

better late than never?

Why does the OH leave in the 4th step? why does this need to happen? 

 

thanks in advance

[hypervalent_iodine]

Because otherwise how do you form the ester? You could also consider reaction kinetics and thermal stability, but I think that might be a more complicated answer than you're looking for.

[nancy9494]

I just don't understand why it can't chill in 3rd form. I can memorize the mechanism but I'd prefer to know. 

[hypervalent_iodine]

Because there's acid present. The OH is easily able to coordinate a H+ from this, which turns the OH into water (a very good leaving group). 

[BabcockHall]

@OP, Which is a better leaving group, hydroxide ion or water, and why?

[nancy9494]

The water because it is under acidic conditions?

[BabcockHall]

All else held equal, stronger bases make poorer leaving groups than weaker bases.  Strong bases are less able to stabilize negative charge than weaker bases.  Which is the stronger base?

[nancy9494]

the OH 

[BabcockHall]

I would put a minus sign on it, to indicate that it is hydroxide ion.  And we can compare the pK_a values of the conjugate acids of any two leaving groups to determine which base is stronger.

[nancy9494]

ahh yes thank you