Acid or base conditions for orgo

[LuckMC11]

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I wanted to know if anyone had like a chart or anything online showing what reactions take place ONLY in acidic or ONLY in basic conditions.

thanks

 

[orgohacks]

I wanted to know if anyone had like a chart or anything online showing what reactions take place ONLY in acidic or ONLY in basic conditions.

thanks

I don't know of any tables, that would be quite an undertaking.

Most reactions tend to boil down to "nucleophile attacks electrophile". Nucleophiles are lewis basic. Furthermore, the conjugate base of a species is a better nucleophile than the conjugate acid. Therefore the set of reactions that take place under basic conditions is larger than that which occur under acidic conditions - Eg. - the SN2, many additions to aldehydes/ketones/esters, etc.

The major class of exceptions are those where you add acid to activate a functional group. Look at it like this:

weak Nucleophile + acid (electrophile) --> new electrophile (more reactive)
new electrophile + 2nd nucleophile ----> product

3 major classes of functional groups that are activated are
1) alkenes (will not react with nucleophiles without prior activation, e.g. with acid. Acid reacts with alkene to give carbocation, which is stronger electrophile, which then reacts with another nucleophile (e.g. water).
2) carbonyls in certain cases (protonation of carbonyls makes carbonyl carbon better electrophile. Major example is carboxylic acid + alcohol ---> ester (Fischer esterification) which requires acid. Another example is imine formation. A final example is where protonation of a carbonyl gives the enol, like in the acid-catalyzed Aldol reaction.
3) alcohols ---> protonation makes them a better leaving group, can leave to form a carbocation (eg.. E1, SN1).

Also, certain oxidation reactions (e.g. Jones) are promoted by acid.

The key point is that acid acts like the "setup man" to create a strong electrophile from a weak nucleophile. This sets up a second reaction, which creates the product you're interested in.

Possibly relevant posts:
"The conjugate base is always a better nucleophile": http://is.gd/d4qW1
"The power of acid catalysis": http://is.gd/d4qPE

hope this helps - James

 

[LuckMC11]

wow James thanks soo much for your post! clarified a lot for me!

your key point makes the rest of your post make sense lol