Chemoselectivity selective reactions and protection

Most organic molecules contain more than one functional group, and most functional groups can react in more than one way, so organiochemists often have to predict which functional group will react, where it will react, and how it will react. These questions are what we call selectivity. 

Selectivity comes in three sorts chemoselectivity, regioselectivity, and stereoselectivity. Chemoselectivity is which group reacts regioselectivity is where it reacts. Stereoselectivity is how the group reacts with regard to the stereochemistry of the product. 

We talked a lot about regioselectivity two chapters ago, when you learned how to predict and explain which product(s) you get from electrophilic aromatic substitution reactions. The functional group is the aromatic ring where it reacts is the reaction s regioselectivity. Going back further, one of the first examples of regioselectivity you came across was nucleophilic addition to an unsaturated ketone. Addition can take place in a 1,2- or a 1,4-fashion—the question of which happens (where the unsaturated ketone reacts) is a question of regioselectivity, which we discussed in Chapters 10 and 

We shall leave all discussion of stereoselectivity until Chapters 31-34. 

But with just one equivalent of acetic anhydride in the presence of a base (pyridine) only the NH2 group is acylated, and paracetamol is the product. This is chemoselectivity, and it is to be expected that the NH2 group is more nucleophilic than the OH group. It is even possible to hydrolyse the doubly acetylated product to paracetamol with aqueous sodium hydroxide. The ester is more reactive than the amide and hydrolyses much more easily (Chapter 12). 

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Reduction chapter 24 (Chemoselectivity Selective Reactions and Protection). 

Background Needed for this Chapter Reference to Clayden, Organic Chemistry Chapter 24 Chemoselectivity Selective Reactions and Protection. 

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