Chp. 14 Alpha Substitution Reactions of Carbonyls.pdf

Previously... In our prior chapters, we discussed reactions in which a nucleophile attacks the carbonyl (C=O) carbon. If Y ¹ H or C (NOT aldehydes or ketones) quench (usually H2O or H+) If Y = H or C (aldehydes or ketones) Previously... In our prior chapters, we discussed reactions in which a nucleo...

Previously... In our prior chapters, we discussed reactions in which a nucleophile attacks the carbonyl (C=O) carbon. If Y ¹ H or C (NOT aldehydes or ketones) quench (usually H2O or H+) If Y = H or C (aldehydes or ketones) Previously... In our prior chapters, we discussed reactions in which a nucleophile attacks the carbonyl (C=O) carbon. If Y ≠ H or C (NOT aldehydes or class ketones) of reactions In this chapter we’ll discuss another that carbonylbearing molecules do: reactions that involve the alpha-hydrogen! quench (usually H2O or H+) If Y = H or C (aldehydes or ketones) What is an α-Hydrogen? An α-hydrogen (alpha-hydrogen) is a hydrogen bonded to an α-carbon (alphacarbon). An α-carbon is a carbon that is immediately adjacent to a carbonyl (C=O) carbon: What is an α-Hydrogen? An α-hydrogen (alpha-hydrogen) is a hydrogen bonded to an α-carbon (alphacarbon). An α-carbon is a carbon that is immediately adjacent to a carbonyl (C=O) carbon: α-carbon α-hydrogens What is an α-Hydrogen? The negative charge that forms when α-hydrogens are deprotonated can resonancedelocalize. Because of this, α-hydrogens are fairly acidic: What is an α-Hydrogen? The negative charge that forms when α-hydrogens are deprotonated can resonancedelocalize. Because of this, α-hydrogens are fairly acidic: enolate The compound represented by these two resonance contributors is called an enolate. What is an α-Hydrogen? The negative charge that forms when α-hydrogens are deprotonated can resonancedelocalize. Because of this, α-hydrogens are fairly acidic: Just so you know, the major resonance contributor is the one with the – charge on the oxygen. However, for the DAT, it’s easier to just imagine it being the one with the – charge on the carbon. enolate Why do we care? Once an α-hydrogen is deprotonated, the resulting negatively-charged enolate can act as a nucleophile to attack stuff and form bonds with it: enolate This will be the repeating theme throughout this chapter. What if we add acid? If we add acid to our original carbonyl compound, we form an enol: base enol Why do we care? (Part II) Enols can also attack stuff: enol Enol vs. Enolate Even though enols can attack stuff, enolates are better/more reactive attackers (better nucleophiles) because they have a negative charge: better nucleophile Keto-Enol Tautomerization Now, just so you know, all carbonyl-bearing compounds with α-hydrogens interconvert between their keto and enol forms: keto form enol form >99.9%