CHEM302 S09 CA6a key

CHEM302 S09 CA6a key - CHEM302, Spring 2009 Dr. Ruder 1...

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CHEM302, Spring 2009 Dr. Ruder 1 Class Activity 6A Acidity of the α -Carbon of Carbonyls Model 1 : Acidity of an aldehyde C O C NH 2 Li+ H C H H H H H 1. In the reaction shown in Model 1, H 2 N - can act as a nucleophile and as a base. Consider the reaction of H 2 N - as a base which will remove an acidic H. (a). How many different kinds of H’s are there in propanal? Label each H as A, B, C etc. 3 (b). Draw the conjugate bases that result from removal of H A , H B and H C etc. (from the acid base reaction between H 2 N - and propanal). C O C NH 2 Li+ H C H H H H H A B C C O C C H H H H H C O C H C H H H H C O C H C H H H H - Ha -Hb -Hc (c). Draw all possible resonance structures of these conjugate bases. Only one form has a resonance structure (from removal of Hb). C O C H C H H H H C O C H C H H H H (d). Circle the conjugate base that would have the lowest energy. Explain your choice. Loss of Hb leads to the conjugate base with the lowest energy. This conjugate base is stabilized by resonance while the others are not. (e). Circle the H on propanal that takes the least energy to remove. This is the “most acidic” H. Hb is most acidic 2. Carbon atoms in a carbonyl compound are labeled according to how far they are from the carbonyl carbon. The carbon next to the carbonyl is the α position, then each adjacent position is labeled β , γ , δ (a). Label the carbons in pentanal as α,β, γ, δ . Recall the factors that stabilize a conjugate base: 1. Formal charge (charged or neutral?) 2. Electronegativity effect (C, N, O, X?) 3. Resonance effect (resonance stabilization?) 4. Inductive effect (inductive stabilization?)
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CHEM302, Spring 2009 Dr. Ruder 2 C C C C H O H H H H H H H H H α β γ δ (b). Any H attached to the
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CHEM302 S09 CA6a key - CHEM302, Spring 2009 Dr. Ruder 1...

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