Question

3. Draw all the aldol addition products that are formed when a mixture of acetaldehyde and pentanal is treated with aqueous sodium hydroxide.

4. A reaction, such as described in question 3, which produces a mixture of products is generally not useful for organic synthesis. Suggest a change in the reaction starting materials for question 3 that would produce a single, similar product.

5. Draw a generic mechanism for aldol condensation.

6. The last step for an aldol condensation involves hydroxide acting as a leaving group. As you have learned previously, hydroxide is NOT a good leaving group. What makes this step possible? (i.e. what is the driving force for this step?)

3. Draw all the aldol addition products that are formed when a mixture of acetaldehyde and pentanal is treated with aqueous sodium hydroxide. 4. A reaction, such as described in question 3, which produces a mixture of products is generally not useful for organic synthesis. Suggest a change in the reaction starting materials for question 3 that would produce a single, similar product. 5. Draw a generic mechanism for aldol condensation. 6. The last step for an aldol condensation involves hydroxide acting as a leaving group. As you have learned previously, hydroxide is NOT a good leaving group. What makes this step possible? (i.e. what is the driving force for this step?)

Answer 1

3. ehzone Acetaldehyde + CH3 CH2 CHE Chert Pentanal o Jaq. NaOH . HO CH₂ í H + CH3 H / Cross-coupling 0 o anggo foreverything sem hann er con esta como pring Homo- CH - сно Mechanism : pe H at RH - aq. I. R. "ERTH NaOH [- H₂o] СНО CHO R. R H2O lH*] OH Aldol addition / / product t. If the and aldehyde also has d-H atom then 4 - possible products can be generated, 7 by cross-coupling (s/o different molecules) et by homo-coupling (between same molecules

1 . As already mentioned at to the carbony groups are acidic and hence easily abstracted by the base and induce the formation of several products. If we take one reactant with a - H and the other without any x-H, no side products will be formed, only . one product is formed. e.g. R3 C- & + Ruth USO no enolisable drogens kordagens een echte R₃C - CHO OHO Only R product Pome OH H aq. NaOH - R a co of oldel cine y te hangen) aldol (B-hy Hroxy-carbonyl) RD compound H

The last step of the aldol condensation goes via a E1-cB mechanism: meaning an alpha-hydrogen (alpha to the carbonyl group) is abstracted forming a conjugate base (enolate ion). This enolate ion can be formed due to the conjugation it can have with the carbonyl carbon. This conjugate base then eliminates the -OH group which is not a good leaving group generally, but here the negative charge forces the -OH group to leave (cause molecules don't like to be charged you know! they wanna quench that and become neutral eliminating the not-so-good leaving group in the process).

Note: Question 5 has been covered with the question 3.