Question

5. Show the acid catalyzed mechanism for acetic anhydride reacting with 2-Methylpropanol to form an ester (reference Fischer Esterification in 21.9 and 21.10 of your Klein text.). The main reaction (substituting a general alcohol instead of the 2-Methylpropanol) is shown below O R-OH H-SO4 (cat.) OR OH 6. Print and analyze the 'H NMR spectra (unknown ester NMRs) for 6 different Acetate Ester products. Potential alcohol starting materials include: Ethanol, 1-Propanol, 1-Butanol, 2-Butanol, 2- Methylpropanol, 3-Methylbutanol, 1-Hexanol, 1-Octanol, and Benzyl Alcohol. Determine the identity of each of the 6 esters (and hence, the starting alcohol) by analysis of the 'H NMR spectra. As much as possible, you should assign each of the NMR signals to their corresponding protons in your proposed structure, and determine the chemical shifts, splitting patterns, and integrations for each signal. You can show all your work on each spectrum. In the same sequential order (File names: Hancock, Ryan and Jacob, Ester DG, jda, Kelsey, Miskanek) as the unknown ester NMR pdf file, staple the 6 NMR spectra to the back of this MOW. Draw each of the 6 Ester products on the appropriate spectrum.

Answer 1

To show the acid-catalyzed mechanism for acetic anhydride reacting with 2-methylpropanol to form an ester, we can follow the Fischer Esterification process as mentioned in the Klein text (sections 21.9 and 21.10). Here's a step-by-step explanation of the mechanism:

1. Protonation of the carbonyl oxygen: The oxygen of the acetic anhydride's carbonyl group is protonated by the sulfuric acid (H2SO4) catalyst. This increases the electrophilicity of the carbonyl carbon.
O=C(OC(O)C)R + H2SO4 -> O=C(+)(OC(O)C)R-OH2 + HSO4(-)

2. Nucleophilic attack by 2-methylpropanol: The oxygen from the 2-methylpropanol molecule acts as a nucleophile, attacking the carbonyl carbon of the protonated acetic anhydride.
O=C(+)(OC(O)C)R-OH2 + CH3CH(OH)CH3 -> O=C(-)(OCH(CH3)CH3)R-OH2 + CH3CH(OH2+)CH3

3. Proton transfer: A proton is transferred from the protonated 2-methylpropanol to the anhydride oxygen.
CH3CH(OH2+)CH3 -> CH3CH(OH)CH3 + H+

O=C(-)(OCH(CH3)CH3)R-OH2 + H+ -> O=C(OCH(CH3)CH3)R-OH3+

4. Removal of the leaving group: The protonated anhydride oxygen acts as a leaving group, breaking the bond between the carbonyl carbon and the oxygen.
O=C(OCH(CH3)CH3)R-OH3+ -> O=C(OCH(CH3)CH3)R + H2O

5. Deprotonation of the carbonyl oxygen: The carbonyl oxygen gets deprotonated by the bisulfate ion (HSO4-) to regenerate the H2SO4 catalyst and form the ester product.
O=C(+)(OCH(CH3)CH3)R + HSO4(-) -> O=C(OCH(CH3)CH3)R + H2SO4

The final ester product is formed by the reaction between acetic anhydride and 2-methylpropanol through the acid-catalyzed Fischer Esterification mechanism.

As for the second part of your question, since I cannot physically print, analyze, or staple the NMR spectra to your MOW, I recommend you do so with the provided files. Once you have printed the spectra, analyze the chemical shifts, splitting patterns, and integrations for each signal to identify the starting alcohol and draw the corresponding ester products on the appropriate spectrum.