Organic Chemistry Lab 318 Spring, 2012

1. Organic Chemistry Lab 318 Spring, 2012

2. DUE DATES • Today • Aldehyde/Ketone Qualitative Analysis Report at beginning of lab • At end of lab -- copy of laboratory notebook pages for today's experiment • Next Week -Synthesis of Banana Oil Report

3. Synthesis of Isoamyl Acetate • carboxylic acid + alcohol  ester + H2O • Reaction is acid-catalyzed • much too slow in the absence of H+ • Reaction is an equilibrium • equilibrium must be shifted to favor product • Acetic acid used in excess

4. Synthesis of Isoamyl Acetate • Mechanism is Nucleophilic Acyl Substitution • The H2SO4 can protonate both the alcohol and the acetic acid, but only some molecules will be protonated in an acid-base equilibrium

5. Synthesis of Isoamyl Acetate • The H+ is transferred to the oxygen of the carboxyl group which makes the C=O more electrophilic. • After the nucleophilic alcohol attacks the protonated carboxyl in the addition step, a new bond is formed in the tetrahedral intermediate

6. Synthesis of Isoamyl Acetate • The tetrahedral intermediate has protons removed and added by other bases and acids in solution (individual steps are not shown). • The oxonium ion –OH2+ is a good leaving group

7. Synthesis of Isoamyl Acetate • The carbon-oxygen bond breaks • The cation that results is stabilized by resonance with both oxygens (only one resonance contributor is shown)

8. Synthesis of Isoamyl Acetate • The proton is removed by bases that are present, such as the alcohol (ROH) or the carboxylic acid (RCO2H) or H2O. • The product is the ester and the byproduct is water • The acid catalyst is regenerated and the cycle continues

9. The Experiment • Reflux apparatus • Liquid boils in round bottom flask • Vapors ascend into the condenser • The cold inside glass condenses the vapor to a liquid, which drips back into the flask • Clamp at r.b. flask, not condenser

10. The Experiment • Have reflux apparatus ready, but add liquids to flask away from heat source • Weigh vial + isopentyl alcohol (±.001 g) • Pour alcohol into r.b. flask using a funnel • Weigh empty vial • Measure volume of acetic acid using grad. cylinder and add to r.b. flask (you know from reading that it will be in excess). • Add 1 ml H2SO4 and SWIRL the flask to mix

11. The Experiment • Assemble the reflux apparatus • Remember the drying tube • Remember the boiling stone • Heat to boiling for 60 minutes • Cool to room temp.

12. The Experiment • Transfer product mixture to sep funnel, using a glass funnel • Add 10 ml H2O • Gently swirl, invert funnel, vent • Separate lower layer and set aside

13. The Experiment • Extract organic layer with 5% NaHCO3 (aq.) • HCO3– is a base • What does it extract (separate)? • CAUTION: gas build-up in funnel – vent after every shake

14. The Experiment • Extract organic layer with saturated NaCl (aq.) • Always keep the upper organic layer in the funnel • Drain the lower aq. layers into the beaker (combined) • After the last extraction, pour the organic layer through the top of the funnel into a small Erlenmeyer flask

15. The Experiment • Add Na2SO4 as drying agent (anhydrous). • After absorbing H2O from the organic layer, it becomes Na2SO4 ∙ nH2O • See Pavia text on drying agents and drying, p. 696-697. • After ester is dried, transfer liquid to small distilling flask clean, labeled vial. • Set up distillation • Distill product into small pre-weighed vial over ice

16. The Experiment • Take an IR and refractive index if there is time this week – otherwise, take it next week • Report is due next week. • Be sure to follow the instructions for writing a Synthesis report • Make sure you understand how the ester product is separated from the reaction mixture

17. Clean-up: Remy SchoenemannClean vials containing NMR unknown and benzoic acid(put in waste jars)