Organic Chemistry Lab 318 Spring, 2012

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

Synthesis of Isoamyl Acetate carboxylic acid + alcohol  ester + H 2 O 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

Synthesis of Isoamyl Acetate Mechanism is Nucleophilic Acyl Substitution The H 2 SO 4 can protonate both the alcohol and the acetic acid, but only some molecules will be protonated in an acid-base equilibrium

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

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 –OH 2 + is a good leaving group

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)

Synthesis of Isoamyl Acetate The proton is removed by bases that are present, such as the alcohol (ROH) or the carboxylic acid (RCO 2 H) or H 2 O. The product is the ester and the byproduct is water The acid catalyst is regenerated and the cycle continues

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

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 H 2 SO 4 and SWIRL the flask to mix

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

The Experiment Transfer product mixture to sep funnel, using a glass funnel Add 10 ml H 2 O Gently swirl, invert funnel, vent Separate lower layer and set aside

The Experiment Extract organic layer with 5% NaHCO 3 (aq.) HCO 3 – is a base What does it extract (separate)? CAUTION: gas build-up in funnel – vent after every shake

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

The Experiment Add Na 2 SO 4 as drying agent (anhydrous). After absorbing H 2 O from the organic layer, it becomes Na 2 SO 4 ∙ nH 2 O See Pavia text on drying agents and drying, p 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

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

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