Organic Chemistry Lab 318 Spring 2012 DUE DATES

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% Na. HCO 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 Na. Cl (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 ∙ n. H 2 O • 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

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)