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 + 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

CH3 C O

O

H CH3 CH

CH3

CH2 CH2 O HH2SO4

CH3 C O

O

CH2CH2CHCH3

CH3

H OH

acetic acid 3-methyl-1-butanol(isoamyl alcohol isopentyl alcohol)

isopentyl acetate

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

CH3 CH

CH3

CH2 CH2 O HH2SO4

CH3 CH

CH3

CH2 CH2 O H

H

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

CH3 C O

O

H

CH3 CH

CH3

CH2 CH2 O H

HCH3 C O

O

H

H

CH3 CH

CH3

CH2 CH2 O H

The C=O is now moreelectrophilic

The OH acts as a nucleophileand attacks the C=O

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

CH3 C O

O

H

H

CH3 CH

CH3

CH2 CH2OH

The tetrahedral intermediate undergoesproton shifts. The two OH groups are equivalent -- either can be protonated.

CH3 C O

O

H

H

HCH3 CH

CH3

CH2 CH2O

The proton shiftsare intermolecular, not intramolecular

The protons shown in color areequivalent, but they are being "formally" kept account of here.

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)

CH3 C O

O

H

H

HCH3 CH

CH3

CH2 CH2O

H2O leaves, forming a cationthat is stabilized by resonance

CH3 C

OH

CH3 CH

CH3

CH2 CH2O

CH3 C

OH

CH3 CH

CH3

CH2 CH2O

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

CH3 C

OH

CH3 CH

CH3

CH2 CH2O

CH3 C

O

CH3 CH

CH3

CH2 CH2O H+

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

1 1 1

2

3

45 6

7

8

9

1 10

2

3

45 6

7

8

9

1 1 0

Drying tube containing cotton

Hot plate

Al block or sand bath

Round bottom flask

Water in

Water out

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

Round bottom flask

The Experiment

• Assemble the reflux apparatus

• Remember the drying tube

• Remember the boiling stone

• Heat to boiling for 60 minutes

• Cool to room temp.

1 1 1

2

3

45 6

7

8

9

1 10

2

3

45 6

7

8

9

1 1 0

Drying tube containing cotton

Hot plate

Al block or sand bath

Round bottom flask

Water in

Water out

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

CH3 C O

O

H

CH3 CH

CH3

CH2 CH2 O HH2SO4

CH3 C O

O

CH2CH2CHCH3

CH3

H OH

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

CH3 C O

O

H

CH3 CH

CH3

CH2 CH2 O HH2SO4

CH3 C O

O

CH2CH2CHCH3

CH3

H OH

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 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

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)