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
O
CH3 C O H
acetic acid
CH3
CH3 CH CH2 CH2 O H
O
H2SO4
CH3
CH3 C O CH2CH2CHCH3
3-methyl-1-butanol
(isoamyl alcohol
isopentyl alcohol)
H OH
isopentyl acetate
• 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 H2SO4 can protonate both the alcohol and
the acetic acid, but only some molecules will be
protonated in an acid-base equilibrium
CH3
CH3 CH CH2 CH2 O H
H2SO4
CH3
CH3 CH 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.
CH3
H
CH3 CH CH2 CH2 O H
O
H
O
CH3 C O H
CH3 C O H
The C=O is now more
electrophilic
CH3
CH3 CH CH2 CH2 O H
The OH acts as a nucleophile
and attacks the C=O
• 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).
H
O
CH3
CH3 C O H
CH3 CH CH2 CH2
O
H
The tetrahedral intermediate undergoes
proton shifts. The two OH groups are
equivalent -- either can be protonated.
The proton shifts
are intermolecular,
not intramolecular
H
O
CH3
CH3 C O H
CH3 CH CH2 CH2
O
H
The protons shown in color are
equivalent, but they are being
"formally" kept account of here.
• The oxonium ion –OH2+ is a good leaving group
Synthesis of Isoamyl Acetate
• The carbon-oxygen bond breaks
H
H
O
CH3
O
CH3 C O H
CH3 CH CH2 CH2 O
H
CH3
CH3 C
H
CH3 CH CH2 CH2 O
H2O leaves, forming a cation
that is stabilized by resonance
O
CH3
CH3 C
CH3 CH CH2 CH2 O
• 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 (RCO2H) or
H2O.
H
CH3
O
O
CH3 C
CH3 C
CH3 CH CH2 CH2 O
CH3
CH3 CH CH2 CH2 O
H+
• The product is the ester and the byproduct is water
• The acid catalyst is regenerated and the cycle continues
The Experiment
• Reflux apparatus
Drying tube containing cotton
– 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
Water out
Water in
Round bottom flask
Al block or sand bath
5
4
6
5
7
4
3
8
3
2
9
2
1
10
1
6
7
8
9
1
10
Hot plate
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
Drying tube containing cotton
• Remember the drying tube
Water out
• Remember the boiling stone
• Heat to boiling for 60 minutes
Water in
Round bottom flask
• Cool to room temp.
Al block or sand bath
5
4
6
5
7
4
3
8
3
2
9
2
1
10
1
6
7
8
9
1
10
Hot plate
The Experiment
• Transfer product mixture to sep
funnel, using a glass funnel
• Add 10 ml H2O
O
O
CH3 C O H
CH3
CH3 C O CH2CH2CHCH3
CH3
H2SO4
CH3 CH CH2 CH2 O H
H OH
• Gently swirl, invert funnel, vent
• Separate lower layer and set
aside
The Experiment
• Extract organic layer with 5%
NaHCO3 (aq.)
• HCO3– is a base
O
O
CH3 C O H
CH3
CH3 C O CH2CH2CHCH3
CH3
H2SO4
CH3 CH CH2 CH2 O H
H OH
• 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 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)