Background:
In the early nineteenth century, people wore
exclusively natural fibers (e.g. cotton, wool,
silk, etc.). The only dyes available at the
time were from natural sources, such as
fruits and flowers, and most of these lacked
light stability and/or water fastness.
For centuries, the color purple has been
associated with royalty. Tyrian purple, was
a rare and expensive dye made from certain
mollusks. The extraction process was
complicated and the results were variable.
In 1856, William Perkin, an eighteen-yearold student at London’s Royal College of
Chemistry, created an intense purple dye
later called mauveine, from a mixture of
aniline, o- and p-methylaniline, and an
oxidant. Mauveine (a mixture of several
components), was the first of the aniline
dyes.
Perkin had the foresight to patent his
procedure and sent samples of his dye to a
textile manufacturer. It dyed silk well (if
unevenly), but trials with cotton were not so
successful as the necessary mordants
(chemicals that fix dyes in or on a substance
by forming an insoluble compound) were
not yet discovered. Perkin later found that
dying silk in a soap bath resulted in even
color and that using tannins as one of the
mordants allowed the dying of cotton with
any shade of color intensity possible.
In 1862, Queen Victoria wore a mauve
gown to the Royal Exhibition spurring
public demand for the new synthetic organic
dyes. The dye industry was the first
commercial use of synthetic chemicals (not
found in nature).
A few years later, the triphenylmethane dyes
were discovered. Another class of dyes, the
azo dyes, were developed in Germany and
led to the discovery of the first antibiotics,
the sulfa drugs.
What property makes a molecule colored
(absorb in the visible region of the
electromagnetic spectrum)?
Chromophore – the extended pi system of a
molecule over which pi electrons can be
delocalized and which is responsible for the
absorption of light
The longer the extended pi system, the
higher the wavelength of light that is
absorbed.
Representative dyes based on the
triphenylmethyl (trityl) carbocation.
Why is the reduced form of malachite green
colorless?
The extensive delocalization of the pi
electrons is responsible for the absorbance
of the triphenylmethyl cations in the visible
region.
Steric hinderance between the ortho hydrogens makes it
impossible for the three phenyl rings to be coplanar. Instead
the triphenylmethyl carbocation assumes a “propeller”
conformation with the phenyl rings pitched at a 32° angle.
The triphenylmethyl cation can be prepared by reaction of a
Grignard reagent with benzophenone to form triphenylmethyl
alcohol. Treatment with fluoboric acid results in the trityl
carbocation.
Safety Notes for Expt. 30 – Synthesis of
Triphenylmethanol and the Trityl Carbocation
Physical Properties of the Reactants and Reagents in
Expt. 30
IR Spectra of bromobenzene and benzophenone
If you do not do a prelab for this experiment,
you will be asked to leave and receive a
zero for this lab.
Step 1: Preparation of the Grignard reagent.
Any trace of water (including in the air) will
react with the Grignard reagent. What does
it form?
Setup of the round-bottom flask, Claisen adapter,
West condenser and addition funnel
A drying tube is added to the top of the condenser
Caution: Use diethyl ether only in the
hood. It is extremely volatile and
flammable. It can form vapor trails across a
room and if ignited, will flash back to its
source. Be careful not to have ether come in
contact with a hot surface.
Note: There will not be any oven or flame
drying as described in the book.
Set up your apparatus (must be dry) for the
preparation of the Grignard reagent (flask,
Claisen adapter, condenser, addition funnel);
your TA will come around and inspect it
before giving you 60-70 mL of anhydrous
ether.
Weigh 22.0 mmol of dry magnesium
turnings into your round-bottom flask.
Weigh 22.0 mmol of bromobenzene into a
dry Erlenmeyer flask and dissolve it in 5 mL
of anhydrous ether. Transfer this solution to
the addition funnel and add to the Mg
turnings in the reaction flask. Carefully rub
the Mg turnings with the flat end of your
glass stir rod. The reaction will have started
when you see tiny bubbles forming on the
Mg surface and a cloudiness forms in the
solution. If the reaction doesn’t start after 510 minutes, you may add a grain of iodine
(I2) to your solution.
When the solution boils on its own without
heating, start adding ~30 mL of anhydrous
ether dropwise with the addition funnel.
After the addition (boiling is nearly
stopped), maintain a gentle reflux for
another 15 minutes using a water bath. Add
more ether if necessary to maintain the
volume.
Dissolve 20.0 mmol of benzophenone in 10
mL anhydrous ether in a dry Erlenmeyer
flask and add to the addition funnel. Add a
magnetic stir bar to your reaction flask and
add the Grignard reagent dropwise to the
solution when it is no longer boiling. Add
the benzophenone solution rapidly enough
to keep the solution boiling without external
heating. After the addition, gently reflux the
reaction mixture for another 15 minutes and
then allow to cool to room temperature.
Add 5 mL of water dropwise through the
addition funnel and then 15 mL of 5% HCl.
Use a spatula to break up any chunks of
undissolved solid and add ether and/or 5%
HCl to dissolve the solids.
If any undissolved Mg remains, remove it by
gravity filtration. Transfer the ether/water
mixture to a separatory funnel, shake
carefully (venting frequently), then let the
layers settle before draining and discarding
the aqueous layer.
Carefully wash the ether layer with 5%
sodium bicarbonate.
What are the bubbles that form?
Wash the ether layer once with sat. sodium
chloride, before drying with sodium or
magnesium sulfate. Filter the drying agent
and evaporate the solvent under vacuum.
Add ~10 mL of petroleum ether (PE) to the
solid residue and use a spatula or glass stir
rod to triturate the solid for 2-3 minutes.
Collect the solid by vacuum filtration and
wash with fresh PE. Let the solid air-dry on
the filter.
What is the purpose of this trituration?
Recrystallize the crude triphenylmethanol
with a 2:1 mixture of ligroin (high bp
hexanes) and ethanol. This procedure can
be done the following week, if necessary.
Use a minimal amount of solvent and let the
solution sit undisturbed for at least 30
minutes as the crystals form slowly. Filter,
dry and weigh the crystalline solid.
During the second part of the experiment,
triphenylmethyl alcohol will be converted
into triphenylmethyl fluoborate by mixing
1.00 g of the alcohol with 7.9 mL of acetic
anhydride in a dry Erlenmeyer flask. Add
1.0 mL of 48% fluoboric acid (HBF 4) and
swirl to dissolve all solids. Let sit for 15
minutes, stopper the flask and cool in an ice
bath. Collect the precipitated solid by
vacuum filtration, rinse with cold anhydrous
ether and let it air-dry on the filter. Weigh
and calculate your yield.
What is the purpose of the acetic anhydride?