Aldehydes and ketones show a high degree of reactivity with respect to nucleophilic attack of the
carbonyl carbon. Ammonia and its derivatives such as primary amines, hydrazine (N2H4) or
phenylhydrazines (C6H5NHNH2) all react by initially attacking the carbonyl carbon, followed by
hydrogen transfers to give an amine where the nitrogen is bonded to a carbon that also contains a
hydroxy group. Under acidic conditions, the hydroxyl group is protonated, followed by loss of water to  give the conjugate acid of the observed product.

"Schiff Base" or also known as an "imine" Hydrogen ion transfer then gives the Schiff base product. The condensation step to give water is acidcatalyzed and is the rate-determining step of the sequence. All of the steps of the above sequence are reversible. In principle, the reaction of water with the Schiff base will give back the aldehyde or ketone  and the amine from which it was formed. However, here, we are using aromatic reagents, which form products that are more stable than analogous alkyl-containing reagents. Ketones do not react as readily as do aldehydes.
The Importance of Imines From a synthetic perspective, imines are important in the syntheses of complicated amines.
The Schiff base can be reduced with hydrogen to give the amine.
From a biological perspective, imines also play an important role in the chemistry of vision: 11-cisretinal is bonded to opsin (the entire complex is called rhodopsin) by the formation of an imine with the NH2 of an amino acid residue of the opsin protein. The cis linkage of cis-retinal isomerizes when struck by a photon to form the trans-isomer. The trans isomer does not fit as well in the pocket formed by the opsin, thus the imine linkage hydrolyzes to give the trans-retinal and opsin. Enzymatic isomerization gives back the cis isomer, and the cycle is ready to begin again.

Precautions
Aniline is highly toxic and is an irritant; it is also an anticipated carcinogen. Avoid breathing fumes or
fumes from solutions that contain aniline. Wear gloves and keep the reaction in the hood at all times.
PreLab
Be sure to include all sections of the PreLab that accompany the Synthetic Experiments. In the
Chemical Data Table, you should include your best guess for the structure of the product. If you are
correct, the data for your product is in Aldrich (use the formula index).
PreLab Questions
1. More than one geometric isomer is possible for the imine product. Draw them and predict which one
will be most favored.
2. Predict the structure of the product.
3. How could you determine the ratio of the possible products? For that matter, how could you easily
separate the different isomers?
TLC
You are required to run a TLC to monitor the progress of the reaction. Plates should have three spots
(or lanes) on the origin: one for the main organic starting material that is being transformed, one for a
cospot (starting material and the reaction mixture), and one for the reaction mixture.
Procedure
Obtain a sample of fresh pure benzaldehyde prepacked in a shorty vial from the hooded shelf or the
stockroom. To a tared 13 x 100 test tube, add 2.05 mmoles of benzaldehyde and 2.00 mmol of aniline.
Add 1.0 mL of ethanol and 3 boiling chips. Add 1 drop of 5% HCl solution. (If it is not available, make
a small amount of your own by mixing 0.1 mL of 10% HCl and 0.1 mL of water.) Gently reflux the
mixture for 1 hr. After 1 hr, let the mixture cool to room temperature. Add 2 mL of water to the mixture,
with stirring. At this point, you should have a medium to very cloudy water layer and a yellow to
brownish-yellow oil on the bottom of the test tube. Warm the solution up gently, just until the upper
aqueous layer becomes rather clear. (This step is designed to remove benzoic acid from the mixture,
which is always present to some degree in benzaldehyde; if it's not removed here, the chance of
forming oils later is greater.) Remove the top layer. Now add 2 mL of ethanol, stir to dissolve the oil
completely, and transfer to another test tube, leaving the boiling chips behind. Use another 2 mL of
ethanol to rinse the first tube and add the rinsing the fresh test tube. Place the tube (which now
contains a total of 4 mL of solution) in an ice water bath for 10 min. Once it is very cold, add one small
chunk of ice (it has to be small to go into the test tube) and scratch the inner wall of the test tube. Keep
the tube chilled and continue to add a few more small chunks of ice. (This step allows the water
content of the solution to slowly increase, rather than the sudden change that occurs when water is
added; it also helps to chill the solution.) Once crystals begin to form, let the mixture stand for another
5 minutes in ice water. Filter the product using pipet filtration. Wash the solid twice with 1 mL of water.
Recrystallization
The product can be recrystallized with a two solvent system using ethanol and water. This can be
accomplished by dissolving the solid in a minimum amount (the smallest amount it takes to dissolve the
solid) of warm ethanol (heat the solution on a sand bath with a boiling stick.) Once all the crystals have
dissolved, take solution off the heat and while cooling, add water dropwise with intermittent scratching
on the inside walls of the container with a stir rod. Collect the crystals by pipette filtration or on the
Hirsch funnel. Once all the liquid has been removed, dump the pasty solid onto a piece of filter paper
and allow the solid to dry thoroughly before it is weighed, the melting point obtained, and the product
analyzed.
Cleaning Up
Dispose of all filtrates and wastes in the non-halogenated organic waste containers.
Analyses
In addition to TLC analysis, the product may be analyzed by IR, NMR, UV-VIS GC or GC-MS; see your
assignment strip for the specific analysis you are assigned. Analyze your sample according to your
Assignment sheet and the instructions on Sample Preparation in Lab Guide. If you analyze the product
by IR, the C=N stretch common to imines appears between 1615 and 1700 cm-1; note also that
conjugation will shift the absorbance to lower wavenumbers, just as the case with C=O stretching when
the carbonyl is conjugated.
Questions to be answered in the Final Report
1. Use Aldrich to determine the structure of 1,2-phenylenediamine. Predict the structures of the
products that form from the reactions of one mole of 1,2-phenylenediamine and
a. one mole of benzaldehyde b. two moles of benzaldehyde
2. Use resonance structures of your product to rationalize the decreased wavenumber of absorbance
of the C=N stretch of a conjugated imine.