A Gallery of Acetoacetanilide Photomicrographs

(using polarized light)

by Brian Johnston   (Canada)

Acetoacetanilide is not a chemical that normally would be found in the household environment.  It is an "intermediate" used in the production of organic dyes and pigments, (particularly yellow ones), and in the formulation of some pesticides. 

The white crystalline solid has a melting point of about 85 degrees Celsius and therefore makes an ideal candidate for study with a polarizing microscope.  A melt specimen is easily prepared by placing a few crystals of the compound on a microscope slide, covering with a cover-glass, and heating very gently over the flame of a candle or alcohol lamp.  Since candle flames produce soot, I much prefer the latter method.  It may be of interest to note that all of the photomicrographs in this article were taken of a single slide heated only once!  Note that acetoacetanilide is poisonous and is harmful if swallowed.  It may also cause allergic reactions in some individuals.

The molecular structure of the compound can be seen below.  "HyperChem" software was used to visualize the molecule.

When viewed under the microscope between crossed polars, acetoacetanilide often produces very colourful fields that tend to look rather "flame-like".  The two images below were produced using a lambda/4 compensator in addition to polars in their extinction position.

Without the compensator, the colours appear darker and have greater contrast, as can be seen in the example below.

As the compensator is rotated, the colours of various sections of the field change.  Notice how one particular field looks completely different as the angle changes.

If, for some reason, the layer of crystals between slide and cover-slip changes in thickness, (for example if the cover-slip is not parallel to the slide), completely different formations may appear in thinner regions.  The colours in thinner areas are less brilliant and tend towards gray.  The image on the left shows a field from a "thicker" region, and the one on the right from a "thinner" one.

The colours however, can be determined by other factors.  Immediately after the melt re-solidifies, large sections of the slide have the following appearance.

Within minutes however, colourful fan-shaped formations seem to grow into the darker areas.  Notice the process occurring in the top right corner of the two images below.  The right image was photographed about thirty seconds after the left one.  I believe that as the second (colourful) form grows, it is actually converting the molecular packing type from one form to another.  In the end there is only one molecular arrangement.

After this process has completed, the fields look like the one below.  Both images were produced with crossed polars, but the one on the left used two lambda/4 compensators, while the one on the right used lambda/4 and lambda compensators.

The image below shows a curious formation right at the edge of the crystal melt (where the cover-glass ends).  The same compensators were used as in the image on the right, above.

The hair-like formations below were also observed right at the edge of the crystal melt.  A polarizing condenser with crossed polars and no compensators was used to produce the image.  Beneath it is another taken at a higher magnification utilizing a phase contrast condenser to show the detail.

Acetoacetanilide certainly produces a wide range of interesting crystal formations.  A single attempt at producing a melt specimen can provide a rewarding evening of study and many potential photomicrographic subjects!
The photomicrographs in the article were taken with a Nikon Coolpix 4500 connected to a Leitz SM-Pol microscope.

 All comments to the author Brian Johnston are welcomed.

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Published in the March 2006 edition of Micscape.
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