[Emofreak]

Hi,

Recently i was task to synthesis Dibenzylideneacetone by adding a solution of sodium hydroxide in water and ethanol into another solution of benzaldehyde and acetone in ethanol. However, something when wrong during the recrystallization step, the initial product (Dibenzylideneacetone, yellow) was dissolved in a solution of ethanol and heated, the solution turn into dark brownish in colour and the final product was light brown powder instead the expected pale yellow crystal.

Could someone help in identifying the problem, i could not seem to find a reason for my final product being light brown powder instead the expected pale yellow crystal. Any help will be appreciated. Thank you

[Babcock_Hall]

What is the melting range of your crystals compared with the literature?

[pyngjls]

It's possible the ethanol reacted with the double bonds on the carbon chain to form a diethyl ester through electrophilic addition.
Perhaps try to run a 1H-NMR of your sample. If there are peaks between 2.00 - 4.00 ppm, then that may be the case. The pure product should only have peaks between 7.00 - 8.00 ppm.

[hollytara]

did you thoroughly rinse your crystals to get rid of all remaining hydroxide? 

[Emofreak]

What is the melting range of your crystals compared with the literature?

Unfortunately due to some technically difficulty, we couldn't ascertain the melting point range accurately. However, we were certain that it was below the range of the literature melting point (110–111°C) by at least 15 °C

[pgk]

No, it is not possible that ethanol reacted with the double bonds on the carbon chain to form a diethyl ester through electrophilic addition.
Probably, you have partially oxidized (and/or polymerized) dibenzalacetone during recrystallization and thus, you also have got (gum?) impurities with similar structure that significantly decrease the melting point. Try to recrystallize it with ethyl acetate (“100 ml of solvent for each 40 g of material”).
DIBENZALACETONE, Organic Syntheses, Collective Volume 2, 167, (1943)
http://www.orgsyn.org/demo.aspx?prep=cv2p0167

[Emofreak]

It's possible the ethanol reacted with the double bonds on the carbon chain to form a diethyl ester through electrophilic addition.
Perhaps try to run a 1H-NMR of your sample. If there are peaks between 2.00 - 4.00 ppm, then that may be the case. The pure product should only have peaks between 7.00 - 8.00 ppm.

appreciate the suggestion but my IR spectrum do not suggest there is a C-O stretching, thus i do not think this it the case.

[Emofreak]

did you thoroughly rinse your crystals to get rid of all remaining hydroxide? 

Based on my recollection, no, there were barely any rinsing during the vacuum filtration before the recrystallization step. May i asked what could the presence of sodium hydroxide lead to?

[pgk]

1). Case 1: Due to the extended conjugation, dibenzalacetone IR spetrum has the carbonyl band at lower wavenumbers and notably at 1671 (all s-trans) and/or 1651 (s-trans, s-cis) and/or 1626 (all s-cis) cm-1 that can easily be confused with the double bond/aromatic absorptions.
Synthesis of Dibenzalacetone, ETH Web-text, Zurich, (2007)
http://n.ethz.ch/~nielssi/download/5.%20Semester/Praktikum%20Organische%20Chemie%20I%20f%FCr%20Biol.%20Pharm.Wiss./Reports/Old%20Reports/Praktikum-OCI-Teil-21/Praktikum%20OCI%20(Teil%202)/Dibenzalacetone.pdf
2). Case 2: If you don’t see absorptions bellow 1630-1620 cm-1, you probably have fully polymerized the product (favored by the presence of NaOH?).

[Babcock_Hall]

It seems possible that you made the compound but in very impure form.  Have you tried a second recrystallization?  Is it possible to purify it in some other manner?

[Emofreak]

1). Case 1: Due to the extended conjugation, dibenzalacetone IR spetrum has the carbonyl band at lower wavenumbers and notably at 1671 (all s-trans) and/or 1651 (s-trans, s-cis) and/or 1626 (all s-cis) cm-1 that can easily be confused with the double bond/aromatic absorptions.
Synthesis of Dibenzalacetone, ETH Web-text, Zurich, (2007)
http://n.ethz.ch/~nielssi/download/5.%20Semester/Praktikum%20Organische%20Chemie%20I%20f%FCr%20Biol.%20Pharm.Wiss./Reports/Old%20Reports/Praktikum-OCI-Teil-21/Praktikum%20OCI%20(Teil%202)/Dibenzalacetone.pdf
2). Case 2: If you don’t see absorptions bellow 1630-1620 cm-1, you probably have fully polymerized the product (favored by the presence of NaOH?).

Thank you for your reply and i appreciate your patience with me as i am still new to organic chemistry.

i do have absorption around the range mention in case 1: notably at 1670 & 1653 wavenumber

At the same time i do not have absorption between 1630- 1620 with the next absorption at 1602 wave number which could have been a result from the nujol oil solvent that was used for IR spectrum purpose. In that case, would that suggest polymerization.

[Emofreak]

It seems possible that you made the compound but in very impure form.  Have you tried a second recrystallization?  Is it possible to purify it in some other manner?

Seem possible, a second recrystallisation was not done due to time constraint i had for the lab access.

[pgk]

No!
Absence of absorption between 1630-1620 cm-1, simply means absence of the “all s-cis” geometrical isomer.

CORRECTION
Case 2: If you don’t see absorptions "above" (higher than) 1630-1620 cm-1, you probably have fully polymerized the product.
I apologize for any confusion, due my wrong use of the "above/below/bellow" terms.

[pgk]

OBVIOUS CONCLUSION:
If you have absorptions at 1670 and 1653 cm-1 and you simultaneously get a -15oC melting point, you have prepared your product but not in a very pure form.

[Emofreak]

No!
Absence of absorption between 1630-1620 cm-1, simply means absence of the “all s-cis” geometrical isomer.

CORRECTION
Case 2: If you don’t see absorptions "above" (higher than) 1630-1620 cm-1, you probably have fully polymerized the product.
I apologize for any confusion, due my wrong use of the "above/below/bellow" terms.

no worries, thank a lot i do understand better now