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In the school laboratory, a friend tried the famous Cannizzaro reaction for benzaldehyde. Some $\ce{NaOH}$ was added to benzaldehyde, and the solution was stirred for 30 minutes. What came as a surprise was that, after this time interval, a big, bright yellow, gooey-looking blob appeared in the flask.

As can be seen, the blob does not dissolve upon shaking. Whatever reaction occurred here was probably reversible, though, as the blob burst after the solution was transferred into a separation funnel, and the experiment went on to give normal products and reasonable yields, without any obvious anomaly.

Does anyone have any idea about what this strange intermediate product might be, or what might have caused it?

enter image description here

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2 Answers 2

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Robert DiGiovanni's answer comes close. Benzyl alcohol can indeed precipitate during the intermediate stage, but it is colorless. Because benzyl alcohol has some weak acidity, it can be partially deprotonated by a base as strong as sodium hydroxide, giving $\ce{C6H5-CH2-O^-Na^+}$. This indeed can impart a yellow color according to Sigma-Aldrich, which sells this salt as a solution in the alcohol. When the sodium hydroxide is neutralized upon workup, the benzyl alcohol fully resumes its neutral form $\ce{C6H5-CH2-OH}$, as if nothing untoward had happened.

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  • $\begingroup$ @RobertDiGiovanni thanks for the correction. $\endgroup$ Dec 12, 2022 at 22:40
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    $\begingroup$ pKa benzyl alcohol is 15.4 as read here. But this is plausible. Perhaps the O- would be on the surface of the blob, into the aqueous layer. $\endgroup$ Dec 12, 2022 at 22:45
  • $\begingroup$ The O- from benzyl alkoxide does not conjugate with the ring. The Cannizzaro reaction involves a deprotonated acetal and hydride transfer. It is possible that a doubly deprotonated acetal with the negative ions effectively ion paired to Na+ could enable an electron pair from the ring to supply the impetus for the hydride transfer. This would result in a quinoid compound that would revert to benzoic or possibly hydroxybenzoic acid upon neutralization. This scenario fits the indicator story given above. $\endgroup$
    – jimchmst
    Dec 13, 2022 at 5:51
  • $\begingroup$ @jimchmst would such a quinoid compound be stable enough to form in this case? Would be nice if there are similar examples in the literature, I don't quite understand the mechanism described in the comment. $\endgroup$ Dec 13, 2022 at 6:58
  • $\begingroup$ I missed the impact if the intervening CH2 group. Last paragraph removed. $\endgroup$ Dec 13, 2022 at 9:17
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Benzyl alcohol has a lower solubility (4 g/100 ml) than sodium benzoate (63 g/100 ml) or benzaldehyde (7 gram/100 ml) in water.

The presence of NaOH contributed to the "salting out" of the more nonpolar benzyl alcohol, plus some other impurities, forming your "blob".

Benzaldedyde and benzyl alcohol are also (surprisingly) slightly denser than water.

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  • $\begingroup$ Would a difference of 3g/ml (between benzyl alcohol and benzaldehyde) result in such a drastic change in colour and texture? $\endgroup$ Dec 13, 2022 at 17:51
  • $\begingroup$ @jonohuang that certainly has to do with what was added to the flask and whether or not they reran it more dilute (solubility issues). But I agree with Oscar Lanzi that there may be some deprotonation of benzyl alcohol. $\endgroup$ Dec 13, 2022 at 18:09
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    $\begingroup$ Since benzyl alkoxide is colorless the yellow color must be an intermediate in the reaction or possibly something that can be formed to make the alkoxide yellow probably from oxidation. The colors are possibly related but might not be. Step one: repeat the reaction identically; same chemicals. Step two: Repeat the reaction with carefully purified benzaldehyde and NaOH. If it was an impurity then there is an additional product or an analysis of the original benzaldehyde is in order. $\endgroup$
    – jimchmst
    Dec 13, 2022 at 19:09
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    $\begingroup$ "In the school laboratory", yes, some additional optimization work may be in order, lest the "blob" escape the laboratory and ... $\endgroup$ Dec 13, 2022 at 19:50

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