5
$\begingroup$

I am doing a malonic synthesis shown below, and I wonder why I should distill ethanol off before adding water in the first step of the reaction (First I add absolute ethanol and solid sodium).enter image description here

$\endgroup$
5
  • $\begingroup$ I see no reason to distill off EtOH before adding water for the base hydrolysis step. The EtOH facilitates the reaction as the butyl malonate is readily soluble in it but not in water. $\endgroup$
    – Waylander
    Sep 11, 2018 at 15:25
  • 1
    $\begingroup$ I think it is because of this passmyexams.co.uk/GCSE/chemistry/… i.e alcohol will react with your product (carboxylic acid) to form an ester. $\endgroup$
    – user43021
    Sep 12, 2018 at 0:06
  • 3
    $\begingroup$ Could you clear up the confusion and detail steps in more detail. Your reactions don't indicate when you boil off the ethanol... $\endgroup$
    – MaxW
    Sep 12, 2018 at 0:29
  • $\begingroup$ I think the $\ce{KOH}$ hydrolysis is not necessary. Acid hydrolysis would do it. It is necessary to remove ethanol to prevent resterification (Le Chatelier's principle). $\endgroup$ Jun 4, 2020 at 18:52
  • $\begingroup$ related: chemistry.stackexchange.com/a/37195/102629 $\endgroup$
    – cngzz1
    Jan 31, 2021 at 13:08

2 Answers 2

1
$\begingroup$

I assume this is a real procedure for actual synthesis.

Distilling ethanol off does have an effect on the workup of this reaction. Usually one needs to separate the reaction product and educts or side products afterwards by separation between an aqueous and an organic phase. Having ethanol in the mixture prevents the two phases from separating. In order to isolate your product you will need to distill ethanol off before adding water, because water would also be distilled off, if you added it first.

$\endgroup$
4
  • 1
    $\begingroup$ I disagree. You remove the ethanol after the base hydrolysis. $\endgroup$
    – Waylander
    Sep 11, 2018 at 18:36
  • $\begingroup$ Where does it say the ethanol is distilled off after base hydrolysis? $\endgroup$
    – Chris
    Sep 11, 2018 at 18:45
  • $\begingroup$ It doesn't. My point is that it makes little sense to remove the EtOH before the base hydrolysis, all you need do is add some water and an excess of KOH to conduct the hydrolysis, then concentrate as the first part of the workup. $\endgroup$
    – Waylander
    Sep 11, 2018 at 18:49
  • $\begingroup$ If you distill it off before, then do the hydrolysis, you can isolate your acid by acidifying your solution and extraction with apolar solvent. I think you just have a different procedure that you like to stick to, but OP describes the procedure like i explained. $\endgroup$
    – Chris
    Sep 11, 2018 at 18:58
0
$\begingroup$

The synthesis adds two carbon units from malonic acid to butyl bromide, followed by a de-carboxylation step, to form caproic acid.

There are 2 reasons:


1. Water and alcohol form an azeotrope, why not recover some absolute ethanol before the second step?
2. Reflux temperature for de-esterification will be higher with a greater percentage of water as ethanol is removed.

Butyl bromide boils around 101 C, so it can be distilled off during KOH reflux. A possible by-product, n-butanol, which boils at 117 C but forms an azeotrope with H2O at 92 C, can also be removed with higher temperatures.

So, alcohols and butyl bromide could be fractionally distilled off during the KOH de-esterification (this technique would be practiced on a lab scale to perfect it), leaving the water soluble malonate adduct for subsequent decarboxylation.

Interestingly, after decarboxylation step, caproic acid could then be extracted back into non-polar solvent for further purification.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.