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I'm performing the reaction between acetone and 2-nitrobenzaldehyde catalysed by (L)-Proline, as answered to one of my questions a while ago: Simple experiments involving enantiomer synthesis

However, while the literature specified using 1 mmol of (L)-Proline in 20ml of solution, due to the cost of the materials and the budget available, I've only been able to use a fraction of that, around 0.01mmol. I've scaled down the entire experiment to 5 ml to try and compensate for this.

The issue is, I've left this solution reacting for a long period of time (12+ hours) and I'm not seeing any optical activity at all. With the reduced concentrations, I'd have expected to see a lot less activity than the original literature specifies, but to see at least some indication that my reaction is working.

Are the concentrations of (L)-Proline simply too small for any significant reaction to take place? Or can there be some other reason why my reaction isn't working? Note: I'm a chemistry 'newbie', so don't assume anything!

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    $\begingroup$ How are you measuring the optical activity? How are you monitoring the progress of the reaction? $\endgroup$
    – jerepierre
    Commented Mar 6, 2015 at 12:02
  • $\begingroup$ I am using a polarimeter, and I was expecting to see a progressive change in optical rotation with which I could determine the rate $\endgroup$
    – Robin McCorkell
    Commented Mar 6, 2015 at 15:43
  • $\begingroup$ Can you measure the optical rotation of proline itself? Based on the optical rotation of the product, can you calculate the expected change in optical rotation for these conditions? $\endgroup$
    – jerepierre
    Commented Mar 6, 2015 at 16:08
  • $\begingroup$ That's just the problem - there isn't any optical rotation, both before the reaction (aka the pro line) or afterwards $\endgroup$
    – Robin McCorkell
    Commented Mar 6, 2015 at 16:19
  • $\begingroup$ Based on the concentration of proline and the reported optical rotation, should it be measurable using your instrument? You may be below the limit of detection for the polarimeter. $\endgroup$
    – jerepierre
    Commented Mar 6, 2015 at 16:46

1 Answer 1

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  • The original "Rutgers' reference" deploys $2\,\mbox{mmol}$ aldehyde in $20\,\mbox{mL}$ solvent (i.e. starting at $0.1\,\mbox{mol/L}$, or $0.1\,\mbox{M}$) and recommends a stirring of $3\,\mbox{h}$.

Subsequently, you mention $1\,\mbox{mmol}$ per $20\,\mbox{mL}$ solvent (i.e. starting at $0.05\,\mbox{M}$), but without an indication of reaction time.

  • But if the starting concentration is lowered further ($0.01\,\mbox{mmol}$ in $5\,\mbox{mL}$ equals to $0.002\,\mbox{M}$ or $2\,\mbox{mM}$), not only the detection of the product becomes difficult.
    Did you take into consideration that by diluting the reaction you over-proportionally slowed down the advancement of the intended reaction? Compared to the initially mentioned starting concentration, your reaction starts at a dilution by a factor of 50 (fifty). This is more than a big factor for process involving a bimolecular reaction.

By the way, $0.01\,\mbox{mmol}$ of proline, molar weight of $115.13\,\mbox{g/mol}$ as stated here would correspond to little more than $1.1\,\mbox{mg}$.

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