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In the synthesis of DMAD, we eliminate two bromides from dibromosuccinic acid in basic environment to get acetylenedicarboxlyic acid (Step 2). dmad

However, this reaction could also happen: alternate_mech

The reaction in Organic Syntheses (http://www.orgsyn.org/demo.aspx?prep=cv2p0010) went with a yield of 72-88 %, which means that these pathways probably do happen, but only in a minority of cases.

Why is that so, given that the elimination of CO2 is entropically favoured?

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  • $\begingroup$ iI there is any propargylic acid or acetylene formed, the yield is not an indication. Most OrgSyn yields are less than 100%. Bromination of methacrylic acid and heating in pyridine gives loss of CO2, pyridine hydrobromide and 2-bromopropene by the mechanism you suggest. $\endgroup$ – user55119 Nov 3 '19 at 3:33
  • $\begingroup$ @user55119 Any references for the methacrylic acid? $\endgroup$ – FusRoDah Nov 3 '19 at 22:26
  • $\begingroup$ E. A Braude and E. A. Evans, J. Chem. Soc., 3333 (1956). I ran the reaction about 50 years ago. In your example the rate of the observed elimination is faster than the one you question. $\endgroup$ – user55119 Nov 3 '19 at 23:39
  • $\begingroup$ Is it possible that the solvent is the determining factor? $\endgroup$ – FusRoDah Nov 4 '19 at 5:30
  • $\begingroup$ Something like that just doesn't happen - you need stabilised carbanion as intermediate. Having some leaving group nearby simply isn't enough for carboxylate instability. $\endgroup$ – Mithoron Nov 4 '19 at 21:02
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This elimination is unfavorable because C--C bonds are generally quite stable. Decarboxylation would involve the formation of an unstable carbanion before proceeding to the elimination as you have outlined. (Note, this is more favorable under different functional group conditions, namely when a neighboring carbonyl can stabilize the carbanion through the formation of an enol https://www.masterorganicchemistry.com/tips/decarboxylation/ ) So, the formation of the alkyne is favored here over the elimination of CO2.

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  • $\begingroup$ But C-H bonds are stronger than C-C bonds and when eliminating CO2, the newly formed C=O double bond is also very strong. $\endgroup$ – FusRoDah Nov 5 '19 at 16:53
  • $\begingroup$ You not only need to look at the strength of the bonds broken and formed but the stability/energetics of the intermediates the reaction would have to pass through. Since this would necessitate the formation of an unstable (and unstablized) carbanion intermediate, this process is unfavorable energetically. $\endgroup$ – Jeff Gustafson Nov 5 '19 at 18:12
  • $\begingroup$ The elimination could be concerted, avoiding the carbanion intermediate. $\endgroup$ – FusRoDah Nov 5 '19 at 18:42
  • $\begingroup$ And also, in the comments above, it was mentioned that methacrylic acid gives the CO2 elimination product, even though the carbanion intermediate would be less stable than in this case. $\endgroup$ – FusRoDah Nov 5 '19 at 18:43
  • $\begingroup$ This isn’t entirely true; the eliminations can be concerted if the atoms are aligned properly (think of the mechanism of the Swern oxidation, especially the breakdown of oxalyl chloride). I suspect that alignment is the problem here. $\endgroup$ – Jan Nov 6 '19 at 4:55

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