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I am trying to get into chemistry and I saw a video by Nile Red in which powdered sodium benzoate and sodium hydroxide are mixed and heated and a reaction proceeds as follows:

$\ce{NaOH (s) + C6H5COONa (s) ->[200^\circ C] C6H6 (g) + Na2CO3}$

I don't understand how the bond between the benzene ring and the carboxyl group is being broken because a benzene anion seems very unstable, and I also don't get how the hydroxide is being ripped apart considering the OH bond is very strong. Can someone explain a plausible mechanism for how this reaction might take place?

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  • $\begingroup$ Probably two step substitution + proton exchange (kinda like haloform reaction, with benzene instead). It happens so looking unlikely is irrelevant. $\endgroup$
    – Mithoron
    Apr 16, 2023 at 13:15
  • $\begingroup$ Ok, so the carboxylate carbon undergoes nucleophilic attack by the hydroxide and forms a bicarbonate ion whose proton is then transferred to the newly formed aromatic anion? Is that a reasonable possible mechanism? I want to understand this stuff so I can predict reaction products and invent syntheses. $\endgroup$ Apr 17, 2023 at 0:57
  • $\begingroup$ Probably something like that. Proton transfer could be concerted, so there may be no separate phenyl anion involved. $\endgroup$
    – Mithoron
    Apr 17, 2023 at 0:59
  • $\begingroup$ @EatingTechnobladesRemainsAt3am it would not be bicarbonate. With the carbon in the $\ce{CO3}$ moiety also bound to the hydrocarbon group it would be an orthocarboxylate. $\endgroup$ Apr 17, 2023 at 20:06

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I have viewed the presumed Nile Red video to which the OP refers on the thermal decomposition of sodium benzoate in the presence of sodium hydroxide. An intimate, powdered mixture of sodium benzoate (0.69 mole) and sodium hydroxide (1.5 moles) contained in a metal paint can and fitted with a distillation apparatus was heated over a camp burner.[1] Although the OP indicates that the temperature of the reaction was 200oC, there is no evidence that the temperature of the reaction was determined. At these elevated temperatures, it is clear that the entropy (TΔS) outweighs enthalpy (ΔH).

Two previous Answers to this question offered variations on an ionic fragmentation of the carbon-carbon bond in question of sodium benzoate to afford benzene and carbon dioxide. The prospect of a radical fragmentation may also be considered. One source of hydrogen atoms is surreptitious water with a bond dissociation energy (BDE) of 119 kcal/mol. Alternatively, hydroxide has a BDE of 110 kcal/mol. There is the possibility that, under the vigorous conditions of the reaction, addition of hydroxide to the carbonyl group of the benzoate anion 1 occurs to form adduct 2. Bond dissociation of 2 leads to phenyl radical 4 and radical 3. continued...


The BDE of the O-H bond in 3 is expected to be much weaker than that in the other candidates. The BDE of the C-H bond in formic acid 7, although not formate, is 96 kcal/mol. However, the BDE for the same bond in the radical 8 is only 12 kcal/mol. Presumably, a radical cage effect would be operable prior to diffusion.

BDE's are taken from S. J. Blanksby and G. B. Ellison, Acct. Chem. Res., 2003, 36, 255.

  1. My advice, don't try this at home.
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An alternative possibility is proposed here, which does not require formation of a free phenyl anion. Instead the phenyl-anion moiety is incorporated into a delocalized bonding in an intermediate state.

In this alternative, the hydroxide ion first adds to the carboxyl carbon to from the orthocarboxylate salt on the left. Ordinarily, we would not expect a carboxylate ion to act as an electrophile, but here the addition of heat energy and the strong basicity of the hydroxide ion may enable such an addition. The orthocarboxylate then undergoes an electronic rearrangement to separate the carbonate portion and leave the decarboxylated hydrocarbon behind.

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    $\begingroup$ Did you intend for your mechanism arrows to move electrons as shown? $\endgroup$
    – user55119
    Apr 17, 2023 at 21:56
  • $\begingroup$ Yes. One pair moves into the C-H bond region and the other becomes the second bond in the carbonyl portion of the sodium carbonate. $\endgroup$ Apr 17, 2023 at 22:02
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    $\begingroup$ I meant have the arrows go in the opposite direction. $\endgroup$
    – user55119
    Apr 18, 2023 at 1:05
  • $\begingroup$ I don't have the nice apps to edit easily, could you show me? Thanks. $\endgroup$ Apr 18, 2023 at 1:29
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    $\begingroup$ OL: If this diagram is unsuitable, feel free to go back to the original. $\endgroup$
    – user55119
    Apr 18, 2023 at 3:17
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Decarboxylations tend to follow the mechanistic path I've drawn below. You're right in that this is a bold transformation--partly this is driven by the high heating. Decarboxylations are also driven by entropic and enthalpic forces.

Decarboxylation

Entropically, you are starting with one compounded (sodium benzoate) and producing two (phenyl carbanion and CO2 gas). Entropy is frequently described as "disorder" which you can quantify as how many microstates are possible. If you have a loaf of bread and a four-shelf book case, you can put in on the first, second, third or fourth shelf. If you tear the loaf of bread in half, you can for example, put one loaf half in the third shelf and the other loaf in the first shelf—you can see there is many more combinations of options or "microstates". Replace "bread" with molecules and "book shelf" with energy level and you have entropy. Breaking the molecule in two increases entropy, especially because the CO2 is a gas, which has more entropy than a solid.

Enthalpic means, put simply, how strong the bonds are. While you do raise legitimate enthalpic concerns, this will be offset in part by the strong double bond you form making CO2 gas (a double bond is stronger than a single bond). If you make more strong bonds than you break, this is enthalpically favoured.

CO2 becomes NaCO3H and Na2CO3 readily. In fact this happens in your blood and is why people used to recommend breathing into a brown bag—increase the CO2 concentration which pushes towards NaCO3H. On that same point, decarboxylations are also an extremely common reaction in the body, albeit not with sodium benzoate.

Finally, the proposed reaction proceeds through phenyl carbanion (all other proposed mechanisms involve the same), though the mechanism may be concerted. This is a bit strange, but I'll just reference one or two previous posts on this species and point out that the aromaticity is not disturbed, as shown in the second figure. Typically you may see phenyl carbanions stabilised as lithiated species (i.e., using extremely strong organolithium bases to generate), which I think points to why such extreme heating was required by NileRed. Also, knowing his channel is sort of "garage chemistry", heating is much accessible than unusual reagents.

Orbital

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  • $\begingroup$ Looks like 2 people have suggested 2 possible mechanisms. The one you suggested involved the dissociation of sodium benzoate into CO2 and a benzene anion as the rate limiting step, and the other commenter suggested that nucleophillic attack of hydroxide on the carbon in the carboxyl group would be the rate limiting step. Could we test these 2 hypotheses by measuring the effect that the lye concentration has on the reaction rate? $\endgroup$ Apr 17, 2023 at 19:11
  • $\begingroup$ Sodium hydroxide is introduced as a solid, not a solution species. So we can't define a variable sodium hydroxide concentration to follow this approach. Quenching with an alkyl halide and seeing if an orthoester is recovered might work, or it might not if the sodium hydroxide consumes the alkyl halide first. $\endgroup$ Apr 17, 2023 at 20:23
  • $\begingroup$ That's a good question regarding distinguishing mechanisms. One possibility would be to do computational studies, which may distinguish which is more energetically reasonable--attacking a carboxylate anion with a hydroxide anion, or the presumably difficult generation of the phenyl anion. You can potentially monitor for CO2 gas evolution; "capture" the phenyl anion intermediate; or try a second reaction with a strong but nucleophilic base. Phenyl anions are generated all the time (though unusual looking) and partially stabilised so I'm not sure proposing an alternative mechanism is motivated $\endgroup$
    – sat0ri
    Apr 17, 2023 at 20:50

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