Wikipedia:Reference desk/Archives/Science/2024 June 12

= June 12 =

Weigeltisaurus species clarification
Is this Weigeltisaurus reptile in this family Rhynchocephalia. Would that statement be true to say. Its for this article Johannes Weigelt. There is source that states it but I don't know how accurate it is.  scope_creep Talk  21:23, 9 June 2024 (UTC)


 * While Weigeltisaurus flourished in the Late Permian, our article on the order Rhynchocephalia states that the oldest record of the group is dated to the Middle Triassic. According to this chronology, a gap of several million years separates them. --Lambiam 07:21, 10 June 2024 (UTC)
 * Right. I'll leave the Rhynchocephalia bit out and only mention the Weigeltisaurus bit, since I don't understand it. Thanks    scope_creep Talk  08:52, 10 June 2024 (UTC)

Grignard Reagent with Haloalkane
The reaction between a Grignard reagent and an alkyl halide  gives what product? I see some sources saying there would be substitution reaction thus forming, whereas other source says there would be transmetallation thus forming. Thanks for your time, Exclusive Editor  Notify Me! 12:19, 12 June 2024 (UTC)


 * The WP article on Grignard compounds itself states that "Grignard reagents do not typically react with organic halides, in contrast with their high reactivity with other main group halides. In the presence of metal catalysts, however, Grignard reagents participate in C-C coupling reactions." Also in this article there is a diagram of different reactions that show the reaction between R'-X and RMgX to yield the C-C coupling product (where R' should be a carbonyl-like moiety such as benzyl or allyl). Logically, the MgX+ cation should strongly bond with the halogen atom of R'-X during the reaction, resulting in a very stable salt MgX2. The more stable the products are, the more favorable a reaction pathway is. 2402:800:639D:9B0A:C9C4:BA57:2B8:DC89 (talk) 14:01, 16 June 2024 (UTC)
 * Yes, but as you state, it happens in presence of metal catalyst (transition metal). My question doesn't mention our reaction having any such catalyst. Exclusive Editor  Notify Me! 19:11, 21 June 2024 (UTC)
 * On otherhand, the transmetallation reaction requires that both two of the reactants must be organometallic compounds (i.e, contains at least a metal-carbon bond), which is not the case with one of your reactants R'-X.Vanadium-3065 (talk) 14:08, 16 June 2024 (UTC)
 * When forming a Grignard reagent by the standard method of adding an alkyl halide like EtBr to Magnesium turnings suspended in ether, controlling the exotherm by the rate of addition, there will be a point where the halide is being added to material that contains substantial amounts of EtMgBr. If coupling (i.e. the substitution reaction) were fast, the main product would be butane and it would be impossible to form a good yield of the Grignard reagent! My trusty "Advanced Organic Chemistry" by Jerry March says that you can deliberately encourage the coupling reaction by adding thallium (I) bromide, Cr, Co or Cu chlorides or various other reagents, depending on the type of Grignard reagent you have already formed. The mechanism is via the R-Metal intermediate which decomposes via free radicals. Mike Turnbull (talk) 15:08, 17 June 2024 (UTC)
 * You may be right. But then the question is still half answered. It still does not say if R-R' will be formed or not. If not transmetalation, there could be a metal-halogen exchange as stated by DMacks below. Exclusive Editor  Notify Me! 19:13, 21 June 2024 (UTC)

To clarify, I am not an advanced chemistry student, so I don't realize things which really require deep insights. However one thing I would mention is that the statement that R-R' would not be formed but transmetallation would happen is written in Clayden (available at pg 189 at pdfs available online.). Maybe this would help clear any confusion, or if the publisher should be informed that the strict language that R-R' would not be formed is wrong. Reply appreciated, Exclusive Editor  Notify Me! 16:13, 21 June 2024 (UTC)


 * The Clayden statement is correct and corresponds to what I said above. In order to get R-R' you need to add a transition metal. Mike Turnbull (talk) 17:22, 21 June 2024 (UTC)
 * I just saw Kumada coupling and read that page of Clayden again. So we need some transition metals like Ni/Pd to act as catalyst to form carbon-carbon bond (R-R'). If it is not available, then transmetalation takes place in equilibrium? Exclusive Editor  Notify Me! 18:18, 21 June 2024 (UTC)
 * The logic of that Clayden statement seems incorrect (or circular at best). It says that substitution "does not work because of transmetallation. The two alkyl bromides and their Grignard reagents will be in equilibrium with each other so that, even if the coupling were successful, three coupled products will be formed." (my underlining). That doesn't sound like a reason why it doesn't work, just potential evidence that it doesn't work and not even stating that the evidence is true. And isn't this really metal–halogen exchange (swapping M and X on the R groups) rather than transmetallation (M and M', like the Schlenk equilibrium)? DMacks (talk) 18:19, 21 June 2024 (UTC)

So I think my question could be summarized as, what product do we get, if we conducted Kumada coupling without any catalyst, and the answer supposedly is that we get transmetalation (or metal–halogen exchange as said by DMacks) per Clayden. It is only if we use some transition metal like Ni/Pd as catalyst, that we get a carbon-carbon bond (R-R'). Exclusive Editor Notify Me! 18:41, 21 June 2024 (UTC)


 * Yes but.... like many things in practical organic chemistry, the issue is not simply whether a particular product forms. Considerations of side-products, rate of reaction and hazard (among others) also come into play. The Wurtz reaction takes R-Hal with sodium and gives coupled products R-R but as our article says is "of little value". That's because in general side-reactions like elimination and rearrangement lead to low yields. The free-radical intermediates are too difficult to control in practice. Hence chemists have developed milder and more specific methods for coupling, of which Kumada coupling is one and there are many others e.g. listed in the "see also" section of that article. These are now a key group of important C-C bond-forming reactions finding extensive general use. I agree with DMacks that Clayden could have expressed himself better. He was arguing that even if the coupling were successful, three coupled products will be formed as if it were the main reason not to try the reaction R1CH2MgBr + R2CH2Br -> R1CH2CH2R2 (i.e. because you would also get R1CH2CH2R1 and R2CH2CH2R2) whereas in fact the main reason this sort of coupling is unsuccessful is that the rate of any useful reaction is too low. Mike Turnbull (talk) 11:16, 22 June 2024 (UTC)

Check a SpringerLink reference
http://doi.org/10.1007/978-94-017-9861-7_22

Does anyone have access to this? If so, does it make any reference to the diet or hunting behaviour of Euthyrhynchus floridanus? Such claims were added without reference to this insect's article four years ago, and later someone dumped in a reference to this book. Nyttend (talk) 21:15, 12 June 2024 (UTC)
 * I don't think so. The closest I can find is "However, only a few notes about its life history on the ﬁeld are known (Avila-Núñez et al. 2009 )" The reference is "Avila-Núñez JL, Ortega LDO, Pisarelli MPC (2009) Un caso de depredación de adulto de Gonodonta pyrgo Cramer 1777 (Lepidoptera: Noctuidae) por Euthyrhynchus ﬂoridanus(Linnaeus 1767)(Heteroptera: Pentatomidae: Asopinae). Entomotropica 23:173–175". (Btw, you should be eligible for Wikipedia Library; that's how I could access the book).--Wrongfilter (talk) 21:46, 12 June 2024 (UTC)
 * You can get its access at Wikipedia Library, publications at springer are available there. However, for this one there is no mention of things that were claimed, in that chapter at least, so there removal is legitimate. Exclusive Editor  Notify Me! 16:41, 21 June 2024 (UTC)