What makes $\ce{R-MgX}$ a more reactive Grignard reagent than $\ce{R-Cu}$? Has it to do with solvation, bond enthalpy, electronegativity?

  • $\begingroup$ I'm really not sure, but it could be because of the size of the Copper versus Magnesium? The alkyl mostly does nucleophilic attack, so it might need the space. $\endgroup$ – Eljee Dec 18 '14 at 14:38

The difference in electronegativity between copper (1.9) and magnesium (1.3) is the key difference.

Since copper is more electronegative than magnesium its electronegativity is much closer to that of carbon (2.55). This results in carbon-copper bonds being less polarized and more covalent than carbon-magnesium bonds. The electrons in a carbon-copper bond are better shared between the two atoms, whereas the electron distribution in a carbon-magnesium bond places more electron density on carbon.

As a result of this difference in electron density on the carbon atoms the carbon portion of a cuprate is a softer nucleophile, while the carbon portion of a Grignard is a harder nucleophile. These difference in nucleophilicity produce different reactivity patterns for the two reagents. For example, the softer cuprate will undergo 1,4 addition to an $\alpha,\beta$-unsaturated carbonyl compound (a softer electrophile), while the Grignard adds directly to the carbonyl (which is a harder electrophile).

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    $\begingroup$ Would you mind explaining what you mean by a soft/hard nucleophile please? Thanks for the great answer by the way. $\endgroup$ – RobChem Dec 18 '14 at 16:48
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    $\begingroup$ In a nutshell, hard nucleophiles are generally small, highly charged atoms. Because of this they tend to be more reactive. They prefer to react with similar (hard) electrophiles. Their reactions are usually kinetically controlled with early transition states and are governed by electrostatic interactions. Soft nucleophiles are generally larger systems with a more diffuse charge. Because of this they tend to be less reactive (more discriminating). They prefer to react with similar (soft) electrophiles. $\endgroup$ – ron Dec 18 '14 at 17:00
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    $\begingroup$ Because they are more discriminating their reactions are usually thermodynamically controlled with later transition states and are governed more by orbital interactions. $\endgroup$ – ron Dec 18 '14 at 17:01
  • $\begingroup$ The question is if it is actually less reactive or more reactive. Addition of $\ce{RMgX}$ catalized by copper (I) salts is an 1,4 addition (characteristic for organocopper) meaning that active particle of $\ce{R2CuM}$ is MORE reactive than $\ce{RMgX}$ Ron, care to refine your answer? $\endgroup$ – permeakra Jan 6 '17 at 19:53

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