# What makes R-MgX a more reactive Grignard reagent than R-Cu?

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

• 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. – Eljee Dec 18 '14 at 14:38

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).
• 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? – permeakra Jan 6 '17 at 19:53