orgo question

[jlt0530]

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Can someone help me why this rxn is unlikely to happen?

I get the part that Iodide is a better LG than Cl becuz it's weaker base. But isn't I- better nucleophile than Cl- becuz it's bigger? and i thought since this rxn is taking place in polar aprotic solvent, which indicates SN2, having a good nucleophile weighs more than having a good LG.

I thought the answer should be the mixture of both reactants and products at the end of the rxn, but the answer says that this rxn scheme is incorrect.

Much appreciated for your help!

 

[loveoforganic]

This is not an SN2! Look closely at your reactants - your substrate is sp2 hybridized. This is a nucleophilic acyl substitution. Chloride is more electronegative than iodide, and thus makes the carbonyl carbon more electron deficient, and more reactive.

 

[jlt0530]

oh right..it was a stupid mistake.
but even so, like you said, chlorine is more electronegative which makes carbonyl carbon more electron deficient, then wouldn't it be more likely to be attacked by iodide, thus rxn proceeds to right? the part i don't understand is how come this rxn doesn't take place while acyl chloride is very reactive reactant.

thanks for your help loveoforganic!
u indeed love orgo and are good at it! 🙂

 

[loveoforganic]

I'm not sure, that's one topic I can never seem to remember. That is, whether bromide will replace chloride, or chloride will replace bromide, etc.

 

[sixkiller]

oh right..it was a stupid mistake.
but even so, like you said, chlorine is more electronegative which makes carbonyl carbon more electron deficient, then wouldn't it be more likely to be attacked by iodide, thus rxn proceeds to right? the part i don't understand is how come this rxn doesn't take place while acyl chloride is very reactive reactant.

thanks for your help loveoforganic!
u indeed love orgo and are good at it! 🙂

The strength of halide nucleophicity in polar aprotic solvents is reversed.

I- < Br- < Cl- < F-

In polar PROTIC solvents, Iodide ion is the strongest nucleophile because it is solvated to the LEAST extent; therefore, less energy is required to break the solvation bonds and reach the transition state corresponding to a lower activation energy.

Empirically, we do find that in polar APROTIC solvents that basicity correlates strongly with nucleophicity and less so with electron polarizability.

 

[sixkiller]

Solvents and activation energy (transition state stability) should always be considered.....

 

[jlt0530]

it makes much more sense now.
i didn't think of the reverse nature of nucleophilicity in aprotic solvent!
Thanks!! 👍

 

[pauly11235]

Just think of it like this. I is a better leaving group than Cl, so if I acts as a nucleophile and attacks the carbonyl, the carbonyl will reform and the I- will leave and not the CL after the attack.