O Chem - leaving group and reactivity

[RCT PC CRN]

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Why would -CN as LG and I as nucleophile NO REACTION and -Cl as LG and I as nucleophile, Reaction goes forward and replace Cl with I?

I is the best LG in all three (CN, Cl, I). I'm ok with first part where it says NO REACTION cause I is better LG then CN but then if I use the same stretagy isn't the later one is NO REACTION as well? 😕

 

[UndergradGuy7]

Why would -CN as LG and I as nucleophile NO REACTION and -Cl as LG and I as nucleophile, Reaction goes forward and replace Cl with I?

I is the best LG in all three (CN, Cl, I). I'm ok with first part where it says NO REACTION cause I is better LG then CN but then if I use the same stretagy isn't the later one is NO REACTION as well? 😕

Sounds tricky.

My guess...
CN- is conjugate base of HCN. HCN is not a strong acid, so CN- is not that weak. HI is a strong acid, and I- is a very weak base.

The stronger the base, the less likely to leave. So CN- > I- in basicity and I- won't replace it.


In the I-, and Cl- example. HCl and HI are both strong acids. HI is stronger than HCl. So we can't reason it the same way as above. But maybe their differences in basicity don't differ that much and the reaction can happen?

Any other comments?

Also where is this question from?

 

[FredDurst]

Does it give reaction conditions? Like NaI in acetone added to an alkyl chloride? This reaction will give you the alkyl iodide for a different reason than the one in the previous post. NaI is soluble in acetone, while NaCl pretty much isn't. The precipitation of NaCl will drive the reaction toward the alkyl iodide product.

 

[RCT PC CRN]

Does it give reaction conditions? Like NaI in acetone added to an alkyl chloride? This reaction will give you the alkyl iodide for a different reason than the one in the previous post. NaI is soluble in acetone, while NaCl pretty much isn't. The precipitation of NaCl will drive the reaction toward the alkyl iodide product.

That's it. It was NaI. So I think reaction goes cause of some other reason then simply comparing LG strength. Any other exception like this worth knowing?

 

[RCT PC CRN]

Undergrad...this was part of one the question in destroyer.

FredDrusy explained it well.

 

[Troyvdg]

Why would -CN as LG and I as nucleophile NO REACTION and -Cl as LG and I as nucleophile, Reaction goes forward and replace Cl with I?

I is the best LG in all three (CN, Cl, I). I'm ok with first part where it says NO REACTION cause I is better LG then CN but then if I use the same stretagy isn't the later one is NO REACTION as well? 😕

I is a better nucleophile than Cl.

nucleophilicity increases down the periodic table (as size and polarizability increase and electronegativity decreases)

I is able to stabilize the transition state of the SN2 reaction better than Cl...so basically its electrons are more loosely bound and are able to form bonds easier

 

[orgohacks]

Does it give reaction conditions? Like NaI in acetone added to an alkyl chloride? This reaction will give you the alkyl iodide for a different reason than the one in the previous post. NaI is soluble in acetone, while NaCl pretty much isn't. The precipitation of NaCl will drive the reaction toward the alkyl iodide product.

Exactly.

The reaction is an equilibrium - but precipitation of NaCl drives it forward toward the alkyl iodide.

wikipedia: http://en.wikipedia.org/wiki/Finkelstein_reaction

 

[FredDurst]

That's it. It was NaI. So I think reaction goes cause of some other reason then simply comparing LG strength. Any other exception like this worth knowing?

The only other exception that pops into my head is neighboring group participation. Just know I haven't taken or plan on taking the DAT so I don't know if this is on there, but I've seen these used before to trick students during OChem tests. If you have a nucleophilic functional group like a carboxylate or nitrogen near the leaving group, what can happen is first the nucleophilic neighboring group kicks off the leaving group, then the added nucleophile kicks off the funtional group to add to the molecule.

So, a question where this would come up is maybe with a non-allylic or benzylic primary carbon. You see the primary carbon leaving group replaced with the nucleophile to give a compound with the same stereochemistry. Well how would this happen with an SN2 reaction? The neighboring group kick off the leaving group to give an inverted center and then the added nucleophile comes in and kicks off the neighboring group to invert the stereochemistry again.