Nucleophilic halide strength

[chiddler]

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Isn't Cl(-) a stronger nucleophile than Br(-)?

Original question: If have acid + 1-pentanol and then add NaCl + NaBr, one equivalent of each, product?

Cl is smaller, more electronegative...perfect for Sn2, no? It's also a worse leaving group implying that it latches onto its carbon more tightly.

Book says that "Bromine is a better nucleophile."

 

[indianjatt]

Polarizability. Bromine is larger so when Bromine attacks, it will overlap much better with the sp3 orbital. This creates greater stability of the intermediate state.

 

[MedPR]

Isn't Cl(-) a stronger nucleophile than Br(-)?

Original question: If have acid + 1-pentanol and then add NaCl + NaBr, one equivalent of each, product?

Cl is smaller, more electronegative...perfect for Sn2, no? It's also a worse leaving group implying that it latches onto its carbon more tightly.

Book says that "Bromine is a better nucleophile."

Br is a better leaving group because it is larger and can delocalize electrons better than Cl.

Br- is a better nucleophile because it is more polarizable than Cl-. You might also think of it in terms of hard/soft. Fluorine is very hard, that is, it is very small (compared to the other halogens) and therefore must get very close to something in order to bond to it. Iodine is soft, since it has the largest cloud and can more easily reach out to something to bond to (like an electrophile). As you go down a column, you get "softer" and it is easier for you to bond to something.

 

[mcloaf]

You are right that the smaller size will make Cl a better nucleophile, but this is offset by a couple of factors. First is that Cl's higher electronegativity will make it a poorer nucleophile than Br because it will hold the electrons tighter and make them less available to react. Second, polarizability, or the size and malleability of the electron cloud, makes for a more reactive nucleophile--in this case Br has a much larger, more flexible electron cloud. So in a protic solvent, nucleophilicity is I > Br > Cl > F.

Though annoyingly enough, it's the complete reverse in aprotic solvent, making F the best nucleophile, etc.


edit: damn, got to this ochem party a little late. 🙂

 

[chiddler]

thanks i understand

except

You are right that the smaller size will make Cl a better nucleophile, but this is offset by a couple of factors. First is that Cl's higher electronegativity will make it a poorer nucleophile than Br because it will hold the electrons tighter and make them less available to react. Second, polarizability, or the size and malleability of the electron cloud, makes for a more reactive nucleophile--in this case Br has a much larger, more flexible electron cloud. So in a protic solvent, nucleophilicity is I > Br > Cl > F.

Though annoyingly enough, it's the complete reverse in aprotic solvent, making F the best nucleophile, etc.


edit: damn, got to this ochem party a little late. 🙂

why is it reversed in an aprotic solvent?

 

[mcloaf]

http://books.google.com/books?id=QV...EwAw#v=onepage&q=nucleophilicity size&f=false

 

[chiddler]

http://books.google.com/books?id=QV...EwAw#v=onepage&q=nucleophilicity size&f=false

ty very much

 

[411309]

The general trend for strength of a nucleophile is it increases going from right to left on a period table, and top to bottom.

 

[gettheleadout]

The general trend for strength of a nucleophile is it increases going from right to left on a period table, and top to bottom.

AKA the opposite of the electronegativity trend.