Sn2 - Leaving Groups

[skyisblue]

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Why do weak bases make better leaving groups?

 

[laundry]

because your leaving group has to be a worse nucleophile than the attacking nucleophile.

 

[jdmsamurai]

lol @ laundry for not really explaining anything

so i will answer for him

the weaker the basicity of a group the better it is as a leaving group. The reason leaving ability depends on basicity is weak bases are stable bases, they readily bear the electrons they formerly shared with the protons

in other words they like having their own set of electrons...

think of conjugate acid/conjugate base pairs

 

[lilchinoboy03]

If I was a reaction, I would be SN2 so I could attack your backside.

 

[jdmsamurai]

If I was a reaction, I would be SN2 so I could attack your backside.

that by far is the best pick up line ever!

 

[darksky]

basicity measures the stability of charge(thermodynamic)
nucleophilicity measures how fast a nucleophile attacks something(kinetic)
weak base = charge is stable(less reactive) check the electronegativity value
if the leaving group is a weak base then it would be more willing to leave carbon atom. another word the leaving group is less reative to the carbon atom or more stable by itself. hope this helps you. good luck!

 

[skyisblue]

basicity measures the stability of charge(thermodynamic)
nucleophilicity measures how fast a nucleophile attacks something(kinetic)
weak base = charge is stable(less reactive) check the electronegativity value
if the leaving group is a weak base then it would be more willing to leave carbon atom. another word the leaving group is less reative to the carbon atom or more stable by itself. hope this helps you. good luck!

Check the electronegativity value? Please elaborate.

 

[darksky]

Check the electronegativity value? Please elaborate.

C N O F
increasing electronegativity from C to F. neg. charge on Oxygen atom will be more stable than negative charge on Carbon atom. comparing electronegativity for atoms in the same row.
F

Cl

Br

I

increasing atom size from F to I. Iodine atom >>> Flourine atom. neg charge will be more stable on Iodine than neg charge on Flourine atom. comparing size for atoms in the same column.

 

[chreesteene]

In addition to what everyone else has already said, for Sn2, you always want to think about the transition-state as being like a trigonal bipyramidal sort of shape. Weak bases are more stable, less electronegative, larger in size (think of going down the halides), and therefore more polarizable, generally.

The polarizability goes with the electronegativity -- if the leaving group holds onto the electrons less tightly, it can still provide electron density to the transition state. But it is stable once it leaves, as well.