distinguishing base and nucleophile help

[kies89]

i have trouble distinguishing nucleophile and base

i know base gets stronger up and left in periodic table
and nucleophile gets stronger up and left if aprotic, left and down if protic

but when i have like NaOCH3, NaN3, NaOEt for example

i do not know how to distinguish if they are strong nucleophile and strong base
or strong nucleophile but weak base etc..


can i get some tips??

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[Neil45]

Strong nucleophiles are those with a negative charge and have lone pair of electrons. All of those compounds you listed (N3-, OCH3, OEt) would be strong nucleophiles since all have a negative charge and have lone pair of electrons. Examples of a weak nucleophiles are H2O and CH3OH. Even though they both have lone pair of electrons, they lack a negative charge.

Anytime you see an alkyl group attached to an oxygen that has a negative charge (-OR where R means alkyl groups), it is usually a strong base. Strong bases have an overall negative charge. So, both -OCH3 and -OEt would be strong bases.

N3- (azide) is a weak base. I think the reason is due to its resonance forms which can delocalize its electrons; this makes it more stable, hence, acidity of the compound increases. The other two compounds (OCH3 and OEt) does not have any resonance forms to delocalize the charge, so it is more basic than azide.

 

[Neil45]

Also, nucleophiles HAVE to be small. They have to be small enough to be able to attack the electrophilic substrate. All of the compounds you listed would be good nucleophiles because they're linear, small molecule capable of "precision striking".

Strong, bulky bases such as t-butoxide ions are horrible nucleophiles because they just can't get to the electrophile, so they are more likely to act as a base plucking off hydrogens.

 

[jwest88]

Also, it will depend on the site of attack(primary, secondary, or tertiary LG), -OEt or -OMe will act more like bases or nucleophiles. If it's a primary LG, they will be exclusively SN2, however, if it is a secondary LG, E2 will compete while tertiary will be exclusively E2.