/sci/ - Science & Math

>>7979389
>If the solvent was protic

So, if the solvent was protic, both trimethylphosphine (pka 8.65) and trymethylamine (pka 9.81) would act as bases and steal the acidic proton from the solvent.

hence they would both be shit nucleophiles, because they are both protonated like a lazy fat fucks, and thus have no lone pairs of electrons left to participate in a nucleophilic substitution

see 1)

> P would be a better nucleophile because it's larger, and I could see how P might act as a better leaving group

remember that in general, the bigger the molecule, the more steric hindrance comes into play

Now, P cant be both a nucleophile AND a leaving group. thats not how SN2 works.

you might be thinking this: a larger molecule or atom can more effectively delocalize a negative charge over a give surface area, thus making it a GOOD leaving group. YES. this is true, refer to Cl, Br, I, tosyl esters, etc.

but P is not acting as a nucleophile, then leaving.

>N should be the stronger one
Basicity is not always indicative of nucleophilicity

A lot of stuff comes into play here, steric hindrance, choice of solvent, electronegativity and stuff. Lets focus on electronegativity. it means elements are electron greedy and want to hold on their electrons.

N is more electronegative than P, therefore its going to hold on to its electrons more strongly than P. P is also electron rich (because higher atomic number) so P doesnt care about its lone pairs going off and making bonds and shit. P is more likely to donate its electrons and form a bond with a partial positively charged carbon atom in a substrate like for ex, 1-propyl bromide.

remember, I can displace Br in an SN2 even though its fat as fuck

in general, nuclophilicity increases with increasing electron density on an atom. think of the posters childs of good nuclophiles (alkoxides and stuff), they have an abundance of electron

so, yes size matters (kek) but so do a bunch of other factors that must be considered.