confusion on nucleophile...

[gomawum]

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1.I was doing this one question in destroyer and got really confused on some basic trends regarding SN2 stuffs..

So Acidity increase across period and down the group (right, down)
and Basicity is direct opposite to it. (left, up)

and nucleophile is generally 'strong base, strong nucleophil'
so it increase leftward in period, but polarizability takes important part, so down the group, better nucleophile (left, down)

but the nucleophile in SN2 really depends on the types of solvent,
even thought it is favored in aprotic solvent, if it is aprotic, down the group is favored and when it's in protic solvent, upward in the group is favored.




can someone explain me why?
(I think since SN2 is favored in aprotic, and nucleophile (in general) is favored in down the group, nucleophile have to be favored in down the group, but it's actually opposite(upward is favored, why?)


2. I know that strong and bulky base favors E2 reaction, and E2 is favored in 3'>2'>1' order, but sometimes I see it's anti-markovnikov. (usally with 2' and 3' leaving group)

so can I assume that if I see bulky base with 2'/3' nucleophile would end up in anti-markovnikov?

such as 2-bromo-butane + Na-OCH2CH2CH3 ---> but-1-en, not but-2-ne?

 

[PSU SHC 414]

I think you're confusing the h*ll out of yourself and everyone that has read this post so far...

SN2 is, as you said, is favored in aprotic solvents, which means nucleophilicity is correlated with basicity (so the stronger the base, the better the nucleophile for SN2 ... i.e. F- is a better nucleophile in an aprotic solvent than I-).

This is not the case in SN1 reactions, which preferably occur in protic solvents. Since protic solvents will tend to solvate (strong) bases, nucleophilicity is more closely dependent on size of the molecule.

Someone correct me if I'm wrong...

 

[PSU SHC 414]

I think you're confusing the h*ll out of yourself and everyone that has read this post so far...

SN2 is, as you said, is favored in aprotic solvents, which means nucleophilicity is correlated with basicity (so the stronger the base, the better the nucleophile for SN2 ... i.e. F- is a better nucleophile in an aprotic solvent than I-).

This is not the case in SN1 reactions, which preferably occur in protic solvents. Since protic solvents will tend to solvate (strong) bases, nucleophilicity is more closely dependent on size of the molecule.

Someone correct me if I'm wrong...

Clarification on SN1... the larger the basic site (atom), the more delocalized the negative charge will be.

 

[gomawum]

so If i interpreted correctly..

across period, strong base is favored (<----- leftward)

in groups, polarizability is counted so..
Protic -> down the group (case of SN1)
Aprotic -> upward in the group (case of SN2)

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I was bit confused because when I googled on wikipedia, this was what I found...

In general, in a row across the periodic table, the more basic the ion (the higher the pKa of the conjugate acid), the more reactive it is as a nucleophile. In a given group, polarizability is more important in the determination of the nucleophilicity: the easier it is to distort the electron cloud around an atom or molecule, the more readily it will react. e.g., the iodide ion (I&#8722😉 is more nucleophilic than the fluoride ion (F&#8722😉. <----- this didn't say this is the case of protic solvent.... said just 'In general'