Ochem question

[yellowjellybean]

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Why is an alkoxide a stronger base than hydroxide? And are tertiary alkoxides > secondary > primary? Why

Also, why is CN- such a good nucleophile?

 

[cloak25]

Alkoxides are very unstable due to extra negative charge donated by the R groups. Unstable = lots of electrons = wants to donate. As you know, bases are electron pair donors, so the more unstable the alkoxide (i.e. the more R groups attached to the O-) the stronger it is as a base. same thing with CN-, very unstable on its own (very negative) wants to attack an electrophile (+ charge).

 

[yellowjellybean]

Thank you, but how does the R group give oxygen the extra negative charge?

 

[cloak25]

alkoxides are conjugate bases of alcohols. For instance CH3OH. CH3O- is the alkoxide. CH3 is an electron donating group so it basically dumps more negative charge onto the central atom (O). in hydroxide, there is no electron donating going on

 

[UTsksk]

Would it also have to do w/ the electronegativity of the O since is more negative than the C so it'll get the electrons. Hah, correct me if I'm wrong.

 

[gettheleadout]

alkoxides are conjugate bases of alcohols. For instance CH3OH. CH3O- is the alkoxide. CH3 is an electron donating group so it basically dumps more negative charge onto the central atom (O). in hydroxide, there is no electron donating going on

Would it also have to do w/ the electronegativity of the O since is more negative than the C so it'll get the electrons. Hah, correct me if I'm wrong.

In this case, yes. CH3 (alkyl) groups are electron donating to other carbons through hyperconjugation, but when only connected to an oxygen atom, yes, the extra electron density in the oxygen's area is a result of the electron-withdrawing ability of the oxygen (as a result of its greater electronegativity), not any specific electron-donating ability of the CH3 in this case.

 

[yellowjellybean]

In this case, yes. CH3 (alkyl) groups are electron donating to other carbons through hyperconjugation, but when only connected to an oxygen atom, yes, the extra electron density in the oxygen's area is a result of the electron-withdrawing ability of the oxygen (as a result of its greater electronegativity), not any specific electron-donating ability of the CH3 in this case.

I don't understand how this has to do with hyperconjugation, can anyone explain further? If it does, wouldn't a tertiary be more stable and thus, less reactive base?

And is the electronegativity part the inductive effect?

 

[gettheleadout]

I don't understand how this has to do with hyperconjugation, can anyone explain further? If it does, wouldn't a tertiary be more stable and thus, less reactive base?

And is the electronegativity part the inductive effect?

Hyperconjugation *doesn't* have anything to do with it in this case, because hyperconjugation only occurs between carbon groups, and we only have one carbon here connected to an oxygen. Compared to a hydrogen atom, the carbon has more electrons which the oxygen atom can draw density away from by the inductive effect (which is indeed a result of the electronegativity difference between C and O.)