[Nescafe]

Hi,

I know this topic has been discussed in this forum but I could not find the answer that helps untangle my confusion.

My understanding is that primary amine is less nucleophilic than a secondary amine (let us say we are discussing Me-NH2 v.s. Me-NH-Me), while tertriary ((CH3)3N) would be the least nucleophilic due to hindrance while it is the strongest base (the +I effect of alkyl groups raises the energy of the lone pairs of electrons thus elevating the basicity). Although I have read articles debating that the secondary amine is the stronger base due to better solvation as tertiary amine does not allow for proper solvation simply due to hindrance.

Can someone confirm my train of thought?

Thanks in advance,

Nescafe.

[kriggy]

Isnt nucleophility and basicity directly connected? Like if its strong nucleophile, its also strong base?
We were told in lectures that sec.amines are stronges bases becase the tertialy amines are stericaly hindered. I could imagine the sterical hinderance affecting the reactivity of tertiary amines in other reactions too (lowering the nucleophility due to steric hinderance)

[Nescafe]

Isnt nucleophility and basicity directly connected? Like if its strong nucleophile, its also strong base?
We were told in lectures that sec.amines are stronges bases becase the tertialy amines are stericaly hindered. I could imagine the sterical hinderance affecting the reactivity of tertiary amines in other reactions too (lowering the nucleophility due to steric hinderance)

I think people have tried correlating nucleophilicty with Pkah and it does not always hold true. Although, yes you are right, as we go from left to right in the periodic table (in a row) electronegativity increases, basicity decreases and so does the nucleophilicity. But it is not always true to say that stronger bases are stronger nucleophiles. Iodine is a good exception(maybe the only?) as it is the weakest base among the halogens yet the strongest of nucleophiles. You can same the same thing about amines, tertiary amine is a stronger base than a secondary amine yet the weakest nucleophile due to hindrance. I have read many articles where basicity and leaving group ability has been correlated but less evidence correlating basicity with nucleophilicity.

I am just sharing what I think and read so it would be good to get some perspective from some of the more hardcore organic chemists in this forum.

Thanks in advance,

Nescafe.

[Babcock_Hall]

Why would primary amines be less nucleophilic than secondary amines?

[clarkstill]

Why would primary amines be less nucleophilic than secondary amines?

Because the effect of neighbouring electron-rich C-H and C-C bonds is to raise the energy of the N lone pair by hyperconjugation, making it more reactive.

Also, basicity and nucleophilicity are absolutely not the same effect.  Nucleophilicity is a kinetic parameter, a measure of the rate constant in a nucleophilic substitution, whereas basicity is thermodynamic and derived from equilibrium constant values.  The fact that they are often correlated (by the Hammond Postulate, I suppose) doesn't mean they are causally linked... As well as iodide (which Nescafe mentioned), the basicity of second-row elements like sulfur and phosphorus is generally low, whilst their nucleophilicities are generally high.

[Nescafe]

Why would primary amines be less nucleophilic than secondary amines?

Because the effect of neighbouring electron-rich C-H and C-C bonds is to raise the energy of the N lone pair by hyperconjugation, making it more reactive.

Also, basicity and nucleophilicity are absolutely not the same effect.  Nucleophilicity is a kinetic parameter, a measure of the rate constant in a nucleophilic substitution, whereas basicity is thermodynamic and derived from equilibrium constant values.  The fact that they are often correlated (by the Hammond Postulate, I suppose) doesn't mean they are causally linked... As well as iodide (which Nescafe mentioned), the basicity of second-row elements like sulfur and phosphorus is generally low, whilst their nucleophilicities are generally high.

Thank you sir, you are a gentleman and a scholar =)

Nescafe.