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Nucleophilicity and basicity relationship

  • Thread starter Puchinita5
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Homework Statement



This isn't so much a homework question as a question to help me undertstand how to do my homework.

The book says that nucleophilicty roughly parallels basicity. Which makes sense since bases are electron pair donors, and an electron pair makes a nucleophile.

but then it says nucleophilicity increases going down a column of the periodic table. I understand that because of the larger valence shells going down a column means the electrons will be held less tightly so they will be more reactive. But this also goes against my intuition because I know that acidity increases going down a column as well??? And since nucleophilicity should increase with basicity, this seems to be a conflicting idea to me?

Can anyone explain?

Homework Equations





The Attempt at a Solution

 

Answers and Replies

  • #2
Im not entirely sure I understand what you're asking but acidity increases down a group because the bond between the conjugate base and the hydrogen gets weaker. Think of it in terms of orbital overlap. HF should be a stronger acid than HI because F is the most electronegative atom but the orbital overlap between H and F is better than H and I so H can dissociate from I more easily.

Hope this helps.
 
  • #3
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Nucleophilicity is more of a kinetic description while acidity/basicity has a lot to do with thermodynamics. That is, in organic chemistry, proton transfers occur so readily and rapidly that it's pointless to discuss the rate of reaction, just the ratio of products to reactants.

Nucleophilicity, on the other hand, speaks to how readily something will donate electrons in a nucleophile/electrophile reactions (Sn1, Sn2, Carbonyl addition, etc.). The rates of these reactions can easily be measured and are affected by things such as the electronegativity, polarizability, and geometry of a nucleophile. Cyanide and azide ions are both very good nucleophiles due to their geometry: they are small, and linear, sort of bullet-shaped.

Tertiary amines are good bases but also poor nucleophiles. They readily accept protons because of their lone electron pair, but are reluctant to add to, for example, carbonyl groups. This is one reason why they are used as acid scavengers in reactions that generate HCl, for example, but that also involve molecules that could undergo nucleophilic attack. An example off the top of my head is:

PCl3 + 3 ROH --(tertiary amine or pyridine)--> (RO)3P + 3 HCl

Really though, there are a lot of shades of grey. Nucleophilicity does parallel basicity, roughly, but one should not necessarily be used as an indicator of the other. That is, when you're looking at a compound and deciding its nucleophilicity or basicity, you should really evaluate the two independently by their separate criteria.
 

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