Dec 12, A nucleophile donates electrons to an electrophile. All nucleophiles are Lewis bases , but not all Lewis bases are nucleophiles. All electrophiles are Lewis acids , but not all Lewis acids are electrophiles.
A Lewis base after having donated an electron pair, involved in Bronsted-basic activity, just wants a proton and is thermodynamic behavior. Both want to donate electrons to accomplish their purposes. A Lewis acid after having accepted an electron pair, involved in Bronsted-acidic activity, just wants to give up a proton and is thermodynamic behavior. Both want to accept electrons to accomplish their purposes. Another great question! Acid-base reactions tend to be fast , relatively speaking.
But it will greatly depend on the situation. More soon, I assure you. Thank you for all great resources. Good question. Lewis acidity is essentially electrophilicity. Acidity Bronsted acidity is what we call it when the electrophile is a proton. What is the link between being too basic and the hydride in NaH not being able to carry out the nucleophilic attack?
Thank you. NaH is generally not observed to add to aldehydes and ketones, although it will add to Lewis acids such as borane BH3. You can think about the rates of competing reactions — 1 addition of hydride to aldehyde slow versus 2 deprotonation of the alpha-carbon by hydride fast and the latter reaction prevails. NaH acts as a non nucleophilic base so basically involves in deprotonation rather in addition of the hydride to the carbonyl.
In general all non nucleophilic base acts by the same mechanism. The best guide to basicity is by looking at a pKa table. The pka of water is This was not a clear cut example where one could point to periodic trends. However with nitrogen being coordinated to C in -CN using that principle, they would draw the wrong conclusion.
Because multiple variables are in play [we are changing the basic atom as well as the substituents connected to that atom] the only recourse is to check a pKa table because the effect of changing two variables at once is not easily predictable. This site is not deficient in describing why certain species are stronger acids and bases according to a set of principles.
I have a whole series of articles where I discuss acidity trends and refer to electronegativity, polarizability, resonance, adjacent electron withdrawing groups, and even aromaticity. The point of the current article is to mention that basicity is measured by pKa — it is an equilibrium — whereas nucleophilicity is measured by rate. So only a brief treatment of basicity was given here, with reference to the series on acid-base reactions.
I like your explanation. If pKa is an equilibrium description that makes it thermodynamic in nature; nucleophilicity is a description of a rate, which makes it kinetic behavior. But cn- is a good nucleophile than oh- On basis of electonegativity Is this correct?? You are correct in all of your assumptions. O being more electronegative would make it the weaker base IF the N was holding the negative charge however, the base you listed actually has the negative charge on the carbon molecule, so in this case your comparison is not applicable.
Please check and revert. Nucleophilicity is measured by rate, Lewis basicity can be measured by equilibria given the right conditions. Which is a stronger nucleophille R3P or R3N?
In general how do i find out which is a stronger nucleophille? Going down the periodic table, the electrons will be held more loosely, phosphorus should be more nucleophilic than nitrogen just like sulfur is more nucleophilic than oxygen.
What about the basicity of R3P and R3N? Also can you please provide examples of species which are good nucleophiles but bad bases and vice versa. Obviously it depends on the context whether you would call a particular species a nucleophile or a base but are the two terms largely synonymous or is there a difference?
The two are related, in that most nucleophiles are Lewis bases and vice versa. Some good nucleophiles are also strong bases, e. However, a species can be a good nucleophile and a weak base, e. How can we separate this behavior? Nucleophilicity is most often defined based on the relative rate of the reactions of nucleophiles with a standard substrate in a standard solvent.
There is a difference indeed: basicity is a particular kind of nucleophilicity. A nucleophile is a chemical species that donates an electron pair to an electrophile. A nucleophile can also be called a base when this donation occurs towards a particular electrophile, which is an hydrogen ion a proton.
When the size of a negatively charged molecule is small then it behaves as a nucleophile, but when size of a negatively charged molecule is big then it behaves as a base. This is because of crowding around the central atom of other reactant species. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more.
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