All through the series on understanding where electrons are, and how they flow, we’ve been talking about how the basis of chemistry is that opposite charges attract and like charges repel, and that in reactions, electrons flow from “electron rich” areas to “electron poor” areas.

Today, we’ll officially give a name to the types of species that are considered electron rich and “electron poor”.

They’re called nucleophiles and electrophiles. 

Let’s start with “nucleophiles”  (from “nucleus loving”, or “positive-charge loving”). A nucleophile is a reactant that provides a pair of electrons to form a new covalent bond. 

Sound familiar? It should!  This is the exact definition of a Lewis base. In other words, nucleophiles are Lewis bases.

When the nucleophile donates a pair of electrons to a proton, it’s called a Brønsted base, or simply, “base”.

Here are some examples of Lewis bases you’re probably familiar with. As you can see, nucleophiles all have pairs of electrons to donate, and tend to be rich in electrons. [Moving ahead, there are actually three classes of nucleophiles you'll meet in organic chemistry, but let's focus on the simple examples for now.]


Now let’s talk about electrophilicity (from “electron-loving”, or “negative-charge loving”). An electrophile is a species that accepts a pair of electrons to form a new covalent bond.

Again, this should sound familiar: this is the definition of a Lewis acid!

An electrophile that accepts an electron pair at hydrogen is called a Brønsted acid, or just “acid”.

Here are some examples of Lewis acids you’re familiar with.

Two more things: 

We can vaguely define “nucleophilicity” as “the extent to which a species can donate a pair of electrons”. [There's actually a more precise definition we'll discuss in the next post, but this will do for now.]

Similarly, the extent to which a species can accept a lone pair of electrons is called “electrophilicity”.

Example:

Let’s look at an example we’re familiar with: hydroxide ion.

When hydroxide ion donates a pair of electrons to an electrophilic atom (such as carbon here) to form a new covalent bond, it is acting as a nucleophile.

And as we’ve seen before, when hydroxide ion donates a pair of electrons to an (acidic) proton to form a new covalent bond, we say it’s acting as a “base”.

So species can be both nucleophiles and bases? Yes!!! In fact, the “basicity” we’ve been talking about is just a subset of “nucleophilicity” – the special case where the electrophile is a proton!

As well, species can be both electrophiles and acids. And “acidity” is just a subset of “electrophilicity”.

Let’s go even further here: the vast majority of the reactions you’ll see (>95%) – will be reactions where a nucleophile donates a pair of electrons to an electrophile. Nucleophile attacks electrophile. There are very few exceptions!

This is why understanding where electrons are, and how electrons flow is so important – because the electron richness (or poorness) of an atom (or molecule) determines its nucleophilicity or electrophilicity, which in turn determines its reactivity. 

It’s not an exaggeration to say that nucleophilicity and electrophilicity are the fundamental basis of chemical reactivity. They are truly the yin and the yang of chemistry.

Next Post: Nucleophilicity Versus Basicity


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{ 9 comments… read them below or add one }

mamid June 6, 2012 at 11:21 am

The picture of Lewis acids appears twice in the post, the 2nd time for no apparent reason.
Great job, by the way.

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james June 7, 2012 at 6:48 pm

Fixed. Thanks for spotting, as always.

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Ola June 7, 2012 at 7:28 pm

Hi. I am wondering whether the kind of nuclephile has an impact on whether we will get Sn1, Sn2, E1 or E2 reactions?
I know that solvent and steric effects do matter but what about “nucleophile species”.
Thank you for all great resources.
Greetings.

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james June 8, 2012 at 4:58 pm

Yes, it absolutely has an impact. That’s coming up in the series, but you might find this post helpful: http://masterorganicchemistry.com/2010/10/29/trapped-in-sn1sn2e1e2-hell-some-resources/

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Agustin July 28, 2013 at 4:25 pm

Now I’ve read this explanation seems much easier! But i’m having problems generalizing. I was asked to build a table featuring EWG and EDG of common functional groups , arranging them by its ” electrophilic power”. First a table of decreasing “Inductive effect”; secondly a table of decreasing mesomeric effect and at last a table comparing them and defining “decreasing” Electron Withdrawal Power.

The groups to arrange where:

-COOH/-COOR/-NO2/-CONH2/-CN/-SO2OH/-COCR/-COH/-COCl/-OCH3/-COCOCH3/-OH/-F/-Br/-I/-NH2/-F/-H/-CH3

Yes I Know is weird, but this “teacher” is kinda strange; I know that those things are kinda complicated to measure in first place. I’m really drowning here so if you can help me to order this thins according at least to electrophylicity ‘ld be gratefull.

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Fatunbi, David January 21, 2014 at 6:42 am

Can i assume that all compounds formed in the homologous series of ether(ROR),amines(RNH2), RO, ROH etc are nucleophiles

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James January 21, 2014 at 12:28 pm

Yes, they’re all Lewis acids bases and therefore can all act as nucleophiles.[thanks to brenda for spotting my typo]

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Brenda January 22, 2014 at 5:35 pm

Do you mean Lewis bases…?

“A nucleophile is a reactant that provides a pair of electrons to form a new covalent bond.
Sound familiar? It should! This is the exact definition of a Lewis base. In other words, nucleophiles are Lewis bases.”

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James January 22, 2014 at 5:40 pm

yes, thanks for spotting the typo. Sorry

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