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A Primer On Organic Reactions

By James Ashenhurst

Nucleophiles and Electrophiles

Last updated: June 1st, 2019 |

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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Comments

Comment section

25 thoughts on “Nucleophiles and Electrophiles

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

  2. 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.

  3. 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.

    1. hey augustin,
      basically it is happens lyk..the grp with a large hydrocarbon part comes out to b a good electron donating grp. In general, electron donating grps such as -ch3 , -och3, -nh2 etc. increase the basicity while electron withdrawing substituents such as -NO2 , CN ,-COOH , -X , decrease the basicity .

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

      1. 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.”

          1. Simple question but I’m having trouble. Does Hydrogen act as an electrophile or nucleophile in H30+? I first expected the Hydrogen atoms to act as nucleophiles seeing that H3O+ is positively charged; however, when looking at an electrostatic potential map even though the molecule is positively charged it still shows the Oxygen atom as partially negative while the Hydrogen atoms are partially positive leading me to believe the Hydrogen atoms are acting as electrophiles. Also what about NaH? I would expect the Hydrogen to be nucleophilic seeing that in this ionic bond the electron dense region leans towards the Hydrogen atom. Any clarification would be greatly appreciated

          2. Hydrogen is the electrophile – the difference in electronegativity means that hydrogen is electron poor, oxygen is electron rich. Formal charge is not always a reliable guide to electron density.
            You are right that the H in NaH is nucleophilic.

  5. There is one thing to mention in the first reaction, “Hydroxide Ion as a Nucleophile”, the solvent is H2O. In case of alcohol, an Alkene would be formed.
    Great Job!

    1. They are all electrophilic. The proton of H3O+ is electrophilic. The nitrogen of NO2+ is electrophilic. And the empty p orbital of BH3 is electrophilic. Don’t know where you got this question.

  6. Yes, there are. Many molecules can be both nucleophiles and electrophiles. How they behave, determines whether they are electrophile or neutrophile.
    Ex : Water/H2O
    If water is reacted with an electrophile,
    the water will behave as a nucleophile.
    If water is reacted with an neutrophile,
    the water will behave as a electrophile.

    (* The oxygen atom of water has two lone pairs and a d- charge (oxygen is more electronegative than hydrogen). This suggests that water can behave an a nucleophile. Each hydrogen atom bears a d+ charge, so the molecule can behave as an electrophile as well.) *Quoted

    1. Well, the “most electrophilic” site would be the atoms directly connected to the most electronegative atom, which is nitrogen. so the carbons will be partially delta positive as will be the hydrogen attached to nitrogen. However in practice diethylamine is an extremely poor electrophile. The only way to make it behave like an electrophile is to add a Lewis acid to the nitrogen or to convert it into an alkylammonium salt with something like CH3I, which will make the nitrogen a much better leaving group.

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