Electrophiles and Nucleophiles

We study general organic chemistry, which talks about all the general things that are backbones of organic chemistry. One of these is the reaction mechanism.

So, what is the reaction mechanism?

Let’s consider a chemical equation, where a reactant transforms into a product. The following reaction occurs:

                                 S + R → P

Here, S is the substrate or a reactant (a molecule on which the reaction is occurring).

R is the attacking reagent that can be an electrophile or a nucleophile.

This nucleophile makes changes to form a product P.

So, under the reaction mechanism, we are going to study the electrophile and nucleophile.

What are Electrophiles and Nucleophiles?

There are two types of attacking reagents, which are:

• Electrophile 

• Nucleophile

The word ‘phile’ in electrophile means lover, so the word electrophile means electron lover.

Similarly,

The word ‘phile’ in nucleophile means a nucleus lover.

Let’s understand these two terms one-by-one:

Electrophile

Electron-deficient species are electrophiles. They are denoted by E+.

These attacking reagents attack at the electron-rich center, i.e., at the substrate.

Let’s discuss the three types of electrophiles:

1. Positively Charged Electrophile

Electrophile examples: H⊕, Cl⊕, Br⊕, CH3⊕ 

All positively charged species are not electrophiles. For example,

NH4⊕, H3O⊕  is not electrophiles. It’s because all the orbitals of N are O and are occupied by H-atoms. Let’s understand how.

The configuration of N = 2s22p3

2s                                        2p                                 2p                                       2p

There are three H+ atoms, each combined with 2p-orbitals of N, and the fourth H+ atom combines with 2s-orbital of N, to take a lone pair of electrons and fill its octet.

So, electrons don’t find any place for them. A similar thing happens in H3O⊕.

That’s why all positively charged species aren’t electrophiles.

However, if NH4⊕, H3O⊕ is in an aqueous medium, i.e., when allowed to dissociate. It becomes an electrophile. The following reaction occurs:

NH4⊕ (aq) → NH3 + H+.

H3O⊕(aq) → H2O + H+

We get an H+ electrophile.

2. Incomplete Octet Electrophile

Let’s take examples: BF3, BCl3, BBr3, BI3, BeCl2, AlCl3.

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Through this structure, we can see that BF3 is an electron-deficient species. In this structure, what happens is, Boron has empty p-orbital combines with p-orbital of fluorine, where fluorine donates its lone pair to Boron and the bonding formed between them is the back bonding, as you can see in the image below:

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Let’s find the order of the strength of an electrophile among BF3, BCl3, BBr3, BI3.

In all these compounds, back bonding occurs but what happens here is that the size of F is smaller, as it gets additional electrons from Boron, it gives the electron back to Boron through back bonding. Also, the π-bond between B and F is stronger, which makes it unstable, while BI3 is the strongest because the size of I is bigger.

That’s why the order becomes BI3 > BBr3 > BCl3 > BF3.

Other examples of incomplete octet electrophiles are Cl⊕, CH3⊕, CH•3, AlCl3, FeCl3, SnCl4, BF3.

3. Polar Functional Group

For example, Oxygen captures electrons from the C-atom, and forms partial charge, as shown below:

                               Cδ+ = Oδ-

Here, C is electron-deficient, so it is an electrophile, while O is electron-rich, so it is a nucleophile.

So, polar functions are electrophiles and nucleophiles.

Nucleophile

These species have lone pairs of electrons. They attack at the electron-deficient center because of the excess of electrons.

Let’s discuss the types of nucleophiles:

Negatively Charged Nucleophile

The nucleophiles that are negatively charged are electron-rich species, and they attack at the electron-deficient center.

The examples of these nucleophiles are Cl⊝, Br⊝, CH-3, OH-, NH2-, O22-, C2H5O-

Neutral Nucleophiles

Nucleophiles that become neutral because of the lone pair of electrons are neutral.

           H₂O••, NH, R - ••O•• - H, R - NH, •P•H3, •PCl3, C2H5-••O••- H

Organometallic Compounds

The compounds having organic and metallic compounds joined together are organometallic compounds. This means that compounds in which metal is bonded with C-atom are organometallic compounds.

Let’s take examples of such compounds:

 R - MgX (Grignard’s Reagent)

Here, R is an alkyl group (we can use methyl, ethyl, or any alkyl group here), X is a halogen, such as C2H5 - Mg - Cl

As the bond breaks, a partial charge on this compound forms in the following way:

                               C2H5⊝ - Mg⊕ - Cl

So, C2H5⊝ is electronegative (nucleophile), and Mg⊕ is electropositive. 

R2Zn (Frankland Reagent)

For R2, we can methyl-methyl, ethyl-ethyl, or ethyl-ethyl; such as (C2H5)2 - Zn

R2  - CuLi (Gillman’s Reagent)

R - Li

Such as C2H5- Li

Here, Li is directly bonded with C-atom.

The partial charge on C2H5 - Li forms in the following way: 

C2H5 δ- - Liδ+

This happens because C is electronegative as compared to Li.

So, here C2H5 became a nucleophile.

This means electrophile Mg⊕ and Liδ+ are electrophiles, however, this is not true because alkali and alkaline earth metals are the worst electrophiles.

Let’s understand why Mg⊕ and Liδ+ are not electrophiles.

Since Li already has a half-filled stable configuration and on losing an electron, i.e., Liδ+ builds a stable configuration of He. Similarly, Mg⊕ makes a configuration of Ne, which has a stable configuration.

This is the reason why Mg⊕ and Liδ+ are not electrophiles.

We've been talking about how the cornerstone of chemistry is that opposite charges attract and like charges repel, and that in reactions, electrons flow from "electron rich" locations to "electron poor" parts throughout the series on knowing where electrons are and how they flow.

We'll give the types of species that are deemed "electron rich" and "electron poor" a name today.

Nucleophiles and electrophiles are two types of nucleophiles.

What is the definition of a nucleophile?

The electron-rich atoms in nucleophilic functional groups can transfer a pair of electrons to establish a new covalent bond. The most important nucleophilic atoms in both laboratory and biological organic chemistry are oxygen, nitrogen, and sulfur, while the most common nucleophilic functional groups are water, alcohols, phenols, amines, thiols, and carboxylates.

In laboratory reactions, the anions halide and azide (N3-) are frequently encountered functioning as nucleophiles.

Of course, carbons can also be nucleophiles; otherwise, how could big organic molecules like DNA or fatty acids generate new carbon-carbon bonds? The most prevalent carbon nucleophiles in biochemical reactions are enolate ions (section 7.5), while the cyanide ion (CN-) is merely one example of a carbon nucleophile commonly utilized in the laboratory.

These are some examples of nucleophiles:-

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Nucleophiles Add to the Carbonyl Group Hydroxide,

A nucleophile adds to CO2's electrophilic carbon. A pair of electrons migrate as indicated by the purple arrows. The nucleophile (hydroxide) transfers a pair of electrons to the electrophile first (carbonyl carbon). A pair of electrons connected with the C=O unit must shift to the oxygen atom since carbon can only have 8 total electrons around it.

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Alcohols, too, are nucleophiles that can attach to an aldehyde's electrophilic carbon.

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There is an acid/base reaction after the nucleophile is added to the electrophile. The basic, negatively charged oxygen absorbs the acidic proton from the positively charged oxygen.

Nucleophiles come in a variety of shapes and sizes.

The following are the different types of nucleophiles:

1. Nucleophiles with Negative Charges:

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2. Every Lewis base that has lone pairs:

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The atom that donates electrons is called a donor atom.

The atom that gives electrons to the substrate is indicated by a star (*).

3. Ambident Nucleophile (Ambident Nucleophile):

Nucleophiles with two electron-rich centers or two or more atoms that share an unshared pair of electrons.

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Ambient nucleophiles are also resonating structures.

4. Amphiphile Nucleophile: Molecules with numerous carbon-to-more electronegative-atom linkages can behave as both electrophiles and nucleophiles.

Consider the following scenario:

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Electrophiles

The electrophilic atom is usually a carbon that is bound to an electronegative element, such as oxygen, nitrogen, sulfur, or a halogen, in the vast majority of nucleophilic substitution reactions you will read in this and other organic chemistry texts. Electrophilicity is a basic concept: an electron-poor atom is an attractive target for an electron-rich atom, such as a nucleophile. However, the influence of steric hindrance on electrophilicity must also be considered. Furthermore, we must consider how the electrophilic carbon's nature, specifically the stability of a putative carbocationic intermediate, affects the SN1 vs. SN2 character of a nucleophilic substitution process.

An electrophile is a molecule that accepts both bonding electrons from its reaction partner (the nucleophile) in order to create a bond. Lewis acids are electrophilic reagents.

These are some examples of electrophiles:-

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Electrophiles come in a variety of shapes and sizes.

Electrophiles can be categorized into the following categories:

1. Electrophiles with a Positive Charge:

H+, SO3H+, NO+, NO2+, X+, R+, C6H5N2+, C+2H-OH, CH3 C+, SO3H+, SO3H+, SO3H+, SO3H+, SO3H+, SO3H+, SO3H+, SO3H+, SO3H+, SO3H+ =O

2. Electrophiles with a neutral charge: These exhibit electron shortage.

BF3, AlCl3, SO3, ZnCl2, BeCl2, FeCl3, SnCl2, CO2, SnCl4 (a) All Lewis acids: BF3, AlCl3, SO3, ZnCl2, BeCl2, FeCl3, SnCl2, CO2, SnCl2.

(b) The electron-accepting neutral atom in the substrates:

> *C = O, R *COCl, R – * Mg – X, *I – Cl, CH3 – *CN, R*–Cl, R*–O, R*–Cl, R*–O, R*–O, R*–O, R*–O, R*–O, R*–O, R*–O, R*–O, R*–O, R*–

Important electrophile information:

Although the hydrogen ion has a positive charge, it does not qualify as an electrophile since it has entire empty orbitals in its outer shell.

The ammonium ion, on the other hand, does not have any unoccupied orbitals to attract electrons. As a result, ammonium ions are not considered electrophiles.