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.
There are two types of attacking reagents, which are:
The word ‘phile’ in electrophile means lover, so the word electrophile means electron lover.
The word ‘phile’ in nucleophile means a nucleus lover.
Let’s understand these two terms one-by-one:
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.
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-
Nucleophiles that become neutral because of the lone pair of electrons are neutral.
H2••O••, NH, R - ••O•• - H, R - NH, •P•H3, •P•Cl3, C2H5-••O••- H
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.
Q1: Are all Lewis Acids Electrophiles?
Ans: We know that electron-deficient species are lewis acids. These acids accept a lone-pair of electrons. So, we can say that all lewis acids are electrophiles.
Q2: Are all Electrophiles Lewis Acids?
Ans: No. All electrophiles are not lewis acids. Let’s understand this with an example of NO (CH•3).
Here, free-radical (CH•3) has 7 electrons that require an electron. This means that (CH•3) is an electrophile. However, it requires only a single electron, not a lone pair. That’s why we can say that all electrophiles (like we took an example of CH•3) are not lewis acids.
Q3: Are Carbocations Lewis Acids?
Ans: Yes. A carbocation is an example of lewis acids. When carbocation reacts with water, one of the electron pairs of O2 is used to form a new σ-bond to the central carbon in the carbocation.
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Q4: Write the Differences Between Electrophile and Nucleophile.
Ans: The difference between an electrophile and nucleophile is discussed below:
These are atoms that accept a lone-pair of electrons.
The species that donate electrons.
Either positively or neutrally charged.
Either negatively or neutrally charged.
They undergo electrophilic addition and substitution reactions.
They undergo nucleophilic addition and substitution reactions.
They are called Lewis acids.
Also called Lewis bases.