The key difference between nucleophilic and electrophilic addition is that, in nucleophilic addition reactions, an electron-rich component combines with a molecule whereas, in electrophilic addition reactions, either an electron-deficient species or a neutral compound with empty orbitals combines with a molecule.
A nucleophile is an electron-rich chemical species that can donate an electron pair to an electron-deficient species. An electrophile, on the other hand, is either positively charged or neutral. If it is neutral, then it should have empty orbitals to accept electrons from another species.
CONTENTS
1. Overview and Key Difference
2. What is a Nucleophilic Addition
3. What is an Electrophilic Addition
4. Side by Side Comparison – Nucleophilic vs Electrophilic Addition in Tabular Form
5. Summary
What is a Nucleophilic Addition?
The nucleophilic addition is the process of adding a nucleophile to either an electron-deficient species or a pi bond in a molecule (we call it substrate). The added nucleophile forms a single bond (a sigma bond) with the substrate. Let us consider some examples to understand the process of nucleophilic addition.
Addition of Nucleophiles to Carbonyl Carbon
Carbonyl groups are polar because they have a carbon atom double bonded to an oxygen atom. This polarity arises due to the high difference between electronegativity values of carbon and oxygen. That means oxygen has a higher affinity for bond electrons than carbon. Then the carbon atom of the carbonyl group gets a partial positive charge. This carbon is a better position for a nucleophile to attack the molecule. The nucleophile donates its electrons to this carbon and forms a single bond with the carbon atom. Thus, this is a nucleophilic addition. Furthermore, this type of reactions takes place commonly in aldehydes and ketones.
Figure 1: Nucleophilic Addition to Carbonyl Carbon
Addition of Nucleophiles to Nitriles
A nitrile is a compound containing a carbon triple bonded to a nitrogen atom. This bond is very polar because the electronegativity of nitrogen is higher than carbon. Then the carbon atom becomes partially positively charged. As a result, this carbon can undergo nucleophilic addition. The nucleophile combines with the carbon atom. The resultant molecule has a double bond between carbon and nitrogen, instead of a triple bond.
Addition of Nucleophiles to Double Bonds
A double bond has a pi bond and a sigma bond. Double bonds are present in alkenes. When an alkene undergoes a nucleophilic addition, the unsaturated molecule becomes saturated with the nucleophile and combines with one of the vinyl carbon atoms (double bonded carbon atoms) via a covalent bond.
What is an Electrophilic Addition?
The electrophilic addition is the process of adding an electrophile to the pi bond of an alkene. At the end of the reaction, this pi bond breaks down, forming two new sigma bonds. The molecule should contain a double bond or a triple bond to receive an electrophile. It occurs in two steps. Let us consider an example to understand the mechanism of electrophilic addition.
Figure 2: Electrophilic Addition
Here, we have positively charged electrophile. Further, the unsaturated bond or the double bond is rich with electrons. Therefore, it can donate electrons to the electron-deficient electrophile. Then the positive charge is transferred to the C-C bond while a sigma bond forms between a carbon atom and the electrophile. This results in a carbocation. Since this is unstable, the positively charged carbon atom gets electrons from an anion, forming another sigma bond.
What is the Difference Between Nucleophilic and Electrophilic Addition?
Nucleophilic vs Electrophilic Addition |
|
| Nucleophilic addition is the process of adding a nucleophile to either an electron-deficient species or a pi bond in a molecule. | Electrophilic addition is the process of adding an electrophile to the pi bond of an alkene. |
| Species being Added | |
| A nucleophile combines with a molecule. | An electrophile combines with a molecule. |
| Substrate | |
| Either a positive charge chemical species or a pi bond in a molecule. | An alkene or alkyne |
| Process | |
| A nucleophile forms a sigma bond with a carbon atom in the substrate | An electrophile forms a sigma bond with a vinyl carbon atom in the double bond. |
Summary – Nucleophilic vs Electrophilic Addition
Both nucleophilic addition and electrophilic addition are two important chemical reactions used in synthesizing saturated compounds from unsaturated compounds. The key difference between nucleophilic and electrophilic addition is that in nucleophilic addition reactions, an electron-rich component is added to a molecule, whereas in electrophilic addition, an electron-deficient species is added to a molecule.
Reference:
1. “Nucleophilic Addition.” Wikipedia, Wikimedia Foundation, 14 Apr. 2018, Available here.
2. Libretexts. “19.4 Nucleophilic Addition Reactions of Aldehydes and Ketones.” Chemistry LibreTexts, Libretexts, 25 Aug. 2017, Available here.
3. “Electrophilic Addition.” Wikipedia, Wikimedia Foundation, 14 Apr. 2018, Available here.
Image Courtesy:
1. “NucleophilicAdditionsToCarbonyls” By V8rik at English Wikipedia (CC BY-SA 3.0) via Commons Wikimedia
2. “ElectrophilicAdditionmechanism” By V8rik at English Wikipedia (CC BY-SA 3.0) via Commons Wikimedia