6.4:
Electrophiles
Recall that in a nucleophilic substitution reaction, a nucleophile donates its electrons to an electrophile.
Electrophiles are electron-seeking reagents — either neutral or positively charged — containing an empty atomic orbital or a low-energy antibonding orbital.
A positive electrophile, like the proton — with a vacant, low-energy 1s orbital — is very reactive. Consequently, a nucleophile like the hydroxide ion attacks the proton, neutralizing the charge and forming water.
Another positive electrophile — the carbocation — has a vacant p orbital, making it reactive towards a nucleophilic attack.
A neutral electrophile, like the Lewis acid boron trifluoride, has an empty p orbital that can accept electrons from the nucleophile, thus forming a bond and resulting in a stable complex.
In a neutral molecule like chlorobutane, the electrophilic center results from the electron-withdrawing inductive effect of the more electronegative substituent attached to the molecular chain.
In an organic electrophile with a double-bonded electronegative atom — like the carbonyl group — the C=O bond dipole renders a partial positive charge to the carbon atom.
In a reaction, the nucleophile deposits its electrons into the lower energy antibonding π orbital of the electrophile. As a result, the C–O π bond breaks, and the electrons move on to the oxygen atom.
In an electrophile, like HCl, which consists of a single-bonded electronegative atom, the dipole of the σ bond forces the nucleophilic electrons to move into the lower energy HCl antibonding σ orbital, and thus breaks the bond.
Molecules like halogens, with σ bonds and no dipoles, also make good electrophiles. In bromine, for example, poor overlaps between the atomic orbitals of bromine atoms result in a weak Br–Br bond.
Thus, a nucleophile attacks the lower energy σ antibonding orbital, breaking the Br–Br bond and making a new bond.
6.4:
Electrophiles
This lesson explains the definition, classification, and characteristic features of an electrophile that are key features of nucleophilic substitution reactions. An analysis of their charge and orbital picture helps understand their reactivity for seeking electrons. Electrophiles can be classified into positive and neutral species. Other classes include free radicals and polar functional groups.
While a positive electrophile, like a proton, reacts due to its vacant, low-energy 1s orbital, the other positive electrophiles, like carbocations, are reactive due to their vacant p orbital.
On the other hand, neutral electrophiles, analogous to Lewis acids, possess empty p orbitals that can accept electrons from the nucleophile to generate stable complexes. An electrophilic center can often be formed in a neutral molecule due to the electron-withdrawing inductive effect in the presence of a more electronegative substituent attached to the molecular chain. This explains the partial positive charge on the carbon atom in a carbonyl group.
In the context of a chemical reaction, a comprehensive picture of the electron transfer in this process is necessary. A nucleophile deposits its electrons into the lower energy antibonding π orbital of the electrophile. In contrast, the dipole of the σ bond forces the nucleophilic electrons to move into the lower energy antibonding σ orbital, resulting in bond breaking. These phenomena are often explained using examples of carbonyl groups and HCl. Typically, the lowest occupied molecular orbitals (LUMOs) in organic electrophiles are antibonding orbitals with low energy, as they are associated with electronegative atoms. These happen to be either π* orbitals or σ* orbitals.
Some molecules, such as halogens, also make good electrophiles. Here, despite the absence of a dipole, a poor overlap between the atomic orbitals of the two halides weakens the bond making it more prone to a nucleophile attack. This leads to the other classification of strong versus weak electrophiles, covered in later lessons. Usually, molecules with a single or double bond linked to an electronegative atom like O, N, Cl, or Br make good electrophiles.
Suggested Reading
- Brown, W.H., & Iverson, B.L., & Anslyn, V.E., & Foote S.C. (2014). Organic Chemistry. Mason, Ohio: Cengage Learning, 118-123.
- Klein, D. (2017). Organic Chemistry. New Jersey, NJ: Wiley, 183-188.
- Clayden, J., & Greeves, N., & Warren, S. (2012). Organic Chemistry. Oxford: Oxford University Press, 300-305.
- Solomons, G., & Fryhle, C. & Snyder, S. (2015). Organic Chemistry. New Jersey, NJ: Wiley, 192-194.
- Loudon, M., & Parise, J. (2016). Organic Chemistry. New York, NY: Macmillan Publishers, 230-234.