Electrophilic Reagents

Introduction-

Table 1 presents the structures of a variety of electrophilic reagents and lists some physical properties of these compounds.

Table 1

Getting to Know You

Entry	Compound	Name	b.p., oC	Comments
1	HCl	hydrogen chloride	-85	Concentrated hydrochloric acid is a saturated solution of HCl in water. It contains 37% HCl by weight. It is 12 M.
2	HBr	hydrogen bromide	-67	Concentrated hydrobromic acid is a saturated solution of HBr in water. It contains 48% HCl by weight. It is 8.8 M.
3	BrBr	dibromine	60	Dibromine is a dark red liquid. Its density is 3.10 g/mL.
4	ClCl	dichlorine	-34	Dichlorine is a pale yellow gas.
5	CH3Cl	chloromethane	-24	Chloromethane is a suspected carcinogen.
6		acetyl chloride	52	Acetyl chloride is a lachrymator. It reacts rapidly with water forming acetic acid and HCl vapor, which irritate the eyes.
7		acetic anhydride	139	Acetic anhydride is a mild lachrymator. It reacts with water, forming acetic acid, which irritates the eyes.
8		chlorosulfonic acid	152	ClSO3H is a colorless liquid. It reacts violently with water forming sulfuric acid and hydrogen chloride. Nasty.
9		nitric acid	---	Concentrated nitric acid is a saturated solution of HNO3 in water. It contains 71% HNO3 by weight. It is 16 M.
10		sulfur trioxide	45	SO3 is sometimes called sulfuric anhydride. It reacts rapidly with water forming sulfuric acid. When this happens in the atmosphere, it produces acid rain.
11	BF3	boron trifluoride	-100	BF3 is available as a complex with diethyl ether. The complex has a bp of 126oC.
12	AlCl3	aluminum chloride	190 (m.p.)	AlCl3 is a white solid. It reacts vigorously with water, giving off HCl vapor.
13	FeBr3	ferric bromide	684	FeBr3 is a yellow solid.

What structural features do these reagents have in common that makes them all electrophilic? The most obvious feature is that the electrophilic atom, which is highlighted in blue, is attached to one or more electronegative atoms. This reduces electron density around the highlighed atom, making it electron deficient and, therefore, reactive towards compounds that contain an electron-rich center, i.e. nucleophiles. In the case of the first five compounds in Table 1, Coulombic attraction between the electrophilic center and a nucleophilic center leads to substitution reactions that may be generalized as shown in Scheme 1.

Scheme 1

Electrophiles With Saturated Electrophilic Centers

Electrophiles of this type undergo nucleophilic substitution because the electrophilic center has a filled valence shell. In order to avoid violating the filled shell rules, formation of the Y-E bond must be accompanied by cleavage of the E-X bond; a 1,2-punch, if you will. Equations 1 and 2 present familiar examples of this type of reaction.

Equation 1 depicts a simple acid-base (electrophile-nucleophile) reaction, while Equation 2 is your prototypical Sn2 reaction.

Identify the electrophile in each reaction by entering its formula in the appropriate text field:

Scheme 2

Electrophiles With Unsaturated Electrophilic Centers

Equations 3 and 4 provide specific examples of the general process outlined in Scheme 2.

Equation 3 illustrates the comments in Table 1 about the lacrymatory properties of acetyl chloride. From one perspective reaction 4 is a nucleophilic acyl substitution reaction where the pi electrons of the aromatic ring act as the nucleophile. More commonly this transformation is described as an electrophilic aromatic substitution. From that perspective the chlorosulfonic acid is an electrophilic reagent that replaces a hydrogen atom on the aromatic ring.

Using Equation 4 as an example, write an equation depicting the reaction between a molecule of benzene and a protonated molecule of nitric acid. Label the bonding interactions 1-4. Draw the structure of the cyclohexadienyl cation intermediate as well as that of the final product.

Exercise 3 Sulfuric acid may be used as a "sulfonating agent" to replace an H atom with an SO₃H group in an aromatic compounds that contain activating substituents such as a methoxy group, OCH₃:

Write an equation similar to the one in Exercise 2, but depicting the protonation of one molecule of sulfuric acid by another. Then write an equation depicting the reaction between a molecule of methoxybenzene and a protonated molecule of sulfuric acid. Label the bonding interactions 1-4. Draw the structure of the cyclohexadienyl cation intermediate.

Scheme 3

Electrophiles that Undergo Addition

Equations 5 and 6 describe specific examples of the general reactivity pattern presented in Scheme 3. Mixing diethyl ether with boron trifluoride produces the commercially available boron trifluoride etherate complex mentioned in Table 1, while adding dibromine to a sample of ferric bromide results in the formation of a complex in which the bromine atom that is initially nucelophilic develops a positive charge, thereby becoming electrophilic.

The complex in reaction 6 is regarded as an electrophilic species because of the presence of the positively charged bromine atom. Having a positive charge on an electronegative atom is energetically unfavorable, i.e. the complex has a high potential energy. It is highly reactive. Equation 7 indicates the reaction pathway that is followed when the complex is formed in the presence of a (weakly) nucleophilic species, specifically benzene.

Exercise 5 Following the format shown in Equations 5 and 6, show how reactions a-e would occur:

Exercise 6 Exercise 2 suggested that in the nitration of benzene the nitrating agent was a protonated molecule of nitric acid. An alternative suggestion proposes that the following equilibrium is coupled to the protonation of nitric acid:

According to this theory, the active nitrating agent is the nitronium ion, ⁺NO₂. Following the format shown in Equations 5 and 6, show how the nitronium ion would react with a molecule of benzene. Draw the structure of the cyclohexadienyl cation that would be formed as an intermediate.