Question

Give structures and names of the principal products expected from the ring monobromination of each of the following compounds. In each case, tell whether bromination will occur faster than with benzene itself. (a) iodobenzene (b) sec-butylbenzene (c) acetophenone \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{3}\right)\) (d) phenetole \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OC}_{2} \mathrm{H}_{5}\right)\) (e) diphenylmethane \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{C}_{6} \mathrm{H}_{5}\right)\) (f) benzotrifluoride \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CF}_{3}\right)\)

Short answer

(a) 1-bromo-4-iodobenzene; bromination slower than benzene. (b) 1-bromo-2-sec-butylbenzene (ortho) and 1-bromo-4-sec-butylbenzene (para); bromination faster than benzene. (c) 1-bromo-3-phenylpropanone (or 3'-bromoacetophenone); bromination slower than benzene. (d) 1-bromo-2-methoxybenzene (ortho) and 1-bromo-4-methoxybenzene (para); bromination faster than benzene. (e) 1-bromo-2-(3'-phenylpropyl)benzene (ortho) and 1-bromo-4-(3'-phenylpropyl)benzene (para); bromination faster than benzene. (f) 1-bromo-3-(trifluoromethyl)benzene; bromination slower than benzene.

Step-by-step solution

(a) Iodobenzene

: Iodobenzene is a halogen as substituent and is weak electron-withdrawing due to its slightly lower electronegativity than bromine. However, since it is a bulkier group, it will hinder the ortho position especially when compared to bromine. Hence, the main product from monobromination will be in the para position and have the name 1-bromo-4-iodobenzene. Bromination will be slower than with benzene itself due to the weak electron-withdrawing effect of the iodine group.

(b) sec-Butylbenzene

: The sec-butyl group is an alkyl group which is electron-donating due to its inductive effect. Additionally, it also participates in hyperconjugation which makes it a strong electron-donating group. The main product from monobromination will be in the ortho and para positions due to its electron-donating nature. The names of the products will be 1-bromo-2-sec-butylbenzene (ortho) and 1-bromo-4-sec-butylbenzene (para). Bromination will occur faster than with benzene itself due to the strong electron-donating effect of the sec-butyl group.

(c) Acetophenone \((\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{3})\)

: The carbonyl group in acetophenone is an electron-withdrawing group due to the polarity of the C=O bond. The main product from monobromination will be in the meta position due to its electron-withdrawing nature. The name of the product will be 1-bromo-3-phenylpropanone (also called 3'-bromoacetophenone). Bromination will be slower than with benzene itself due to the electron-withdrawing effect of the carbonyl group.

(d) Phenetole \((\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OC}_{2} \mathrm{H}_{5})\)

: The methoxy group in phenetole is an electron-donating group due to its +M effect. The main product from monobromination will be in the ortho and para positions. The names of the products will be 1-bromo-2-methoxybenzene (ortho) and 1-bromo-4-methoxybenzene (para). Bromination will occur faster than with benzene itself due to the electron-donating effect of the methoxy group.

(e) Diphenylmethane \((\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{C}_{6} \mathrm{H}_{5})\)

: For diphenylmethane, the phenyl group behaves as a weak electron-donating group. The main product from monobromination will be in the ortho and para positions. The names of the products will be 1-bromo-2-(3'-phenylpropyl)benzene (ortho) and 1-bromo-4-(3'-phenylpropyl)benzene (para). Bromination will occur faster than with benzene itself due to the electron-donating effect of the phenyl group.

(f) Benzotrifluoride \((\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CF}_{3})\)

: The trifluoromethyl group in benzotrifluoride is an electron-withdrawing group due to the electronegativity of fluorine atoms. The main product from monobromination will be in the meta position due to its electron-withdrawing nature. The name of the product will be 1-bromo-3-(trifluoromethyl)benzene. Bromination will be slower than with benzene itself due to the electron-withdrawing effect of the trifluoromethyl group.

Key concepts

Electrophilic Aromatic Substitution

Electrophilic Aromatic Substitution (EAS) is a fundamental type of reaction that occurs when an aromatic ring, like benzene, reacts with an electrophile to form a new product. The overall process can typically be broken down into two main steps: the generation of the electrophile and the aromatic substitution mechanism.

In the mechanism, the aromatic ring, known for its stability due to delocalized pi electrons, briefly gives up this stability. It forms a high-energy intermediate called a sigma complex or arenium ion, where the electrophile has added to one of the carbon atoms. The ring's pi electrons help stabilize this intermediate by dispersing the positive charge around the ring. Eventually, a base removes a hydrogen from the sigma complex, restoring the aromaticity and yielding the substituted aromatic compound.

EAS is influenced by the nature of substituents already attached to the benzene ring. Substituents can be classified as either electron-donating or electron-withdrawing, and this influences the reactivity of the benzene ring as well as the position where new substituents will be added during the reaction.

Electron-Donating Groups

Electron-Donating Groups (EDGs) are substituents that release electron density into the benzene ring through inductive or resonance effects. Examples of EDGs include alkyl groups (like methyl, ethyl, or in our exercises sec-butyl) and groups containing atoms with lone pairs that can resonate with the ring, such as methoxy (-OCH₃), as seen in phenetole.

These groups increase the electron density of the ring, making it more reactive towards electrophiles. They also influence the regioselectivity of the EAS reaction, typically directing the incoming electrophile to the ortho and para positions relative to themselves. This behavior is a result of the stable resonance structures that can be drawn when the electrophile attacks these positions. Therefore, electron-donating substituents are also known as ortho-para directors.

Electron-Withdrawing Groups

Electron-Withdrawing Groups (EWGs) pull electron density away from the benzene ring, either by inductive effect—usually due to electronegative atoms like halogens—or by resonance as is the case with groups like carbonyls (-CO-), nitriles (-CN), and nitro (-NO₂) groups.

Such groups make the aromatic ring less reactive towards electrophiles because they decrease the electron density, which is essential for the aromatic ring to interact with an electrophile. EWGs typically direct incoming electrophiles to the meta position. This pattern is due to the instability of the charged intermediate that would occur if the electrophile added to the ortho or para position. Given this, compounds with electron-withdrawing groups, such as acetophenone and benzotrifluoride, react slower with electrophiles than benzene itself, and the major product is usually found at the meta position.

Ortho-Para Directors

Ortho-Para directors are substituents that, as their name suggests, direct incoming electrophiles to the ortho and para positions of the aromatic ring during an electrophilic substitution reaction. These are typically electron-donating groups, and their effect makes the ortho and para positions more electrophilic than the meta position.

Substituents such as alkyl groups and atoms with lone pairs that can participate in resonance (e.g., -OH, -OCH₃) are examples of ortho-para directors. When present on a benzene ring, they not only enhance the reactivity of the ring towards electrophiles but also influence the substitution pattern to favor the ortho and para positions. This is eloquently illustrated in our exercise with sec-butylbenzene and phenetole, which both have groups that direct the incoming bromine to the ortho and para positions.

Meta Directors

Meta directors are groups that direct incoming electrophiles to the meta position of the aromatic ring during EAS reactions. These groups are typically strong electron-withdrawing groups that stabilize the positive charge of the intermediate when the substitution occurs at the meta position.

Given their electron-withdrawing nature, they decrease the electron density at the ortho and para positions more than at the meta position, which explains the preference for meta substitution. Examples of meta-directing groups include the carbonyl group in acetophenone and the trifluoromethyl group in benzotrifluoride, both from the exercise. Their presence on an aromatic ring will slow the reaction with electrophiles and result in substitution occurring predominantly at the meta position.