Benzene Additions - Organic Chemistry

In the molecule shown, the nitro group will direct incoming substituents to positions meta to it, and the methoxy group will direct incoming substituents ortho or para to it. The only product option shown in which the chlorine substituent is meta (1,3) to the nitro group AND either ortho (1,2) or para (1,4) to the methoxy group is option I.

In the molecule shown, the aldehyde group will direct incoming substituents to positions meta to it, and both the the hydroxyl group AND the fluorine group will direct incoming substituents to positions ortho or para to them. Options I and III show the incoming substituent meta to the aldehyde group, as well as either ortho or para to both the fluorine group and hydroxyl group. Therefore, they are both possible products.

Note: because option III shows attachment at a carbon that is slightly less sterically hindered than that of option I, option III will be produced in a slightly greater quantity.

The reagents shown will add a bromine to an aromatic ring through an electrophilic aromatic substitution mechanism. A nitro group is a strong electron withdrawing group and a benzene deactivator. All benzene deactivators (with the exception of halogens) direct incoming substituents to the meta positions. Therefore, option III is the major product.

is the only electron-donating group listed. Therefore, it will add to the ortho and para positions on phenol. The rest of the substituents are highly electron-withdrawing groups and will add to the meta positions on phenol.

is the correct answer. In most cases, the deactivating substituents direct "meta." Halogens deactivate benzene rings, but are the exception, as they direct "ortho" or "para." A substituent is deactivating when it has a low electron concentration on the atom directly attached to the benzene ring. Carboxylic acid provides resonance, a delocalization of electrons. However, the amine group, for example, sees a lone pair on the nitrogen.

The functional group that is already on the phenyl is the group that dictates where any other substituent will be directed on the ring. In this case, we have a carboxylic acid as our director. Due to resonance, carboxylic acid deactivates (is an electron-withdrawing group) the benzene ring and directs the bromine to the meta position.

Here we have a classic EAS reaction.

If we examine the molecule, we see that the benzene ring already has two substituents on it: , an activating group, and , a deactivating group. Remember, when assigning the position of a new substituent to a benzene ring, activating groups always dictate the position of the new substituent.

We know that activating groups direct new substituents to the ortho or para positions. However, because of sterics, a new substituent will not be directed to the ortho position in between two substituents. Thus, the new substituent will be directed to the para position (or the other ortho position, but that is not one of our answer choices), and the correct answer is position 2.

With the exception of halogens, meta directors deactivate a benzene ring. In other words, they make the benzene ring less reactive, especially in an electrophilic aromatic substitution (EAS) reaction. Meta directors have little electron density at the point of contact with the benzene ring. For example, a carboxylic acid is a meta director because it experiences resonance, a delocalization of electrons. All of the answer choices in this problem have a lone pair of electrons on the point of contact with the benzene ring and they all are ortho/para directors.

Phenol contains the hydroxide group, which is an electron donor, puts electron density into the benzene ring. Resonance structures are drawn as follows

Carbonyls (as in 4 and 5) are always electron withdrawing due to the Oxygen's electronegativity. Similarly, the oxygens on the nitrate (1) are electron withdrawing.

is the only electron-withdrawing substituent because it contains two electronegative oxygen atoms which pull electrons from the benzene ring towards itself. This effect is electron-withdrawing and makes the ring slightly positive in charge. All the other substituents are electron-donating groups, which activate the ring for electrophilic addition.

A tert-butyl functional group is electron donating and will therefore activate the ortho and para positions. However, the ortho positions are sterically hindered by the bulky tert-butyl group. Therefore, the para position will be favored.

This is an example of a Friedel-Crafts acylation reaction, which will add an acyl group (alkyl group containing a carbonyl () group) to a benzene ring at the carbonyl carbon (shown below).

This reaction proceeds using an electrophilic aromatic substitution reaction mechanism. The presence of the hydroxyl group on the benzene will affect where the incoming acyl group will attach. A hydroxyl group is a strong benzene activator, which will direct incoming substituents to positions ortho or para to it. Option I is the only option shown where the incoming substituent is attached at either the ortho or para position.

This reaction proceeds using an electrophilic aromatic substitution reaction mechanism. The presence of the amino group on the benzene will affect where the incoming acyl group will attach. An amino group is a strong benzene activator, which will direct incoming substituents to positions ortho or para to it. Option III is the only option shown where the incoming substituent is attached at either the ortho or para position on a benzene ring.

Several resonance structures can be drawn for the molecule phenol. These are shown below.

Because the overall charge distribution puts partial negative charges on carbons , , and , these carbons have an increased nucleophilic character. Therefore, these carbons are more likely than the other carbons to accept an incoming electrophilic substituent, making these positions more likely to be substituted. Carbons and are known as the ortho positions, and carbon is known as the para position.

Several resonance structures can be drawn for the molecule nitrobenzene. These are shown below.

From these resonance structures, an overall molecular electronic distribution can be determined:

Because the overall charge distribution puts partial positive charges on carbons , , and , these carbons have an increased electrophilic character. Therefore, these carbons are less likely than the other carbons to accept an incoming electrophilic substituent, making these positions less likely to be substituted. By default, carbons , and , known as the meta positions are the only ones nucleophilic enough to carry out this reaction.

When speaking of the arrangemts of two substituents on a benzene, their positions can be ortho, meta, or para. Ortho substituents are bound to adjacent carbons (C1 and C2). Meta substituents are bound to carbons with one intermediary carbon (C1 and C3). Para substituents are opposite one another across the benzene ring (C1 and C4).

The bromines in the given reaction result in a para product because bromine is ortho/para directing. There is a significant amount of ortho product, but because of steric hinderance, the major product is para.

This is a trick question on Friedal-Craft alkylation. If you didn't pick the starting material, that is because you forgot the rule that alkylation cannot happen on a highly deactivated aromatic ring. Since is a highly deactivating group the answer is no reaction, or:

This question is essentially testing an understanding of electrophilic aromatic substitution and substituent placement. first acts as a lewis acid by accepting the chloride from the reagent, which generates . This, in turn, is susceptible to nucleophilic attack from one of the double bonds in the benzene ring. Due to the disruption of aromaticity in the ring, this double bond is quickly restored by abstraction of an adjacent proton on the ring.

But we also need to be aware of what position this substituent will be directed to. Since the original molecule, toluene, consists of a benzene ring bonded to an electron-donating methyl group, this directs the substitution reaction towards the ortho or para position on the ring. Of the choices shown, only the para position is shown. The ortho position is not shown as an answer choice, and direction to the meta position would not be likely.

While both the bromide and the alkyl group are ortho/para directing groups, the alkyl group is more activating and therefore the carbonyl will be added ortho to that substituent. The and work to reduce the carbonyl to an .