Stereochemistry - Organic Chemistry

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.

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.

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.

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.

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.

Hydrogen and a catalyst like paladium reduce the double bond to a single bond. There is no equal steric hinderance on each side. The hydrogen can bond from either side. That means the methyl group can either be oriented into the page or out of the page. One form is cis, and one form is trans.

For a molecule to be chiral, it must have a stereocenter and no axis of symmetry. An atom with a stereocenter has no identical bonds; it is a carbon atom with four unique substituents. There are two stereocenters in each of the three molecules. Notice that if you take the second molecule and draw a line connecting the top carbon and the point between the the two carbons with hydroxy groups, it has an axis of symmetry and therefore cannot be chiral. There is no way to draw that axis of symmetry for molecules one and three.

A stereocenter exists when the central atom is bound to four unique substituents. In the given molecule, the carbons are numbers from left to right. Carbons 1, 3, 5, 6, and 8 are all bound to at least two hydrogen atoms and cannot be stereocenters. Similarly, the carbons in the two methyl groups (bound to C4 and C7) do not qualify as stereocenters. Carbon 7 has two identical methyl substituents. This leaves only C2 and C4. The molecule has two stereocenters.

Carboxylic acid is highest priority, so carbon chain labelled from right to left. Since highest priority groups are on the same side of the double bond, it's given the "Z" designation.