Stereochemistry - 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.

Carbon number : Hydroxyl is , group going up is , group going down is . Therefore it's S. Carbon number : Group going to chloride is , group going to hydroxyl is , methyl is . Therefore it's R. Carbon number : Chloride is , group going up is , group going down is . Therefore it's R.

Numbering goes from right to left, so double bond is attributed to carbon #. The double bond is E, because higher priority groups are across the double bond from each other. Carbon # is S.

Enantiomers are stereoisomers that have non-identical mirror image conformations and they are chiral as well. They are also non-superimposable which means, when placed on top of each other, they do not look the same.

This molecule contains two stereocenters, at carbon number (containing the hydroxyl group), and carbon number (containing the methyl group).

For carbon , the hydroxyl group is first priority, the alkane group going across the top of the molecule is second priority, the alkane group going down the left side of the molecule is third priority, and the hydrogen (not drawn) is the fourth (lowest) priority. Placing the fourth priority away from the viewer, into the plane of the page/screen (on a dash), the order of the substituents from highest to lowest priority is , which follows a counterclockwise directionality, so the absolute configuration is S.

For carbon , the alkane group moving across the top of the molecule toward the hydroxyl group is first priority, the alkane group going down the right side of the molecule is second priority, the methyl group is third priority, and the hydrogen (not drawn) is the fourth (lowest) priority. Placing the fourth priority away from the viewer, into the plane of the page/screen (on a dash), the order of the substituents from highest to lowest priority is , which follows a counterclockwise directionality, so the absolute configuration is S.

This molecule has no stereocenters (usually identified as a carbon atom attached to four different substituents. Therefore, the molecule is described as achiral.

A molecule is meso if it contains at least two stereocenters, but is rendered optically inactive by internal structural symmetry. In other words, a meso compound may be split in half in some way such that portions on either side of an imaginary line are mirror images. Note: The absolute configurations of a meso compound with two stereocenters are opposite (R/S). The internal symmetry that makes molecule III a meso compound is best conveyed through a Haworth projection:

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.