Question

Each molecule in this problem can be drawn as a hybrid of five contributing structures: two Kekulé structures and three that involve creation and separation of unlike charges. Draw these five contributing structures for each molecule. (a) Chlorobenzene (b) Phenol (c) Nitrobenzene

Short answer

Question: Draw five contributing structures for each of the following molecules: (a) Chlorobenzene, (b) Phenol, and (c) Nitrobenzene. Include two Kekulé structures and three structures involving the creation and separation of unlike charges for each molecule. Answer: (a) Chlorobenzene: 1. Kekulé structure 1 2. Kekulé structure 2 3. Charge-separated structure 1 4. Charge-separated structure 2 5. Charge-separated structure 3 (b) Phenol: 1. Kekulé structure 1 2. Kekulé structure 2 3. Charge-separated structure 1 4. Charge-separated structure 2 5. Charge-separated structure 3 (c) Nitrobenzene: 1. Kekulé structure 1 2. Kekulé structure 2 3. Charge-separated structure 1 4. Charge-separated structure 2 5. Charge-separated structure 3

Step-by-step solution

(a) Chlorobenzene

To draw the contributing structures for chlorobenzene, start by drawing the basic structure of the benzene ring with a chlorine atom attached to one of the carbons. To find the different contributing structures, move the pi electrons within the ring to create the necessary double bonds and charge separation. 1. Kekulé structure 1: Start with alternate single and double bonds around the ring, with the chlorine atom attached to a carbon with a single bond. 2. Kekulé structure 2: Keep the chlorine atom attached but switch the position of double bonds in the ring, so all the single bonds in Structure 1 are now double bonds, and vice versa. 3. Charge-separated structure 1: Now, create a resonance form where the pi bond between two carbons next to the carbon with the chlorine atom breaks. The chlorine atom gains the electron pair and assumes a negative charge, while the adjacent carbon loses those electrons and gains a positive charge. 4. Charge-separated structure 2: Do the similar thing as in structure 3 but now with the carbon on the opposite side of the carbon with the chlorine atom. 5. Charge-separated structure 3: Now, create a structure where both adjacent carbons have a positive charge, and the carbon with the chlorine atom has a negative charge.

(b) Phenol

To draw the contributing structures for phenol, draw the basic structure of the benzene ring with a hydroxyl group (OH) attached to one of the carbons. 1. Kekulé structure 1: Start with alternate single and double bonds around the ring, with the phenol group (OH) attached to a carbon with a single bond. 2. Kekulé structure 2: Keep the phenol group attached but switch the position of double bonds in the ring, so all the single bonds in Structure 1 are now double bonds, and vice versa. 3. Charge-separated structure 1: Create a resonance form where the pi bond between two carbons next to the carbon with the phenol group breaks. The hydroxyl group gains the electron pair and assumes a negative charge, while the adjacent carbon loses those electrons and gains a positive charge. 4. Charge-separated structure 2: Do the similar thing as in structure 3 but now with the carbon on the opposite side of the carbon with the hydroxyl group. 5. Charge-separated structure 3: Now, create a structure where both adjacent carbons have a positive charge, and the carbon with the hydroxyl group has a negative charge.

(c) Nitrobenzene

To draw the contributing structures for nitrobenzene, draw the basic structure of the benzene ring with a nitro group (NO_2) attached to one of the carbons. 1. Kekulé structure 1: Start with alternate single and double bonds around the ring, with the nitro group (NO_2) attached to a carbon with a single bond. 2. Kekulé structure 2: Keep the nitro group attached but switch the position of double bonds in the ring, so all the single bonds in Structure 1 are now double bonds, and vice versa. 3. Charge-separated structure 1: Create a resonance form where the pi bond between two carbons next to the carbon with the nitro group breaks. The nitro group gains the electron pair and assumes a negative charge, while the adjacent carbon loses those electrons and gains a positive charge. 4. Charge-separated structure 2: Do the similar thing as in structure 3 but now with the carbon on the opposite side of the carbon with the nitro group. 5. Charge-separated structure 3: Now, create a structure where both adjacent carbons have a positive charge, and the carbon with the nitro group has a negative charge.