Question

Mark each chiral center in the following molecules with an asterisk. How many stereoisomers are possible for each molecule? (a) CC(O)C(O)C(=O)O (b) [O]C(C(=O)O)C(CC(=O)O)C(=O)O (c) CC1=CCC(C(C)C)CC1 (c) CC1CCC(C(C)C)CC1 (f) CC(C)CC(=O)CC(C)C1CC=C(C(=O)O)CC1 (g) OC1CCOC1O (h) CC12CCCCC1CC(=O)CC2

Short answer

Question: Identify the number of stereoisomers for each of the listed molecules by marking each chiral center with an asterisk and using the 2^n rule method. (a) CC(O)C(O)C(=O)O (b) [O]C(C(=O)O)C(CC(=O)O)C(=O)O (c) CC1=CCC(C(C)C)CC1 (d) CC1CCC(C(C)C)CC1 (e) CC(C)CC(=O)CC(C)C1CC=C(C(=O)O)CC1 (f) OC1CCOC1O (g) CC12CCCCC1CC(=O)CC2 Answer: (a) 4 stereoisomers (b) 2 stereoisomers (c) 2 stereoisomers (d) 1 stereoisomer (e) 4 stereoisomers (f) 1 stereoisomer (g) 2 stereoisomers

Step-by-step solution

(a) CC(O)C(O)C(=O)O

Identify chiral centers by looking for carbon atoms with four different groups bonded to them. There are two chiral centers at the carbon atoms bonded to the two -OH groups in this molecule. Number of stereoisomers = 2^n = 2^2 = 4

(b) [O]C(C(=O)O)C(CC(=O)O)C(=O)O

Identify chiral centers by looking for carbon atoms with four different groups bonded to them. There is one chiral center in this molecule, at the carbon atom bonded to the -OH group and the -C(=O)O group. Number of stereoisomers = 2^n = 2^1 = 2

(c) CC1=CCC(C(C)C)CC1

Identify chiral centers by looking for carbon atoms with four different groups bonded to them. There is one chiral center in this molecule, at the carbon atom bonded to the -C(C)C group within the ring. Number of stereoisomers = 2^n = 2^1 = 2

(d) CC1CCC(C(C)C)CC1

Identify chiral centers by looking for carbon atoms with four different groups bonded to them. There are no chiral centers in this molecule. Number of stereoisomers = 2^n = 2^0 = 1

(e) CC(C)CC(=O)CC(C)C1CC=C(C(=O)O)CC1

Identify chiral centers by looking for carbon atoms with four different groups bonded to them. There are two chiral centers in this molecule, at the carbon atom bonded to the -C=C(C(=O)O)CC1 group and another one carbon atom away from the -C(=O)O group. Number of stereoisomers = 2^n = 2^2 = 4

(f) OC1CCOC1O

Identify chiral centers by looking for carbon atoms with four different groups bonded to them. There are no chiral centers in this molecule. Number of stereoisomers = 2^n = 2^0 = 1

(g) CC12CCCCC1CC(=O)CC2

Identify chiral centers by looking for carbon atoms with four different groups bonded to them. There is one chiral center in this molecule, at the carbon atom bonded to the -C(=O)C group. Number of stereoisomers = 2^n = 2^1 = 2

Key concepts

Chiral Centers

Understanding chiral centers is crucial in the realm of organic chemistry, where they represent a specific type of asymmetry in molecules. A chiral center, commonly a carbon atom, is attached to four different groups, giving the molecule a non-superimposable mirror image. Think of your hands; they are mirror images but cannot be perfectly overlaid on each other, making them chiral.

To identify a chiral center, examine each carbon atom to determine if it's bound to four distinct groups. If the answer is yes, you've found a chiral center, which can greatly influence the molecule's physical and chemical properties, especially how it interacts with polarized light and other chiral molecules such as enzymes.

Stereochemistry

Stereochemistry delves into the three-dimensional arrangement of atoms within molecules and how this spatial configuration impacts their behavior and reactions. Stereoisomers are molecules that have the same molecular formula and sequence of bonded atoms (constitution), but differ in the 3D orientations of their atoms in space.

The concept transcends just chiral centers; it includes geometric isomerism (cis/trans isomerism) and conformations (like chair and boat forms in cyclohexanes). Stereochemistry is pivotal in pharmacology, as the different arrangements can lead to drastically different biological activities.

Organic Molecules

Organic molecules form the backbone of organic chemistry and consist primarily of carbon atoms bonded together, with hydrogen, oxygen, nitrogen, and other elements. They can vary vastly in size, shape, and the elements they contain, leading to an immense variety of compounds with unique chemical properties.

The complexity of organic molecules, with their potential for forming rings, chains, and branches, as well as containing functional groups like hydroxyls (-OH) or carbonyls (C=O), results in a wide range of possible interactions and reactions, making organic chemistry a rich and diverse field of study.

Isomerism

Isomerism is the phenomenon where compounds with the same chemical formula exist in different forms due to variance in the arrangement of their atoms. In stereochemistry, this primarily refers to stereoisomers, as discussed. However, isomerism can also be structural, known as constitutional isomerism, where the connectivity of atoms differs between isomers.

When considering isomerism in the context of the number of possible isomers, remember the formula for calculating the number of stereoisomers: \(2^n\), where \(n\) is the number of chiral centers. Variations in structure can have profound implications on the chemical behavior of a molecule, illustrating why the study of isomerism is key in understanding organic molecules.