Question

Draw mirror images for these molecules. Are they different from the original molecule? (a) CC(O)C(=O)O (b) O=CC(O)CO (c) CC(N)C(=O)O (d) OC1CCCO1 (e) OC1CCCCC1 (f) CC1CCCCC1O (g) CC1CCCCC1O (h) CC1CC(C)C(O)C1O

Short answer

(a) Lactic acid (2-hydroxypropionic acid) (b) Glyoxylic acid (c) Alanine (d) Tetrahydrofuran-2-ol (e) Phenol (f) 4-methylphenol (Cresol) (g) 4-methylphenol (Cresol) (h) Menthol Answer: The mirror images that are different from the original molecules belong to (a) Lactic acid, (c) Alanine, and (h) Menthol. These molecules have chiral centers, making their mirror images non-superimposable.

Step-by-step solution

Draw the original molecules and their mirror images

Draw the molecules from the given SMILES codes and then draw their mirror images. It is important to identify any chiral centers in the molecule so that we can determine if the molecules are chiral or not.

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

Lactic acid (2-hydroxypropionic acid): Original molecule: H3C-CH(OH)-C(=O)OH Mirror image: H3C-CH(OH)-C(=O)OH This molecule has one chiral center: the carbon atom that is connected to the -OH group. The mirror image is non-superimposable on the original molecule, so they are different.

(b) O=CC(O)CO

Glyoxylic acid: Original molecule: O=C-C(O)-C=O Mirror image: O=C-C(O)-C=O This molecule doesn't have any chiral center, so the mirror image is superimposable with the original molecule. They are not different.

(c) CC(N)C(=O)O

Alanine: Original molecule: H3C-CH(NH2)-C(=O)OH Mirror image: H3C-CH(NH2)-C(=O)OH This molecule has one chiral center: the carbon atom that is connected to the -NH2 group. The mirror image is non-superimposable on the original molecule, so they are different.

(d) OC1CCCO1

Tetrahydrofuran-2-ol: Original molecule: O-1-CH2-CH2-CH2-CH2-1-OH Mirror image: O-1-CH2-CH2-CH2-CH2-1-OH This molecule doesn't have any chiral center, so the mirror image is superimposable with the original molecule. They are not different.

(e) OC1CCCCC1

Phenol: Original molecule: OH-1-CH-CH2-CH2-CH2-CH2-1 Mirror image: OH-1-CH-CH2-CH2-CH2-CH2-1 This molecule doesn't have any chiral center, so the mirror image is superimposable with the original molecule. They are not different.

(f) CC1CCCCC1O

4-methylphenol (Cresol): Original molecule: H3C-1-CH-CH2-CH-CH2-CH2-1-OH Mirror image: H3C-1-CH-CH2-CH-CH2-CH2-1-OH This molecule doesn't have any chiral center, so the mirror image is superimposable with the original molecule. They are not different.

(g) CC1CCCCC1O

4-methylphenol (Cresol): Original molecule: H3C-1-CH-CH2-CH-CH2-CH2-1-OH Mirror image: H3C-1-CH-CH2-CH-CH2-CH2-1-OH This molecule doesn't have any chiral center, so the mirror image is superimposable with the original molecule. They are not different.

(h) CC1CC(C)C(O)C1O

Menthol: Original molecule: H3C-1-CH-CH(C)(CH(OH))-CH2-1-OH Mirror image: H3C-1-CH-CH(C)(CH(OH))-CH2-1-OH This molecule has one chiral center: the carbon atom that is connected to the -OH group, -CH3 group, and the ring. The mirror image is non-superimposable on the original molecule, so they are different.

Key concepts

Chiral Centers

Chiral centers are the focal point of understanding chirality in organic chemistry. A chiral center, also known as a stereocenter, is a carbon atom that is bonded to four different groups or atoms. When such a center exists within a molecule, it has the potential to exist in two non-superimposable mirror images, making the molecule chiral.

The presence of a chiral center means that a molecule can exist as two stereoisomers, called enantiomers, which are mirror images of each other. It is crucial when analyzing molecules to identify these chiral centers as they contribute to the molecule's overall stereochemistry and can influence a compound’s physiological properties and behavior in biological systems. For example, lactic acid, mentioned in the original exercise as (a), has a chiral center at the carbon atom connected to the -OH group, making it chiral and resulting in two enantiomers.

Superimposable Mirror Images

The concept of superimposable mirror images is fundamental in distinguishing chiral molecules from achiral ones. Two objects are superimposable if, when placed over each other, they match perfectly in all dimensions. In the context of chiral molecules, if the original molecule and its mirror image cannot be superimposed, then they are different; this distinction confirms the molecule's chirality.

Enantiomers are an interesting case of non-superimposable mirror images. They might appear identical at a glance, but their spatial arrangement prevents them from being superimposable. This lack of superimposability is what makes them enantiomers and can lead to differing reactions with other chiral substances, such as enzymes in the body. For instance, alanine and menthol, indicated as (c) and (h) respectively, illustrate non-superimposable molecules due to their respective chiral centers.

Stereochemistry

Stereochemistry is the study of the three-dimensional arrangement of atoms within molecules and how this arrangement affects the physical and chemical properties of these molecules. It encompasses concepts such as chiral centers and superimposability to predict the behavior of molecules in three-dimensional space.

This branch of chemistry is important in a variety of fields, especially in the development of pharmaceutical drugs, as the spatial configuration of a molecule can greatly influence its effectiveness and interactions with biological systems. Proper understanding of stereochemistry is essential for predicting how different enantiomers will interact with the body, as they can produce different physiological effects despite having the same chemical formula.

As seen in the exercise solutions, molecules without chiral centers, like in glyoxylic acid (b) and tetrahydrofuran-2-ol (d), are identical to their mirror images and exhibit symmetry that characterizes achirality. Meanwhile, stereoisomers arise from molecules with chiral centers, leading to significant differences in biochemical behavior, making stereochemistry a key aspect of organic chemistry worth mastering.