Question

Draw and specify as $R$ or $S$ the enantiomers of: (a) 3 -chloro-1-pentene (b) 3-chloro-4-methy1-1-pentene (c) HOOCCH ${}_2$ CHOHCOOH, malic acid (d) ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{CH}\left({\mathrm{CH}}_3\right){\mathrm{NH}}_2$

Short answer

The enantiomers of the given compounds are as follows: (a) 3-chloro-1-pentene: R-enantiomer (H on the top, Cl on the bottom); S-enantiomer (H on the bottom, Cl on the top) (b) 3-chloro-4-methyl-1-pentene: R-enantiomer (H on the top, Cl on the bottom, CH${}_3$ on the right); S-enantiomer (H on the bottom, Cl on the top, CH${}_3$ on the right) (c) Malic acid: R-enantiomer (hydrogen on the bottom, hydroxyl group on the right); S-enantiomer (hydroxyl group on the bottom, hydrogen on the right) (d) C${}_6$H${}_5$CH(CH${}_3$)NH${}_2$: R-enantiomer (C${}_6$H${}_5$ on the left, CH${}_3$ on the top); S-enantiomer (C${}_6$H${}_5$ on the left, CH${}_3$ on the bottom)

Step-by-step solution

(a) Draw the structure of 3-chloro-1-pentene

First, draw a pentene molecule, which is a 5-carbon chain with a double bond between the first and second carbons. Then, add the chlorine atom to the third carbon in the chain. The structure will look like this: ``` H H - C = C - C - C - H H | H Cl ```

(a) Identify the chiral center and draw enantiomers of 3-chloro-1-pentene

The carbon at position 3 has four different substituents (a hydrogen, a chlorine, and two different carbon chains). This makes it a chiral center. Draw the enantiomers as R and S. R-enantiomer: The Fischer projection will be as follows: ``` C=C | Cl-C-H | H ``` S-enantiomer: The Fischer projection will be as follows: ``` C=C | H-C-Cl | H ```

(b) Draw the structure of 3-chloro-4-methyl-1-pentene

First, draw a pentene molecule. Then, add the chlorine atom to the third carbon and a methyl group to the fourth carbon in the chain. The structure will look like this: ``` H H - C = C - C - C - H H | | H Cl CH_3 ```

(b) Identify the chiral center and draw enantiomers of 3-chloro-4-methyl-1-pentene

The carbon at position 3 has four different substituents (hydrogen, chlorine, methyl, and ethene), making it a chiral center. Draw the enantiomers as R and S. R-enantiomer: ``` C=C | Cl-C-CH_3 | H ``` S-enantiomer: ``` C=C | H-C-CH_3 | Cl ```

(c) Draw the structure of HOOCCH${}_2$CHOHCOOH (malic acid)

The structure of malic acid is: ``` HOOC-CH_2-CHOH-COOH ```

(c) Identify chiral center and draw enantiomers of malic acid

The carbon at the third position has four different substituents (H, OH, CH${}_2$COOH and COOH), making it a chiral center. Draw the enantiomers as R and S. R-enantiomer: ``` HOOC-CH_2-C | COOH | OH | H ``` S-enantiomer: ``` HOOC-CH_2-C | COOH | H | OH ```

(d) Draw the structure of C${}_6$H${}_5$CH(CH${}_3$)NH ${}_2$

The structure is: ``` NH_2 | C_6H_5 - CH - CH_3 ```

(d) Identify the chiral center and draw enantiomers of C${}_6$H${}_5$CH(CH${}_3$)NH${}_2$

The carbon in the middle has four different substituents (C${}_6$H${}_5$, CH${}_3$, NH${}_2$, and H), making it a chiral center. Draw the enantiomers as R and S. R-enantiomer: ``` C_6H_5 - C | | CH_3 NH_2 ``` S-enantiomer: ``` C_6H_5 - C | | NH_2 CH_3 ```

Key concepts

Chiral Center

A chiral center is a specific type of carbon atom that plays a crucial role in the field of stereochemistry. This carbon atom is bonded to four different substituents or groups. Because of these unique attachments, the carbon can exist in two non-superimposable mirror image forms called enantiomers. Think of it like your left and right hands – similar in shape but cannot be perfectly aligned if placed on top of each other.

When identifying a chiral center, the key requirement is that the carbon atom must be bonded to four different groups. If even one group is the same, the carbon is no longer considered a chiral center.

• For example, in 3-chloro-1-pentene, the carbon at the third position has four different groups: hydrogen, chlorine, and two different carbon chains, making it a chiral center.
• Similarly, in the structure of malic acid, the central carbon atom is attached to hydroxyl (OH), hydrogen (H), a carboxyl group (COOH), and a methylene group (CH₂COOH), making it another example of a chiral center.

Understanding chiral centers is critical for predicting the behavior of molecules and their interactions in chemical reactions.

Fischer Projection

The Fischer projection is a simplified way to depict the three-dimensional structure of molecules in a two-dimensional drawing. It is especially useful for representing molecules with chiral centers.

In this projection, the molecule is viewed with the chiral center in the front, and horizontal lines represent bonds coming out of the plane towards the viewer. Vertical lines show bonds going into the plane, away from the viewer. This method is handy in visualizing the orientation of groups around the chiral center.

• For example, in the Fischer projection of the 3-chloro-1-pentene enantiomers, the hydrogen (H) and chlorine (Cl) are drawn horizontally to show they are coming out of the plane, while the remaining groups are arranged vertically.
• The same goes for malic acid, where the arrangement of functional groups can be interpreted directly using Fischer projections.

Fischer projections are not only helpful in determining the spatial arrangement of atoms but also in assigning the R/S configuration to chiral centers.

Stereochemistry

Stereochemistry is the study of the spatial arrangement of atoms within molecules. It is vital in understanding how molecules interact, react, and transform in chemical processes, impacting everything from biological systems to material science.

When we talk about stereochemistry, we're often referring to the differences between forms of the same molecular formula, such as isomers and enantiomers. For molecules with chiral centers, stereochemistry helps determine whether they are the right-handed (R) or left-handed (S) enantiomer.

• In the solution set, each molecule's stereochemistry is explored by establishing its enantiomers, like for the 3-chloro-1-pentene molecule.
• R enantiomers and S enantiomers, as seen in the various exercises, exhibit differences in physical properties and reactivity, despite having the same molecular formula.

Grasping the basics of stereochemistry is crucial for anyone looking to delve into organic chemistry, as it dramatically affects how molecules function, especially in pharmaceuticals and biology.