Question

Which group or atom in each of the following pairs gets priority according to CIP rules used for assigning \(\mathrm{R}\) and \(\mathrm{S}\) configuration for chiral molecules? (a) \(-\mathrm{CH}_{2} \mathrm{OH}\) or \(-\mathrm{CH}_{2} \mathrm{Cl}\) (b) \(-\mathrm{CH}=\mathrm{CH}_{2}\) or \(-\mathrm{CH}_{2} \mathrm{CH}_{3}(\mathrm{c})-\mathrm{NH}_{2}\) or \(-\mathrm{OH}\) (d) \(-\mathrm{CH}_{2} \mathrm{OH}\) or \(-\mathrm{CHO}\) Choose from the options given below \(\begin{array}{llll}\text { (a) } \mathrm{A}:-\mathrm{CH}_{2} \mathrm{OH}, & \mathrm{B}:-\mathrm{CH}=\mathrm{CH}_{2}, & \mathrm{C}:-\mathrm{NH}_{2}, & \mathrm{D}:-\mathrm{CHO}\end{array}\) \(\begin{array}{llll}\text { (b) } \mathrm{A}:-\mathrm{CH}_{2} \mathrm{Cl}, & \mathrm{B}:-\mathrm{CH}=\mathrm{CH}_{2}, & \mathrm{C}:-\mathrm{OH}, & \mathrm{D}:-\mathrm{CHO}\end{array}\) \(\begin{array}{llll}\text { (c) } \mathrm{A}:-\mathrm{CH}_{2} \mathrm{OH}, & \mathrm{B}:-\mathrm{CH}_{2} \mathrm{CH}_{3}, & \mathrm{C}:-\mathrm{NH}_{2}, & \mathrm{D}:-\mathrm{CHO}\end{array}\) \(\begin{array}{llll}\text { (d) } \mathrm{A}:-\mathrm{CH}_{2} \mathrm{Cl}, & \mathrm{B}:-\mathrm{CH}_{2}-\mathrm{CH}_{3}, & \mathrm{C}:-\mathrm{OH}, & \mathrm{D}:-\mathrm{CH}_{2} \mathrm{OH}\end{array}\)

Short answer

(a) \(\mathrm{-CH_2OH}\) or \(\mathrm{-CH_2Cl}\) (b) \(\mathrm{-CH=CH_2}\) or \(\mathrm{-CH_2CH_3}\) (c) \(\mathrm{-NH_2}\) or \(\mathrm{-OH}\) (d) \(\mathrm{-CH_2OH}\) or \(\mathrm{-CHO}\) Answer: (b) The priority groups are: (a) \(\mathrm{-CH_2Cl}\) (b) \(\mathrm{-CH=CH_2}\) (c) \(\mathrm{-OH}\) (d) \(\mathrm{-CHO}\)

Step-by-step solution

Comparing Pair (a)

We have \(\mathrm{-CH_2OH}\) and \(\mathrm{-CH_2Cl}\). Since both groups have the same carbon atom directly attached, we need to consider the next set of connected atoms. Oxygen has an atomic number of 8, while chlorine has an atomic number of 17. According to CIP rules, higher atomic number takes priority. Thus, in pair (a), \(\mathrm{-CH_2Cl}\) has priority.

Comparing Pair (b)

We have \(\mathrm{-CH=CH_2}\) and \(\mathrm{-CH_2CH_3}\). The first carbon atom in both groups is directly attached, so we need to consider the next set of connected atoms. In the first group, we have a double bond to another carbon, which is counted as two connections to that carbon. In the second group, we have only a single bond. So, in pair (b), \(\mathrm{-CH=CH_2}\) has priority.

Comparing Pair (c)

We have \(\mathrm{-NH_2}\) and \(\mathrm{-OH}\). Nitrogen is directly bound and has an atomic number of 7, while oxygen has an atomic number of 8. So, in pair (c), \(\mathrm{-OH}\) has priority according to CIP rules.

Comparing Pair (d)

We have \(\mathrm{-CH_2OH}\) and \(\mathrm{-CHO}\). The first carbon atom in both groups is directly connected, so we need to consider the next set of connected atoms. Oxygen in \(\mathrm{-CH_2OH}\) has an atomic number of 8, and the same oxygen in \(\mathrm{-CHO}\) has atomic number 8 but with a double bond which counts as two connections to the carbon. So, in pair (d), \(\mathrm{-CHO}\) has priority.

Choose correct option

The correct combination of priorities in pairs is: (a) \(\mathrm{-CH_2Cl}\) (b) \(\mathrm{-CH=CH_2}\) (c) \(\mathrm{-OH}\) (d) \(\mathrm{-CHO}\) Thus, the correct option is (b) in the given exercise.

Key concepts

R and S Configuration

Understanding R and S configuration is a fundamental aspect of stereochemistry in organic chemistry. These configurations are used to describe the spatial arrangement of atoms around a chiral center. A chiral molecule has four different substituents attached to its chiral center, which can be arranged in two non-superimposable forms like left and right hands, known as enantiomers. Each enantiomer is assigned either an R (from the Latin rectus, meaning right) or S (from the Latin sinister, meaning left) configuration based on the priority of the substituents.

To determine the configuration, we look at the chiral center and identify the priority of groups attached to it following the Cahn-Ingold-Prelog (CIP) priority rules. Starting with the highest priority group, we then visualize the order in which the groups are attached from highest (1) to lowest (4) priority. When the lowest priority group is pointing away from us, if the order of the remaining groups is clockwise, the chiral center is given an R configuration, and if it's counterclockwise, an S configuration. The exercise solution above demonstrates how to apply these rules to find the corresponding R or S configuration.

Chiral Molecules

Chiral molecules play a critical role in many biological processes and are central in the study of organic chemistry. A chiral molecule contains at least one carbon atom connected to four different atoms or groups, leading to non-superimposable mirror images, just as one's left hand is not superimposable on the right hand.

These two mirror image forms are called enantiomers, and they can have vastly different behaviors in biological systems, despite having the same molecular formula. Enantiomers can only be distinguished in a chiral environment, for example, when interacting with another chiral molecule, making this concept extremely important in the synthesis of pharmaceuticals and understanding biomolecular interactions. The distinguishing of groups in the CIP rules, as highlighted in the exercise, is essential for distinguishing these enantiomers as either R or S.

Atomic Number Priority

The atomic number priority is a key component of the CIP rules and is fundamental in determining group priority for the R and S configurations. Here's the principle: the higher the atomic number of the atom directly attached to the chiral center, the higher its priority. For example, iodine (atomic number 53) has a higher priority than carbon (atomic number 6) due to its greater atomic number.

In the context of the exercise, understanding that chlorine (atomic number 17) has a higher priority than oxygen (atomic number 8) because of its higher atomic number is crucial to correctly solve part (a) of the problem. As seen in the step-by-step solution, this rule is systematically applied to determine the priority sequence for all the pairs of groups given.

Functional Group Priority

In addition to atomic number priority, functional group priority becomes particularly important when the same atom is present in different functional groups. Functional groups with multiple bonds, such as carbonyls (-CHO), have higher priority than those with single bonds (-CH2OH), as the multiple bonds effectively count as being bonded to more atoms of the same kind.

This is evident in the step-by-step solution for pair (d), where the carbonyl group with a double bond to oxygen (-CHO) is given priority over a single bonded hydroxyl group (-CH2OH). This rule ensures that the priority is correctly assigned, respecting the presence and nature of the functional groups attached to the chiral center. Considering such nuances is imperative for accurate determination of R and S configurations in organic molecules, demonstrating the importance of understanding functional group priority in stereochemistry.