Question

Assign priorities to the groups in each set. (a) \(-\mathrm{CH}_{2} \mathrm{OH}\) and \(-\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) (b) \(-\mathrm{CH}_{2} \mathrm{OH}\) and \(-\mathrm{CH}=\mathrm{CH}_{2}\) (c) \(-\mathrm{CH}_{2} \mathrm{OH}\) and \(-\mathrm{C}\left(\mathrm{CH}_{3}\right)_{3}\)

Short answer

Answer: In all three sets, the group \(-\mathrm{CH}_{2}\mathrm{OH}\) has a higher priority.

Step-by-step solution

Set (a): Comparison of \(-\mathrm{CH}_{2}\mathrm{OH}\) and \(-\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{OH}\)

First, we compare the first atom in each group, which is carbon in both cases. Since both are carbon, they have the same atomic number, and we have a tie. Now, we move on to comparing the connected atoms. In the first group, carbon is bonded to an oxygen, while in the second group, the carbon is bonded to another carbon. Since oxygen has a higher atomic number (8) than carbon (6), the group \(-\mathrm{CH}_{2}\mathrm{OH}\) has higher priority.

Set (b): Comparison of \(-\mathrm{CH}_{2}\mathrm{OH}\) and \(-\mathrm{CH}=\mathrm{CH}_{2}\)

Again, the first atoms in each group are both carbon, so it results in a tie. For the first group, carbon is bonded to an oxygen atom, while for the second group it is bonded to another carbon atom with a double bond. Since oxygen has a higher atomic number (8) than carbon (6), the group \(-\mathrm{CH}_{2}\mathrm{OH}\) has a higher priority.

Set (c): Comparison of \(-\mathrm{CH}_{2}\mathrm{OH}\) and \(-\mathrm{C}\left(\mathrm{CH}_{3}\right)_{3}\)

The first atoms are carbon in both cases, creating a tie. The first group has carbon bonded to oxygen, while the second group has carbon bonded to three other carbon atoms. Since oxygen has a higher atomic number (8) than carbon (6), the group \(-\mathrm{CH}_{2}\mathrm{OH}\) has higher priority.

Key concepts

Alkyl Group Priority

In organic chemistry, alkyl group priority becomes an important consideration when we are comparing molecules or groups of atoms attached to a central atom, particularly in stereochemistry and naming organic compounds using the IUPAC nomenclature. An alkyl group consists of carbon and hydrogen atoms arranged in a chain. The priority of an alkyl group is typically determined by its size and the number of carbon atoms it contains.

As seen in the provided exercise, when comparing \( -\mathrm{CH}_2\mathrm{OH} \) with \( -\mathrm{CH}_2\mathrm{CH}_2\mathrm{OH} \), even though the latter contains more carbon atoms, priority goes to the group with a functional group, like OH, that has atoms with higher atomic number attached directly to the stereo-center. It's essential to consider the whole group and not just the immediate alkyl chain when assigning priority.

Atomic Number Priority

Atomic number priority is a fundamental rule in assigning precedence between different atoms attached to a chiral center or in determining the order of groups in an isomeric structure. It is a simple yet powerful guideline based on the atomic number of the elements involved. The higher an atom's atomic number, the higher its priority. In each part of the given exercise, the atomic number priority rule is applied after finding a tie with the first comparison point. In the example involving \( -\mathrm{CH}_2\mathrm{OH} \) and \( -\mathrm{CH}_2\mathrm{CH}_2\mathrm{OH} \), both attached carbons have the same atomic number, but the oxygen in the first group has a higher atomic number than the additional carbon in the second, thereby giving the first group higher priority.

Functional Group Priority

Functional group priority is closely tied with atomic number priority, but it specifically deals with the priority of functional groups over alkyl chains in a molecule. Functional groups are the reactive parts of molecules and often determine the molecule's properties and reactivity. They are prioritized over mere carbon chains due to their influence on a molecule's chemical behavior. For instance, in the exercise, the hydroxyl (OH) group is a functional group and is given higher priority over an alkyl chain because oxygen has a higher atomic number than carbon, making \( -\mathrm{CH}_2\mathrm{OH} \) a higher priority compared with \( -\mathrm{C}(\mathrm{CH}_3)_3 \) despite the latter having more carbon atoms.

R/S Configuration

The R/S configuration is a notation used to describe the absolute configuration of chiral centers in molecules. 'R' stands for 'rectus', which is Latin for 'right', indicating a clockwise configuration, while 'S' stands for 'sinister', Latin for 'left', representing a counterclockwise configuration. To determine the R/S configuration, first assign priorities to the groups attached to the chiral center based on atomic number priority, functional group priority, and alkyl group priority. Then, orient the molecule so that the lowest-priority group is pointed away from you. Trace a path from the highest to the lowest ranked group according to the assigned priorities; the direction of this path determines the R or S configuration. The step-by-step exercise provided helps students learn to assign these priorities, which is crucial when it comes to determining the R/S configuration of a chiral center.