Question

Arrange the following in decreasing order of priority according to Cahn-Ingold Prelog (CIP) system. (I) \(-\mathrm{CH}_{3}\) (II) \(-\mathrm{CH}_{2} \mathrm{~F}\) (III) \(-\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Cl}\) \((\mathrm{IV})-\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}\) (A) \(\mathrm{I}>\mathrm{II}>\mathrm{III}>\mathrm{IV}\) (B) IV > III > II > I (C) II > III > IV > I (D) IV \(>\) II \(>\) III \(>\) I

Short answer

The short answer for the problem is: \(IV > III > II > I\) (Option B).

Step-by-step solution

Determine the atomic numbers

Look up the atomic numbers of the atoms that are directly bonded to Carbon in every group: - Hydrogen: 1 - Carbon: 6 - Fluorine: 9 - Chlorine: 17 - Bromine: 35

Assign the first layer priority

The Carbon atom in each group is bonded to a different atom. Using the atomic numbers, assign the priority: - Group I: Carbon is bonded to three Hydrogen atoms. - Group II: Carbon is bonded to a Fluorine atom. - Group III: Carbon is bonded to a Carbon atom, which is bonded to a Chlorine atom. - Group IV: Carbon is bonded to a Carbon atom, which is bonded to another Carbon atom, and that third Carbon atom itself is bonded to a Bromine atom.

Priority order based on Atomic numbers

Observe that in the first layer (directly bonded atoms), highest atomic number gets the highest priority. Following this, the order will be II > I. For groups III and IV, since Carbon is the next atom, we need to move to the next layer. Here, group III has Chlorine and group IV has Bromine. Following again the atomic number rule, we will have IV > III. So, in the end, the decreasing order of priority is: IV > III > II > I. Therefore, choice B is the correct answer.

Key concepts

Organic Chemistry

Organic chemistry is the branch of chemistry that deals with the structure, properties, and reactions of compounds that contain carbon. It plays a crucial role in the development of everyday products such as pharmaceuticals, plastics, and fuels. Central to organic chemistry is the study of the carbon atom, which is versatile due to its ability to form four covalent bonds with other atoms, allowing for the creation of complex molecules.

Understanding the types of bonds and the molecular structure of organic compounds is essential for predicting how they will react. This includes recognizing functional groups like alcohols, ketones, and carboxylic acids, which possess distinctive reactivity patterns. One of the complexities of organic molecules is their ability to exist in different forms, which may look similar but have distinct physical and chemical properties. This leads us to the concept of stereochemistry.

Stereochemistry

Stereochemistry involves the study of the spatial arrangement of atoms in molecules and how this influences the properties and reactivity of the molecules. An understanding of stereochemistry is essential for appreciating the behavior of many biological molecules and pharmaceuticals that exhibit different effects based on their three-dimensional arrangement.

Isomerism is a key aspect of stereochemistry; isomers are compounds with the same molecular formula but different structures. Specifically, stereoisomers are isomers that differ only in the spatial orientation of their atoms. Within stereoisomers, there are enantiomers, which are mirror images of each other and cannot be superimposed, and diastereomers, which are not mirror images.

The challenge in stereochemistry is not just identifying these isomers but also predicting how each will behave in chemical reactions. This challenge extends to assigning priority to substituents attached to a chiral center, which is where the Cahn-Ingold-Prelog (CIP) system comes into play.

Atomic Number Priority

The Cahn-Ingold-Prelog system is the standard used for determining the priority of substituents attached to a chiral center, based on the concept of atomic number priority. In organic molecules, atoms are replaced by various substituents, which can impact the molecule's reactivity and physical properties. To understand reactions or properties of these molecules, chemists need a reliable method to denote the configuration around chiral centers.

The priority rule of the Cahn-Ingold-Prelog system states that the higher the atomic number of the immediate atom bonded to the chiral center, the higher the priority. For instance, a substituent bonded to the carbon atom with an iodine atom (which has a higher atomic number) will be given a higher priority than a substituent bonded to a chlorine atom. This rule can extend to the next atoms in line if the immediately bonded atoms are identical. It's a systematic way to distinguish between stereoisomers, especially when defining absolute configurations as 'R' (rectus) or 'S' (sinister).