Isopentane is allowed to undergo free-radical chlorination, and the reaction mixture is separated by careful fractional distillation. (a) How many fractions of formula \(\mathrm{C}_{5} \mathrm{H}_{11} \mathrm{CI}\) would you expect to collect? Draw structural formulas, stereochemical where pertinent, for the compounds making up each fraction. Specify each enantiomer as \(\mathrm{R}\) or \(\mathrm{S}\). (b) Which if any, of the fractions, as collected, would show optical activity? Account in detail for the optical activity or inactivity of each fraction.

Isopentane chlorination is a fundamental reaction in organic chemistry where the hydrogen atoms of isopentane (2-methylbutane) are replaced by chlorine atoms through a free-radical mechanism. During this reaction, various positions on the isopentane molecule can be chlorinated, leading to different chlorinated products. Specifically, chlorination can occur at both primary and secondary hydrogens, yielding 2-chloro-2-methylbutane, 2-chloro-3-methylbutane, and 1-chloro-3-methylbutane.

This distinction between primary and secondary chlorinations is important because it affects the reactivity and the type of products formed. Primary chlorination occurs at the end methyl groups, while secondary chlorination takes place at the middle carbon, which is connected to two other carbons. The secondary chlorination is particularly noteworthy as it introduces a chiral center into the molecule, leading to the creation of enantiomers.

Furthermore, because the reactivity of primary and secondary hydrogens differs, the amount of each chlorinated product formed varies, influencing the output of the chlorination reaction. Careful fractional distillation can then be used to separate the different chlorinated isomers based on their boiling points.

Enantiomers are mirror-image isomers of a molecule that cannot be superimposed on one another, much like a person's left and right hands. This phenomenon occurs in compounds that have at least one chiral center—an atom, usually carbon, with four different substituents attached to it. When isopentane undergoes chlorination, it can form a chiral center, leading to the formation of enantiomers depending on where the chlorination occurs in the molecule.

The R (from Latin 'rectus', meaning right) and S (from Latin 'sinister', meaning left) configuration of enantiomers is determined by the Cahn-Ingold-Prelog priority rules, which assign a hierarchical sequence to the substituents attached to the chiral center. When writing out chemical structures, specifying each enantiomer as R or S helps chemists understand the three-dimensional arrangement of the atoms in the molecule. This configuration is crucial for understanding the behavior of molecules in biological systems and for the synthesis of pharmaceuticals, as the different enantiomers of a drug can have very different effects on the body.

In the context of isopentane chlorination, 2-chloro-3-methylbutane can exist as R or S enantiomers due to the chiral center at the second carbon atom. If a mixture contains equal amounts of R and S enantiomers, it is called a racemic mixture and shows no optical activity. However, an unequal mixture of these enantiomers will display optical activity because they rotate plane-polarized light differently.

Fractional distillation is a technique commonly used in organic chemistry to separate mixtures of compounds based on their boiling points. This process is essential for purifying chemicals and isolating different components from a chemical reaction, as is the case with the chlorination of isopentane.

The procedure involves heating a mixture until the component with the lowest boiling point vaporizes. This vapor passes up a distillation column and is cooled, causing it to condense into a liquid that is collected separately. The process leverages the fact that different compounds boil at distinct temperatures.

In the case of separating chlorinated isopentane, fractional distillation would be employed to separate the different fractions—2-chloro-2-methylbutane, 2-chloro-3-methylbutane, and 1-chloro-3-methylbutane—owing to their different boiling points. The efficiency of the separation depends on the lengths of the distillation column and the boiling point differences of the compounds. Non-chiral and chiral compounds can be effectively separated using this process. However, fractional distillation cannot separate enantiomers, as they have identical physical properties such as boiling points. For enantiomer separation, specific optical resolution techniques are required.