Question

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.

Short answer

We expect to collect three fractions of C5H11Cl from the free-radical chlorination of isopentane. The compounds in these fractions are 2-chloro-2-methylbutane, R-2-chloro-3-methylbutane and S-2-chloro-3-methylbutane, and 1-chloro-3-methylbutane. Only fraction 2, containing the R and S enantiomers of 2-chloro-3-methylbutane, has the potential to show optical activity depending on the proportion of each enantiomer. Fractions 1 and 3 do not possess chiral centers and will not show optical activity.

Step-by-step solution

Draw the structure of isopentane (2-methylbutane)

Isopentane, also known as 2-methylbutane, has the molecular formula C5H12. Its structure has a methyl group attached to the second carbon of a butane chain: ``` CH3 | CH3-CH-CH2-CH3 ```

Identify possible chlorination positions for C5H11Cl

In a free-radical chlorination reaction, a hydrogen atom is replaced by a chlorine atom. There are three possible positions for chlorination of isopentane: 1. A hydrogen atom from the CH3 group attached to the second carbon atom (primary position). 2. A hydrogen atom from the central CH group (secondary position). 3. A hydrogen atom from any of the terminal CH3 groups (primary position).

Determine the number of fractions of C5H11Cl collected

Since there are three different positions for chlorination, we expect to collect three fractions of C5H11Cl.

Draw structural formulas for the compounds in each fraction, including stereochemistry (R/S)

Now we will draw the structures for the three fractions corresponding to the three chlorine substitution positions: Fraction 1 - Chlorination at CH3 group attached to the second carbon atom: ``` Cl | CH3-C-CH2-CH3 | CH3 ``` This compound is 2-chloro-2-methylbutane and has no chiral centers, so there is no need to assign R/S configuration. Fraction 2 - Chlorination at the central CH group: ``` CH3 | CH3-CH-C-CH3 | Cl ``` This compound is 2-chloro-3-methylbutane and has one chiral center at the second carbon atom. Since it has a chiral carbon, it can exist as two enantiomers: R-2-chloro-3-methylbutane and S-2-chloro-3-methylbutane. Fraction 3 - Chlorination at any terminal CH3 group: ``` Cl | CH3-CH2-C-CH3 | CH3 ``` This compound is 1-chloro-3-methylbutane and has no chiral centers, so there is no need to assign R/S configuration.

Identify fractions showing optical activity and provide reasoning

Optical activity is observed in compounds having chiral centers when there is not an equal mixture of R and S enantiomers (a racemic mixture). Now we will analyze optical activity in the fractions: Fraction 1: The 2-chloro-2-methylbutane compound has no chiral centers, so it cannot show optical activity. Fraction 2: Since this fraction contains R-2-chloro-3-methylbutane and S-2-chloro-3-methylbutane enantiomers, it can show optical activity if there is an unequal mixture of R and S enantiomers. However, we cannot determine the exact proportion of each enantiomer from the information given in the question. Fraction 3: The 1-chloro-3-methylbutane compound has no chiral centers, so it cannot show optical activity. In conclusion, only fraction 2 has the potential to show optical activity, depending on the proportion of R and S enantiomers. Fractions 1 and 3 will not show optical activity.

Key concepts

Isopentane Chlorination

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 R and S Configuration

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 in Organic Chemistry

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.