Question

In the presence of light, chlorine can substitute for one (or more) of the hydrogens in an alkane. For the following reactions, draw the possible monochlorination products. a. 2,2 -dimethylpropane \(+\mathrm{Cl}_{2} \stackrel{\mathrm{hv}}{\longrightarrow}\) b. 1,3 -dimethylcyclobutane \(+\mathrm{Cl}_{2} \stackrel{\text { hv }}{\longrightarrow}\) c. 2,3 -dimethylbutane \(+\mathrm{Cl}_{2} \stackrel{\mathrm{hv}^{-}}{\longrightarrow}\)

Short answer

a. 2,2-dimethylpropane: The monochlorination product is 2-chloro-2,2-dimethylpropane (or 1-chloro-2,2-dimethylpropane). b. 1,3-dimethylcyclobutane: The monochlorination products are 1-chloro-1,3-dimethylcyclobutane and 1-chloro-3,3-dimethylcyclobutane. c. 2,3-dimethylbutane: The monochlorination products are 1-chloro-2,3-dimethylbutane, 2-chloro-2,3-dimethylbutane (or 2-chloro-3,3-dimethylbutane), and 1-chloro-3,3-dimethylpentane.

Step-by-step solution

a. 2,2-dimethylpropane with Cl2 in the presence of light

First, draw the structure of 2,2-dimethylpropane (also known as neopentane): It has a central carbon atom bonded to three methyl groups and one hydrogen atom. All the methyl groups are identical, so there is only one type of hydrogen to be substituted. Due to the symmetry, all the methyl groups will give the same product when chlorinated. Now, draw the product formed when one H in any of the methyl groups of 2,2-dimethylpropane is replaced by a chlorine atom (Cl). You will end up with 2-chloro-2,2-dimethylpropane or 1-chloro-2,2-dimethylpropane (both names represent the same structure).

b. 1,3-dimethylcyclobutane with Cl2 in the presence of light

First, draw the structure of 1,3-dimethylcyclobutane: It has a cyclobutane ring with methyl groups on carbons 1 and 3. There are two types of hydrogens present: those attached to carbons 1 and 3 (secondary), and those attached to carbons 2 and 4 (primary). Now, draw two different products formed by replacing one hydrogen atom on carbon 1 or 3 with a chlorine atom, and another product formed by replacing one hydrogen atom on carbon 2 or 4 with a chlorine atom. You will have 1-chloro-1,3-dimethylcyclobutane, and 1-chloro-3,3-dimethylcyclobutane as products.

c. 2,3-dimethylbutane with Cl2 in the presence of light

First, draw the structure of 2,3-dimethylbutane: It has a linear carbon chain with four carbon atoms, and methyl groups are attached to carbons 2 and 3. There are three types of hydrogens present: those attached to carbon 1 (primary), those attached to carbon 4 (primary), and those attached to carbons 2 and 3 (secondary). Now, draw three different products formed by replacing one hydrogen atom on carbon 1, carbon 4, and on carbon 2 or 3 with a chlorine atom. You will have 1-chloro-2,3-dimethylbutane, 2-chloro-2,3-dimethylbutane (or 2-chloro-3,3-dimethylbutane, both names represent the same structure), and 1-chloro-3,3-dimethylpentane as products.

Key concepts

Free Radical Halogenation

Free Radical Halogenation is a fascinating reaction in organic chemistry that involves breaking molecules into radicals and forming new bonds. This process occurs when a halogen, such as chlorine (\(Cl_2\)), reacts with an alkane in the presence of light or heat. The reaction progresses through a series of steps, beginning with the homolytic cleavage of the halogen bond. This means the bond between two halogen atoms breaks evenly, forming two radicals. Radicals are atoms or molecules that have an unpaired electron, which makes them highly reactive.
- **Initiation:** Light or heat initiates the breakdown of \(Cl_2\) into two chlorine radicals. These radicals are ready to react with other molecules to form new radicals.
- **Propagation:** This involves the stepwise reaction between the newly formed chlorine radicals and the alkane, resulting in the formation of a new radical and more stable molecules. For example, a chlorine radical can abstract (remove) a hydrogen atom from the alkane, creating a carbon radical and hydrochloric acid (HCl). The carbon radical can then react with another chlorine molecule, stabilizing the radical by forming a new carbon-halogen bond.
- **Termination:** This stage involves the combination of two radicals to form a stable, non-radical product. Since radicals are short-lived and reactive, they tend to combine to stabilize the reaction mixture again. This step prevents further reaction and finalizes the product formation.

Alkane Substitution Reaction

Alkane Substitution Reactions are a key concept in organic chemistry, particularly involving saturated hydrocarbons like alkanes. These reactions allow one atom to replace another atom in a compound, specifically focusing on hydrogen atoms in alkanes.
- In substitution reactions, a halogen atom is introduced to an alkane, leading to the replacement of one hydrogen atom in the alkane with a halogen atom. This is a transformation that is commonly done through halogenation processes.
- The reaction typically involves a direct interaction between the alkane and a diatomic halogen, like \(Cl_2\), often facilitated by light or heat. This interaction permits a chlorine atom to replace hydrogen, transforming the alkane into a haloalkane (alkyl halide).
- The alkane substitution reaction is characterized by its selectivity. The position at which substitution occurs in the alkane depends on the type of hydrogens present, with tertiary hydrogens generally reacting more readily than secondary or primary ones due to the stability of the resulting radicals. This stability affects the distribution of resulting products, especially in molecules with different types of hydrogen atoms.

Monochlorination

Monochlorination refers to the process wherein only one hydrogen atom in an alkane is replaced by a chlorine atom. It's one of the simpler forms of halogenation reactions, facilitating the study of alkane reactivity and structure elucidation.
- **Understanding Symmetry:** In alkanes where all hydrogen atoms are equivalent, such as in 2,2-dimethylpropane, monochlorination produces a single product. The symmetry simplifies the reaction as each hydrogen atom substitution results in the same product.
- **Diversity in Products:** In molecules like 1,3-dimethylcyclobutane, monochlorination can yield different products depending on which hydrogen atom is replaced due to the presence of different types of hydrogen atoms—primary, secondary, or tertiary.
- This reaction is controlled by the stability of the potential radicals, affecting the reaction path and products. For instance, hydrogen atoms attached to secondary carbon atoms are more prone to get replaced as they lead to more stable radical intermediates relative to primary carbon atoms.

Light-initiated Reactions

Light-initiated Reactions are a significant principle in organic reactions where light energy is used to trigger a chemical reaction. This phenomenon is particularly effective in reactions involving halogens like chlorine.
- **Role of Light:** The energy from light initiates the reaction by inducing the homolytic cleavage of the chlorine molecule, leading to the formation of two chlorine radicals. These radicals further engage in the reaction, leading to product formation. Without the presence of light or heat, the reaction would not proceed efficiently due to the lack of radical formation at room temperature.
- **Catalytic Role:** In many reactions, light is akin to a catalyst that helps overcome the activation energy barrier. It doesn't get consumed in the reaction; instead, it facilitates the reaction process.
- **Practical Implications:** In organic synthesis, such reactions are advantageous because they allow specific and controlled transformations under relatively mild conditions. For students, understanding light's role in initiating reactions enhances their grasp of how energy influences chemical processes, enabling mastery over reaction manipulations in lab settings.