Question

Name and draw structural formulas for all monochlorination products formed by treatment of 2 -methylpropane with \(\mathrm{Cl}_{2}\). Predict the major product based on the regioselectivity of the reaction of \(\mathrm{Cl}_{2}\) with alkanes.

Short answer

Answer: The monochlorination products of 2-methylpropane are 1-chloro-2-methylpropane and 2-chloro-2-methylpropane. The major product is 2-chloro-2-methylpropane, due to the higher stability of the tertiary carbocation intermediate formed during the reaction.

Step-by-step solution

Identify the structure of 2-methylpropane

First, we need to draw the structure of 2-methylpropane. 2-methylpropane is a four-carbon alkane with a methyl group attached to its second carbon atom. Its structure is as follows: CH\(_{3}\)-C(CH\(_{3}\))\(_{2}\)-CH\(_{3}\).

Determine the types of hydrogen atoms in 2-methylpropane

Now, we need to identify the types of hydrogen atoms in the structure, based on their bonding environment. In 2-methylpropane, there are two types of hydrogens: primary (1\(^{\circ}\)) and tertiary (3\(^{\circ}\)) hydrogen atoms. Primary hydrogen is bonded to the first carbon atom, whereas tertiary hydrogen is bonded to the central carbon atom. There are 9 tertiary hydrogens (3 on each central methyl group) and 3 primary hydrogens (all on the end methyl group).

Monochlorination of 2-methylpropane

Since there are two types of hydrogen atoms (primary and tertiary), monochlorination of 2-methylpropane can produce two different products: 1-chloro-2-methylpropane (1\(^\circ\)) and 2-chloro-2-methylpropane (3\(^\circ\)). The structural formulas for these products are: 1-chloro-2-methylpropane: CH\(_{3}\) - CH(Cl)CH(CH\(_3\))\(_2\) 2-chloro-2-methylpropane: CH\(_3\) - C(Cl)(CH\(_3\))\(_2\) - CH\(_3\)

Consider regioselectivity to predict the major product of monochlorination

Inhalogenation reactions of alkanes, regioselectivity is determined by the stability of carbocation intermediates formed during the reaction. In the case of 2-methylpropane, the tertiary carbocation intermediate is more stable than the primary carbocation intermediate. As a result, the reaction prefers to replace the more stable, tertiary hydrogen atoms with chlorine. Therefore, the major product of monochlorination of 2-methylpropane is 2-chloro-2-methylpropane.

Key concepts

2-Methylpropane Structure

Understanding the structure of 2-methylpropane is crucial before delving into chemical reactions. 2-methylpropane, also known as isobutane, is an alkane with the chemical formula (C₄H₁₀). It contains four carbon atoms arranged so that one of the central carbon atoms is connected to three other carbons, making it a branched molecule. The central carbon atom, being quaternary, carries three methyl groups (CH₃), while the fourth, a primary carbon, is part of the main chain terminating in a methyl group. This molecule's structure defines its physical and chemical properties, including its reaction pathways during halogenations like monochlorination.

Visualizing the molecule, you might imagine a central carbon atom with three other carbon atoms sprouting from it like branches on a tree. Each of these 'branches' is a methyl group. Because of its symmetrical nature, certain reactions on 2-methylpropane can lead to fewer distinct isomers compared to its unbranched counterparts.

Regioselectivity in Halogenation

Regioselectivity is a key concept in organic chemistry that refers to the preference of a chemical reaction to occur at a particular position or region within a molecule over others. When dealing with halogenation, regioselectivity plays a significant role because halogens can potentially attach to multiple locations on an alkane.

Inhalogenation reactions, such as the monochlorination of alkanes, the formation of intermediates called carbocations largely determines regioselectivity. Carbocations are positively charged carbon atoms that occur transiently during the reaction process. Their stability varies depending on the molecular environment, with tertiary carbocations being more stable than secondary, which in turn are more stable than primary carbocations. Therefore, the carbon bonded to the greatest number of alkyl groups (which in 2-methylpropane's case is the quaternary central carbon) is the most favorable site for halogenation because it can form the most stable tertiary carbocation intermediate.

This preference is due to hyperconjugation and the inductive effect of alkyl groups stabilize the positive charge. Understanding regioselectivity helps in predicting the major product of a halogenation reaction, making it a valuable tool for chemists.

Primary and Tertiary Hydrogens

Primary (1^o) and tertiary (3^o) hydrogens in alkanes refer to the hydrogen atoms bonded to primary and tertiary carbons respectively. Their reactivity towards halogenation is considerably different due to differences in the stabilizing effects they confer to carbocations.

• Primary hydrogen atoms are attached to a carbon that is bonded to only one other carbon. These hydrogens are less reactive towards halogenation because their removal would result in a less stable primary carbocation.
• Tertiary hydrogen atoms, on the other hand, are bonded to a carbon that is connected to three other carbons. This makes them more reactive because their removal leads to the formation of a much more stable tertiary carbocation intermediate.

Monochlorination of 2-methylpropane primarily targets the tertiary hydrogen atoms due to their tendency to stabilize the resulting carbocation. The presence of nine tertiary hydrogens in the molecule increases the likelihood that they will be the site for chlorine attachment, as opposed to the three primary hydrogens. Consequently, this affects the outcome of the reaction, favoring the formation of 2-chloro-2-methylpropane as the majority product.