Question

A hydrocarbon \(\mathrm{C}_{5} \mathrm{H}_{10}\) does not react with chlorine in dark but gives a single monochloro compound \(\mathrm{C}_{5} \mathrm{H}_{9} \mathrm{Cl}\) in bright sunlight. Identify the hydrocarbon.

Short answer

The hydrocarbon is 2-methyl-2-butene.

Step-by-step solution

Understand the Given Information

The hydrocarbon has the formula \(\mathrm{C}_{5}\mathrm{H}_{10}\), which indicates it might be an alkene (unsaturated hydrocarbon with a double bond) since the formula fits \(\mathrm{C}_{n}\mathrm{H}_{2n}\). We are given that it produces a single monochloro compound \(\mathrm{C}_{5}\mathrm{H}_{9}\mathrm{Cl}\) when exposed to bright sunlight.

Analyze Reactivity with Chlorine

The compound does not react with chlorine in the dark, indicating it is likely unsaturated, because saturated hydrocarbons (alkanes) typically react with chlorine even in the dark. The reactivity in sunlight forms a single monochloro compound, suggesting a unique position of the double bond.

Consider Possible Hydrocarbons

Acyclic alkenes that fit the \(\mathrm{C}_{5}\mathrm{H}_{10}\) formula include 1-pentene, 2-pentene, and 2-methyl-2-butene. Each can be isomerized based on the position of the double bond and branching.

Determine the Most Likely Isomer

Since only one monochloro compound is produced, the structure likely has symmetrical elements or a specific double bond location that limits the number of chlorination products. The symmetrical structure of 2-methyl-2-butene only allows for substitution at one type of carbon (primary), which leads to a single chloroproduct.

Key concepts

Understanding Hydrocarbons

Hydrocarbons are organic compounds composed entirely of hydrogen and carbon atoms. They form the backbone of organic chemistry. Depending on their bonding and structure, hydrocarbons can be classified into several types. Here are a few essential aspects about them:

• Saturated Hydrocarbons (Alkanes): These contain only single bonds between carbon atoms. They are quite stable and generally exhibit fewer chemical reactions.
• Unsaturated Hydrocarbons: This category includes alkenes (with double bonds) and alkynes (with triple bonds). Their unique bonding makes them more reactive than alkanes.
• Aromatic Hydrocarbons: These are hydrocarbons that contain conjugated pi-bond systems, and they often form rings. Benzene is a classic example.

Understanding the type of hydrocarbon is crucial to predicting its chemical behavior and reactivity, especially in reactions involving elements like chlorine.

Exploring Alkenes

Alkenes are a vital class of hydrocarbons characterized by the presence of at least one carbon-carbon double bond. This feature sets them apart from alkanes and allows them diverse chemical reactivity.

The general formula for alkenes is \(C_{n}H_{2n}\), indicating that for every n carbon atoms, there are 2n hydrogen atoms. This ratio reflects the presence of unsaturation due to the double bond. Alkenes are considered unsaturated because they contain fewer hydrogen atoms than the saturated hydrocarbons.

• Reactivity: The double bond is electron-rich, making alkenes nucleophilic and reactive in electrophilic addition reactions. Typical reactions include hydrogenation, halogenation, and hydrohalogenation.
• Isomers: Alkenes can have structural isomers depending on the position of the double bond. For example, \(C_{5}H_{10}\) can be represented as 1-pentene, 2-pentene, or 2-methyl-2-butene.
• Geometric Isomers: The rigidity of the double bond can give rise to cis-trans isomerism (or E-Z isomerism), adding another layer of complexity to alkenes.

Understanding alkenes aids in predicting the outcomes of reactions such as chlorination, especially under varying conditions like sunlight exposure.

The Chlorination Reaction

Chlorination is a type of halogenation reaction where chlorine atoms are added to a molecule. This process can vary significantly based on the conditions under which it occurs.

When chlorination involves alkenes, the double bond opens up, allowing chlorine atoms to add across the bond, forming chlorinated hydrocarbons.

• Conditions: Chlorination can occur in the presence of light or heat. In the dark, alkanes will react slowly, while alkenes typically require energy input from light to promote reaction.
• Selectivity: In reactions like those involving \(C_{5}H_{10}\), sunlight enables selective chlorination, often resulting in a single type of monochloro compound due to unique structural features. For example, in 2-methyl-2-butene, the symmetrical structure ensures only one position for chlorination, limiting the number of chlorinated products.

The chlorination of alkenes is an important transformation in organic chemistry allowing for the synthesis of various useful chlorinated compounds.

Identifying Isomers

Isomers are compounds that share the same molecular formula but differ in structure. The concept of isomerism is crucial for understanding the diverse properties and behaviors of organic molecules, including alkenes.

In the context of alkenes, two major types of isomerism can occur:

• Structural Isomerism: This involves variation in the connectivity of atoms. An example is changing the location of a double bond in a hydrocarbon chain, such as between 1-pentene and 2-pentene.
• Geometric Isomerism: When possible, the spatial orientation around the double bond can differ, resulting in cis (same side) or trans (opposite side) configurations.

In certain reactions, only one type of product may be formed if the starting material possesses symmetry or selective reactivity. For instance, 2-methyl-2-butene’s structure allows for only one monochlorinated product, illustrating how isomerism influences chemical behavior.
Understanding isomers is essential for grasping the nuanced outcomes in reactions like chlorination, as it ensures that predictions about the products are accurate and complete.