Question

Directions: This section contains 3 paragraphs. Based upon the paragraph, 3 multiple choice questions have to be answered. Each question has 4 choices (a), (b), (c) and (d), out of which ONLY ONE is correct. Passage I The three main classes of hydrocarbons are alkanes, alkenes and alkynes. Alkanes having cyclic structure are called cycloalkanes. They exhibit chain and ring chain isomerism. Alkenes and alkynes are more reactive than alkanes. A certain hydrocarbon has the molecular formula \(\mathrm{C}_{5} \mathrm{H}_{8} .\) Which of the following is not a structural possibility for this hydrocarbon? (a) a cycloalkane (b) contains a ring and a double bond (c) contains two double bond and no ring (d) an alkyne

Short answer

Answer: (a) a cycloalkane

Step-by-step solution

Identify the Hydrocarbon Class

Based on the molecular formula C5H8, we can determine that it isn't an alkane because it doesn't follow the general formula CnH2n+2. It could be an alkene or alkyne, as they have the general formulas CnH2n and CnH2n-2, respectively.

Analyze the Given Choices

Now we will analyze each choice to see which one cannot be a structural possibility for this hydrocarbon. (a) a cycloalkane: Cycloalkanes have the general formula CnH2n, which matches the given molecular formula C5H8. This is a possible structural possibility. (b) contains a ring and a double bond: A hydrocarbon containing a ring and one double bond would have the general formula CnH2n-2, which also matches the given molecular formula C5H8. This is a possible structural possibility. (c) contains two double bonds and no ring: A hydrocarbon containing two double bonds and no rings would have the general formula CnH2n-2. This also matches the given molecular formula C5H8. This is a possible structural possibility. (d) an alkyne: Alkynes have the general formula CnH2n-2, and in this case, the given molecular formula C5H8 also matches this general formula. This is a possible structural possibility. By analyzing each choice, we can see that all of them are structural possibilities for the given hydrocarbon C5H8 except for Option (a) which is a cycloalkane. The cycloalkane molecular formula doesn't match the given molecular formula. Answer: (a) a cycloalkane

Key concepts

Alkanes

Alkanes are the simplest type of hydrocarbons and consist of carbon and hydrogen atoms with single bonds between them. The general molecular formula for alkanes is CnH2n+2. This class of hydrocarbons is known for being saturated, meaning they have the maximum number of hydrogen atoms per carbon atom.

Examples of alkanes include methane (CH₄), ethane (C₂H₆), and propane (C₃H₈). Alkanes are not as reactive as other classes of hydrocarbons due to the stability of their single bonds. They can undergo reactions such as combustion and halogenation but usually require a higher amount of energy to react compared to alkenes and alkynes.

Alkenes

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond, which gives them different properties than alkanes. Their general molecular formula is CnH2n, indicating they have two fewer hydrogen atoms than an alkane with the same number of carbon atoms. This unsaturation makes alkenes more reactive.

Typical reactions involving alkenes are addition reactions where the double bond opens up to connect with other atoms or molecules. For example, the addition of hydrogen (hydrogenation) turns alkenes into alkanes. Common examples of alkenes are ethene (C₂H₄) and propene (C₃H₆).

Alkynes

Alkynes are characterized by the presence of a carbon-carbon triple bond and have the general molecular formula CnH2n-2. This triple bond makes alkynes even more reactive than alkenes. Alkynes can also participate in addition reactions similar to alkenes but the reactions are typically more exothermic due to the higher energy of the triple bond.

Alkynes are used in organic synthesis and can be hydrogenated to become alkanes or partially hydrogenated to form alkenes. Acetylene (C₂H₂), the simplest alkyne, is widely used as a fuel in welding torches.

Cycloalkanes

Cycloalkanes are hydrocarbons that form a cyclic structure, with carbon atoms joined in a ring. Despite being saturated with single bonds, their general molecular formula is CnH2n, sharing the same formula as alkenes due to the ring structure.

Cycloalkanes are named after the number of carbon atoms in the ring, such as cyclopropane (C₃H₆) and cyclohexane (C₆H₁₂). Unlike their straight-chain counterparts, cycloalkanes may exhibit different types of strain due to the ring structure, which can influence their reactivity.

Structural Isomerism

Structural isomerism occurs when two or more compounds share the same molecular formula but differ in the structure of their atoms. For hydrocarbons, this means that the arrangement of carbon and hydrogen atoms is different, leading to variations in physical and chemical properties.

There are several types of structural isomerism including chain isomerism where carbon chains can be branched differently, and positional isomerism where the position of functional groups or double/triple bonds shifts. This concept clarifies why a given molecular formula, such as C₅H₈, may correspond to multiple structures, be they linear, cyclic or containing different types of bonds.

Hydrocarbon Reactivity

Hydrocarbon reactivity refers to the tendency of a hydrocarbon to engage in chemical reactions. Various factors, including the type of carbon bonds (single, double, triple) and the overall structure of the molecule, affect this reactivity. Alkanes are the least reactive due to their stable single bonds. Alkenes and alkynes, with their double and triple bonds respectively, are more reactive.

The presence of a ring structure can also influence reactivity. For instance, cycloalkanes, despite being saturated, can be more reactive than their acyclic counterparts. External factors, such as the presence of catalysts, light, and heat, as well as the type of reaction (substitution, addition, elimination) also play crucial roles in determining the reactivity of hydrocarbons.