Problem 25 - Chemistry The Molecular Science

Question

From their molecular formulas, classify each of these straight-chain hydrocarbons as an alkane, an alkene, or an alkyne. (a) \(\mathrm{C}_{5} \mathrm{H}_{8}\) (b) \(\mathrm{C}_{24} \mathrm{H}_{50}\) (c) \(\mathrm{C}_{7} \mathrm{H}_{14}\)

Step-by-Step Solution

Answer

(a) Alkyne, (b) Alkane, (c) Alkene.

Step 1: Understanding Alkane, Alkene, and Alkyne

Alkanes are saturated hydrocarbons with the general formula \( C_nH_{2n+2} \). Alkenes are unsaturated hydrocarbons with one double bond, following the formula \( C_nH_{2n} \). Alkynes, having one triple bond, follow the formula \( C_nH_{2n-2} \). Understanding these formulas is crucial to classifying the hydrocarbons.

Step 2: Classifying Part (a)

Given \( \text{C}_5\text{H}_8 \), we first determine if it fits any of the general formulas. For an alkane, using \( n=5 \), the formula becomes \( C_5H_{12} \), which does not match. For an alkene, \( C_5H_{10} \) which also doesn't fit. For an alkyne, it matches \( C_5H_8 \) using \( C_nH_{2n-2} \). Thus, \( C_5H_8 \) is an alkyne.

Step 3: Classifying Part (b)

Given \( \text{C}_{24}\text{H}_{50} \), we apply the formulas. For an alkane, \( C_{24}H_{50} \) corresponds with the formula \( C_nH_{2n+2} \) when \( n=24 \), because \( 2(24) + 2 = 50 \). Thus, \( C_{24}H_{50} \) is an alkane.

Step 4: Classifying Part (c)

For \( \text{C}_7\text{H}_{14} \), we check the alkene formula \( C_nH_{2n} \), and \( n=7 \). It satisfies \( 2(7) = 14 \), matching the given formula. Therefore, \( C_7H_{14} \) is classified as an alkene.

Key Concepts

Alkane

Alkanes are the simplest type of hydrocarbons, known for being saturated, which means they contain only single bonds between carbon atoms. This saturation makes them very stable, and they follow a specific molecular formula:

• General formula: \( C_nH_{2n+2} \)
• Characteristics: Only single bonds (\( C-C \))
• Simplest example: Methane (\( CH_4 \))

These substances are often found in natural sources such as crude oil and are commonly used as fuels. To classify a compound as an alkane, such as in Part (b) of the exercise where we have \(C_{24}H_{50}\), you substitute \(n = 24\) into the formula: \(2(24) + 2 = 50\) . It matches perfectly, confirming \(C_{24}H_{50}\) is indeed an alkane. In daily life, hydrocarbons like alkanes are critical in creating energy, as they burn to release energy, making fuels like gasoline and natural gas crucial in powering engines and heating homes.

Alkene

Alkenes are unsaturated hydrocarbons that contain at least one carbon-to-carbon double bond. This double bond gives alkenes unique reactivity characteristics, making them more flexible and reactive than alkanes. Their general formula is:

• General formula: \( C_nH_{2n} \)
• Characteristics: Contains one double bond (\( C=C \))
• Simplest example: Ethene (\( C_2H_4 \))

For these hydrocarbons, the presence of a double bond allows for reactions such as polymerization, leading to materials like plastics. In Part (c) of the exercise, \( C_7H_{14} \) fits the alkene formula \( n = 7 \), and thus \( 2(7) = 14 \), making \( C_7H_{14} \) an alkene.

Alkyne

Alkynes are another class of unsaturated hydrocarbons, characterized by the presence of a carbon-to-carbon triple bond. This triple bond imparts even greater reactivity compared to alkenes. The general formula for alkynes is:

• General formula: \( C_nH_{2n-2} \)
• Characteristics: Contains one triple bond (\( C≡C \))
• Simplest example: Ethyne (commonly known as acetylene, \( C_2H_2 \))

They are often used in welding torches because the combustion of alkynes such as acetylene produces a very hot flame. In Part (a) of the exercise, for \( C_5H_8 \), the molecular formula fits an alkyne as \( n = 5 \), with \( 2(5) - 2 = 8 \), confirming that \( C_5H_8 \) is classified as an alkyne.