Problem 24 - Chemistry The Central Science

Question

Give the molecular formula of a cyclic alkane, a cyclic alkene, a linear alkyne, and an aromatic hydrocarbon that in each case contains six carbon atoms. Which are saturated and which are unsaturated hydrocarbons?

Step-by-Step Solution

Answer

The molecular formulas for the four types of hydrocarbons with six carbon atoms are: - Cyclic Alkane: C₆H₁₂ (Saturated) - Cyclic Alkene: C₆H₁₀ (Unsaturated) - Linear Alkyne: C₆H₁₀ (Unsaturated) - Aromatic Hydrocarbon: C₆H₆ (Unsaturated)

Step 1: Determine the molecular formula for a cyclic alkane

A cyclic alkane is composed of carbon and hydrogen atoms connected in a ring structure with single bonds. Based on the general formula for alkanes (C_nH_2n+2), a cyclic alkane has six carbons and follows the formula C_nH_2n. Therefore, a cyclic alkane with six carbon atoms will have the molecular formula C₆H₁₂.

Step 2: Determine the molecular formula for a cyclic alkene

A cyclic alkene is composed of carbon and hydrogen atoms connected in a ring structure with one double bond. The general formula for alkenes is CnH2n, making a cyclic alkene with six carbons have the molecular formula C₆H₁₀.

Step 3: Determine the molecular formula for a linear alkyne

A linear alkyne is composed of carbon and hydrogen atoms connected in a straight chain with at least one triple bond. The general formula for alkynes is CnH2n-2. Therefore, a linear alkyne with six carbon atoms will have the molecular formula C₆H₁₀.

Step 4: Determine the molecular formula for an aromatic hydrocarbon

An aromatic hydrocarbon consists of a ring structure with alternating single and double bonds. The most common example of an aromatic hydrocarbon is benzene, which contains six carbon atoms. Thus, the molecular formula for an aromatic hydrocarbon with six carbon atoms is C₆H₆.

Step 5: Classify the hydrocarbons as saturated or unsaturated

Saturated hydrocarbons contain only single bonds between carbon atoms and are mostly found in alkanes. Unsaturated hydrocarbons contain at least one double or triple bond between carbon atoms and are mostly found in alkenes and alkynes. - Cyclic Alkane (C₆H₁₂): Saturated - Cyclic Alkene (C₆H₁₀): Unsaturated - Linear Alkyne (C₆H₁₀): Unsaturated - Aromatic Hydrocarbon (C₆H₆): Unsaturated

Key Concepts

Saturated Hydrocarbons

Saturated hydrocarbons are types of hydrocarbons where all carbon atoms are bonded with single bonds. This full saturation means each carbon atom has the maximum possible number of hydrogen atoms attached to it. A common example of saturated hydrocarbons are alkanes.
A specific case is a cyclic alkane, such as cyclohexane, which has the formula \(C_6H_{12}\). In cyclic alkanes, the carbon atoms join to form a ring, but they maintain single bonds only. This makes them part of the family of saturated hydrocarbons.

• All carbons are connected by single bonds.
• Structurally stable and typically less reactive.
• Include structures like linear alkanes and cyclic alkanes.

The distinction of being 'saturated' implies that these compounds tend to have lower reactivity compared to unsaturated hydrocarbons. This characteristic makes them valuable as fuels, like in gasoline.

Unsaturated Hydrocarbons

Unsaturated hydrocarbons have at least one double or triple bond between carbon atoms. This configuration means there are fewer hydrogen atoms bonded to the carbon atoms when compared to saturated hydrocarbons. There are different types of unsaturated hydrocarbons: alkenes, alkynes, and aromatic hydrocarbons.
For instance, a cyclic alkene such as cyclohexene has the formula \(C_6H_{10}\) and contains one double bond. Further, a linear alkyne such as hexyne has a formula \(C_6H_{10}\) with one triple bond. Aromatic hydrocarbons such as benzene have the formula \(C_6H_6\), featuring alternating single and double bonds arranged in a planar ring structure.

• Cyclic alkene and linear alkyne: contain double or triple bonds respectively.
• Aromatic hydrocarbons like benzene: possess unique stability despite unsaturation.
• Higher reactivity due to presence of multiple bonds compared to saturated hydrocarbons.

These hydrocarbons react more readily due to the unsaturation, making them useful for chemical synthesis and as precursors to polymers.

Molecular Formula

The molecular formula of a compound reveals the number and type of atoms present in a molecule. For hydrocarbons, the molecular formula mainly includes carbon (C) and hydrogen (H) atoms. Understanding the formula helps to determine whether a hydrocarbon is saturated or unsaturated.
For example:

• Cyclic Alkane: \(C_6H_{12}\) - all single bonds indicate saturation.
• Cyclic Alkene: \(C_6H_{10}\) - one double bond shows unsaturation.
• Linear Alkyne: \(C_6H_{10}\) - presence of a triple bond confirms unsaturation.
• Aromatic Hydrocarbon: \(C_6H_6\) - alternating bonds in a ring suggest aromatic stability and unsaturation.

The molecular formula can guide the classification of hydrocarbons, clarifying their chemical behavior and potential applications. Unguided experiments often use these formulas as a preliminary check to understand the nature of the compound.