Problem 76 - Chemistry: The Central Science

Question

(a) What ending is used for the names of alkanes? (b) Hexane is an alkane whose structural formula has all its carbon atoms in a straight chain. Draw the structural formula for this compound, and determine its molecular and empirical formulas. (Hint: You might need to refer to Table 2.6.)

Step-by-Step Solution

Answer

(a) Alkanes use the ending "-ane." (b) Hexane's structural formula is a straight chain with six carbon atoms, each bonded to the adjacent carbon and sufficient hydrogen atoms for a total of four bonds per carbon. Hexane's molecular formula is \(C_6H_{14}\) and its empirical formula is \(C_3H_7\).

Step 1: Part (a): Identify Alkane Ending

Alkanes are a class of hydrocarbons, which only contain single bonds between their carbon atoms. The general formula for alkanes is \(C_nH_{2n+2}\), where \(n\) is the number of carbon atoms. Alkanes have names that end with the suffix "-ane."

Step 2: Part (b): Draw Hexane's Structural Formula

Hexane is an alkane that has six carbon atoms in a straight chain. Its name starts with "hex-" (meaning six) and ends with "-ane" (indicating it is an alkane). To draw the structural formula for hexane, follow these steps: 1. Draw a straight chain of six carbon atoms. 2. Connect each carbon atom with a single bond to the adjacent carbon atom. 3. Attach hydrogen atoms to each carbon atom to complete the tetravalency of carbon. This means that each carbon atom should have four bonds in total.

Step 3: Part (b): Determine Molecular and Empirical Formulas of Hexane

Using the general formula of alkanes, \(C_nH_{2n+2}\), we can find hexane's molecular formula: 1. Replace \(n\) with 6 (the number of carbon atoms in hexane): \(C_6H_{2(6)+2}\) 2. Calculate the molecular formula: \(C_6H_{14}\) So, hexane's molecular formula is \(C_6H_{14}\). To find the empirical formula, we need to simplify the ratio of carbon to hydrogen atoms: 1. Divide the number of carbon atoms (6) and hydrogen atoms (14) by their greatest common divisor, which is 2 in this case. 2. The simplified ratio becomes \(C_3H_7\). So, hexane's empirical formula is \(C_3H_7\).

Key Concepts

Hydrocarbons

Hydrocarbons are organic compounds that consist entirely of hydrogen and carbon atoms. These compounds are fundamental to organic chemistry and serve as the backbone of many structures. They are categorized based on the type of bonds between the carbon atoms:

• *Alkanes*: These are the simplest types of hydrocarbons and consist only of single bonds. Each carbon atom in alkanes shares a single bond with other carbon atoms, forming a saturated hydrocarbon. The general formula is \(C_nH_{2n+2}\).
• *Alkenes*: Hydrocarbons that contain at least one double bond, making them unsaturated. Their formula is \(C_nH_{2n}\).
• *Alkynes*: Similar to alkenes but have at least one triple bond. They are also unsaturated with the formula \(C_nH_{2n-2}\).

Alkanes, such as hexane, are among the most stable hydrocarbons due to their single bonds, making them less reactive than alkenes and alkynes.

Structural Formula

A structural formula is a representation of the arrangement of atoms within a molecule. While molecular formulas only show the number and type of atoms, structural formulas depict how these atoms are connected.
For alkanes like hexane, a structural formula is essential for understanding its straight-chain structure:

• **Carbon Backbone**: Display each carbon atom in a line, indicating a chain.
• **Bond Representation**: Use single lines to show the carbon-carbon bonds. Each carbon atom should be connected to four other atoms or groups, representing its tetravalency.
• **Hydrogen Atoms**: Ensure each carbon atom that can is surrounded by sufficient hydrogen atoms to make four bonds total.

This gives a visual guide to the complexity of a molecule beyond what's shown in its molecular formula. Structural formulas are crucial in chemistry to deduce properties and predict reactions.

Molecular Formula

The molecular formula of a compound provides specific information about the types and numbers of atoms in a molecule, but not their arrangement. For hexane, the molecular formula is \(C_6H_{14}\).
To arrive at this:

• Identify the type of alkane: With hexane being an alkane, it follows the general alkane formula \(C_nH_{2n+2}\).
• Substitute the number of carbon atoms: In hexane, \(n=6\), hence substituting gives \(C_6H_{14}\).

Knowing the molecular formula allows for the calculation of molecular weight and gives insight into possible structural variations. It provides a quantitative perspective on the compound's composition.

Empirical Formula

The empirical formula of a compound is a simplified version that shows the ratio of elements by the smallest whole numbers. Unlike the molecular formula, which gives the actual number of atoms, the empirical formula provides a reduced ratio.
For hexane, the process to find the empirical formula is:

• Start with the molecular formula \(C_6H_{14}\).
• Determine the greatest common divisor of the subscripts, here it's 2.
• Divide each subscript by this divisor: Carbon remains 3, and hydrogen reduces to 7, resulting in \(C_3H_7\).

The empirical formula \(C_3H_7\) indicates a smaller, consistent ratio within the structure. It's especially useful in cases where the molecular formula is unknown, like in analytical chemistry, helping deduce information about the compound's basic makeup.