Problem 77 - Chemistry The Molecular Science

Question

Write a Lewis structure for each molecule. Which formulas represent alkanes? Which are probably aromatic? Which fall into neither category? (a) \(\mathrm{C}_{8} \mathrm{H}_{10}\) (b) \(\mathrm{C}_{10} \mathrm{H}_{8}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{12}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{14}\) (e) \(\mathrm{C}_{8} \mathrm{H}_{18}\) (f) \(\mathrm{C}_{6} \mathrm{H}_{10}\)

Step-by-Step Solution

Answer

Alkanes: (d) and (e). Aromatic: (b). Neither: (a), (c), and (f).

Step 1: Understand Alkanes

Alkanes are saturated hydrocarbons with only single bonds and follow the generic formula \( C_nH_{2n+2} \). We will compare each molecular formula against this to check if it is an alkane.

Step 2: Determine Aromatic Possibility

Aromatic compounds contain a ring structure with alternating double bonds (like benzene with \( C_6H_6 \)). The general characteristic is cyclic, planar, and follows Hückel's rule (\(4n+2\) \(\pi\) electrons).

Step 3: Analyze (a) \(\mathrm{C}_{8} \mathrm{H}_{10}\)

The formula does not match the alkane pattern (\(2n+2=18\) not 10), suggesting neither an alkane nor an aromatic assuming typical structure lacking the aromatic pi system quantity.

Step 4: Analyze (b) \(\mathrm{C}_{10} \mathrm{H}_{8}\)

The hydrogen deficiency (\(2n+2=22\), not 8) suggests a compound possibly holding aromatic characteristics (similar to naphthalene, an aromatic which could be seen as fused benzene rings).

Step 5: Analyze (c) \(\mathrm{C}_{6} \mathrm{H}_{12}\)

The formula is \(C_nH_{2n}\), suggesting a cycloalkane (rings with single bonds), neither typically aromatic nor an alkane.

Step 6: Analyze (d) \(\mathrm{C}_{6} \mathrm{H}_{14}\)

Here we have \(2n+2=14\), matching the key alkane pattern; thus, it is an alkane.

Step 7: Analyze (e) \(\mathrm{C}_{8} \mathrm{H}_{18}\)

The formula fits \(2n+2=18\). As such, it is an alkane.

Step 8: Analyze (f) \(\mathrm{C}_{6} \mathrm{H}_{10}\)

This deviates from both alkane (\(2n+2=14\)) and typical aromatic expectations, potentially being an alkene or cycloalkene.

Key Concepts

Alkanes

Alkanes are the simplest type of hydrocarbons. They are known for their saturated nature, meaning they only consist of single bonds with maximum hydrogen atoms attached to carbon. The general formula is \( C_nH_{2n+2} \). This formula indicates that for every carbon atom, there are two more hydrogen atoms plus two additional hydrogens.

• Example: Methane (\( CH_4 \)) is the simplest alkane, with 1 carbon and 4 hydrogens.
• Alkanes are sometimes referred to as paraffins and they are usually non-polar and exhibit London dispersion forces as their primary intermolecular force.

Alkanes are considered saturated because no more hydrogen atoms can be added without altering the basic structure. They usually exist in linear or branched forms and are fundamental in organic chemistry. Recognizing alkanes involves checking if a molecular formula matches the \( C_nH_{2n+2} \) format, as seen in exercises with their straightforward single-bond structures.

Aromatic Compounds

Aromatic compounds are a fascinating group of hydrocarbons that are characterized by their cyclic and planar structures. They contain resonance bonds, which means their electrons are delocalized within the ring. This special electron configuration gives them significant stability and unique chemical properties.

• The most common example is benzene (\( C_6H_6 \)), which showcases a ring of six carbon atoms with alternating double bonds.
• Aromatic compounds must satisfy Hückel's rule, specifically having \(4n+2\) \( \pi \) electrons, where \( n \) is a positive integer.

These compounds often have a distinctive smell, the root of the term ‘aromatic,’ and are found naturally in materials like coal and crude oil. They play essential roles in producing dyes, plastics, and pharmaceuticals. Identifying aromatics typically involves looking for molecular formulas that suggest extensive hydrogen deficiency relative to carbon while allowing possible ring structures, as discussed in the exercises.

Hydrocarbon Saturation

Understanding the concept of hydrocarbon saturation is crucial in differentiating several organic compounds. Saturated hydrocarbons contain only single bonds; hence they are 'saturated' with hydrogen. Alkanes fall under this category. Their structure allows no room for additional hydrogen atoms without disrupting the molecule's integrity.

• Unsaturated hydrocarbons, on the other hand, have double or triple bonds, allowing them to react more readily with additional hydrogen.
• Alkenes (with double bonds) and alkynes (with triple bonds) are examples of unsaturated hydrocarbons.

The degree of saturation influences the chemical reactivity and physical characteristics of the compound. For example, unsaturated hydrocarbons can undergo addition reactions, whereas saturated compounds cannot. This property also helps students deduce the correct classification of hydrocarbons using molecular formulas, as exhibited in different parts of the solution steps.

Molecular Formula Analysis

Molecular formula analysis in organic chemistry involves deducing the structure and properties of a compound based solely on the elements and number of atoms present. Each molecular formula provides insights into the potential classification, structure, and even potential reactivity of the compound.

• Start by comparing the number of hydrogen and carbon atoms to known general formulas like for alkanes, \( C_nH_{2n+2} \).
• A deficiency in hydrogen, compared to the expected saturated formula, suggests the presence of double bonds, rings, or aromatics.
• Compounds like cycloalkanes may have the same molecular formula as certain alkenes but different structures and properties due to the closed-ring formation.

By analyzing the exercise step by step, it becomes easier to identify each compound. Practicing such analysis hones the ability to predict not only the structure but also the reactivity of the compound, an essential skill in organic chemistry.