Question

Write a structural formula and a condensed structural formula for each alkane. (a) heptane (b) octane (c) hexane (d) ethane

Short answer

Heptane: Structural formula - CH3-CH2-CH2-CH2-CH2-CH2-CH3, Condensed - CH3(CH2)5CH3. Octane: Structural formula - CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH3, Condensed - CH3(CH2)6CH3. Hexane: Structural formula - CH3-CH2-CH2-CH2-CH2-CH3, Condensed - CH3(CH2)4CH3. Ethane: Structural formula - CH3-CH3, Condensed - CH3CH3.

Step-by-step solution

Understand Alkanes

Alkanes are simple hydrocarbons, where all the carbon atoms are connected through single bonds (saturated hydrocarbons). Each carbon atom makes four bonds. The general formula for alkanes is \( C_nH_{2n+2} \) for an alkane with n carbon atoms.

Draw Heptane (C7H16)

For heptane, which is \( C_7H_{16} \), you have a chain of 7 carbon atoms with each interior carbon bonded to 2 other carbons and 2 hydrogens, while the terminal carbons (at the ends) are bonded to 3 hydrogens. Structural formula is a single chain with no branches, and the condensed structural formula is CH3(CH2)5CH3.

Draw Octane (C8H18)

For octane, the formula is \( C_8H_{18} \). It has 8 carbon atoms in a straight chain, again with no branches. The condensed structural formula is CH3(CH2)6CH3.

Draw Hexane (C6H14)

For hexane, which is \( C_6H_{14} \), draw a chain of 6 carbon atoms. Each carbon atom (except for the terminal ones) has 2 hydrogens. The condensed structural formula is CH3(CH2)4CH3.

Draw Ethane (C2H6)

Ethane, with the formula \( C_2H_{6} \), consists of only two carbon atoms, each bonded to 3 hydrogens, and a single bond between the carbon atoms. The condensed structural formula is CH3CH3.

Key concepts

Alkane Chemistry

Alkanes form a crucial foundation in the study of organic chemistry, representing the simplest class of hydrocarbons. These molecules consist purely of carbon and hydrogen atoms, linked exclusively by single covalent bonds, which classifies them as saturated hydrocarbons. Understanding alkanes involves grasping the concept of homologous series, where each subsequent alkane differs from the previous by a CH₂ unit.

For students embarking on organic chemistry education, recognizing the general alkane formula, \( C_nH_{2n+2} \), is essential. The 'n' represents the number of carbon atoms, and it allows for the determination of the number of hydrogen atoms present in the molecule. As an exercise example, when 'n' equals 7 for heptane, the alkane's chemical formula is expressed as \( C_7H_{16} \).

It's not just about memorizing formulas; visualizing the structure aids in a better understanding. Therefore, sketching the structural formula as a straight chain for heptane, or any other alkane, showcases how the carbons bond in a successive, snake-like fashion with hydrogens filling the available valences.

Hydrocarbon Structures

Delving into the architecture of hydrocarbons like alkanes involves two key depictions: the structural formula, which shows the detailed connections between atoms, and the condensed structural formula, which simplifies the representation of these connections.

In the case of alkanes, visualizing hydrocarbon structures often starts with straight-chain arrangements. For example, octane, \( C_8H_{18} \), is illustrated as a line of eight carbon atoms with appropriate hydrogen attachments, signifying a lack of branching. As the complexity of the alkane increases, so does the intricacy of its possible structures, with the potential for isomers—molecules with the same molecular formula but different arrangements of atoms.

This diversity makes it important for learners to practice drawing both detailed and condensed structural formulas, such as CH₃(CH₂)₆CH₃ for octane, to become comfortable with the structural variety present within organic chemistry.

Organic Chemistry Education

The journey of mastering organic chemistry education extends beyond memorizing formulas and structures; it encompasses developing a conceptual framework that helps in solving complex problems. For instance, exercises like writing structural formulas for alkanes serve as a fundamental skill-building activity.

To assist students in their educational journey, it is beneficial to provide exercises with incremental difficulty, starting with simpler alkanes like ethane (\( C_2H_{6} \) or CH₃CH₃) and progressing towards more complex ones, such as hexane (\( C_6H_{14} \) or CH₃(CH₂)₄CH₃). Likewise, educators should emphasize the importance of pattern recognition, especially the way in which an alkane's carbons increase by one and hydrogens by two in each step of the homologous series.

Through these methods, students can establish a solid foundation that supports their understanding of more advanced organic chemistry topics, including isomerism and functional group identification.

Saturated Hydrocarbons

Saturated hydrocarbons are organic molecules that possess the maximum possible number of hydrogen atoms per carbon; they do not contain double or triple carbon-carbon bonds. Within this category, alkanes represent the most fundamental group. The simplicity in their bond structure makes them an excellent starting point for studying organic reactions and mechanisms.

The term 'saturated' conveys the concept of these hydrocarbons having no room for additional hydrogen atoms—each carbon atom is 'saturated' with hydrogens. As we explore through exercises, every internal carbon atom in an alkane is attached to two other carbons and two hydrogens, while terminal carbons bond with three hydrogens. A saturated hydrocarbon like hexane (\( C_6H_{14} \) or CH₃(CH₂)₄CH₃), exhibits this fully hydrogenated state.

Grasping the concept of saturation is critical because it defines the reactivity and physical properties of these molecules, like their reluctance to undergo addition reactions, their less polar nature, and relative inertness compared to their unsaturated counterparts.