Question

For each of the following organic molecules draw a Lewis structure in which the carbon atoms are bonded to each other by single bonds: (a) ${\mathrm{C}}_2{\mathrm{H}}_6$ (b) ${\mathrm{C}}_4{\mathrm{H}}_{10}$ (c) ${\mathrm{C}}_5{\mathrm{H}}_{12}$ For parts (b) and (c), show only structures in which each $\mathrm{C}$ atom is bonded to no more than two other $\mathrm{C}$ atoms.

Short answer

Draw the structures: (a) Ethane: H3C-CH3, (b) Butane: H3C-(CH2)2-CH3, (c) Pentane: H3C-(CH2)3-CH3.

Step-by-step solution

Determine the Molecular Formula for Each Compound

For each given hydrocarbon, identify the molecular formula: (a) Ethane, (b) Butane, and (c) Pentane.

Calculate Total Valence Electrons

Determine the number of valence electrons for each hydrocarbon. Each carbon atom contributes 4 valence electrons and each hydrogen atom contributes 1 valence electron.

Construct the Basic Carbon Skeleton

For each hydrocarbon, draw a straight carbon chain. (a) Ethane: Connect 2 carbon atoms with a single bond. (b) Butane: Connect 4 carbon atoms in a row. (c) Pentane: Connect 5 carbon atoms in a row. Ensure no carbon is bonded to more than two other carbons.

Add Hydrogen Atoms to Satisfy Valence Requirements

Add hydrogen atoms to each carbon to ensure each carbon has four bonds total. (a) Ethane: Add 3 hydrogens to each carbon. (b) Butane: Add hydrogens so interior carbons have 2 hydrogens each, and end carbons have 3 hydrogens. (c) Pentane: Similar to (b), ensure each end carbon atom has 3 hydrogens and interior carbons have 2 hydrogens.

Draw the Lewis Structures

Using the carbon skeleton and hydrogen placements from previous steps, draw the Lewis structure for each compound indicating all bonds with lines: (a) Ethane: H3C-CH3 (b) Butane: H3C-(CH2)2-CH3 (c) Pentane: H3C-(CH2)3-CH3

Key concepts

Organic Molecules

Organic molecules are fascinating because they form the basis of all living matter. These molecules are primarily made up of carbon atoms, often in combination with hydrogen, nitrogen, oxygen, and sometimes sulfur or phosphorus. The backbone of many organic molecules is a chain of carbon atoms, which allows for the creation of complex structures with diverse properties.
Carbon's ability to form four covalent bonds makes organic chemistry incredibly rich and varied, allowing these molecules to form rings, chains, and branches essential for life's processes.
Organic molecules play a vital role in biological functions, such as energy storage and cellular structure, making their study essential in both chemistry and biology.

Valence Electrons

Understanding valence electrons is crucial for interpreting how atoms bond and form molecules, particularly in organic chemistry. Valence electrons are the electrons in an atom's outermost shell, and they determine the atom's bonding capacity.
For carbon atoms, which are central in organic molecules, there are four valence electrons. This enables carbon to form four covalent bonds with other atoms, allowing for a stable tetrahedral geometry. For hydrogen, the simplest element, one valence electron results in the capability to form a single bond with other elements.
These valence electrons form bonds by overlapping with those of other atoms, creating the molecules that define organic chemistry. As with our hydrocarbon examples, knowing the number of valence electrons helps in drawing accurate Lewis structures.

Carbon Chain

The carbon chain is a fundamental component of many organic molecules, serving as the skeleton to which other elements or functional groups can attach. In hydrocarbons, such as ethane, butane, and pentane, carbon chains are joined by single covalent bonds between carbon atoms.
Carbon's ability to bond with itself allows for the formation of long chains, which can be straight or branched. This flexibility is what contributes to the diversity of organic compounds. Every point where two carbons meet forms a bond that can be combined with up to four atoms, maximizing stability.

• In ethane, the chain is shortest, with two carbons linked.
• Butane extends this to four, providing a longer framework.
• Pentane further extends the chain, showcasing the potential increase in complexity.

This scalability is key to the expansive nature of organic chemistry.

Hydrocarbons

Hydrocarbons are the simplest organic compounds, consisting solely of carbon and hydrogen. They are categorized into different types based on their structure and bond types. In our examples with ethane, butane, and pentane, they are saturated hydrocarbons, meaning they only contain single bonds between carbon atoms, known as alkanes.
Hydrocarbons are significant due to their abundance and utility, being a major component of fossil fuels like natural gas and petroleum. The simple structure of these molecules means they are often fundamental building blocks for more complex substances.
Despite their simplicity, hydrocarbons can range in size and configuration, from small, straight chains to large, complex branched or even cyclic structures, showcasing their versatility and importance in both industrial applications and chemical reactions.