Question

Draw a Lewis structure for each of these organic molecules in which the carbon atoms are bonded to each other by single bonds: \(\mathrm{C}_{2} \mathrm{H}_{6}, \mathrm{C}_{4} \mathrm{H}_{10}, \mathrm{C}_{5} \mathrm{H}_{12}\).

Short answer

Ethane \(\mathrm{C}_{2}\mathrm{H}_{6}\) Lewis structure: \(:\: H- C - C -H\), with a \(H\) atom on the top and bottom of each carbon. Butane \(\mathrm{C}_{4}\mathrm{H}_{10}\) Lewis structure: \(:\: H- C - C - C - C -H\), with a \(H\) atom on the top and bottom of each carbon. Pentane \(\mathrm{C}_{5}\mathrm{H}_{12}\) Lewis structure: \(:\: H- C - C - C - C - C -H\), with a \(H\) atom on the top and bottom of each carbon.

Step-by-step solution

Draw \(\mathrm{C}_{2}\mathrm{H}_{6}\)

Write down two carbon atoms, each bonded to three hydrogen atoms. Carbon atoms will be bonded to each other by a single bond. This is how the Lewis structure for ethane \(\mathrm{C}_{2}\mathrm{H}_{6}\) looks like: \(:\: H- C - C -H\), with a \(H\) atom on the top and bottom of each carbon.

Draw \(\mathrm{C}_{4}\mathrm{H}_{10}\)

Write down four carbon atoms in a row, each bonded to enough hydrogen atoms to make four bonds in total. The carbon atoms at the ends of the row are bonded to three hydrogen atoms each, and the two carbon atoms in the middle are each bonded to two hydrogen atoms. The resulting Lewis structure for butane \(\mathrm{C}_{4}\mathrm{H}_{10}\) is: \(:\: H- C - C - C - C -H\), with a \(H\) atom on the top and bottom of each carbon.

Draw \(\mathrm{C}_{5}\mathrm{H}_{12}\)

Write down five carbon atoms in a row, each bonded to enough hydrogen atoms to make four bonds in total. The carbon atoms at the ends of the row are bonded to three hydrogen atoms each, and the three carbon atoms in the middle are each bonded to two hydrogen atoms. The resulting Lewis structure for pentane \(\mathrm{C}_{5}\mathrm{H}_{12}\) is: \(:\: H- C - C - C - C - C -H\), with a \(H\) atom on the top and bottom of each carbon.

Key concepts

Chemical Bonding

Chemical bonding is the force that holds atoms together in molecules. In organic chemistry, the most common type of bond is the covalent bond, where atoms share pairs of electrons to achieve a stable electron configuration. Understanding the nature of chemical bonds is crucial when examining the properties and behaviors of organic molecules.

When we talk about bonding in alkanes, we're looking at single covalent bonds between the carbon atoms. Each carbon atom has four electrons in its outer shell and needs four more to fill it. By sharing one electron with each hydrogen atom (which only needs one additional electron to be stable) and one with another carbon atom, they form a stable structure known as a tetrahedral configuration.

Covalent bonds in alkanes are nonpolar, as the electronegativities of carbon and hydrogen are relatively similar. This has implications for the physical properties of alkanes, such as their low solubility in water and low boiling and melting points compared to other types of organic compounds.

Alkanes

Alkanes are the simplest family of hydrocarbons and consist only of carbon and hydrogen atoms with single covalent bonds between the carbon atoms. They are also known as saturated hydrocarbons because they contain the maximum number of hydrogen atoms possible on a carbon chain, with no double or triple bonds.

Alkanes have a general formula of \(C_nH_{2n+2}\), where \(n\) is the number of carbon atoms. The structure of an alkane molecule determines its physical and chemical properties. For example, as the chain of carbons gets longer, the boiling point of the alkane increases.

These molecules are also relatively unreactive due to the strong C-C and C-H bonds; however, they can undergo reactions such as combustion, where they react with oxygen to produce carbon dioxide, water, and energy.

Structural Formulas

Structural formulas are graphical representations of molecules that show how atoms are connected in a molecule. They provide more information than the molecular formula, which only states the numbers of each type of atom. In the case of organic molecules, especially alkanes, the structural formula is essential for understanding the shape and connectivity of the compound.

The simplest form of a structural formula is the Lewis structure, which displays the bonds between the atoms as lines and the valence electrons as dots. This structure helps understand molecules' reactivity and physical properties by showing lone pairs and bond angles.

When creating Lewis structures for alkanes, it's important to remember that each carbon atom is at the center of a tetrahedral geometry with 109.5-degree angles between bonds. This is why, in the Lewis structures provided in the exercise, we see the carbon atoms linearly arranged; however, the actual shape of the molecule in three dimensions would reflect the tetrahedral arrangement around each carbon atom.