Consider the simple alkane propane with the molecular formula C3H8. Draw a Lewis dot diagram for this molecule and convert it into a Kekule structure. Identify the hybridization of each carbon atom in the molecule. Provide the value for the bond angle shown in red in the figure below. H H H | | | H-C-C-C-H | | | H H H
The Correct Answer and Explanation is:
Answer:
- Lewis Dot Structure of Propane (C₃H₈):
mathematicaCopyEdit H H H
| | |
H - C - C - C - H
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H H H
- Kekulé Structure of Propane:
The Kekulé structure uses lines to represent bonds:
mathematicaCopyEdit H H H
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H — C — C — C — H
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H H H
- Hybridization of Carbon Atoms:
Each carbon atom in propane forms four sigma bonds and has tetrahedral geometry, so each carbon is sp³ hybridized.
- Bond Angle in Red (Assuming H–C–H or H–C–C):
In sp³ hybridization, the bond angles are approximately 109.5°.
Explanation
Propane (C₃H₈) is a simple alkane, composed of three carbon atoms and eight hydrogen atoms. Alkanes are saturated hydrocarbons, meaning they contain only single bonds between carbon atoms. To construct its Lewis dot structure, we arrange the carbon atoms in a straight chain, each forming four bonds to satisfy the octet rule. Hydrogen atoms are attached such that each carbon has four bonds in total.
In propane, the first and third carbon atoms (terminal carbons) are each bonded to three hydrogen atoms and one neighboring carbon atom. The middle carbon atom is bonded to two other carbon atoms and two hydrogen atoms. Each hydrogen forms one single covalent bond (sigma bond) with carbon.
Once the Lewis structure is constructed, we can convert it into a Kekulé structure, which is a more simplified structural formula that shows bonds as lines rather than dots. Each line represents a pair of shared electrons (a covalent bond), and each atom is shown with its bonding partners.
Now, considering hybridization, each carbon atom forms four sigma (σ) bonds—either with hydrogen or another carbon. To form four sigma bonds, the carbon atom must use one s orbital and three p orbitals, combining into four sp³ hybrid orbitals. Thus, all three carbon atoms in propane are sp³ hybridized, resulting in a tetrahedral geometry.
Due to this tetrahedral arrangement, the bond angles around each carbon atom are approximately 109.5°. This includes the bond angle between hydrogen atoms or between hydrogen and carbon atoms, such as the H–C–H or H–C–C angle, depending on the figure in question. This angle minimizes electron pair repulsion, as described by Valence Shell Electron Pair Repulsion (VSEPR) theory.
