Organic Chemistry MCAT Practice Exam

What is the ideal bond angle in a saturated alkane?

1. 120 degrees

2. 90 degrees

3. 180 degrees

4. 109.5 degrees

The correct answer is: 109.5 degrees

In saturated alkanes, each carbon atom is sp³ hybridized, which promotes the formation of four sigma bonds with a tetrahedral geometry. The bonding scenario in these hydrocarbons involves four bonding pairs of electrons around each carbon atom. According to VSEPR (Valence Shell Electron Pair Repulsion) theory, the ideal bond angle that minimizes repulsion between these four electrons is approximately 109.5 degrees. This tetrahedral shape leads to the characteristic bond angles observed in compounds like methane (CH₄) and other alkanes, such as ethane (C₂H₆) and propane (C₃H₈). Thus, recognizing that the tetrahedral arrangement is consistent with sp³ hybridization allows us to ascertain that the bond angle is indeed close to 109.5 degrees, which is the ideal angle for saturated alkanes.