Consider the methyl cyanide molecule, CH3CN. (i) draw the Lewis structure of CH3CN (ii) predict the approximate bond angles at each carbon atom
The Correct Answer and Explanation is:
(i) Lewis Structure of CH₃CN (Methyl Cyanide)
To draw the Lewis structure of methyl cyanide (acetonitrile, CH₃CN):
- Count valence electrons:
- Carbon (C) = 4 valence electrons × 2 atoms = 8
- Hydrogen (H) = 1 valence electron × 3 = 3
- Nitrogen (N) = 5 valence electrons
→ Total = 8 + 3 + 5 = 16 valence electrons
- Connect atoms:
The structure consists of a methyl group (CH₃) connected to a carbon triple-bonded to a nitrogen atom:
H₃C—C≡N - Arrange electrons:
- Place single bonds between H and the first carbon (CH₃ group).
- Connect this carbon to the second carbon with a single bond.
- The second carbon and nitrogen form a triple bond.
- Add lone pairs on nitrogen to complete its octet.
Final Lewis structure:
H H H
\ | /
C—C≡N
..
- Each H has 1 bond.
- The first carbon (methyl carbon) has 4 single bonds.
- The central (nitrile) carbon forms a single bond with the methyl group and a triple bond with nitrogen.
- Nitrogen has one lone pair.
(ii) Bond Angles at Each Carbon Atom
- Methyl carbon (CH₃ group):
The carbon is bonded to three hydrogen atoms and one carbon — four bonding regions → sp³ hybridized.
Approximate bond angle: 109.5° (tetrahedral geometry) - Nitrile carbon (C≡N group):
This carbon forms a triple bond with nitrogen and a single bond with the methyl carbon — two bonding regions → sp hybridized.
Approximate bond angle: 180° (linear geometry)
Explanation
The methyl cyanide molecule, CH₃CN, exhibits a combination of two carbon atoms with different hybridizations and bond angles due to the distinct electronic environments they occupy. To understand this molecule, we begin by examining its Lewis structure.
The molecule consists of two carbon atoms: one as part of a methyl group (CH₃) and the other in a nitrile group (C≡N). The methyl carbon is bonded to three hydrogen atoms and one carbon, forming four sigma bonds. This geometry corresponds to sp³ hybridization, which naturally results in a tetrahedral shape with bond angles of approximately 109.5°.
The second carbon, part of the nitrile group, forms a triple bond with nitrogen and a single bond with the methyl carbon. This configuration includes two regions of electron density (one sigma and two pi bonds in the triple bond count as one region), resulting in sp hybridization. The electron geometry for sp-hybridized atoms is linear, giving rise to a bond angle of 180° between the central carbon, the nitrile carbon, and the nitrogen atom.
The nitrogen atom completes the molecule with a lone pair, adhering to the octet rule. Its presence slightly affects the molecular dipole, making CH₃CN a polar molecule with a significant dipole moment.
In summary, the methyl carbon is sp³ hybridized with 109.5° bond angles, while the nitrile carbon is sp hybridized with 180° bond angles. This hybridization and geometry directly influence the molecule’s physical properties, including its polarity and linearity at the C≡N end.
