Nitromethane has the formula CH3NO2, with the N bonded to the C and without O-O bonds. Draw its two most important contributing structures. a. What is the hybridization of the C, and how many hybrid orbitals are in the molecule? b. What is the shortest bond? c. Between which two atoms is the strongest bond found? d. Predict whether the HCH bond angles are greater or less than 109.5 and justify your prediction.
The correct answer and explanation is:
Sure! Let’s break down the problem step-by-step for nitromethane (CH₃NO₂).
1. Lewis Structures: Two Most Important Resonance Contributors
Nitromethane’s structure includes a methyl group (CH₃) attached to a nitro group (NO₂). The nitrogen (N) is bonded directly to the carbon (C) of the methyl.
- Structure 1:
- C single bonded to N
- N double bonded to one oxygen (O)
- N single bonded to the other oxygen, which carries a negative charge
- N carries a positive charge
- Structure 2:
- Resonance where the double bond to oxygen switches to the other oxygen
- The charge distribution shifts accordingly (positive on N, negative on other O)
No O-O bonds are present, and the nitro group shows resonance between the two oxygens attached to N.
2. a. Hybridization of Carbon and Number of Hybrid Orbitals
- The carbon in CH₃NO₂ is bonded to three hydrogens and one nitrogen. All are single bonds.
- Carbon is therefore sp³ hybridized (4 sigma bonds).
- This means carbon has 4 sp³ hybrid orbitals, each forming sigma bonds with H or N.
3. b. Shortest Bond
- The shortest bond is typically the bond with the greatest bond order.
- In nitromethane, the N=O double bonds (in resonance, either oxygen can be double bonded) are shorter than single bonds (C-N or N-O single).
- Therefore, the shortest bond is the N=O double bond.
4. c. Strongest Bond
- The strongest bond is generally the one with the highest bond order and strongest overlap.
- The N=O double bond is strongest due to partial double bond character.
- So the strongest bond is between N and O (double bond in resonance).
5. d. H–C–H Bond Angles: Greater or Less Than 109.5°?
- The ideal bond angle for sp³ carbon is 109.5° (tetrahedral).
- However, the carbon is bonded to 3 hydrogens and 1 nitrogen (a more electronegative atom).
- Nitrogen pulls electron density, causing the H–C–H angles to be slightly larger than 109.5° to reduce repulsion between bonded atoms.
- Also, the methyl group is attached to an electron-withdrawing nitro group, which can distort angles.
- So, H–C–H bond angles are predicted to be slightly greater than 109.5°.
Summary:
| Question | Answer |
|---|---|
| a. Hybridization of Carbon | sp³, 4 hybrid orbitals |
| b. Shortest Bond | N=O double bond |
| c. Strongest Bond | N=O double bond |
| d. H–C–H Bond Angles | Slightly greater than 109.5° |
Explanation:
Nitromethane’s structure is dominated by resonance between two nitro resonance forms, which stabilize the molecule by delocalizing charges over the two oxygens. The carbon is sp³ hybridized because it forms four sigma bonds (3 with H, 1 with N). The presence of the nitro group influences the geometry, slightly increasing the H–C–H bond angles from the ideal tetrahedral angle due to electron withdrawal by nitrogen, which reduces electron-electron repulsions among the hydrogen atoms.
The shortest and strongest bonds are the N=O double bonds due to their higher bond order and greater overlap compared to single bonds like C–N or N–O. This also aligns with typical bond length trends where double bonds are shorter and stronger than single bonds.