Draw the Lewis structure for HF. Identify the bond order and discuss the polarity of the molecule based on electronegativity: Draw the molecular orbital (MO) diagram for HF (see Figure 2.14). Compare and contrast the Lewis and MO descriptions of HF. For any orbitals which are nonbonding; describe why they are nonbonding: Explain how an electrophile and a nucleophile would attack HF_
The Correct Answer and Explanation is:
Lewis Structure for HF (Hydrogen Fluoride):
The Lewis structure for HF is:
H : F
- Hydrogen has 1 valence electron
- Fluorine has 7 valence electrons
They share one pair of electrons forming a single covalent bond. Fluorine has 6 remaining electrons arranged as three lone pairs:
markdownCopyEditH — F :
..
..
..
Bond Order:
Bond order is the number of bonds between two atoms:
For HF:
- There is a single covalent bond between H and F
- Therefore, bond order = 1
Polarity Based on Electronegativity:
- Hydrogen’s electronegativity ≈ 2.1
- Fluorine’s electronegativity ≈ 3.98
The large difference means the bonding electrons are pulled closer to fluorine, making:
- Fluorine: partially negative (δ−)
- Hydrogen: partially positive (δ+)
Thus, HF is highly polar, with a dipole pointing toward fluorine.
Molecular Orbital (MO) Diagram for HF:
HF involves overlap between:
- Hydrogen’s 1s orbital
- Fluorine’s 2p orbitals
Key Points:
- The F 2pz orbital aligns with H 1s forming a σ bonding orbital
- Two 2px and 2py orbitals on fluorine do not overlap with H’s 1s orbital
MO diagram (simplified):
makefileCopyEditFluorine: 2px (nonbonding) 2py (nonbonding)
2pz → σ* (antibonding)
2pz → σ (bonding)
Hydrogen: 1s orbital
Nonbonding Orbitals:
- 2px and 2py orbitals remain nonbonding because they lie perpendicular to the bond axis, so no effective overlap occurs with hydrogen’s orbital
- Fluorine’s lone pairs (from Lewis structure) correspond to these nonbonding MOs
Lewis vs MO Description Comparison:
- Lewis structure shows shared electrons in a localized bond, with lone pairs explicitly drawn on fluorine
- MO theory explains bonding through orbital overlap and energy levels, highlighting the presence of nonbonding MOs that cannot interact with hydrogen
Both agree on:
- One strong covalent bond
- Presence of nonbonding electron pairs on fluorine
- Polarity due to electron distribution
Electrophile and Nucleophile Attack on HF:
- HF has electron-rich regions (fluorine lone pairs)
- HF has an electron-deficient hydrogen
Electrophile Attack:
- An electrophile is electron-seeking
- Likely targets the lone pairs on fluorine
Nucleophile Attack:
- A nucleophile is nucleus-seeking (electron-rich)
- Likely attacks the partially positive hydrogen
Example:
- A strong base (nucleophile) abstracts the proton (H+), forming fluoride ion (F−)
- An electrophile can coordinate to fluorine’s lone pairs
In both views, HF’s polarity governs reactivity, with fluorine being a nucleophilic site and hydrogen being an electrophilic site.
