H3 Chemistry 9813 · Study focus: H3 Chemistry: Explain electronegativity and inductive effects on shielding
H3 Chemistry: Explain electronegativity and inductive effects on shielding
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Explain electronegativity and inductive effects on shielding
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Compare chlorine proximity
CH3Cl protons occur near δ3.05, far downfield of ethane near δ0.9.
Chlorine withdraws electron density inductively and deshields nearby protons.
Shielding language
Shielding lowers effective field; deshielding raises effective field and δ.
Downfield means larger δ, not physically lower on the page.
In 1-chloropropane CH2Cl near δ3.4 is more downfield than terminal CH3 near δ1.0.
The inductive effect weakens through successive σ bonds.
| Compound/environment | δ / ppm | Distance from Cl | Controlled inference |
|---|---|---|---|
| CH₃Cl | 3.05 | α | deshielded versus ethane CH₃ |
| CH₃CH₃ | 0.9 | no Cl | matched methyl baseline |
| ClCH₂CH₂CH₃: CH₂Cl | 3.4 | α | largest effect |
| ClCH₂CH₂CH₃: middle CH₂ | 1.8 | β | smaller effect |
| ClCH₂CH₂CH₃: terminal CH₃ | 1 | γ | smallest effect |
Text alternative: Text states each group, distance, shift, order and causal direction.
Distance matters
In 1-chloropropane CH2Cl near δ3.4 is more downfield than terminal CH3 near δ1.0.
The inductive effect weakens through successive σ bonds.
Assign 1-chloropropane
The 2H signal near δ3.4 belongs to CH2Cl; the 3H signal near δ1.0 is terminal CH3.
Both count and proximity support the assignment.
- Assign the two fixed signals.
Open the feedback checkpoint after attempting
- Credit CH2Cl at δ3.4 and terminal CH3 at δ1.0.
Order halomethanes
Using supplied values CH3F δ4.3 and CH3Cl δ3.05, CH3F is more deshielded.
The comparison is matched and does not claim electronegativity is the only shift effect.
- Order the two signals.
Open the feedback checkpoint after attempting
- Credit CH3F further downfield from stronger withdrawal.
Trace distance
For ClCH2CH2CH3, predict CH2Cl > central CH2 > terminal CH3 in δ.
State that other local effects can modify exact values.
- Predict the qualitative order.
Open the feedback checkpoint after attempting
- Credit decreasing δ with distance from chlorine.
Repair shielding direction
Electron withdrawal reduces shielding; it does not add a protective electron cloud.
Therefore the affected signal moves to larger δ.
- Repair: ‘chlorine shields CH2.’
Open the feedback checkpoint after attempting
- Credit withdrawal, deshielding and downfield direction.
Use bounded evidence
Complete twelve fixed comparisons and two different unseen halogen cases.
Next: anisotropy at /learning/h3-nmr-anisotropy-lesson.html.
- Explain the δ3.4/1.0 contrast.
Open the feedback checkpoint after attempting
- Credit inductive withdrawal, distance and cautious assignment.