Chemical and magnetic equivalence

Equivalence

  • Protons in methyl and most methylene (-CH₂-) groups are:

    • Chemically equivalent – same chemical environment.

    • Magnetically equivalent – same coupling constants to other nuclei.

  • The (n+1) rule applies to groups of equivalent nuclei.

🔹 Types of Equivalence

  • Chemically equivalent: same environment.

  • Magnetically equivalent: same coupling constants.

Note: Some protons (e.g. in aromatic rings) may be chemically equivalent but magnetically inequivalent due to differing coupling constants.

🔹 Example: Para-disubstituted Benzene

  • A & A′ and B & B′ are chemically equivalent.

  • Coupling constants differ:

    • Ortho (1,2): 7–9 Hz

    • Meta (1,3): 1–3 Hz

    • Para (1,4): ≤1 Hz

  • Therefore, proton Hₐ gives a doublet of doublets in ¹H NMR.

🔗 Connectivity

🔹 COSY (Correlation Spectroscopy)

  • Reveals which protons are coupled.

  • Useful for complex molecules or similar coupling constants.

  • Can be used for:

    • ¹H–¹H coupling

    • ¹H–¹³C coupling (in separate experiments)

🧠 ¹³C NMR Recap

🔹 Simpler than ¹H NMR:

  • One signal per unique carbon.

  • Position of signal → indicates electronegativity of neighbouring atoms.

  • No integration or multiplicity.

🧪 ¹³C DEPT Spectra

🔹 DEPT135

  • CH and CH₃ → Up

  • CH₂ → Down

🔹 DEPT90

  • CH only

🔹 ¹³C Standard Spectrum

  • All carbons, including quaternary.

📈 Chemical Shift

  • Depends on electronegativity of neighbouring atoms.

  • Similar trend to ¹H NMR.

🧬 Coupling in ¹³C NMR

  • ¹³C signals split by directly attached protons.

  • Usually suppressed using broadband decoupling for simplicity (single signal per carbon).

🔍 DEPT Spectrum Interpretation

🔹 Why it matters:

  • Determines how many protons are attached to each carbon.

  • Helps distinguish CH, CH₂, CH₃.

🧠 Spectrum Interpretation Strategy

🔹 ¹H NMR (4 Key Rules)

  1. Number of hydrogen environments

  2. Chemical shift

  3. Integration (number of protons)

  4. Coupling/multiplicity

🔹 ¹³C NMR (3 Key Rules)

  1. Number of carbon environments

  2. Chemical shift

  3. Use DEPT (to find attached protons)

🔢 Double Bond Equivalents (DBEs)

🔹 Purpose:

  • Indicates rings and π-bonds.

  • Useful for identifying aromaticity or unsaturation.

🔹 Formula:

  • Replace heteroatoms with equivalent CHn groups.

  • Alkanes: CₙH₂ₙ₊₂

  • DBE = ½ × (number of “missing” Hs)

🔹 Example: Hydrocarbon C₁₀H₁₆

  • Alkane equivalent: C₁₀H₂₂

  • Difference = 6 H → DBE = 6 ÷ 2 = 3

🧪 Non-Hydrocarbons

  • Replace other atoms (e.g. N, O, Cl) with hydrocarbon equivalents of the same valency.

🧬 Aromatic Systems

🔹 Disubstituted Benzenes

  • Include para-couplings, even if small (often not visible).

🔹 Monosubstituted Benzenes

  • Predict splitting patterns similarly.

  • Spectrum can become complex due to many couplings.

🔹 Practical Tip:

  • A messy signal in aromatic region integrating to 5H suggests a monosubstituted benzene.