Lecture on Chemical Equivalency and Proton NMR

Introduction

• Discussion on chemical equivalency and predicting proton NMR absorptions.
• Explanation on why only one signal appears in the NMR spectrum for certain compounds.
• Importance of understanding chemical environments and chemical shifts.

Simple Examples of Compounds with One Proton Type

• Objective: Understand why there's only one signal, how many protons each signal is worth, and its chemical environment.
• Method: Replace a proton with an X group and check if the resulting compound is the same.

Cyclohexane Example

• Hydrogens attached to the same carbon are equivalent.
• Use of symmetry to identify equivalent protons.

Methyl Ether (Dimethyl Ether)

• All hydrogens in methyl ether are equivalent.
• Technique of replacing each hydrogen to prove equivalency.

Determining Proton Worth in NMR Signals

• Example compounds: Ethane, Cyclohexane, Methyl Ether.
• Signal worth:
  • Ethane: 6 Hydrogens
  • Cyclohexane: 12 Hydrogens
  • Methyl Ether: 6 Hydrogens
• Use of chemical environment to understand x-axis position.

Chemical Environments and NMR Shifts

• Ethane and Cyclohexane: Simple alkane environments.
• Ether Example: Hydrogen attached to carbon connected to oxygen (ether environment).

Advanced Example: Molecule with Multiple Functional Groups

• Goal: Determine number of proton NMR absorptions.
• Multiple CH groups identified, categorized into signals.
• Technique for identifying non-equivalent protons using the X group method.

Identifying Non-Equivalent Protons

• Use of symmetry and replacement technique to distinguish different signals.
• Example with non-equivalent CH3 groups due to proximity to oxygen.

Chemical Shifts and Expected NMR Ranges

• Reference to tables for expected proton positions on the x-axis.
• Examples of functional groups and their expected chemical shift ranges.

Understanding Parts Per Million (PPM)

• Explanation of TMS (tetramethylsilane) as a standard reference.
• Use of PPM for consistency across different instruments.

Practical Example: Tert-Butyl Acetate

• Integration: Understanding the number of hydrogens from signal peaks.
• Splitting Patterns: Introduction to the n+1 rule for predicting peak splitting.
• Chemical Shifts: Using tables to confirm the presence of functional groups.

Conclusion

• The utility of proton NMR in determining chemical structures.
• Key Components:
  • Chemical shift
  • Integration
  • Splitting (to be covered in future lectures)
• Practical application: Puzzle pieces of integration, chemical shift, and splitting help confirm structures.

Note: This lecture included a practical segment on using NMR tables and identifying functional groups based on chemical shifts.