MCAT Organic Chemistry - Chapter 11: Spectroscopy

Introduction

• Host: Iman
• Chapter: 11
• Topic: Spectroscopy
• Objectives:
  1. Infrared (IR) Spectroscopy
  2. Ultraviolet (UV) Spectroscopy
  3. Nuclear Magnetic Resonance (NMR) Spectroscopy

Objectives for the Chapter

1. Infrared Spectroscopy (IR)

   • Technique introduction
   • Defining intramolecular vibrations and rotations
   • Discussing characteristic absorptions for the MCAT

2. Ultraviolet Spectroscopy (UV)

   • Focuses on conceptual discussions such as electron transitions and conjugated systems
   • No need to interpret spectra for the MCAT

3. Nuclear Magnetic Resonance (NMR) Spectroscopy

   • Focus on Hydrogen NMR (H-NMR)

General Overview

• Spectroscopy: Measures energy differences between molecular states by determining frequencies of absorbed electromagnetic radiation.
• Medical Importance: MRI scanners measure H-NMR Spectra of water molecules in different body environments converting signals into grayscale for visualization.

Infrared Spectroscopy (IR)

• Method: Uses infrared light to detect chemical bonds through vibrational excitations.
• Molecular Vibrations: Bonds stretch, bend, etc., at specific frequencies.
• Energy Gaps and Photon Absorption: Energy gap between vibrational states must match photon's energy for absorption.
• IR Spectrum Analysis: Measures absorbance vs. frequency.
  • Fingerprint Region: 1500 to 400 wave numbers
  • Diagnostic Region: 4000 to 1500 wave numbers

Important IR Peaks to Remember

• Hydroxyl (O-H):

  • Alcohol: ~3300 (broad peak)
  • Carboxylic acids: ~3000 (broad peak)

• Carbonyl (C=O): ~1700 (sharp peak)

  • Different functional groups slightly modify this value

• Amines (N-H): ~3300 (sharp peak, less broad compared to O-H)

• Alkane (C-H):

  • Single bond: Right below 3000
  • Alkene (C=C-H): Right around 3100
  • Alkyne (C≡C-H): Just above 3300

Ultraviolet Spectroscopy (UV)

• Basic Understanding: No spectra interpretation required for the MCAT.

• How It Works:

  • UV light passes through a sample dissolved in a nonabsorbing solvent.
  • Absorbance plotted against wavelength.
  • Wavelength of Maximum Absorbance indicates the degree of conjugation.

• Key Points:

  • More Conjugation: Lower energy transition and greater wavelength of max absorbance.
  • Pi and Non-bonding Electrons: Can be excited to anti-bonding orbitals.
  • Conjugated Systems: Shifts absorption to higher wavelengths (lower frequencies).
  • HOMO-LUMO Gap: Small gap allows absorption of longer wavelengths.

Conclusion and Next Steps

• Review practice problems in next video on IR, UV, and later, NMR spectroscopy
• UV Spectroscopy measures absorption caused by electron transitions
• IR Spectroscopy identifies functional groups through vibrational modes
• Further details and problems in Organic Chemistry playlist, Chapters 14 and 15