Organic Chemistry Lecture 3 PDF

# Proton Nuclear Magnetic Resonance Spectroscopy (¹H-NMR) **It is a technique used to determine a compound's unique structure.** It identifies the carbon-hydrogen framework of an organic compound. It gives information about the number and type of hydrogen atoms in the molecule. # Magnet...

# Proton Nuclear Magnetic Resonance Spectroscopy (¹H-NMR) **It is a technique used to determine a compound's unique structure.** It identifies the carbon-hydrogen framework of an organic compound. It gives information about the number and type of hydrogen atoms in the molecule. # Magnetic Resonance Imaging (MRI) Magnetic resonance imaging (MRI) uses magnetic fields and radio frequencies to create a three-dimensional picture of structures inside the body. An MRI can diagnose diseases of the brain, spine, skeleton, abdomen, and pelvis. An MRI image is more detailed than an X-ray, ultrasound, or CT scan. # Theory **With no external magnetic field** - A spinning proton creates a magnetic field. - The nuclear magnets are randomly oriented. **In a magnetic field** - The nuclear magnets are oriented with or against B₀. **v = Resonance frequency (specific to nucleus)** - ΔE = γHAB₀ - hv = ΔE **Excitation:** Transition between lower. **Relaxation:** Return to thermal. # NMR-Chart: - **Downfield (deshielded)**: Protons in electron poor environments. - **Upfield (shielded)**: Protons in electron-rich environments. # 1. Number of Signals Each group of chemically equivalent protons gives rise to a signal. Signals represent the number of protons types, not the number of protons. **How many signals are there in the ¹H-NMR spectrum for this molecule?** - CH₃OCH₃ - CH₃CH₂Cl - CH₃OCH₂CH₃ # II. Position of Signals The positions of the signals in an NMR spectrum are based on how far they are from the signal of the reference compound (Chemical shift, δ). This information tells us the kind of proton. **Reference compound:** Tetramethylsilane (TMS) is usually used as the reference compound because: - It is inert, volatile and soluble in most organic solvents. - TMS gives intense single peak because it contains 12 equivalent protons. - It is at a lower frequency (highly shielded) than most other signals because its methyl protons are in a more electron dense environment than most protons are because silicon is less electronegative than carbon. **Solvents:** CDCI₃, DMSO, CD₂OD The unit used to measure chemical shift, δ, is a ppm which stands for parts per million. # 2) Anisotropic effect: - **Magnetic anisotropy** is the magnetic field created by pi (π) electrons or rings. - π electrons are held less strongly than sigma (σ) electrons, so π electrons are more able to move in response to the magnetic field. **How this affects the chemical shift?** It depends on the direction of the induced magnetic field relative to the direction of the applied magnetic field. In π electrons found in the benzene ring and an alkene, the magnetic field induced is in the same direction as the applied magnetic field, so the protons feel a larger effective magnetic field. Therefore, the protons undergo resonance at a higher frequency due to the π electrons. If the magnetic field induced is oriented in the opposite direction as the applied magnetic field, the protons will feel a. # 3) Hydrogen Bonding Hydrogen bonding causes deshielding of protons due to lowering of the electron density around the proton. - **(X-H---O-CH-)** - **a) C-H aliphatic:** 1-4 ppm - **b) N-H or O-H:** 1-5 [D₂O exchangeable] - **c) C-H aromatic:** 6.5-9 ppm - **d) H aldehydic:** 9-10 ppm - **e) H (COOH):** 10-12 [D₂O exchangeable] # III. Integration (Intensity) of signals Integration is the area measurement that tells us the relative number of protons that give rise to each signal. **Problem:** Explain how this molecule produces this ¹H-NMR spectra using: - Number of signals: - Position of the signals: - Integration of the signals. CH₃ | CH₃CCH₂Br | CH₃ 7.0 | 1.6 - 7/1.6 = 4.3, 1.6/1.6 = 1 - Ratio = 4.3:1, sum = 5.3 - Total no. of Hs = 11, 11/5.3 = 2 - Total no. of Hs for peak 7 = 4.3 x 2 = 8.6 = 9 Hs - Total no. of Hs for peak 1.6 = 2 x 1 = 2 Hs # IV. Splitting of Signals Splitting of signals tells us the number of protons bonded to adjacent carbons using N+1 Rule. - **N** is the number of equivalent protons that are bonded to the adjacent carbons. **Using the N+1 rule, the signal for a proton with N neighbors is split into N+1 lines.** - If a proton has no neighbors, it is a singlet (s). - If it has one neighbor, it is a doublet (d). - If it has two neighbors, it is a triplet (t). - If it has three neighbors, it is a quartet (q). - If it has four neighbors, it is a pentet. # Spin-Spin Splitting | Number of equivalent hydrogen atoms causing splitting | Multiplicity | Relative peak intensities | |---|---|---| | 1 | Singlet | - | | 2 | Doublet | 1:1 | | 3 | Triplet | 1:2:1 | | 4 | Quartet | 1:3:3:1 | | 5 | Quintet | 1:4:6:4:1 | | 6 | Sextet | 1:5:10:10:5:1 | | 7 | Septet | 1:6:15:20:15:6:1 | # Example **Put the chemical shifts of the nonequivalent protons from the lowest frequency to the highest frequency, CH₃CH₂CH₂NO₂**. **Answer:** CH₃CH₂CH₂NO₂ 1 2 3 1, more shielded 3, more deshielded. 0 1.95 NO 0.94 4.39 5 4 3 2 1 0 PPM # Example 2: CH₂CH₂CH₂Br - Triplet, so n=2 - Deshielded - Triplet, so n=2 - Shielded **3 types of H** Middle methylene appears as 6 lines due to 5 neighbours, 3 + 2 11 10 9 8 7 6 5 4 3 2 1 0 ppm - 6 lines, so n=5 # Examples - CH₃CH₂Cl - BrCH₂CH₂F - CICH₂CHCI₂ # II) Deduce the structure of the following compounds on the basis of their ¹H-NMR spectra and molecular formula: - a. C₄H₁₀O: δ 1.3 (s, 9H), δ 1.35 (s, 1H). - b. C₇H₈O: δ 2.4 (s, 1H), δ 4.6 (s, 2H), δ 7.2 (m, 5H). - c. C₄H₇BrO₂: δ 1.0 (t, 3H), δ 2(m, 2H), δ 4.2 (t, 1H), δ 11.0 (s, 1H). - d. C₈H₈O: δ 2.3 (s, integration line 24), δ 7.3 (m, integration line 24), δ 7.8 (m, integration line 16). - e. C₃H₆Br₂: δ 2.35 (quintet, 2H), 3.55 (t, 4H). # III) Sketch the ¹H-NMR spectrum of each of the following compounds: - a. CH₃-O-CH(CH₃)₂ - b) CH₃COCH=C(CH₃)₂ - c) p-CH₃CH₂CH₂O-Ph-CHO # Examples: - 1) CI-CH₃ - 2) CH₃CH(Br)CH₃ - 3) Ph-CH₂OCH₂CH₃ - 4) CH₃COOCH₃ - 5) CH₃COOCH₂CH₃ - 6) CH₂CH₂OH - 7) CICH₂CH₂CI