Organic chemistry tricks that make spectroscopy — nmr & ir stick

Broad 3200–3550 = O-H stretch. Broad 3100–3500 = N-H. Sharp 2700–2850 = aldehyde C-H. 1710 = ketone C=O. 1735 = ester C=O. 1715 = carboxylic acid. 1680 = amide. Below 1500 = fingerprint region.

Downfield (high ppm) = deshielded. Upfield (low ppm) = shielded. TMS reference = 0 ppm. Alkyl H: 0–2. Allylic: 2–3. Alkyne: 2–3. Alkene: 4.5–6.5. Aromatic: 6.5–8.5. Aldehyde: 9–10. COOH: 10–12.

A proton with n equivalent neighboring H's splits into n+1 peaks. Singlet(1), doublet(2), triplet(3), quartet(4), quintet(5). Integration tells number of H's. Coupling constant J measures peak separation.

M+ peak = molecular weight. M+1 peak: 13C isotope contribution (~1.1% per carbon). M+2 peak: indicates Cl (M:M+2 = 3:1) or Br (M:M+2 = 1:1). Base peak = most abundant fragment.

DEPT (Distortionless Enhancement by Polarization Transfer) distinguishes C types: CH3 and CH give positive peaks, CH2 gives negative peaks, quaternary C gives no signal. Used with 13C NMR.

The area under each NMR peak is proportional to the number of protons responsible for that signal. Integration is reported as a ratio. Example: ethanol (CH₃CH₂OH) shows peaks with ratio 3:2:1 (methyl:methylene:OH). To determine actual number of protons, use molecular formula. Integration + splitting pattern + chemical shift together identify each proton environment. Integration does NOT give absolute numbers — only ratios.

¹³C NMR gives one peak per chemically distinct carbon (due to low natural abundance of ¹³C, C-C coupling is negligible). Key differences from ¹H NMR: NO integration (peak heights are not proportional). NO splitting from adjacent carbons (decoupled in routine spectra). Chemical shifts: 0–50 ppm (alkyl), 50–90 ppm (C-O or C-X), 100–150 ppm (alkene/aromatic), 160–220 ppm (carbonyl). Count peaks = count unique carbon environments. Equivalent carbons give ONE peak.

Not all deshielding is due to electronegativity — ring currents in aromatic compounds create a magnetic field that deshields protons on the periphery → aromatic H appear at 6.5–8.5 ppm (far downfield for C-H). Aldehyde H also deshielded by C=O anisotropy (9–10 ppm). Alkyne H are shielded by the cylindrical pi system → appear upfield (~2–3 ppm) despite being on sp carbon. These are tested on every spectroscopy exam.

Common fragmentation patterns: Alpha cleavage: bond adjacent to carbonyl, heteroatom, or radical breaks. Methyl loss: M-15 (loss of CH₃). Loss of water: M-18 (alcohols). Loss of HCl: M-36 (chloro compounds). McLafferty rearrangement: gamma-H transfer → loss of alkene (M - alkene). Base peak = most stable carbocation fragment. Work backwards: M⁺ minus fragment masses = identify structural pieces. Odd molecular ion → nitrogen present (nitrogen rule).

UV-Vis measures electronic transitions. pi → pi* transitions: conjugated dienes (~217 nm), extended conjugation shifts to longer wavelength (bathochromic shift). n → pi* transitions: lone pair → pi* (carbonyl ~280 nm, low intensity). More conjugation = longer wavelength absorption = color. Chromophore = light-absorbing group (C=C, C=O, aromatic). Auxochrome = group that shifts absorption (NH₂, OH). Beer-Lambert law: A = εlc. Used to determine conjugation and aromatic systems.

Step 1: Calculate DoU from molecular formula (rings + pi bonds). Step 2: Mass spectrum — get molecular weight (M⁺) and fragmentation clues. Step 3: IR — identify functional groups (carbonyl ~1700, OH broad 3300, NH 3100–3500). Step 4: ¹H NMR — count environments (peaks), count H per environment (integration), identify neighbors (splitting), identify type (chemical shift). Step 5: ¹³C NMR or DEPT — confirm carbon types. Step 6: assemble structure consistent with ALL data.

Chemically equivalent protons give ONE NMR signal. Two H atoms are equivalent if they can be interchanged by a rotation axis or mirror plane of symmetry. Examples: CH₄ — 1 NMR signal (4 equivalent H). CH₃Cl — 1 signal. CH₂Cl₂ — 1 signal. CH₃CH₃ — 1 signal. ClCH₂CH₂Cl — 1 signal (the two CH₂ are equivalent). Diastereotopic H (adjacent to chiral center, in different environments) are NOT equivalent — give different signals.

Normal vicinal coupling (³J) is between protons on adjacent carbons (~6-8 Hz for free rotation). Long-range coupling (⁴J and beyond): allylic coupling through C=C (~0-3 Hz, small). W-arrangement: 4 sigma bonds in a specific geometry allows 4-bond coupling. Aromatic coupling: ortho (³J ~8 Hz), meta (⁴J ~2 Hz), para (⁵J ~0-1 Hz). Geminal coupling (²J on sp³): between protons on same carbon (~10-16 Hz). Reported as J-value in Hz.