Spectroscopy and Laboratory Techniques

Core Review

IR Spectroscopy — Approximate Absorption Ranges

IR identifies functional groups by their characteristic bond vibrations. Values below are approximate — the DAT tests ranges, not exact wavenumbers.

Key diagnostic regions:

• >3000 cm⁻¹: sp²/sp C-H, O-H, N-H
• <3000 cm⁻¹: sp³ C-H
• ~1700 cm⁻¹: C=O (the single most diagnostic peak)
• 1500–400 cm⁻¹: Fingerprint region (complex, unique to each compound)

¹H NMR Spectroscopy

Four pieces of information from every signal:

1. Chemical Shift (δ, ppm): Where the signal appears.

• 0–1.5: Alkane C-H (shielded)
• 2.0–2.5: α to carbonyl, allylic, benzylic
• 3.0–4.5: C-H attached to electronegative atom (O, N, X)
• 4.5–6.5: Alkene C-H
• 6.5–8.5: Aromatic C-H
• 9.0–10.0: Aldehyde C-H (very characteristic!)
• 10–12: Carboxylic acid O-H

2. Integration: The area under each signal is proportional to the NUMBER of equivalent protons. Spectra give integration as a ratio (e.g., 3:2:1 = 3H, 2H, 1H).

3. Splitting (Multiplicity): n+1 rule — a proton signal is split into n+1 peaks, where n = number of non-equivalent neighboring protons (³J coupling, three bonds away).

• 0 neighbors = singlet (s)
• 1 neighbor = doublet (d)
• 2 neighbors = triplet (t)
• 3 neighbors = quartet (q)
• Multiple different neighbors = multiplet (m)

OH and NH protons often appear as broad singlets (exchangeable with D₂O — the signal disappears).

4. Number of Signals: Chemically equivalent protons give ONE signal. Protons are equivalent if they are related by symmetry (rotation, reflection) or are in identical chemical environments.

¹³C NMR Spectroscopy

• Number of signals = number of non-equivalent carbon environments.
• Chemical shift ranges: 0–50 ppm (sp³ C), 50–100 ppm (C-O, C-N, C-X), 100–160 ppm (sp² C — alkene, aromatic), 160–220 ppm (C=O carbonyl).
• ¹³C spectra are typically proton-decoupled — each carbon gives a SINGLET (no splitting by attached protons in standard spectra).
• DEPT experiments distinguish CH₃, CH₂, CH, and quaternary carbons.

Mass Spectrometry (MS)

• Molecular ion (M⁺): The peak at the highest m/z (assuming no M+1 or M+2 from isotopes). Gives the molecular weight.
• Base peak: The tallest peak (most abundant fragment), set to 100% relative abundance.
• Isotope patterns — CRITICAL for DAT:
  • Chlorine: ³⁵Cl:³⁷Cl ≈ 3:1. M⁺:M+2 ≈ 3:1 ratio.
  • Bromine: ⁷⁹Br:⁸¹Br ≈ 1:1. M⁺:M+2 ≈ 1:1 ratio.
  • Multiple Cl/Br atoms create characteristic cluster patterns.
• Fragmentation: Common losses — CH₃ (M-15), OH (M-17), H₂O (M-18), C₂H₅ (M-29), etc. α-cleavage and McLafferty rearrangement in carbonyl compounds.

Laboratory Techniques

Extraction (Liquid-Liquid):

• Separates compounds based on solubility differences between two immiscible solvents (typically aqueous and organic).
• Density determines which layer is on bottom — do NOT assume aqueous is always the bottom. Chlorinated solvents (CH₂Cl₂, CHCl₃, d ~1.3–1.5 g/mL) are denser than water and form the bottom layer.
• Acidic compounds (carboxylic acids, phenols) can be extracted into aqueous base (NaOH, NaHCO₃).
• Basic compounds (amines) can be extracted into aqueous acid (HCl).
• Neutral compounds stay in the organic layer.

Distillation:

• Simple distillation: Separates liquids with large boiling point differences (>50°C apart).
• Fractional distillation: Separates liquids with small boiling point differences (<50°C apart). Uses a fractionating column for multiple vaporization-condensation cycles.

Recrystallization:

• Purifies solid compounds based on differential solubility at different temperatures.
• Choose a solvent where the compound is soluble hot but poorly soluble cold.
• Impurities should be either always soluble or always insoluble.

Chromatography:

• Thin Layer Chromatography (TLC): Polar stationary phase (silica gel), nonpolar mobile phase. More polar compounds move slower (lower Rf). Nonpolar compounds move faster (higher Rf). Rf = distance traveled by spot / distance traveled by solvent front.
• Column Chromatography: Same principle, scaled up for purification. Flash chromatography uses air pressure to speed elution.
• "Polar sticks to polar" — polar compounds hydrogen-bond to the silica and move slowly.

Common Traps

• Assuming aqueous layer is always on bottom: Dichloromethane and chloroform are denser than water and form the bottom layer.
• IR values as exact numbers: The DAT gives ranges. Don't memorize one magic number for C=O; know that aldehydes ~1725, ketones ~1715, esters ~1735.
• NMR integration: The ratio gives the relative number of protons, but you must multiply to get the actual number (using the molecular formula).
• Forgetting equivalent protons: Symmetrical molecules give fewer NMR signals than the total number of protons suggests. para-disubstituted benzenes with identical substituents: the four aromatic protons are often equivalent in pairs.
• OH peak disappearing with D₂O shake: Exchangeable protons are replaced by deuterium (no signal in ¹H NMR).