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Organic Chemistry 2 Unit 1: Spectroscopy: IR, NMR and Mass Spec

Spectroscopy lets you deduce a structure from data: the formula gives the degrees of unsaturation, IR shows functional groups, mass spec gives the mass and fragments, and NMR maps the carbon-hydrogen framework.

Covers: Degrees of unsaturation, IR spectroscopy and functional groups, Mass spectrometry and fragmentation, ¹H NMR chemical shift, Integration and spin-spin splitting, ¹³C NMR and DEPT, Combined structure determination

Study guide

Start every structure problem with the molecular formula and the degrees of unsaturation (DoU), also called the index of hydrogen deficiency. DoU = (2C + 2 + N − H − X)/2, where X is the number of halogens; oxygen and sulfur are ignored. Each ring or π bond counts as one, so a C=O or C=C is one, a triple bond is two and a benzene ring is four (one ring plus three π bonds). A DoU of four or more with six or more carbons should make you think of a benzene ring. A DoU of zero means a saturated, acyclic compound such as an alcohol, ether or alkane. The DoU narrows the possibilities before you look at a single spectrum.

Infrared (IR) spectroscopy measures absorptions from bond vibrations, reported in wavenumbers (cm⁻¹). The most useful region is above 1500 cm⁻¹. A strong, sharp band near 1700 cm⁻¹ is a C=O: about 1715 for ketones, 1730 for aldehydes, 1735 to 1750 for esters, and lower (about 1685 to 1690) when the carbonyl is conjugated with a ring or C=C. A broad band from 3200 to 3550 cm⁻¹ is an alcohol O–H; a very broad band from 2500 to 3300 cm⁻¹ with a C=O is a carboxylic acid. N–H appears near 3300 to 3500, C≡N near 2250, and C≡C near 2100 to 2260. Aldehydes also show two weak C–H bands near 2720 and 2820 cm⁻¹.

Mass spectrometry (electron ionization) knocks an electron out of the molecule to make a radical cation, the molecular ion M⁺, whose m/z equals the molecular mass. The tallest peak is the base peak. Isotope patterns identify halogens: chlorine gives an M+2 peak about one third as tall as M (³⁵Cl:³⁷Cl ≈ 3:1), and bromine gives M and M+2 of nearly equal height (⁷⁹Br:⁸¹Br ≈ 1:1). The nitrogen rule says an odd molecular mass means an odd number of nitrogens. Common fragmentations include loss of CH₃ (M − 15), α-cleavage next to a carbonyl to give an acylium ion (CH₃CO⁺ at m/z 43), and the McLafferty rearrangement, in which a carbonyl with a γ-hydrogen loses an alkene.

¹H NMR gives four kinds of information. The number of signals equals the number of chemically distinct hydrogen environments, found by symmetry. Chemical shift (ppm, relative to TMS at 0) tells the environment: alkyl C–H 0.9 to 1.7, H next to C=O or on a benzylic carbon 2.0 to 2.7, H on carbon bearing O or halogen 3.3 to 4.5, vinyl 4.5 to 6.5, aromatic 6.5 to 8, aldehyde 9 to 10, and carboxylic acid 10 to 13. Integration gives the relative number of H in each signal. Splitting follows the n + 1 rule: a signal with n equivalent neighboring H on adjacent carbons splits into n + 1 peaks. A triplet (3H) plus quartet (2H) pattern is the signature of an ethyl group; a 6H doublet and 1H septet signal an isopropyl group.

¹³C NMR shows one line per distinct carbon, normally without splitting. Shifts span 0 to 220 ppm: sp³ carbons 10 to 50, C–O carbons 50 to 90, alkene and aromatic carbons 100 to 150, nitriles near 120, ester and acid carbonyls 165 to 185, and aldehyde and ketone carbonyls 190 to 220. DEPT experiments classify carbons: in DEPT-135, CH and CH₃ point up, CH₂ point down, and carbons with no H disappear; DEPT-90 shows only CH. To solve a combined problem, compute DoU, read IR for functional groups, use mass spec for mass and halogens, then build fragments from NMR shifts, integration and splitting and assemble them so every atom is accounted for.

Key ideas

• Degrees of unsaturation count rings plus π bonds and come straight from the molecular formula.
• IR identifies functional groups, especially C=O near 1700 cm⁻¹ and O–H or N–H above 3000 cm⁻¹.
• Conjugation lowers a carbonyl stretching frequency by about 25 to 30 cm⁻¹.
• Mass spec gives the molecular mass, halogen isotope patterns and characteristic fragments.
• The number of ¹H NMR signals reflects symmetry, and the n + 1 rule describes splitting by neighbors.
• Electronegative atoms and π systems deshield protons and move them downfield (higher ppm).
• ¹³C NMR and DEPT count distinct carbons and classify them as C, CH, CH₂ or CH₃.

Formulas and rules

Common mistakes

• Forgetting to subtract halogens or add nitrogens when calculating degrees of unsaturation.
• Counting hydrogens on the same carbon as neighbors when applying the n + 1 rule.
• Reading integration as absolute numbers instead of ratios that must be scaled to the formula.
• Mistaking a broad alcohol O–H band for the much broader carboxylic acid O–H that overlaps C–H stretches.
• Assuming the base peak is the molecular ion; the molecular ion may be small or absent.

Before the test, make sure you can

• Calculate degrees of unsaturation for formulas containing N, O and halogens.
• Identify carbonyl, hydroxyl, carboxylic acid, amine, nitrile and alkyne bands in an IR description.
• Use M+2 patterns and the nitrogen rule to detect Cl, Br and N in a mass spectrum.
• Explain common fragments such as M − 15, m/z 43 and McLafferty peaks.
• Predict the number of ¹H and ¹³C signals for a structure using symmetry.
• Predict chemical shift, integration and splitting for each proton in a molecule.
• Interpret DEPT-135 and DEPT-90 data.
• Assemble a structure from a formula plus IR, MS and NMR data.

Words to know

Degrees of unsaturation

The number of rings plus π bonds in a molecule, calculated as (2C + 2 + N − H − X)/2.

Wavenumber

The IR frequency unit, cm⁻¹; higher wavenumbers correspond to stronger bonds or lighter atoms.

Fingerprint region

The IR region below about 1500 cm⁻¹, full of complex bands unique to each molecule.

Molecular ion (M⁺)

The radical cation formed when a molecule loses one electron; its m/z equals the molecular mass.

Base peak

The most intense peak in a mass spectrum, set to 100% relative abundance.

Nitrogen rule

A compound with an odd molecular mass contains an odd number of nitrogen atoms.

McLafferty rearrangement

A mass-spec fragmentation in which a carbonyl compound with a γ-hydrogen transfers it and loses a neutral alkene.

Chemical shift

The position of an NMR signal in ppm relative to TMS, reflecting the electronic environment of the nucleus.

Deshielding

Reduction of electron density around a nucleus that moves its signal downfield to higher ppm.

Integration

The area under a ¹H NMR signal, proportional to the number of hydrogens producing it.

n + 1 rule

A proton with n equivalent neighbors on adjacent atoms is split into n + 1 peaks.

Coupling constant (J)

The spacing in Hz between peaks of a split signal; coupled protons share the same J.

Chemically equivalent protons

Hydrogens that are interchangeable by symmetry and therefore give one NMR signal.

TMS

Tetramethylsilane, the reference compound assigned 0 ppm in ¹H and ¹³C NMR.

DEPT

A ¹³C NMR technique that distinguishes CH₃, CH₂, CH and quaternary carbons.

Acylium ion

A resonance-stabilized R–C≡O⁺ cation formed by α-cleavage of carbonyl compounds in mass spec.

Sample practice questions

30 more questions like these, from level 1 to 5, are in the practice set.