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Espectroscopia e Determinação Estrutural · Reading each technique

RMN de próton: deslocamento, integração, desdobramento e acoplamento

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O que você vai aprender

  • Predict and rank ¹H chemical shifts from electronegativity, carbonyl groups, π bonds and the aromatic ring current, and convert between Hz and ppm.
  • Count chemically distinct proton environments and turn integrals into numbers of hydrogens.
  • Predict multiplicities with the n + 1 rule and recognise when equivalent or exchanging protons do not split.
  • Use coupling constants to assign alkene geometry, aromatic substitution patterns and coupled partners.
  • Recognise diastereotopic protons and use a D₂O shake to identify exchangeable O–H protons.
  • Choose the structure that fits a molecular formula and a complete ¹H NMR data set.

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Chemical shift: where a signal appears

Each proton feels the applied field reduced slightly by the electrons around it (shielding). Shifts are reported in ppm relative to tetramethylsilane (TMS, δ 0.00): δ = (signal frequency − TMS frequency in Hz) ÷ (spectrometer frequency in MHz). Electron-withdrawing neighbours remove shielding and move a signal downfield (to larger δ). Typical ranges in CDCl₃: alkyl C–H δ 0.9–1.7; C–H next to C=O or an arene δ 2.0–2.7; C–H on a carbon bearing O or a halogen δ 3.0–4.5 (each extra electronegative atom adds roughly 2 ppm, so CH₃Cl ≈ 3.05, CH₂Cl₂ ≈ 5.30, CHCl₃ ≈ 7.26); alkene C–H δ 4.6–6.5; aromatic C–H δ 6.5–8.5, pushed downfield by the ring current that circulating π electrons set up in the field; aldehyde C–H δ 9.5–10; carboxylic acid O–H δ 10–13. In ethyl acetate (shown) the OCH₂ appears at δ 4.12, the acetyl CH₃ at δ 2.04 and the ethyl CH₃ at δ 1.26.

Equivalence and integration: how many and how many hydrogens

Protons that are interchanged by a symmetry operation or by fast rotation are chemically equivalent and give one signal; a CH₃ group always counts as one environment. p-Xylene (shown) has only two environments: four aromatic H and six methyl H. The area under each signal (its integral) is proportional to the number of hydrogens it represents. Integrals give a ratio, not absolute numbers: divide each integral by the total and multiply by the number of hydrogens in the molecular formula. Integrals of 2 : 3 : 3 for C₄H₈O₂ mean 2H, 3H and 3H.

Splitting: the n + 1 rule

A proton couples to non-equivalent protons on neighbouring atoms, usually those three bonds away (H–C–C–H). With n equivalent neighbours its signal splits into n + 1 lines whose intensities follow Pascal's triangle: doublet 1:1, triplet 1:2:1, quartet 1:3:3:1, septet 1:6:15:20:15:6:1. In 2-bromopropane (shown) the CH has six methyl neighbours and appears as a septet at δ 4.21 (1H), while the six methyl H have one neighbour and appear as a doublet at δ 1.71 (6H). Two rules prevent false splitting: equivalent protons do not split each other (1,2-dichloroethane gives a singlet), and O–H or N–H protons that exchange rapidly between molecules usually appear as broad singlets and do not split their neighbours. When a proton has two different sets of neighbours with different coupling constants, the patterns multiply (a doublet of doublets has four lines).

Coupling constants carry geometry

The spacing between lines of a multiplet is the coupling constant J, measured in Hz; it does not change with the spectrometer field, whereas the separation of two different shifts in Hz grows with the field. Coupled partners always show the same J. Typical values: free-rotating H–C–C–H ≈ 7 Hz; alkene H–C=C–H trans 12–18 Hz, cis 6–12 Hz (up to ≈13 Hz in conjugated alkenes), geminal =CH₂ 0–3 Hz; aromatic ortho 7–9 Hz, meta 1–3 Hz, para under 1 Hz. Within one alkene, trans is always larger than cis. In (E)-cinnamic acid (shown) the two alkene protons are doublets with J ≈ 16 Hz, as expected for trans hydrogens; the (Z) isomer shows J ≈ 12.5 Hz. A para-disubstituted benzene with two different groups gives two 2H doublets with J ≈ 8–9 Hz.

Diastereotopic and exchangeable protons

Two protons on one CH₂ are diastereotopic when replacing each in turn by a test group gives diastereomers. This happens whenever the molecule already contains a stereocentre, as in butan-2-ol (shown). Diastereotopic protons are not equivalent: they can have different shifts, couple to each other (geminal J ≈ 12–17 Hz on sp³ carbon, at the top of that range next to C=O) and couple differently to their neighbours. Protons on oxygen or nitrogen exchange with D₂O. Shaking the CDCl₃ solution with a drop of D₂O replaces O–H by O–D, which is invisible in a ¹H spectrum, so the O–H signal disappears (a new HOD signal appears near δ 4.8). C–H protons do not exchange under these conditions; α-C–H protons of ketones exchange only when a catalyst is added, such as NaOD (through the enolate) or a strong acid (through the enol).

From data to structure

Work in a fixed order. From the formula, compute the degree of unsaturation (2C + 2 + N − H − X) ÷ 2; four or more suggests a benzene ring. Use integrals to fix how many hydrogens each signal holds, and symmetry to explain fewer signals than expected. Use shifts to place each group next to O, C=O or an arene, and splitting to connect neighbours: a 2H quartet with a 3H triplet is an ethyl group, a 1H septet with a 6H doublet is an isopropyl group, and singlets are groups with no H on the next atom. Finally check every signal of your candidate against the data: isomers such as ethyl acetate and methyl propanoate share the same multiplicities and differ only in where the signals fall.

What to carry forward

Read each signal four ways: shift says what the proton is attached to, integration says how many, splitting says how many neighbours, and J says how they are arranged. Then build the candidate and check every signal against it.

Problemas de prática (40)

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Desenhe o mecanismo 2

Stereotopic and exchanging protons

  • Acid-catalysed enolization of pentane-2,4-dioneDraw the acid-catalysed conversion of pentane-2,4-dione into its enol, and explain why its ¹H NMR spectrum shows separate signals for the keto and enol tautomers.
  • Acid-catalysed exchange of the ethanol O–H protonDraw the two proton transfers by which a trace of acid swaps the O–H proton of ethanol for a proton from the medium, and explain why fast exchange removes O–H splitting from the ¹H NMR spectrum.

Preveja o produto 8

Equivalence, integration and splitting

  • Multiplicity of the CH₂ in chloroethanePredict the multiplicity of a CH₂ group with a CH₃ neighbour by the n + 1 rule.
  • Lines in the CH signal of 2-bromopropaneApply the n + 1 rule to a proton with six equivalent neighbours.
  • The middle CH₂ of 1-bromopropanePredict the multiplicity of a CH₂ flanked by two different groups with similar coupling constants.
  • Line intensities of a quartetPredict the relative line intensities of a quartet from Pascal's triangle.

Coupling constants

  • Predict the alkene methyl signal of ethyl crotonatePredict a multiplet from the coupling constants its partners display.

Stereotopic and exchanging protons

  • Propanoic acid after a D₂O shakePredict which signals of a carboxylic acid change after shaking with neutral D₂O.

Structure from data

  • Predict the spectrum of 2-methylpropanalPredict the shifts, integrals and multiplicities of an aldehyde with an isopropyl group.

Chemical shift

  • Which compound has a 3H singlet near δ 3.7?Predict which compound has a methoxy singlet by locating a CH₃ bonded to oxygen.

Classificar 2

Chemical shift

  • What kind of proton is at δ 9.78?Classify a sharp, coupled 1H signal near δ 9.8 as an aldehyde proton.

Stereotopic and exchanging protons

  • The two =CH₂ hydrogens of chloroetheneClassify the geminal hydrogens of a monosubstituted alkene as diastereotopic by the substitution test.

Comparar 5

Chemical shift

  • More chlorines, larger shiftRank the chloromethanes by ¹H shift using the additive deshielding of electronegative substituents.
  • Rank five one-signal compounds by shiftOrder alkyl, α-carbonyl, ether, alkene and aromatic protons by chemical shift.

Stereotopic and exchanging protons

  • Find the diastereotopic CH₂Identify which molecule has diastereotopic CH₂ hydrogens by locating an existing stereocentre.

Equivalence, integration and splitting

  • Rank by number of ¹H environmentsUse molecular symmetry to rank compounds by their number of distinct proton environments.

Coupling constants

  • Rank the three vinyl couplings of styreneRank geminal, cis and trans alkene couplings within one vinyl group.

Calcular 5

Stereotopic and exchanging protons

  • Percent enol of pentane-2,4-dioneCalculate a tautomer ratio from integrals by dividing each integral by the number of hydrogens it represents.

Chemical shift

  • Convert a frequency offset into δConvert a signal's offset from TMS in Hz into a chemical shift in ppm.

Equivalence, integration and splitting

  • Turn integrals into hydrogensConvert relative integrals into numbers of hydrogens using the molecular formula.
  • Count environments in ethyl propanoateCount chemically distinct proton environments in an ester with two ethyl groups.

Coupling constants

  • Measure J from a doubletCalculate a coupling constant in Hz from the ppm positions of two lines and the spectrometer frequency.

Encontre o erro 1

  • Find the flaw in an ester assignmentDiagnose an assignment that matches multiplicities and integrals but ignores chemical shifts.

Interprete as evidências 12

Structure from data

  • Identify a C₄H₈O carbonyl compoundIdentify a methyl ketone from an ethyl pattern and a 3H singlet near δ 2.1 with no aldehyde signal.
  • Two signals for C₄H₁₀ORecognise a symmetrical ether from a 4H quartet and 6H triplet.
  • Which phenyl ester?Distinguish ester isomers with the same multiplicities by the shifts of their CH₂ groups.
  • Where is the isopropyl group?Place an isopropyl group on oxygen or on carbon from the shift of its septet.
  • Two singlets for C₉H₁₂Use symmetry and integration to identify a symmetrically trisubstituted benzene.
  • Identify a C₄H₈O₂ esterChoose between ester isomers using the shifts of an O–CH₃ singlet and an α-CH₂ quartet.
  • One singlet for C₄H₉BrUse a single 9H singlet to identify a tert-butyl group.
  • Identify a C₈H₈O₂ acidCombine an exchangeable acid proton, a para pattern and an aryl methyl singlet to identify a structure.

Coupling constants

  • Cis or trans cinnamate?Assign alkene geometry from the vicinal H–C=C–H coupling constant.
  • Which methoxyacetophenone?Recognise the two-doublet aromatic pattern of a para-disubstituted benzene.
  • Assign the =CH₂ hydrogens of vinyl acetateUse cis and trans coupling constants to assign the two geminal hydrogens of a vinyl group.

Chemical shift

  • Find the acetyl methyl in ethyl acetateAssign the acetyl methyl signal of ethyl acetate from shift, integration and multiplicity.

Planeje uma síntese 1

  • Prove a signal is an O–HChoose the D₂O-exchange experiment to identify an O–H signal.

Explique por quê 4

Chemical shift

  • Why the β-hydrogen of an enone is downfieldExplain the downfield β-alkene proton of a conjugated enone by resonance.
  • Why aromatic H are further downfield than alkene HExplain the extra deshielding of aromatic protons by the ring current.

Equivalence, integration and splitting

  • Why 1,2-dichloroethane gives a singletExplain why coupling between equivalent protons produces no splitting.

Stereotopic and exchanging protons

  • Two 1H signals for one CH₂Explain why the CH₂ next to a stereocentre gives two separate doublets of doublets.

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