Organic Chemistry · Carboxylic Acids and Nitriles
Spectroscopy of Carboxylic Acids and Nitriles
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In 30 seconds
Spectroscopy answers "what is this molecule?" without breaking it apart. For carboxylic acids and nitriles, three techniques matter most:
- Infrared (IR) reports bond vibrations: the acid's broad O–H and C=O stretches, the nitrile's sharp C≡N stretch.
- ¹H NMR reports hydrogen environments: the acid's Downfield Higher δ (higher frequency) region of the spectrum Full entry →, exchangeable O–H proton and the α-CH₂ protons.
- ¹³C NMR reports carbon environments: acid carbonyl δ 178–180, nitrile carbon δ 115–120.
The power comes from combining techniques: IR gives the functional group, NMR the connectivity, mass spectrometry the formula. This topic builds a "spectral fingerprint" for R–COOH and R–C≡N so you can recognize them in a data problem.
Why this matters
- Core exam skill. "Identify the compound" spectral problems appear in every organic course; acid and nitrile signals are among the most recognizable.
- Industry QC: IR confirms functional groups in synthesis and raw materials; NMR confirms structure and purity.
- Identifying unknowns: broad 2500–3300 + strong ~1710 cm⁻¹ → acid; sharp ~2250 cm⁻¹ → nitrile (or alkyne — a classic trap).
- Mechanistic studies: IR tracking of C≡N loss and C=O gain follows nitrile hydrolysis in real time.
- Lab practice: these are measurement techniques; handle samples with standard rules (fume hood for volatiles, gloves, goggles).
The college version
Core Concepts
IR spectroscopy: the carboxylic acid fingerprint
Carboxylic acids show three characteristic IR features:
- Broad O–H stretch, 2500–3300 cm⁻¹. Hydrogen bonding between acid molecules (dimers) broadens the band into a "hill" overlapping the C–H stretches — the single most diagnostic feature of an acid.
- Strong C=O stretch, ~1710 cm⁻¹ for the hydrogen-bonded dimer (monomer near 1760 cm⁻¹). Conjugation lowers it (benzoic acid ≈ 1685–1700 cm⁻¹).
- C–O stretch near 1200–1300 cm⁻¹ (medium), from the C–O single bond.
IR spectroscopy: the nitrile fingerprint
Nitriles show a sharp C≡N stretch near 2250 cm⁻¹ (typically 2240–2260 cm⁻¹ for alkyl nitriles; conjugation lowers it to ~2220 cm⁻¹). The band is medium intensity and sits in a region where few other groups absorb — the main confusion is with alkynes (C≡C stretch, 2100–2260 cm⁻¹), but alkyne C≡C stretches are usually weak and often absent for symmetric alkynes, while the nitrile band is reliable and stronger.
¹H NMR: carboxylic acids
The acid O–H proton appears far downfield, δ 10–13, because of strong hydrogen bonding. It is broad and exchangeable: shaking with D₂O removes it (a standard confirmatory test). The α-CH₂ protons appear near δ 2.2–2.5; more remote protons sit in the usual alkyl region.
¹H NMR: nitriles
The nitrile has no O–H, so there is no downfield Exchangeable proton O–H (or N–H) proton that swaps with D₂O Full entry →. Its α-CH₂ (or CH₃) protons appear near δ 2.3–2.4, similar to the acid's — the presence or absence of the δ 10–13 signal distinguishes the families at a glance.
¹³C NMR
- Carboxylic acid carbonyl: δ 178–182 — upfield of aldehydes (δ ~190–205) and ketones (δ ~195–215) because resonance puts partial negative character on the carbonyl oxygen.
- Nitrile carbon: δ 115–120, far upfield because the carbon is triple-bonded and its resonance forms carry partial negative character on carbon.
- Alkyl carbons: the α carbon of an acid appears near δ 20–30, close to that of a nitrile — the carbonyl or nitrile carbon itself is the distinguishing signal.
Mass spectrometry
Mass spectrometry gives the Molecular ion (M⁺) Intact molecule with one electron removed Full entry →, fixing the molecular mass and formula. The Nitrogen rule Odd nominal mass → odd number of N atoms Full entry → helps: an odd nominal mass implies an odd number of nitrogens, and a nitrile has one N. Acid fragmentation commonly shows loss of OH (M−17) and COOH (M−45); the acylium ion R–CO⁺ is common. These are qualitative guides.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Acid O–H stretch (2500–3300) | Alcohol O–H stretch (3200–3600) | Acid band is broader, lower, and overlaps C–H; acid also shows C=O ~1710 |
| Nitrile C≡N (~2250 cm⁻¹) | Alkyne C≡C (~2100–2260 cm⁻¹) | Nitrile band is sharp, medium, reliable; alkyne stretch is weak and often absent for symmetric alkynes |
| Acid carbonyl δ 178–182 (¹³C) | Ketone/aldehyde carbonyl δ 190–215 | Acids are upfield of ketones because resonance puts partial negative character on the carbonyl oxygen |
| Acid O–H (δ 10–13, exchangeable) | Phenol O–H (δ 5–7), alcohol O–H (δ 1–5) | Hydrogen bonding plus the carbonyl push the acid proton far downfield |
| α-CH₂ of acid vs. nitrile | Both near δ 2.2–2.4 | Nearly identical in ¹H NMR; the O–H signal (or its absence) is the tiebreaker |
| Molecular ion mass | Base peak mass | M⁺ is the intact molecule; the base peak is the most abundant fragment |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Spectroscopy is like identifying a friend by voice, handwriting, and height. IR listens to the "voice" — the broad, low O–H hum of an acid versus the sharp C≡N note of a nitrile. NMR reads the "handwriting" of each hydrogen and carbon, and mass spectrometry measures "height," the molecular mass. Put the three clues together and you can name the molecule without seeing it.
Worked example
Example 1: IR identification of two isomers
Two compounds have the formula C₄H₈O₂. A shows a broad 2500–3300 cm⁻¹ band and strong 1712 cm⁻¹; B shows strong 1735 cm⁻¹ and no O–H band. Identify each.
Step 1 — Interpret A. Broad O–H plus C=O near 1710 cm⁻¹ is the acid signature: A is butanoic acid.
Step 2 — Interpret B. Strong C=O near 1735 cm⁻¹ with no O–H means an ester (ester C=O is typically 1735–1750 cm⁻¹): B is methyl propanoate or ethyl ethanoate.
Step 3 — Check with ¹³C. Acid carbonyl δ 178–180 vs. ester δ 165–175 confirms the assignment.
Example 2: Predicting the ¹H NMR of propanoic acid
Predict the signals, integration, and shifts for CH₃CH₂COOH.
Step 1 — Count unique H environments: three — CH₃ (3 H), CH₂ (2 H), O–H (1 H).
Step 2 — Assign shifts. CH₃ δ 1.1 (triplet); CH₂ δ 2.3 (quartet, pulled downfield by the carbonyl); O–H δ 11.5 (broad singlet).
Step 3 — Integration ratio: 3 : 2 : 1. If the sample is shaken with D₂O, the O–H signal disappears, leaving 3 : 2.
Example 3: Degree of unsaturation for a nitrile
C₄H₇N shows a sharp IR band at 2246 cm⁻¹. Use the degree of unsaturation to confirm the nitrile assignment.
Step 1 — Write the formula for degrees of unsaturation (DBE). For C_cH_hN_nO_oX_x:
DBE = 2c + 2 + n - h - x2
Step 2 — Substitute c = 4, h = 7, n = 1, x = 0:
DBE = 2(4) + 2 + 1 - 7 - 02 = 8 + 2 + 1 - 72 = 42 = 2
Step 3 — Interpret. Two degrees of unsaturation match one C≡N triple bond (one σ + two π) on a saturated butyl chain; the sharp 2246 cm⁻¹ band confirms the nitrile. DBE is unitless (atoms cancel).
Key takeaways
- Acid IR: broad O–H 2500–3300 + strong C=O ~1710 + C–O ~1200–1300 cm⁻¹.
- Nitrile IR: sharp C≡N ~2250 cm⁻¹ (conjugated ~2220); watch the alkyne trap.
- Acid ¹H NMR: O–H δ 10–13, broad, D₂O-exchangeable; α-CH₂ δ 2.2–2.5.
- Nitrile ¹H NMR: no downfield O–H; α-CH₂/CH₃ δ 2.3–2.4.
- ¹³C: acid C=O δ 178–182; nitrile C δ 115–120.
- Nitrogen rule: odd nominal molecular mass → odd number of N (nitriles have one).
- Combine techniques: IR for functional group, NMR for connectivity, MS for formula.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What three IR bands identify a carboxylic acid, and which is the most diagnostic?
Show answer
Broad O–H stretch 2500–3300 cm⁻¹, strong C=O ~1710 cm⁻¹, C–O ~1200–1300 cm⁻¹; the broad O–H is the most diagnostic.
Where does the nitrile C≡N stretch appear, and what group is the classic trap?
Show answer
~2250 cm⁻¹ (2240–2260; conjugated ~2220). The alkyne C≡C stretch (2100–2260 cm⁻¹) is the trap — usually weaker and often absent for symmetric alkynes.
Why does the acid O–H proton appear at δ 10–13, and what happens when D₂O is added?
Show answer
Strong hydrogen bonding (dimer formation) shifts it far downfield; adding D₂O exchanges the proton and removes the signal (confirming it is O–H).
Approximate ¹³C shifts of an acid carbonyl and a nitrile carbon?
Show answer
Acid carbonyl δ 178–182; nitrile carbon δ 115–120.
C₄H₇N shows a sharp 2246 cm⁻¹ band. Degree of unsaturation, and what does it match?
Show answer
DBE = (2(4) + 2 + 1 − 7)/2 = 2; it matches one C≡N triple bond on a saturated chain.
How does the nitrogen rule flag a nitrile from its molecular ion?
Show answer
An odd nominal molecular mass implies an odd number of nitrogens; a nitrile has exactly one N, so M⁺ is odd — an immediate flag.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Infrared spectroscopy
- Measures bond vibrations (stretch/bend frequencies)
- Stretching frequency
- Vibration frequency of a bond, in cm⁻¹
- Chemical shift (δ)
- Position of an NMR signal, ppm from a reference
- Exchangeable proton
- O–H (or N–H) proton that swaps with D₂O
- Downfield
- Higher δ (higher frequency) region of the spectrum
- Molecular ion (M⁺)
- Intact molecule with one electron removed
- Nitrogen rule
- Odd nominal mass → odd number of N atoms
Sources & references
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