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Interpreting
and explaining the infrared
spectrum of cinnamic acid (brief mention of NMR spectra)
[Author
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Dr Phil Brown PhD:
Doc Brown's advanced level organic chemistry exam revision notes
suitable for students of UK A level chemistry courses, IB chemistry and US K12 grade
11, grade 12 and AP honors chemistry courses: Molecular
spectrometry - analysing the IR, 1H NMR and 13C NMR spectra of
cinnamic acid
[spectra page updated
April 3rd 2026 *]
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infrared spectrum of C6H5CH=CHCOOH,
C9H8O2
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Introductory note on the infrared spectrum of cinnamic acid
Students and teachers please note
my explanation of the infrared
spectrum of cinnamic acid is designed
for advanced, but pre-university, chemistry courses.
Based in
the infrared spectrum diagram for cinnamic acid, only some of the most
prominent peaks for particular bond vibrations are discussed,
particularly if cinnamic acid has a functional group with a particular
characteristic wavenumber peak.
The infrared spectrum of
cinnamic acid is
unique and the whole, or selected wavenumbers, can be used to
fingerprint its identity, sometimes analysing a mixture
containing cinnamic acid or following its change of concentration in a
reaction.
Spectra obtained from a KBr disc of cinnamic acid. The right-hand part of the of the
infrared spectrum of cinnamic acid, wavenumbers
~1500 to 400
cm-1 is considered the fingerprint region for the
identification of cinnamic acid and most organic compounds. It is due to a unique set
of complex overlapping vibrations of the atoms of the molecule of cinnamic
acid.
3-phenylprop-2-enoic acid, (cinnamic acid),
C9H8O2,
C6H5-CH=CH-COOH (E/Z isomers cis/trans)
Interpretation of
the infrared spectrum of cinnamic acid
Cinnamic acid has two functional groups, an aliphatic
carboxylic acid group and an alkene group.
The most prominent infrared absorption lines of cinnamic
acid
(wavenumber bands in cm-1)
From ~3400 to ~2300 cm-1 there two broad
overlapping bands from O-H and C-H stretching vibrations.
The bands are complicated by hydrogen bonding
affecting the O-H vibration frequencies (as part of the
carboxylic acid functional group) and the C-H vibrations vary
due to different origins i.e. alkene C-H vibrations from the
CH=CH group and arene C-H vibrations from the benzene ring.
The peak at ~1680 cm-1 is due to the
C=O
stretching vibrations of the carbonyl group (of the carboxylic acid
functional group).
The C=O absorption is clearly distinguished from the
alkene C=C stretching absorption band which peaks at ~1630 cm-1.
According to internet data there are two peaks for
the carbon-carbon arene
(NOT
C=C)
stretching vibrations of the benzene ring at ~1580 and
~1500 cm-1.
The absence of other specific functional group bands
will show that a particular functional group is absent from the
cinnamic acid
molecular
structure.
Brief comments on the NMR spectra of cinnamic acid
C6H5CH=CHCOOH,
C9H8O2
The number of 1H NMR and 13C NMR chemical shifts expected
for both E/Z geometrical isomers.
Number of low resolution
NMR
chemical shift
δ
signal peaks: 6
1H and
7 13C
(email
if disagree?)
1H NMR ratio of peaks: 1 : 2 : 2 : 1 : 1 : 1 (for
equivalent protons, from left to right)
Extra notes on
3-phenylprop-2-enoic acid (cinnamic acid)
C6H5CH=CHCOOH,
C9H8O2
Cinnamic acid shows clear IR absorptions for the carboxylic acid group,
distinctive alkene and aromatic signals in ¹H NMR, and characteristic
carbonyl and aromatic peaks in ¹³C NMR.
Students must avoid common misconceptions such as confusing cis/trans
coupling constants or misassigning overlapping aromatic signals.
It is widely used in flavours, fragrances, pharmaceuticals,
cosmetics, and organic synthesis.
Key Revision Points
for the spectra of cinnamic acid
IR Spectroscopy
- Broad O–H stretch: ~2500–3300 cm⁻¹ (carboxylic acid
hydrogen bonding).
- Sharp C=O stretch: ~1680–1700 cm⁻¹ (carboxylic acid
carbonyl).
- C=C stretch: ~1620 cm⁻¹ (alkene conjugated with
aromatic ring).
- Aromatic C–H stretches: ~3030 cm⁻¹.
¹H NMR Spectroscopy
- Carboxylic acid proton: Broad singlet ~11–12 ppm.
- Aromatic protons: Multiplets ~7.2–7.8 ppm (five
protons).
- Alkene protons: Doublets ~6.5 ppm and ~7.6 ppm.
¹³C NMR
Spectroscopy
- Carbonyl carbon: ~167–170 ppm.
- Alkene carbons: ~118–145 ppm.
- Aromatic carbons: ~125–135 ppm.
Common
Misconceptions
- Confusing cis versus trans cinnamic acid:
- Trans shows large coupling constant (~16 Hz).
- Cis would show smaller (~10–12 Hz).
- Overlapping aromatic signals: Students often misassign
multiplets as separate functional groups.
- Carboxylic acid proton: Sometimes overlooked due to
broadness or exchange effects.
- IR misinterpretation: The broad O–H stretch can be
mistaken for alcohol rather than carboxylic acid.
Exam Revision Tips
- Always link spectral data to structure: e.g.,
conjugation lowers C=O stretch frequency compared to simple acids.
- Use coupling constants in ¹H NMR to distinguish
stereochemistry.
- Cross-check IR and NMR: IR confirms functional groups,
NMR provides detailed structural assignment.
- Practice drawing annotated spectra overlays for
cinnamic acid to reinforce recognition.
- Exam boards 'may' test:
- Assigning peaks to functional groups.
- Distinguishing cis/trans isomers.
- Explaining conjugation effects on IR/NMR shifts.
Uses and
Applications of Cinnamic Acid
- Flavours & Fragrances: Used in fruit essences (apple,
cherry), perfumes, soaps, shampoos.
- Food Additives: Preservative and flavor enhancer.
- Pharmaceuticals: Anti-inflammatory, antimicrobial,
antioxidant properties; precursor in drug synthesis.
- Cosmetics: Sunscreens (UV absorption), skin-lightening
agents.
- Organic Synthesis: Intermediate for esters, polymers,
and fine chemicals.
Key words & phrases:
isomer
of molecular formula C9H8O2 C6H5-CH=CH-COOH E/Z isomers cis/trans
geometric isomers image and diagram explaining the infrared spectrum
of cinnamic acid, complete infrared absorption spectrum of cinnamic acid, comparative spectra of
cinnamic acid, prominent peaks and troughs for identifying functional groups in the infrared spectrum of
cinnamic acid,
important wavenumber values in cm-1 for peaks and troughs in the infrared spectrum
of cinnamic acid, revision study to understand the infrared spectroscopy of
cinnamic acid, fingerprint region analysis of
cinnamic acid, how to identify cinnamic acid from its infrared spectrum, identifying organic
compounds like cinnamic acid from their infrared spectrum, how to analyse the
absorption bands in the infrared spectrum of cinnamic acid, detection of
carboxylic acid functional groups in the cinnamic acid molecule example of the infrared spectrum of a
molecule like cinnamic acid with an alkene
functional group
interpreting interpretation of the infrared spectrum of cinnamic acid shows presence
of IUPAC name
3-phenylprop-2-enoic acid How do you interpret the infrared absorption spectrum of
cinnamic acid How
to interpret the infrared spectrum of cinnamic acid Explanatory diagram of the
infrared spectrum of the cinnamic acid molecule in terms of its molecular
structure. Listing data of the prominent main wavenumber peaks
troughs in the infrared spectrum of cinnamic acid. How to explain the infrared
spectrum of cinnamic acid. Use of the infrared spectrum of cinnamic
acid, identification
of cinnamic acid from its infrared spectrum - fingerprint wavenumber pattern
to identify the cinnamic acid molecule. The uses of the infrared spectrum of
the cinnamic acid molecule. The distinctive features of the infrared spectrum
of the cinnamic acid molecule explained interpretation diagram explaining the
peaks-trough of the transmittance of the infrared spectrum of
cinnamic acid
what does the infrared spectrum tell you about the structure and
properties of the cinnamic acid molecule? How is infrared spectrum of
cinnamic acid used
to identify cinnamic acid?
Links associated with cinnamic acid
Infrared spectroscopy index
ALL SPECTROSCOPY INDEXES
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spectroscopy (1H NMR, 13C NMR and infrared spectra of cinnamic acid) are
suitable for use of pre-university students studying AQA advanced level
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