Discussion Examples for IR Spectra of Organic Compounds | How to Interpret Functional Group Peaks

What Is an IR Spectrum?

An IR spectrum is an analytical method that uses infrared light to investigate the vibrations of bonds within molecules.
Bonds in molecules undergo motions such as stretching and bending vibrations and absorb infrared light of specific energies.
A record of this absorption at each wavenumber is called an IR spectrum.

In an IR spectrum, the horizontal axis shows wavenumber and the vertical axis shows transmittance or absorbance.
Wavenumber is expressed in cm⁻¹, and in general, a range of approximately 4000 to 400 cm⁻¹ is examined when identifying functional groups in organic compounds.

What Can Be Confirmed With an IR Spectrum?

IR spectra are mainly used to confirm the presence or absence of functional groups.
For example, carbonyl groups, hydroxyl groups, amino groups, nitro groups, cyano groups, alkynes, alkenes, and aromatic rings show characteristic absorptions and are therefore relatively easy to discuss.

On the other hand, it is difficult to completely determine the entire molecular structure from an IR spectrum alone.
Compounds containing the same functional groups show similar absorptions, so IR is generally used not as “decisive proof of structure determination,” but as “strong evidence for functional-group identification.”
When necessary, the results are evaluated together with NMR, melting point, boiling point, TLC, mass spectrometry, and other analyses.

Main Items to Include in the Results

It is not necessary to list every peak in the results of an IR spectrum.
In a report, select and describe absorptions that are important for confirming the target compound or the progress of the reaction.
In particular, focusing on peaks corresponding to functional groups that changed from the starting material to the product makes the discussion easier to write.

Main Items to Include in the Results

• Name of the sample measured
• Measurement method
• Wavenumbers of characteristic absorptions
• Absorption intensity
• Absorption shape
• Assignment of functional groups
• Differences from the spectrum of the starting material
• Presence or absence of peaks required for the target product
• Presence or absence of peaks remaining from the starting material
• Possibility of peaks derived from impurities or moisture

Functional Group Region and Fingerprint Region

IR spectra are easier to interpret when broadly divided into the functional group region and the fingerprint region.
The functional group region is mainly around 4000 to 1500 cm⁻¹, where absorptions derived from functional groups such as O-H, N-H, C=O, C-H, and C≡N are observed.
The fingerprint region is a complex region mainly below 1500 cm⁻¹, where many absorptions appear depending on the overall molecular structure.

In student-laboratory discussions, first confirm the important peaks in the functional group region, and then compare the fingerprint region with standard spectra or literature values as necessary.

Approximate Positions of Major Functional Group Peaks

In IR spectra, each functional group tends to appear within a characteristic wavenumber range.
However, the actual wavenumber varies depending on molecular structure, conjugation, ring structure, hydrogen bonding, measurement conditions, and other factors.
Therefore, these ranges should be used as guidelines for discussion rather than as absolute values.

How to Interpret O-H Stretching Vibrations

O-H bonds are contained in alcohols, phenols, carboxylic acids, and similar compounds.
O-H absorptions generally appear on the high-wavenumber side and tend to form broad peaks.
In particular, when hydrogen bonding is present, the absorption becomes broad and its shape becomes gradual.

O-H groups in alcohols and phenols may appear as broad absorptions around 3200–3600 cm⁻¹.
O-H absorptions in carboxylic acids can be even broader and may extend to lower wavenumbers.
Therefore, not only the position but also the width and shape of an O-H absorption are important.

Discussion of O-H Absorption in Carboxylic Acids

The O-H absorption of a carboxylic acid may appear much broader than the O-H absorption of an alcohol.
This is because carboxylic acid molecules readily form strong hydrogen bonds with one another.
If a broad O-H absorption is observed together with a C=O absorption, it becomes easier to discuss the presence of a carboxylic acid.

How to Interpret N-H Stretching Vibrations

N-H bonds are contained in amines, amides, and similar compounds.
N-H absorptions may appear as relatively sharper peaks than O-H absorptions.
In primary amines, two peaks derived from N-H stretching may sometimes be observed, while in secondary amines and amides, the absorption may appear closer to a single peak.

However, N-H absorptions may overlap with O-H absorptions or absorptions derived from moisture.
Therefore, the presence or absence of N-H should be judged together with other functional-group peaks and structural information.

How to Interpret C-H Stretching Vibrations

C-H stretching vibrations are absorptions observed in many organic compounds.
sp³ C-H absorptions may appear slightly below 3000 cm⁻¹, while sp² C-H absorptions may appear slightly above 3000 cm⁻¹.
In compounds containing aromatic rings or alkenes, sp² C-H absorption may be confirmed.

Because C-H absorption is common to many organic compounds, it is difficult to use it as decisive information by itself.
However, it can be used as supplementary information indicating the presence of aromatic rings, alkenes, or alkane portions.

How to Interpret C=O Stretching Vibrations

C=O bonds are contained in aldehydes, ketones, carboxylic acids, esters, amides, acid anhydrides, and similar compounds.
C=O absorption is extremely important in IR spectra and is often observed as a strong peak.
Therefore, it is a useful peak for confirming carbonyl compounds.

The position of C=O absorption changes depending on the type of carbonyl group.
For example, esters, carboxylic acids, ketones, and amides show C=O absorption at slightly different positions.
Wavenumber also changes due to conjugation and hydrogen bonding.

Discussion of C=O Absorption in Esters

In esters, absorptions derived from C-O bonds may also be observed in addition to C=O absorption.
In ester-synthesis experiments, if absorptions derived from the carboxylic acid or alcohol starting materials become weaker and C=O and C-O absorptions characteristic of esters are confirmed, this provides evidence for the formation of the target product.

Discussion of C=O Absorption in Amides

In amides, N-H absorption may also be confirmed in addition to C=O absorption.
In amide-synthesis experiments such as the synthesis of acetanilide, amide carbonyl absorption and N-H absorption provide clues for confirming the target product.
The C=O absorption of an amide may appear at a different position from those of typical ketones or esters.

How to Interpret C=C Stretching Vibrations and Aromatic Rings

C=C bonds are contained in alkenes and aromatic rings.
In aromatic rings, absorptions derived from C=C stretching vibrations may be observed around 1600 cm⁻¹.
In addition, aromatic C-H stretching vibrations may appear slightly above 3000 cm⁻¹.

The presence of an aromatic ring can be confirmed more reliably by considering not only C=C absorption, but also aromatic C-H absorption, patterns in the fingerprint region, and aromatic proton signals in NMR.

How to Interpret C≡N Stretching Vibrations

Nitrile compounds containing C≡N bonds may show a relatively sharp absorption around 2200–2260 cm⁻¹.
Because relatively few functional groups absorb in this region, it is comparatively easy to use for confirming nitriles.

However, C≡C absorptions and other triple-bond absorptions may also appear in the same region.
Therefore, the result must be considered together with the molecular structure and other peaks.

How to Interpret the NO2 Group

Compounds containing nitro groups may show approximately two strong absorptions derived from NO₂.
In synthesis experiments involving aromatic nitro compounds, these absorptions provide important evidence for confirming introduction of the nitro group.
Nitro-group absorptions may be observed around 1500–1600 cm⁻¹ and 1300–1400 cm⁻¹.

Discussion Comparing the Starting Material and Product

An important point in the discussion of an IR spectrum is to examine the differences between the starting material and product.
If a new functional group is introduced by the target reaction, a new absorption appears.
If a functional group in the starting material is consumed in the reaction, its absorption becomes weaker or disappears.

For example, when an alcohol is oxidized to synthesize a ketone, the main points of discussion are that the O-H absorption becomes weaker and a C=O absorption appears.
In esterification, changes in O-H absorption and the appearance of ester C=O absorption are important.

Discussion When a Peak Disappears

If a peak present in the starting material disappears in the product, the corresponding functional group may have been consumed by the reaction.
However, even if a peak appears to have completely disappeared, it may simply have become difficult to observe because of concentration, measurement conditions, or overlap with other peaks.

In a report, it may be safer to use expressions such as “became weaker” or “was hardly observed” rather than definitively stating that a peak “disappeared.”

Discussion When a New Peak Appears

If a new peak appears in the product, a new functional group may have been introduced.
For example, the appearance of a new C=O absorption can be discussed as introduction of a carbonyl group, while the appearance of NO₂ absorption can be discussed as introduction of a nitro group.

However, the new peak may also originate from an impurity or residual solvent.
It is important to confirm whether the peak is expected from the target structure.

Discussion When Peaks Overlap

In IR spectra, multiple peaks may overlap and appear as a single absorption.
In particular, O-H, N-H, and moisture-derived absorptions tend to overlap on the high-wavenumber side, while many absorptions are concentrated in the fingerprint region.
When peaks overlap, it becomes difficult to identify a functional group from a single peak alone.

In such cases, make an overall judgment based on peak shape, intensity, the presence or absence of other related peaks, and comparison with the starting material.

Discussion When a Peak Is Weak

If the peak of the target functional group is weak, possible causes include a small sample amount, inherently weak absorption intensity of the functional group, overlap with other absorptions, a small amount of target product, or a large amount of impurities.
In IR spectra, not all functional groups appear with the same intensity.

Discussion When a Peak Is Broad

When a peak is broad, the influence of hydrogen bonding or moisture can be considered.
O-H absorption tends to broaden because of hydrogen bonding and may appear as a broad absorption in alcohols, phenols, carboxylic acids, moisture, and similar cases.
In particular, when moisture is present in the sample, a broad absorption may appear in the O-H region.

Discussion of Residual Moisture

Moisture in a sample may appear as O-H absorption in an IR spectrum.
Broad absorptions derived from moisture may be observed when drying after extraction is insufficient, drying after recrystallization is insufficient, or the sample is hygroscopic.
Because residual moisture may be mistaken for a product containing O-H groups, caution is necessary.

Discussion of Residual Solvent

Absorptions from solvent remaining in the product may appear in the IR spectrum.
If drying or solvent removal is insufficient, C-H, O-H, C=O, and other absorptions derived from the solvent may be observed.
Care must be taken not to mistakenly interpret solvent peaks as functional-group peaks of the target product.

Discussion When Unreacted Starting Material Remains

If peaks characteristic of the starting material remain in the IR spectrum of the product, the reaction may not have proceeded completely or purification may have been insufficient.
For example, if the O-H absorption of a starting alcohol remains strongly in the product, contamination by unreacted alcohol can be considered.
If the C=O absorption of a starting carbonyl compound remains, incomplete reaction may be possible.

Discussion When By-Products Are Present

If multiple peaks not expected from the target structure are observed, by-products may be present.
By-products may be formed through reactions other than the target reaction, overreaction, decomposition, oxidation, hydrolysis, and similar processes.
It may be difficult to identify by-products from IR alone, but absorptions of functional groups unnecessary for the target product provide important material for discussion.

Relationship Between IR Spectra and Yield

IR spectroscopy is a method for confirming functional groups and impurities in a product and does not directly indicate the yield itself.
Even if the yield is high, if peaks from unreacted starting material or residual solvent are observed in the IR spectrum, the purity of the product may be low.
Conversely, even if the yield is low, if the IR spectrum agrees well with that of the target product, a small amount of relatively pure product may have been obtained.

Relationship Between IR Spectra and Melting Point

For solid products, combining IR spectra and melting-point data makes it easier to confirm the target product and evaluate purity.
If the IR spectrum agrees with the functional groups of the target product, the melting point is close to the literature value, and the melting-point range is narrow, a relatively pure target product may have been obtained.
On the other hand, if extra peaks are present in the IR spectrum and the melting point is low or the range is broad, contamination by impurities is suspected.

Why the Structure Should Not Be Judged From IR Alone

IR spectroscopy is effective for identifying functional groups, but it has limitations in completely determining structure.
Different compounds containing the same functional groups may show similar IR spectra.
In addition, isomers and small differences in the carbon skeleton may be difficult to distinguish using IR alone.

Therefore, in a report, it is more natural to write that the IR spectrum “supports the formation of the target product” rather than that it “proves the target product.”
Make an overall judgment together with NMR, melting point, TLC, yield, and other results.

Discussion When Measured by the ATR Method

The ATR method is an IR measurement method in which the sample is brought into contact with a crystal surface.
Sample preparation is relatively simple, and the method is commonly used in organic chemistry experiments.
However, if contact between the sample and the ATR crystal is poor, peaks may become weak and reproducibility of the spectrum may decrease.

Discussion When Measured by the KBr Method

In the KBr method, the sample is mixed with potassium bromide to prepare a pellet, and the IR spectrum is measured.
Because KBr readily absorbs moisture, water contamination may produce a broad absorption in the O-H region.
Therefore, when an O-H-like absorption is observed using the KBr method, it is necessary to consider whether it originates from the sample or from moisture.

When the IR Spectrum Can Be Considered Good

An IR spectrum can be considered good when absorptions of the functional groups required for the target product are clearly observed, absorptions characteristic of the starting material become weaker, and there are few extra peaks.
Furthermore, if the results do not contradict NMR, melting point, and similar data, the target product can be considered to have been obtained relatively successfully.

Example IR Discussion When the Experiment Did Not Go Well

If the IR spectrum does not match expectations, consider incomplete reaction, contamination by unreacted starting material, formation of by-products, insufficient drying, residual solvent, and problems with the measurement conditions.
Possible causes can be organized from results such as weak peaks required for the target product, remaining peaks from the starting material, O-H absorption not expected from the structure, and extra C=O absorption.

How to Write Points for Improvement

In a discussion of an IR spectrum, including points for improvement in addition to interpretation of the results makes the report easier to organize.
Improvements are easier to write when divided into sample purity, drying, measurement conditions, and combination with other analytical methods.

Improvements to Sample Preparation

• Dry the product sufficiently
• Remove residual solvent as much as possible
• Remove unreacted starting material through purification
• Remove by-products by recrystallization, chromatography, or similar methods
• Handle the sample carefully to avoid contamination

Improvements to Measurement Conditions

• In the ATR method, ensure sufficient contact between the sample and the measurement surface
• In the KBr method, pay attention to the influence of moisture
• Collect an appropriate background before measurement
• If peaks are weak, review the sample amount and contact condition
• Record the measurement conditions

Improvements to the Discussion

• Focus on the peaks of the target functional groups
• Compare the spectra of the starting material and product
• Examine not only peak wavenumbers but also their shapes and intensities
• Do not determine the structure from IR alone
• Judge the result together with NMR, melting point, TLC, and similar data

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of an IR spectrum, simply writing that “there was a peak” or “the functional group was confirmed” results in a superficial discussion.
Relating the wavenumber, peak intensity, peak shape, comparison with the starting material, and functional-group changes caused by the reaction produces a more persuasive discussion.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of IR spectra.
Adjust the necessary parts according to your own experimental results.

• In an IR spectrum, absorptions corresponding to bond vibrations within molecules are observed.
• The absorption observed around ○○ cm⁻¹ is considered to originate from the ○○ group.
• Because an absorption characteristic of the target product was confirmed, the target compound may have been formed.
• Because an absorption characteristic of the starting material became weaker, the starting material is considered to have been consumed by the reaction.
• A strong absorption corresponding to a C=O stretching vibration was observed, suggesting the presence of a carbonyl group.
• The broad O-H absorption may have been caused by hydrogen bonding or residual moisture.
• Because an absorption not present in the target-product structure was observed, contamination by unreacted starting material or residual solvent may have occurred.
• Because the entire structure cannot be completely determined from an IR spectrum alone, the result must be judged together with NMR and melting-point data.
• Because the fingerprint region is complex, comparison with a standard spectrum should be used as supplementary evidence.
• Even if the yield is high, the purity is not necessarily high if absorptions derived from impurities are observed in the IR spectrum.

Points to Check When Discussing an IR Spectrum

Checking the following points before writing the report makes the discussion easier to write.

• Have you confirmed the functional-group peaks required for the target product?
• Have you checked whether peaks characteristic of the starting material remain?
• Have you examined not only peak wavenumbers but also their shapes and intensities?
• Have you distinguished O-H, N-H, C=O, C-H, and similar absorptions?
• Have you considered the possibility of residual moisture or residual solvent?
• Have you considered contamination by by-products or unreacted starting material?
• Have you distinguished between the functional group region and fingerprint region?
• Have you avoided determining the structure from IR alone?
• Have you discussed the result together with NMR, melting point, TLC, yield, and similar data?
• Have you considered errors caused by the measurement method and sample condition?
• Do the peak assignments agree with the structure of the target product?
• Do the points for improvement correspond to the sources of error?

Summary

IR spectroscopy is an important analytical method for identifying functional groups contained in organic compounds.
By confirming absorptions such as O-H, N-H, C=O, C-H, C=C, C≡N, and NO₂, it is possible to discuss whether the functional groups required for the target product are present.
In particular, it is important to compare the spectra of the starting material and product and examine whether new peaks appeared through the reaction and whether peaks from the starting material became weaker.

However, it is not possible to completely determine the entire molecular structure from an IR spectrum alone.
Compounds containing the same functional groups may show similar absorptions, and isomers or small structural differences may be difficult to distinguish using IR alone.
Therefore, IR should be used as evidence for functional-group identification, and the result should be judged comprehensively together with NMR, melting point, TLC, yield, and similar data.

In a report, do not simply write that “a peak was present.”
Explain the wavenumber, peak shape, intensity, differences from the starting material, and correspondence with the target structure.
A persuasive IR-spectrum report can be produced by also discussing the possibilities of residual moisture, residual solvent, unreacted starting material, and by-products.