Ep 05 — Functional Groups and Characteristic Absorption Frequencies (Part 1): C=O, O-H, N-H

Series: Infrared Spectroscopy Encyclopedia: From Principles to Practice
Chapter: Part 1 · Introduction — The Code of Light
Target Audience: High school students, undergraduates, beginners in chemistry/materials/pharmacy
Prerequisites: Ep 04 (How to Read an Infrared Spectrum)
Reading Time: Approximately 22 minutes

Introduction: The "Three Superstars" of Infrared Spectroscopy

On the infrared spectrum stage of 4000–400 cm⁻¹, three functional groups stand out the most—they are strong in intensity, stable in position, and of high diagnostic value, appearing in almost every organic chemistry textbook's spectrum analysis chapter [1][2]:

• C=O Carbonyl (~1700 cm⁻¹): The most "overbearing" peak in IR, strong and sharp
• O-H Hydroxyl (3200–3600 cm⁻¹): The "broadest" peak, so wide it's hard to ignore
• N-H Amino (3300–3500 cm⁻¹): The most "splitting" peak, with a distinctive doublet for primary amines

Today we will dissect the characteristic frequencies, peak shape rules, and influencing factors of these three "superstar functional groups." Master them, and you can solve most spectrum analysis problems.

1. C=O Carbonyl: The "King" of Infrared Spectroscopy

1.1 Why is the C=O Peak So Strong?

The stretching vibration of the carbonyl group (C=O) appears in the 1650–1850 cm⁻¹ region, making it one of the strongest and most easily recognized peaks in IR spectroscopy [1][3].

"A strong and relatively sharp peak in the 1650–1750 cm⁻¹ region strongly suggests the presence of a carbonyl group; further confirmation requires combining peak shape, hydrogen bonding shifts, and fingerprint region cross-validation."
—— LibreTexts Organic Chemistry [1]

Why so strong? Because the C=O bond is a highly polar bond (oxygen electronegativity 3.5, carbon 2.5), the dipole moment change during vibration is large, the transition probability is high, and the absorption intensity is strong [2][3]. Recall from Ep 03 the selection rule: the larger the dipole moment change → the stronger the IR peak.

🔗 Further verification: Complete peak position data, literature sources, and assignment reasons for the carbonyl functional group can be found at ftir.fun carbonyl group page. Want to know which functional groups are near 1700 cm⁻¹? Visit ftir.fun peak page 1700.

1.2 Carbonyl "Family Tree": The CORN Mnemonic

The C=O stretching frequencies of different carbonyl derivatives vary systematically. Pearson's organic chemistry textbook provides an excellent mnemonic—CORN [2]:

Table 1: CORN mnemonic—a rough rule for carbonyl frequencies from high to low (data sources: Pearson [2], LibreTexts [3]). Note: The mnemonic lumps carboxylic acids/esters together at ~1750 for quick memorization; carboxylic acid dimers are actually at ~1710, esters at ~1735, see detailed table below—do not equate mechanically with the mnemonic.

A more detailed frequency table is as follows [3][4][5]:

Table 2: C=O stretching frequencies of various carbonyl compounds (data sources: LibreTexts [3], Heriot-Watt University lecture notes [4], Fiveable [5])

📷 Figure 1: Decreasing trend of C=O frequencies from acyl chloride to amide
Source: Heriot-Watt University Organic Chemistry Lecture Notes [4]
http://www.che.hw.ac.uk/teach…

1.3 Why Do Frequencies Vary? — Inductive Effect vs. Conjugation Effect

The key to understanding C=O frequency variation lies in two opposing electronic effects [3][4][6]:

① Inductive Effect (-I effect) → Increases frequency

Electron-withdrawing groups (e.g., Cl, F, OR) pull electron density away from the carbonyl carbon via inductive effect, enhancing the double-bond character of the C=O bond, shortening the bond length, and increasing the force constant k → frequency increases [3][4][6].

       O                O                O
       ‖                ‖                ‖
   Cl—C—CH₃        CH₃—O—C—CH₃      CH₃—C—CH₃
   Acyl chloride 1810        Ester 1735          Ketone 1715

   Cl strong withdrawing →    O weak withdrawing →     No withdrawing group
   k large, high frequency      k medium                 k small, low frequency

"Electron-withdrawing groups raise the absorption wavenumber of adjacent groups, while electron-donating groups lower it. The stronger the electron-withdrawing ability, the greater the increase."
—— CSDN Infrared Spectrum Characteristic Frequency Review [6]

② Conjugation Effect (+M effect) → Decreases frequency

Lone pairs on electron-donating groups (e.g., -NR₂, -OR) can inject into the π* orbital of C=O via conjugation (resonance), giving the C=O bond partial single-bond character, lengthening the bond, and decreasing the force constant → frequency decreases [3][4].

       O                 O⁻                 O⁻
       ‖                 |                  |
   R—N—C—R'   ←→    R—N⁺=C—R'   ←→    R—N=C—R'
       |                                      |
      (conjugation)       (C=O becomes C-O)        (resonance contributor)

   N lone pair injects into C=O → C=O double bond character weakens → frequency decreases

Amides are the most prominent example of conjugation: the lone pair on N strongly conjugates with C=O, lowering the C=O frequency from 1715 (ketone) to 1650 [3][4].

"Tug-of-war" between the two effects [4]:

| Ketone -R | None | None | — | 1715 (reference) |
| Amide -NR₂ | Weak -I | Strong +M | Conjugation dominant | 1650 (lowest) |

Table 3: "Tug-of-war" between inductive and conjugation effects on C=O frequency (data source: Heriot-Watt lecture notes [4])

1.4 Other factors affecting carbonyl frequency

① Conjugation (with C=C or aromatic rings) [2][4]

When C=O is conjugated with C=C or a benzene ring, the frequency decreases by 20–40 cm⁻¹ [2].

• Acetone (non-conjugated): 1715 cm⁻¹
• 3-Buten-2-one (α,β-unsaturated ketone): 1685 cm⁻¹
• Acetophenone (conjugated with benzene ring): 1685–1690 cm⁻¹

Heriot-Watt University lecture notes compare 4-pentenal vs 2-pentenal [4]:

Table 4: Effect of conjugation on C=O and C=C frequencies (data source: Heriot-Watt lecture notes [4])

② Ring strain [3][4]

In cyclic ketones, the C=O frequency increases as the ring becomes smaller (strain increases) [3][4]:

• Cyclohexanone (6-membered ring): 1715 cm⁻¹ (same as open-chain)
• Cyclopentanone (5-membered ring): 1745 cm⁻¹
• Cyclobutanone (4-membered ring): 1780 cm⁻¹
• Cyclopropanone (3-membered ring): ~1810 cm⁻¹

The Pearson textbook refers to the bent bonds in these small rings as "banana bonds," as their special geometry increases the C=O bond force constant [2].

③ Hydrogen bonding [4][7]

Intermolecular or intramolecular hydrogen bonding decreases the C=O frequency (typically 10–20 cm⁻¹). Carboxylic acid dimers are a classic example [7]:

• Free carboxylic acid monomer: ~1760 cm⁻¹
• Hydrogen-bonded dimer: ~1710 cm⁻¹ (decrease of about 50 cm⁻¹)

1.5 Aldehyde "fingerprint": aldehyde C-H doublet

In addition to C=O stretching, aldehydes (RCHO) have a unique feature — aldehyde C-H stretching appears as two weak peaks near 2720 and 2820 cm⁻¹ [2][4][8].

        O
        ‖
    R—C—H    →    C=O stretch ~1730 cm⁻¹
                 Aldehyde C-H stretch ~2820 + ~2720 cm⁻¹ (doublet)

These two peaks arise from Fermi resonance between C-H stretching and the first overtone of C-H bending [8]. Since ordinary alkyl C-H stretches occur at 2850–2960 cm⁻¹ and do not extend to 2720 cm⁻¹, the peak at 2720 cm⁻¹ is the "gold standard" for identifying an aldehyde group [4].

💡 Quick distinction between aldehyde and ketone: Look at 2720 cm⁻¹ — if there is a weak peak, it is an aldehyde; if no peak, it is a ketone.

🔗 In-depth verification: Detailed peak data for aldehyde groups can be found at ftir.fun aldehyde functional group page.

2. O-H hydroxyl: the "broadest" peak

2.1 Frequency range of O-H stretching

O-H stretching vibration appears in the broad range of 3650–2500 cm⁻¹, with the exact position depending on hydrogen bonding state [7][9][10]:

Table 5: Frequency and hydrogen bonding state of O-H stretching (data sources: Heriot-Watt lecture notes [7], Sigma-Aldrich IR table [9])

2.2 Why does hydrogen bonding broaden the peak?

Heriot-Watt University lecture notes explain it clearly [7]:

"Hydrogen bonding makes the O-H bond 'easier to stretch,' so the absorption occurs at lower wavenumbers (Hooke's law). The hydrogen-bonded O-H stretching peak is broader because the strengths of the hydrogen bonds vary."

Specifically [7][10]:

• Free O-H (dilute solution, nonpolar solvent): All O-H bonds have the same environment → sharp, narrow peak at ~3600 cm⁻¹
• Hydrogen-bonded O-H (concentrated solution/pure liquid): Each molecule's O-H is in a hydrogen bonding network of varying strengths → the absorption frequency is continuously distributed over a range → broad peak at 3200–3550 cm⁻¹

📷 Figure 2: Comparison of O-H peak shapes for the same aromatic alcohol in three states: (a) concentrated solution, (b) dilute solution, (c) carboxylic acid
Source: Heriot-Watt University Organic Chemistry lecture notes [7]
http://www.che.hw.ac.uk/teach…

2.3 Alcohol vs carboxylic acid: how to distinguish?

This is a frequently tested point. LibreTexts and Heriot-Watt both emphasize [7][10]:

Table 6: Differences between alcohol and carboxylic acid O-H (data sources: LibreTexts [10], Heriot-Watt [7])

LibreTexts gives a particularly vivid description [10]:

"In the spectrum of caprylic acid, we see a low, broad absorption band that looks like an alcohol peak, but it is slightly shifted to the right (longer wavelength) and overlaps somewhat with the C-H region. This is characteristic of carboxylic acid O-H stretching."

Key criterion: If the O-H peak is broad enough to cover the C-H peaks near 3000 cm⁻¹, and there is also a C=O peak at ~1710 cm⁻¹ → carboxylic acid [7][10].

🔗 In-depth verification: Detailed data for hydroxyl groups can be found at ftir.fun hydroxyl functional group page; overall features of carboxylic acids can be found at ftir.fun carboxyl functional group page.

2.4 O-H's "neighbor" — water peak interference

When measuring, be aware of [10]:

• Water absorbs at 3400 cm⁻¹ (O-H stretch) and 1640 cm⁻¹ (H-O-H bending)
• KBr pellets absorbing moisture will show a water peak at 3400 cm⁻¹
• Method of distinction: check for the H-O-H bending peak at 1640 cm⁻¹ — if present, it confirms water

3. N-H amino: the "most split" peak

3.1 The "N-H rule" for primary, secondary, and tertiary amines

N-H stretching appears at 3300–3500 cm⁻¹, in the same region as O-H, but with a key difference: N-H peaks are sharper and weaker than O-H [8][11][12].

The most classic rule for distinction [8][11][12]:

Table 7: Rules for discriminating N-H peaks by amine type (Data sources: JoVE [11], UCalgary [12], Pearson [8])

3.2 Origin of the double peak in primary amines: symmetric and antisymmetric stretching

The JoVE analytical chemistry textbook provides a clear explanation [11]:

"When a polyatomic group contains at least two identical atoms, symmetric stretching (in-phase) or antisymmetric stretching (out-of-phase) can occur. Therefore, RNH₂ produces two distinct infrared peaks in the 3300–3500 cm⁻¹ region."

Two stretching modes of primary amine -NH₂:

   H                H
    \              /
     N    ←→      N      ←→
    /              \
   H                H

Symmetric stretch      Antisymmetric stretch
(both H together)      (one H in, one H out)
~3300 cm⁻¹             ~3400 cm⁻¹ (higher frequency)

Why is the antisymmetric stretching frequency usually higher? This is firstly a consequence of normal mode mechanics: the eigenfrequency of the antisymmetric mode is often higher than that of the symmetric mode (the force constants and reduced masses yield different eigenvalues). Additionally, the antisymmetric mode often results in a larger change in dipole moment, hence the peak is stronger — strength and frequency are two different matters; do not confuse them [11].

Professor Ian Hunt of UCalgary provides actual spectral examples [12]:

• n-Propylamine (primary amine): obvious double peak near 3500 cm⁻¹
• Dipropylamine (secondary amine): single peak (broader) near 3500 cm⁻¹
• Tripropylamine (tertiary amine): no peak near 3500 cm⁻¹

📷 Figure 3: Comparative spectra of the N-H region for primary, secondary, and tertiary amines
Source: UCalgary Ian Hunt organic chemistry course [12]
https://www.chem.ucalgary.ca/…

3.3 Amide N-H: Amide I and Amide II

Amides (RCONH₂) contain both C=O and N-H, making their spectra more complex than simple amines [3][13]:

Primary amides (RCONH₂) [3][13]:

• N-H stretching: ~3350 (antisymmetric) + ~3180 (symmetric) cm⁻¹ (double peak)
• Amide I band: ~1650 cm⁻¹ (mainly C=O stretching + some C-N stretching)
• Amide II band: ~1600 cm⁻¹ (N-H bending + C-N stretching coupling)

Secondary amides (RCONHR') [3][13]:

• N-H stretching: ~3300 cm⁻¹ (single peak)
• Amide I band: ~1640 cm⁻¹
• Amide II band: ~1550 cm⁻¹

Tertiary amides (RCONR'₂) [3]:

• No N-H peak
• Amide I band: ~1630 cm⁻¹ (lowest)

📷 Figure 4: Full spectrum of a primary amide (propionamide) with Amide I/II and N-H double peak labeled
Source: UCalgary Ian Hunt [12]
https://www.chem.ucalgary.ca/…

BenchChem's technical guide provides a practical decision tree to distinguish amines from amides [13]:

                  Unknown sample
                     │
        ┌────────────┴────────────┐
    Strong peak at 1690-1630 cm⁻¹?    
        │ Amide I band              │ No C=O
        ▼                         ▼
     Amide                      Amine
        │                         │
    N-H at 3500-3100 cm⁻¹?    N-H at 3500-3100 cm⁻¹?
    Double → primary amide    Double → primary amine
    Single → secondary amide  Single → secondary amine
    None → tertiary amide     None → tertiary amine

Figure 5: Decision tree for distinguishing amines and amides (Source: BenchChem technical guide [13])

🔗 Further reference: Amine data at ftir.fun amine functional group page; Amide data at ftir.fun amide functional group page.

3.4 N-H vs O-H: How to distinguish?

Since N-H and O-H both appear in the 3300–3500 cm⁻¹ region, careful distinction is needed [8][12]:

Table 8: Key points for distinguishing O-H and N-H (Data sources: Pearson [8], UCalgary [12])

IV. Typical spectral examples

4.1 Acetone (classic ketone)

Characteristics from the UCalgary spectral library [12]:

• 1715 cm⁻¹: Strong C=O peak (the "textbook" frequency for ketones)
• ~3000 cm⁻¹: CH₃ stretching
• No O-H or N-H peaks

4.2 Ethyl acetate (ester)

• 1746 cm⁻¹: C=O stretching (~30 cm⁻¹ higher than ketones, due to O inductive effect)
• ~1240 + ~1050 cm⁻¹: C-O-C double peak (asymmetric + symmetric)
• No O-H or N-H peaks

🔗 Further reference: Detailed ester data at ftir.fun ester functional group page.

4.3 Acetic acid (carboxylic acid)

Common features from LibreTexts and UCalgary [10][12]:

• 1710 cm⁻¹: C=O stretching (dimer)
• 2500–3300 cm⁻¹: Very broad O-H stretching (covers C-H region)
• ~2655, ~2560 cm⁻¹: Characteristic small dimer peaks [10]

4.4 Benzamide (primary amide)

• ~1650 cm⁻¹: Amide I band (C=O, lower frequency due to conjugation + N conjugation)
• ~1600 cm⁻¹: Amide II band (N-H bending)
• ~3350 + ~3180 cm⁻¹: N-H double peak (primary amide characteristic)
• ~1600, ~1500 cm⁻¹: Aromatic C=C skeleton

V. Section summary: Quick reference table for three major functional groups

| N-H (primary amine) | ~3400 + ~3300 | relatively sharp | medium | doublet |
| N-H (secondary amine) | ~3300 | relatively sharp | medium | singlet |
| N-H (tertiary amine) | — | — | — | no peak |

Table 9: Quick reference for three major functional groups (compiled from [1][2][3][7][8][12])

Review Questions

1. Why is the C=O of esters usually higher than that of ketones, while that of amides is usually lower?
2. How can you distinguish alcohols from carboxylic acids using the O–H peak shape and whether it is accompanied by ~1710 cm⁻¹?
3. What are the differences in the number of peaks for primary, secondary, and tertiary amines in the 3300–3500 cm⁻¹ region? What is the reason?
4. Find one characteristic frequency range consistent with this episode on ftir.fun carbonyl page and hydroxyl page, respectively.

References

[1] LibreTexts. 15.3: Interpreting IR Spectra. Organic Chemistry. https://chem.libretexts.org/@…

[2] Pearson. Infrared Spectroscopy Table. Organic Chemistry. https://www.pearson.com/chann…

[3] LibreTexts. 10.11: Spectroscopy of Carboxylic Acid Derivatives. https://chem.libretexts.org/@…

[4] Heriot-Watt University. Help for carbonyl compounds: The C=O stretch. http://www.che.hw.ac.uk/teach…

[5] Fiveable. 21.10 Spectroscopy of Carboxylic Acid Derivatives. https://fiveable.me/organic-c…

[6] CSDN. 红外光谱中特征官能团的振动频率. 2023. https://blog.csdn.net/allenkx…

[7] Heriot-Watt University. Help for alcohols: The O-H stretch. http://www.che.hw.ac.uk/teach…

[8] Pearson. IR Spect: Drawing Spectra. Organic Chemistry. https://www.pearson.com/chann…

[9] Sigma-Aldrich. IR Spectrum Table & Chart. https://www.sigmaaldrich.com/…

[10] LibreTexts. 19.3: Spectroscopy and Mass Spectrometry of Carboxylic Acids. https://chem.libretexts.org/@…

[11] JoVE. IR Spectrum Peak Splitting: Symmetric vs Asymmetric Vibrations. Analytical Chemistry Ch. 13.15. https://www.jove.com/science-…

[12] Hunt I. Chapter 13: Spectroscopy - Sample IR Spectra. University of Calgary. https://www.chem.ucalgary.ca/…

[13] BenchChem. FTIR Spectrum Analysis: Distinguishing Amide and Amine Functional Groups. 2026. https://pdf.benchchem.com/315…

Next Episode Preview: Ep 06 — Functional Groups and Characteristic Absorption Frequencies (Part 2): C-H, C≡N, C-O-C, etc. We will continue to learn about C-H stretching (saturated/unsaturated/aromatic), triple bonds (C≡C, C≡N), ether bonds (C-O-C), nitro (NO₂), and halogen bonds (C-X) characteristic frequencies.