3. The IR stretching frequency for C=O stretching in carboxylic acid chlorides, esters
amides, and acid anhydrides generally ranks as:
a. Acid chlorides> esters> acid anhydrides> amides
b. Amides> esters> acid anhydrides> acid chlorides
c. Acid chlorides> acid anhydrides > esters> amides
d. Acid anhydrides> esters> acid chlorides> amides

The correct order of IR C=O stretching frequencies is:
Acid chlorides > acid anhydrides > esters > amides, so option (c) is correct.​​

Introduction

In IR spectroscopy, different carboxylic acid derivatives show distinct C=O stretching frequencies because substituents attached to the carbonyl carbon change bond strength by inductive and resonance effects. For acid chlorides, anhydrides, esters and amides, the typical C=O region spans from about 1815 cm⁻¹ down to about 1640 cm⁻¹, allowing a clear ranking of their stretching frequencies. Understanding this order is essential for identifying functional groups in spectra and solving organic spectroscopy MCQs.​

Correct Order and Reasoning

Experimentally observed approximate C=O stretching ranges are:

• Acid chlorides: ~1790–1815 cm⁻¹ (very high).​

• Acid anhydrides: pair of bands ~1800–1850 and 1740–1790 cm⁻¹.​

• Esters: ~1735–1750 cm⁻¹.​

• Amides: ~1650–1690 cm⁻¹ (lowest among these).​

Inductive withdrawal by electronegative atoms (like Cl or –OCOR) increases C=O bond order and raises frequency, whereas resonance donation (especially from nitrogen in amides) lowers bond order and decreases frequency. Therefore, the general order is:​
Acid chlorides > acid anhydrides > esters > amides, matching option (c).​

Option-by-option Explanation

Option (a)

a. Acid chlorides > esters > acid anhydrides > amides

• Acid chlorides are correctly placed at the highest frequency due to strong –I effect of Cl, which shortens and strengthens the C=O bond.​

• However, esters are incorrectly placed above acid anhydrides; actual data show anhydrides have bands higher than or comparable to acid chlorides and clearly higher than esters, with bands near 1800+ cm⁻¹ and 1760–1790 cm⁻¹.​

• Amides being the lowest is correct, but the misplacement of the anhydride makes the overall order wrong.

Option (b)

b. Amides > esters > acid anhydrides > acid chlorides

• This option reverses the true trend: it puts amides at the highest frequency even though amides actually have the lowest C=O stretching values (~1650–1690 cm⁻¹) because resonance donation from nitrogen lengthens the C=O bond.​

• Acid chlorides, which have the highest frequency around ~1800 cm⁻¹, are incorrectly placed at the lowest.​

• Thus, this sequence is entirely opposite to experimental IR data and is incorrect.

Option (c) – Correct

c. Acid chlorides > acid anhydrides > esters > amides

• Acid chlorides: strong electron-withdrawing Cl increases C=O bond order, giving a sharp, strong band near 1790–1815 cm⁻¹.​

• Acid anhydrides: two C=O bands, both high in frequency (∼1800–1850 and 1740–1790 cm⁻¹), overall higher than esters and clearly above amides.​

• Esters: C=O absorbs around 1735–1750 cm⁻¹, higher than carboxylic acids and amides but lower than acid chlorides and anhydrides.​

• Amides: strong resonance from nitrogen lowers C=O frequency to ~1650–1690 cm⁻¹, making them lowest in this series.​

• This matches the well-accepted IR order, so option (c) is correct.

Option (d)

d. Acid anhydrides > esters > acid chlorides > amides

• Acid anhydrides are reasonably placed high but not properly compared to acid chlorides; acid chlorides typically show a high, characteristic band near ~1800 cm⁻¹ comparable to or above the lower anhydride band.​

• Placing esters above acid chlorides is wrong, as ester C=O is generally lower (around 1735–1750 cm⁻¹) than acid chloride C=O (~1790–1815 cm⁻¹).​

• Amides are correctly at the lowest end, but the misordered ester and acid chloride positions make this option incorrect.

Why Amides Are Lowest and Acid Chlorides Highest

In amides, the lone pair on nitrogen delocalizes into the carbonyl, giving a significant resonance contributor with partial C–N double-bond and partial C–O single-bond character, which weakens and lengthens the C=O bond and thus lowers its stretching frequency. In acid chlorides, the highly electronegative chlorine pulls electron density away inductively without donating back by resonance, strengthening and shortening the C=O bond and raising the stretching frequency. This interplay of inductive and resonance effects underlies the observed IR order tested in such MCQs.​