# Infra-red Spectroscopy

> Chemistry · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9701-u16-infra-red-spectroscopy/

This module covers the principles of infra-red (IR) spectroscopy, a core analytical technique used to identify covalent bonds and functional groups in organic molecules. You will learn how to interpret IR spectra to confirm organic structures for CIE exams.

**Prerequisites:** [Organic functional groups and covalent bonding](https://www.owlsprep.com/study/cie-9701-u10-functional-groups/); [Electromagnetic spectrum fundamentals](https://www.owlsprep.com/study/cie-9701-u1-electromagnetic-radiation/)

## Learning objectives

- Explain the mechanism of IR absorption by covalent bonds
- Identify common functional groups from characteristic IR absorption wavenumbers
- Interpret IR spectra to confirm the structure of organic compounds

## Principles of IR Absorption

**Infra-red Spectroscopy** — An analytical technique that uses IR radiation (wavenumber $400-4000 cm^{-1}$) to identify covalent bonds. Only bonds that undergo a change in dipole moment when vibrating absorb IR radiation.

*Example:* Polar bonds like O-H and C=O absorb IR strongly, while non-polar bonds like O=O do not absorb at all.

When IR radiation passes through a sample, bonds absorb energy at specific wavenumbers matching their natural vibrational frequency. A spectrum plots % transmittance of radiation (y-axis) against wavenumber (x-axis, decreasing from left to right).

$$\tilde{\nu} = \frac{1}{\lambda}$$

Where $\tilde{\nu}$ = wavenumber ($cm^{-1}$), $\lambda$ = wavelength of radiation (cm).

**Worked example:** Explain why N₂ gas does not absorb IR radiation, but HBr does.

1. 1. IR absorption only occurs when bond vibration causes a change in the molecule's net dipole moment.
2. 2. N₂ has a non-polar N≡N bond; symmetric stretching does not change the dipole moment (which remains zero at all times).
3. 3. HBr has a polar H-Br bond; stretching changes the distance between the partial charges, so the net dipole moment changes during vibration.
4. Therefore, only HBr absorbs IR radiation, while N₂ does not.

> **Exam tip:** Highly polar bonds produce strong, clear absorption peaks, while non-polar bonds produce weak or no peaks.

*Calculator:* forbidden

## Characteristic Absorptions and Functional Groups

Each type of covalent bond absorbs IR within a narrow, characteristic wavenumber range that is mostly unaffected by the rest of the molecule. This makes it possible to identify functional groups directly from IR spectra.

**Fingerprint Region** — The region of the IR spectrum between 400 $cm^{-1}$ and 1500 $cm^{-1}$ that contains unique absorptions from bending vibrations of the entire molecule. No two different compounds have the same fingerprint region.

*Example:* Used to confirm an unknown compound matches a known reference sample.

> **tip**
>
> Always analyze the region above 1500 $cm^{-1}$ first to identify functional groups; the fingerprint region is only used for final confirmation of identity in CIE exams.

**Worked example:** An unknown compound has a strong peak at 1720 $cm^{-1}$ and a broad peak between 3200 – 3600 $cm^{-1}$. No peak is seen between 2500 – 3300 $cm^{-1}$. What functional groups are present?

1. 1. A strong peak around 1700 – 1750 $cm^{-1}$ is characteristic of a C=O (carbonyl) bond.
2. 2. A broad peak between 3200 – 3600 $cm^{-1}$ corresponds to an O-H bond in an alcohol. Carboxylic acid O-H would appear as a very broad peak between 2500 – 3300 $cm^{-1}$, which is not observed here.
3. 3. The combination of a carbonyl peak and alcohol O-H peak means the compound contains both a hydroxyl group and a carbonyl group, for example a hydroxyketone or hydroxycarboxylic acid (ruling out a simple carboxylic acid due to the missing broad O-H peak at lower wavenumber).
4. Final answer: The compound contains an alcohol hydroxyl group and a carbonyl group.

*Calculator:* forbidden

## Confirming Organic Structures with IR Spectra

In most CIE exam questions, you will be given a proposed structure for an unknown compound and asked to use the IR spectrum to confirm if the structure is correct. You will rarely need to determine a full unknown structure from the IR spectrum alone.

**Worked example:** A student proposes that an unknown compound is ethyl ethanoate (an ester, formula $CH_3COOCH_2CH_3$). The IR spectrum has a strong peak at 1735 $cm^{-1}$ and a strong peak between 1000 – 1300 $cm^{-1}$. There is no broad peak between 2500 – 3600 $cm^{-1}$. Does this support the student's proposal?

1. 1. Ethyl ethanoate is an ester, so it contains a C=O bond and a C-O bond, but no O-H or N-H bonds.
2. 2. Ester C=O has a characteristic absorption at 1730 – 1750 $cm^{-1}$, which matches the observed peak at 1735 $cm^{-1}$.
3. 3. C-O bonds have a characteristic strong absorption between 1000 – 1300 $cm^{-1}$, which matches the observed peak in this range.
4. 4. The absence of a broad O-H peak between 2500 – 3600 $cm^{-1}$ is consistent with ethyl ethanoate, which has no O-H bond.
5. All observed absorptions match the proposed structure, so the student's proposal is supported.

**Check your understanding**

Test your understanding of key peaks:

1. Which peak would you expect to see in an amine but not an alkane?

   - Peak at 1700 $cm^{-1}$
   - Peak at 3400 $cm^{-1}$
   - Peak at 1200 $cm^{-1}$
   - No peak difference

   *Why:* Correct! Amines have an N-H bond that absorbs at 3300-3500 $cm^{-1}$, which alkanes do not have.

*Calculator:* forbidden

## Common pitfalls

- **Wrong:** Confusing the O-H peak positions of carboxylic acids and alcohols
  - Why it fails: Both have broad O-H peaks, but they appear in very different wavenumber ranges
  - Correct: Alcohol O-H = broad at 3200-3600 $cm^{-1}$; Carboxylic acid O-H = very broad at 2500-3300 $cm^{-1}$
- **Wrong:** Claiming all covalent bonds absorb IR radiation
  - Why it fails: Only bonds that produce a change in dipole moment when vibrating absorb IR
  - Correct: Non-polar bonds like O=O and N≡N do not absorb IR radiation, as their vibration does not change dipole moment
- **Wrong:** Trying to identify functional groups from the fingerprint region first
  - Why it fails: The fingerprint region has many overlapping peaks from whole-molecule vibrations, making functional group identification difficult
  - Correct: Always analyze the region above 1500 $cm^{-1}$ first to find key functional group peaks
- **Wrong:** Assuming a peak at ~1700 $cm^{-1}$ always means a carboxylic acid
  - Why it fails: All carbonyl groups (ketones, aldehydes, esters, carboxylic acids) have a C=O peak around 1700 $cm^{-1}$
  - Correct: Check for other peaks (O-H for carboxylic acid, C-O for ester) to distinguish between carbonyl functional groups

## Cheatsheet

| Bond/Functional Group | Wavenumber Range ($cm^{-1}$) | Peak Characteristics |
| --- | --- | --- |
| O-H (alcohol, H-bonded) | 3200 – 3600 | Broad |
| O-H (carboxylic acid) | 2500 – 3300 | Very broad |
| C=O (all carbonyls) | 1640 – 1750 | Strong |
| C-O | 1000 – 1300 | Strong |
| N-H (amine/amides) | 3300 – 3500 | Medium, sharp |
| C=C (alkene) | 1620 – 1680 | Weak |

## What's next

Infra-red spectroscopy is a core technique for confirming organic functional groups, and it is almost always used alongside other analytical methods to determine the full structure of unknown compounds. Mastery of IR peak interpretation is essential for both practical assessments and written exam questions in CIE A-Level Chemistry. Next, you can explore other key analytical techniques covered in the syllabus to build a full understanding of structure determination.

- [Chromatography basics](https://www.owlsprep.com/study/cie-9701-u16-chromatography-basics/)
- [Further chemical energetics](https://www.owlsprep.com/study/cie-9701-u17-overview/)
- [Lattice enthalpy](https://www.owlsprep.com/study/cie-9701-u17-lattice-enthalpy/)

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