Chemistry (9701) | Infrared spectroscopy

Infrared (IR) Spectroscopy: Finding the Molecular Fingerprint

Hello future chemists! Welcome to the fascinating world of Infrared Spectroscopy. This technique might sound complex, but it's essentially like giving an unknown molecule a quick physical exam to see what functional groups it has. It’s one of the most powerful tools organic chemists use to identify unknown substances.

In this chapter, we will learn how to analyse an infrared spectrum (a graph) to determine which functional groups are present in a simple organic molecule, using the data provided in your syllabus booklet (Section 22.1).

What is IR Spectroscopy?

Imagine a molecule is a collection of balls (atoms) connected by springs (chemical bonds). These bonds are constantly vibrating—stretching and bending—even at room temperature.

IR spectroscopy works by shining infrared radiation onto the sample. If the energy of the IR radiation exactly matches the natural vibrational frequency of a specific bond, the bond absorbs that energy and vibrates much more intensely. This absorption gives us a signal.

• Key Concept: A molecule absorbs IR radiation when the frequency of the radiation matches the frequency of a bond's vibration.
• Important Requirement: For a molecule to absorb IR radiation, the vibration must cause a change in the molecule’s dipole moment. Symmetric molecules (like O₂ or N₂) do not absorb IR.

Analogy: Think of pushing a child on a swing. You only need a small push if you match the natural rhythm (frequency) of the swing. If you push at the wrong time, it won't move much!

Key Takeaway 1:

IR spectroscopy measures the absorption of IR energy due to the characteristic vibrations of bonds within a molecule. This tells us what kinds of bonds, and therefore what functional groups, are present.

Understanding the IR Spectrum

An IR spectrum is a graph that plots the amount of radiation transmitted through the sample against the frequency (or energy) of the radiation.

1. The Axes:

• Y-axis: Transmittance (%). High transmittance means most light passed through (little absorption). A low percentage (a trough or valley) indicates strong absorption.
• X-axis: Wavenumber (\(\bar{\nu}\)), measured in units of cm\textsuperscript{-1}. Wavenumber is directly proportional to frequency and energy.

2. The Peaks (Absorption Bands):

When a bond absorbs energy, the % Transmittance drops, creating a trough or "peak" (or absorption band) pointing downwards on the graph. The position (\(\text{cm}\textsuperscript{-1}\)) of this peak identifies the bond type (e.g., C=O, O-H).

The Two Main Regions of the Spectrum

An IR spectrum is typically divided into two key areas, but only one is useful for functional group identification at this level.

(A) The Functional Group Region (\(\mathbf{4000}\) to \(\mathbf{1500}\) cm\textsuperscript{-1}):

This is where the most important peaks for identifying functional groups appear, such as O-H, C=O, N-H, and C-H. The peaks here are usually distinctive and relatively easy to assign using the data table.

(B) The Fingerprint Region (\(\mathbf{1500}\) to \(\mathbf{500}\) cm\textsuperscript{-1}):

This region contains many complex bending and stretching vibrations, resulting in a dense cluster of peaks.

• Why it's called the Fingerprint Region: Just like every person has a unique fingerprint, every single molecule (even isomers) has a unique pattern of peaks in this region.
• Syllabus focus: We usually don't use this region for simple functional group identification because it is too complex. We focus on the region above \(\text{1500}\) cm\textsuperscript{-1}.

Quick Review: The Map

• High Wavenumber (\(>1500\) cm\textsuperscript{-1}) = Functional Groups.
• Low Wavenumber (\(<1500\) cm\textsuperscript{-1}) = Fingerprint Region (Unique to the molecule, too complex to analyze directly).

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Interpreting the Spectrum: The Syllabus Focus

The main skill required is taking an IR spectrum and identifying the presence (or absence) of key functional groups by matching the wavenumber of the troughs to the values in your data booklet.

Don't worry if you don't memorize the numbers! These values are provided in the data section of the syllabus, but you need to know how to use them effectively.

Factors Affecting Wavenumber (\(\bar{\nu}\))

The exact position of an absorption peak depends primarily on two factors:

1. Bond Strength: Stronger bonds vibrate faster and absorb at higher wavenumbers (higher energy). Example: C\(\equiv\)C absorbs higher than C=C, which absorbs higher than C-C.
2. Mass of Atoms: Bonds between lighter atoms vibrate faster and absorb at higher wavenumbers. Example: C-H bonds absorb at higher frequencies than C-Cl bonds.

Step-by-Step Analysis Guide

When faced with an IR spectrum, follow these steps:

1. Ignore the Fingerprint Region: Focus only on absorptions above \(\text{1500}\) cm\textsuperscript{-1}.
2. Check for the Big Three Heteroatom Groups: O-H, C=O, and N-H. These are the most distinctive peaks.
3. Use the Data Table: Match the position (\(\text{cm}\textsuperscript{-1}\)) and appearance (broad/sharp) of the peak to the functional group listing.
4. Confirm C-H Bonds: Almost all organic molecules show C-H stretching around \(\text{2850}\)–\(\text{3000}\) cm\textsuperscript{-1}.

Struggling Student Tip: Start with C=O. If you can identify that peak, you immediately know the molecule is an aldehyde, ketone, carboxylic acid, or ester. Then look for O-H to narrow it down further.

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The Critical Functional Groups to Identify

You must be able to distinguish between the absorption bands associated with the following key functional groups (refer to your data table for precise ranges):

1. Carbonyl Groups (\(\mathbf{C=O}\))

• Bond: Carbonyl stretch (C=O).
• Wavenumber Range: Typically \(\text{1680}\) – \(\text{1750}\) cm\textsuperscript{-1}.
• Appearance: This is the most reliable peak! It is usually very strong and very sharp.
• Did You Know? This peak is so strong because the C=O bond has a large dipole moment, meaning it interacts strongly with the electric field of the IR light.

2. Hydroxyl Groups (\(\mathbf{O-H}\))

The appearance of the O-H peak depends heavily on the presence of hydrogen bonding, which is crucial for identification.

(A) Alcohol O-H (in Hydrogen-Bonded systems):

• Wavenumber Range: Typically \(\text{3200}\) – \(\text{3600}\) cm\textsuperscript{-1}.
• Appearance: Very broad and round, like a "thumb" or a "fat U". This broadening is characteristic of hydrogen bonding.

(B) Carboxylic Acid O-H:

• Wavenumber Range: Very broad, starting much lower, typically \(\text{2500}\) – \(\text{3300}\) cm\textsuperscript{-1}.
• Appearance: Extremely broad, often merging completely with the C-H region below \(\text{3000}\) cm\textsuperscript{-1}. If you see a massive absorption covering everything in the 2500-3300 range, and you also see a C=O peak, it is almost certainly a carboxylic acid.

3. N-H Groups (Primary Amines)

• Bond: N-H stretch.
• Wavenumber Range: Typically \(\text{3200}\) – \(\text{3500}\) cm\textsuperscript{-1}.
• Appearance: Amines often show one or two sharp, medium-strength spikes in this region (depending on whether it is a primary (\(\text{NH}_2\)) or secondary (\(\text{N-H}\)) amine). If it’s a primary amine, look for two "rabbit ears" or spikes.

4. C-H Bonds

While almost everything has C-H, its exact position helps confirm the environment:

• C-H (Saturated, \(sp^3\)): Below \(\text{3000}\) cm\textsuperscript{-1} (e.g., \(\text{2850}\) – \(\text{2970}\) cm\textsuperscript{-1}).
• C-H (Unsaturated, \(sp^2\)): Above \(\text{3000}\) cm\textsuperscript{-1} (e.g., Alkenes, Arenes).

Example Analysis and Common Mistakes

Example 1: Identifying an Alcohol

If a spectrum shows a strong, broad peak at \(\text{3350}\) cm\textsuperscript{-1} and no significant peak around \(\text{1700}\) cm\textsuperscript{-1}, you can confidently deduce that the molecule contains an alcohol group (R-OH) and is not a carboxylic acid or carbonyl compound.

Example 2: Identifying an Aldehyde/Ketone

If a spectrum shows a very strong, sharp peak at \(\text{1715}\) cm\textsuperscript{-1} and NO broad O-H peak above \(\text{2500}\) cm\textsuperscript{-1}, the molecule is a ketone or aldehyde (R-C=O-R or R-C=O-H).

Common Mistakes to Avoid

1. Confusing O-H and C-H: The general C-H stretch is always present. Don't confuse it with the distinctive O-H or N-H signals which appear higher up and often have very different shapes (broad vs. sharp).
2. Ignoring the C=O Peak: The C=O is often the easiest and most defining peak. Always check for a sharp signal around \(\text{1700}\) cm\textsuperscript{-1} first.
3. Misidentifying Carboxylic Acids: Remember that Carboxylic acids must have BOTH the broad O-H absorption (\(\text{2500}\)–\(\text{3300}\)) AND the sharp C=O absorption (\(\text{1700}\) range). If one is missing, it's not a carboxylic acid.

Key Takeaway 2:

To analyze an IR spectrum, focus on the wavenumber (\(\text{cm}\textsuperscript{-1}\)) and the shape (sharp, broad, strong, weak) of the peaks in the functional group region (\(>1500\) cm\textsuperscript{-1}) and compare them rigorously with the provided data table.