IR Spectroscopy - Basic Introduction

Summary

This video provides an introduction to infrared (IR) spectroscopy and how it can be used to identify functional groups in organic molecules. The presenter explains key IR absorption characteristics for various functional groups, including carboxylic acids, alcohols, aldehydes, ketones, esters, ethers, amines, and amides, as well as hydrocarbons like alkanes, alkenes, and alkynes. Emphasis is placed on understanding wave numbers, bond strength, and the impact of conjugation on IR absorption. The video offers a comprehensive foundation for interpreting IR spectra in organic chemistry, with practical insights to help students distinguish between similar functional groups.

Takeaways

• 😀 Carboxylic acids have a broad OH stretch between 2500-3300 cm⁻¹ and a strong CO stretch around 1700 cm⁻¹, distinguishing them from alcohols.
• 😀 Alcohols show a strong OH stretch between 3200-3600 cm⁻¹, with no significant CO stretch like carboxylic acids.
• 😀 Aldehydes and ketones both exhibit a CO stretch around 1700 cm⁻¹, but aldehydes can be identified by a unique CH stretch around 2700 cm⁻¹.
• 😀 Esters have a CO stretch around 1700 cm⁻¹ like aldehydes and ketones, but they also show additional resonance effects due to the sp² carbonyl group.
• 😀 Ethers lack a carbonyl group, instead showing a single bond CO stretch between 1000-1150 cm⁻¹, making them distinct from esters.
• 😀 Primary amines have a double peak for NH stretches between 3300-3500 cm⁻¹, while secondary amines show a single peak in the same range.
• 😀 Amides contain both a CO stretch around 1700 cm⁻¹ and an NH stretch, helping distinguish them from amines.
• 😀 Alkanes show a CH stretch around 2900 cm⁻¹, alkenes show a C=C stretch around 1660 cm⁻¹, and alkynes feature a C≡C stretch between 2100-2200 cm⁻¹.
• 😀 The CH stretch in alkenes occurs around 3000-3100 cm⁻¹, while terminal alkynes have a CH stretch around 3300 cm⁻¹.
• 😀 Bond strength increases wave number: triple bonds have higher wave numbers than double bonds, and double bonds have higher wave numbers than single bonds.

Q & A

What is the key feature that distinguishes a carboxylic acid from an alcohol in IR spectroscopy?

The key feature is the broad, strong OH stretch in carboxylic acids, which appears between 2500-3300 cm⁻¹. Alcohols also show an OH stretch, but it occurs in the range of 3200-3600 cm⁻¹, which is generally narrower and less broad than that of carboxylic acids.

How can an aldehyde be distinguished from a ketone in IR spectroscopy?

Aldehydes and ketones both show a strong CO stretch around 1700 cm⁻¹. However, aldehydes can be distinguished by the presence of a CH stretch around 2700 cm⁻¹, which ketones do not have.

What is the difference between the CO stretch in esters and ethers?

Esters exhibit a CO stretch around 1700 cm⁻¹, similar to aldehydes and ketones, because they have a carbonyl group. Ethers, on the other hand, show a CO stretch in the range of 1000-1150 cm⁻¹, which is due to a single bond CO stretch.

How does hybridization affect the CH stretch in IR spectroscopy?

As the s character of the carbon in a C-H bond increases, the wave number for the CH stretch increases. An sp hybridized carbon (like in alkynes) will have a CH stretch at a higher wave number (around 3300 cm⁻¹) compared to an sp³ hybridized carbon (like in alkanes) which appears around 2900 cm⁻¹.

What is the significance of conjugation on the IR absorption of carbonyl compounds?

Conjugation lowers the IR absorption frequency for carbonyl compounds. A conjugated ketone absorbs at a lower wave number (~1680 cm⁻¹) compared to a non-conjugated ketone, which absorbs slightly higher (~1720 cm⁻¹). This is because conjugation reduces the double bond character of the carbonyl bond.

What role does atomic mass play in IR spectroscopy?

In IR spectroscopy, as the atomic mass of the atoms involved in a bond increases, the wave number decreases. For example, the C-H stretch in organic molecules appears around 2900 cm⁻¹ for hydrogen but decreases as the atomic mass of the substituent increases (e.g., bromine causing a much lower absorption frequency).

How does the strength of a bond affect the wave number in IR spectroscopy?

Bond strength is directly related to the wave number. Stronger bonds, such as triple bonds, absorb at higher wave numbers compared to weaker bonds like single bonds. For example, C≡C stretches in alkynes occur around 2100-2200 cm⁻¹, while C-C stretches in alkanes occur closer to 2900 cm⁻¹.

What is the primary distinction between a primary and secondary amine in IR spectroscopy?

Primary amines show a double peak in the 3300-3500 cm⁻¹ region due to two hydrogen atoms attached to nitrogen. Secondary amines, with only one hydrogen atom attached to nitrogen, produce a single peak in the same region.

How does a conjugated alkene compare to a non-conjugated alkene in IR absorption?

Conjugated alkenes absorb at lower wave numbers than non-conjugated alkenes. A non-conjugated alkene typically absorbs around 1650-1660 cm⁻¹, whereas a conjugated alkene absorbs around 1600 cm⁻¹.

Why do triple bonds have higher wave numbers than double bonds in IR spectroscopy?

Triple bonds are stronger than double bonds, requiring more energy to stretch. Therefore, they absorb IR radiation at a higher wave number. This is why alkynes (C≡C) absorb around 2100-2200 cm⁻¹, while alkenes (C=C) absorb at a lower wave number, around 1660 cm⁻¹.