How would each of the following pairs of compounds differ in their IR spectra?

Name: Problem 40, Chapter 13: Organic Chemistry
Uploaded: 2019-05-17T03:27:43-08:00
Duration: 4 min 26 s
Channel: Ian Kaigh
Description: How would each of the following pairs of compounds differ in their IR spectra?

How would each of the following pairs of compounds differ in...

Key Concepts

Characteristic Absorption Bands

Different bonds and functional groups absorb IR radiation at specific frequency ranges, leading to characteristic absorption bands. For example, carbonyl groups (C=O) typically show strong absorptions in the region around 1700 cm?¹, while hydroxyl groups (O–H) present broad absorptions around 3200-3600 cm?¹. These characteristic bands serve as diagnostic tools for comparing the IR spectra of different compounds, allowing for the identification of even subtle differences in molecular structure.

Molecular Vibrations

Infrared (IR) spectroscopy is based on the concept of molecular vibrations. When molecules are exposed to infrared light, they absorb energy that causes changes in their vibrational states. This absorption occurs at specific frequencies that correspond to the vibrational motions of bonds within the molecule, such as stretching, bending, twisting, or scissoring. Understanding these vibrational modes is fundamental to interpreting how compounds will appear in an IR spectrum.

Functional Group Analysis

One of the most powerful applications of IR spectroscopy is the identification of functional groups within a molecule. Each functional group has a characteristic set of vibrational frequencies due to the unique bond strengths and masses of the atoms involved. By analyzing the positions and intensities of absorption bands, chemists can deduce the presence or absence of particular functional groups and, consequently, the differences between similar compounds.

Effects of Molecular Structure

Variations in molecular structure, such as differences in conjugation, hydrogen bonding, or steric effects, can cause shifts in absorption frequencies and changes in band shapes. Conjugated systems, for example, often display shifted and sometimes less intense absorption bands relative to their non-conjugated counterparts. These structural effects are critical in explaining why pairs of compounds may exhibit distinct IR spectral features even if they contain similar functional groups.

Transcript

00:03 This is the answer to chapter 13, problem number 40 from the smith organic chemistry textbook.

00:10 And so this problem is giving us six pairs of molecules and asking us to identify what the differences would be in their ir spectrums.

00:20 And so for a, we have a cyclopentine versus this alkyne.

00:33 And so the cyclopentine is going to show carbon -carbon -carbon double bond at roughly 1650 and c -sp2 hybridized hydrogen, pardon me, yeah, so sp2 hybridized carbon hydrogen bond.

00:50 And so that's going to show between 3 ,030150.

00:55 The al -kine is obviously going to show the carbon -carbon -triple bond at 2250, and the c -sp -h bond at 3 ,300.

01:09 For b, the main difference in these molecules is that the first one, the carboxylic acid, is going to have that oh bond.

01:19 And remember, in a carboxylic acid, it's hard to say exactly where that will show, but it will show above 3 ,000 wave number.

01:28 The second molecule for b does not have an oh bond in it, and so there should be no signal above 3 ,000.

01:37 And so that makes that pretty straightforward to identify.

01:42 Moving to c, c is also very straightforward.

01:46 There should be the distinctive ketone signal in the first molecule at around 1 ,700 wave number.

01:53 The second molecule in c should show a carbon -carbon -carbon double bond at about 1650.

01:59 The c -sp2h bond, again between 3 ,000 and 31.

02:04 150 and the oh bond.

02:07 And this ohh is not a carboxylic acid, as the one in b was.

02:11 And so we know it should show between 3 ,200 and 3 ,600 wave number.

02:18 And it should be a nice broad peak...

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