What are the major IR absorptions in the functional group region for each compound?

Name: Problem 18, Chapter 13: Organic Chemistry
Uploaded: 2019-05-09T02:13:28-08:00
Duration: 5 min 39 s
Channel: Ian Kaigh
Description: What are the major IR absorptions in the functional group region for each compound?

What are the major IR absorptions in the functional group...

Key Concepts

Absorption Bands

Absorption bands in an IR spectrum are the specific frequencies at which a molecule absorbs infrared light, corresponding to the vibrational transitions of its bonds. The position and intensity of these bands provide meaningful insights into the types of bonds present and their molecular environments, thereby facilitating the assignment of functional groups in the compound.

Vibrational Modes

Molecular vibrations such as stretching (changes in bond lengths) and bending (changes in bond angles) are responsible for the absorption bands observed in an IR spectrum. Different functional groups display unique vibrational modes, which result in specific absorption frequencies that serve as fingerprints for molecular identification.

Functional Group Region

The functional group region of an IR spectrum typically spans from about 4000 cm?¹ to 1500 cm?¹ and includes absorptions associated with key functional groups such as O–H, N–H, C=O, and C–O. This region is critical for identifying characteristic bonds that help determine the overall structure of organic molecules.

IR Spectroscopy

Infrared spectroscopy is an analytical technique that uses the absorption of infrared radiation by molecules to identify and study chemical bonds. By measuring the frequencies at which bonds absorb energy, the technique provides detailed information about the molecular structure and functional groups of compounds.

Transcript

00:03 This is the answer to chapter 13, problem number 18, from the smith organic chemistry textbook.

00:12 And this problem is asking us to look at six molecules and to predict what kinds of ir signals we're going to see.

00:22 And so to start, so each of these molecules has csp3h bonds.

00:31 So sp3 carbons bonded to hydrogens.

00:35 And so all of these molecules are going to have peaks at 2850 to 3 ,000 wave number.

00:44 And so for a, that's actually all the peaks that it's going to have.

00:49 Remember, ir tells us about functional groups, and there are no functional groups in molecule a.

00:54 So it's only going to have those csp3h peaks.

01:00 So then when we look at b, the functional group that we see in b is an alcohol.

01:06 And so alcohol is going to be a broad signal at 3 ,200 to 3 ,600 wave numbers.

01:14 And so i've just, i've written where the peak will be in red, and i've written the functional group that it represents or the bond that it represents in blue.

01:24 And so moving on to c, the only functional group in c is a functional group in c is a functional group.

01:29 An alkene, a double bond.

01:32 And so remember, as long as there are hydrogens on any of the carbons involved in a double bond, you're going to get two signals.

01:43 You're going to get the signal for the carbon -carbon double bond, which is going to show at 1 ,350 wave number.

01:51 And you're also going to get the signal somewhere between 3 ,0301 .50 wave number for the csp2 hydrogen bond.

02:04 And so that's for c.

02:06 Again, there will be two major peaks in addition to the peak, you know, that all of these have for the csp3h bonds.

02:16 So moving on to d, the functional group in d is a ketone.

02:21 And we know ketones show somewhere in the vicinity of 1700 wave number.

02:27 And so that's the answer to d.

02:29 And then moving on to e, e is a little bit more complex of a molecule.

02:34 And there are two functional groups in a...

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