Brief Summary
Alright, so this video is all about practicing IR spectroscopy problems. The tutor goes through six different problems, showing how to identify key functional groups from IR spectra. Key takeaways include recognizing carbonyl, hydroxyl, alkyne, and amine stretches, and using their presence or absence to narrow down the molecular structure.
- Identifying functional groups using IR spectroscopy.
- Recognizing key stretches like carbonyl, hydroxyl, and alkyne.
- Using the process of elimination to find the correct molecule.
Problem 1: Identifying a Carboxylic Acid
The first problem presents an IR spectrum, and the task is to identify the corresponding molecule. The spectrum shows a carbonyl (C=O) stretch near 1700 cm⁻¹, and a broad, strong O-H stretch between 2700 and 3300 cm⁻¹. The presence of both these stretches indicates a carboxylic acid. By eliminating options that don't have a hydroxyl group or have a triple bond signal at 2200 cm⁻¹ (which isn't present), the correct answer is determined to be the molecule with the carboxylic acid functional group.
Problem 2: Identifying an Aldehyde with an Alkene
In the second problem, the IR spectrum shows a carbonyl stretch at 1700 cm⁻¹, indicating either an aldehyde or a ketone. A signal around 2700 cm⁻¹ is identified as the aldehyde C-H stretch, which helps eliminate ketone options. The absence of a broad O-H signal rules out alcohols. A C=C stretch around 1600 cm⁻¹ and an alkene C-H stretch around 3100 cm⁻¹ confirm the presence of an alkene. Thus, the molecule is identified as an aldehyde with an alkene.
Problem 3: Distinguishing Terminal and Internal Alkynes
The third problem involves identifying an alkyne. The spectrum shows a C≡C stretch at 2200 cm⁻¹, confirming the presence of an alkyne. To differentiate between a terminal and an internal alkyne, the presence of a signal at 3300 cm⁻¹ is checked. This signal corresponds to the alkyne C-H stretch, which is only present in terminal alkynes. Since the signal is present, the molecule is identified as a terminal alkyne.
Problem 4: Identifying a Ketone
In the fourth problem, the task is to identify a functional group from the given IR spectrum. The spectrum shows an alkane C-H stretch around 2900 cm⁻¹ and a carbonyl C=O stretch at 1700 cm⁻¹. By systematically eliminating options based on the absence of expected signals, such as the aldehyde C-H stretch at 2700 cm⁻¹ (for aldehydes), the N-H signal around 3300-3500 cm⁻¹ (for amides), and the C-O signal between 1000 and 1300 cm⁻¹ (for esters), the molecule is identified as a ketone.
Problem 5: Identifying an Amine with a Benzene Ring
The fifth problem requires identifying a molecule containing an amine and possibly a benzene ring. The spectrum shows a double peak at 3300-3500 cm⁻¹, indicating an NH₂ group. This eliminates options without a primary amine. A C=C signal close to 1600 cm⁻¹ suggests either an alkene or a benzene ring. The absence of an alkane C-H stretch at 2900 cm⁻¹ helps to identify the correct molecule as a benzene ring with an amine group.
Problem 6: Identifying an Ether
In the sixth problem, the task is to identify a molecule from the given IR spectrum. The absence of a carbonyl group signal at 1700 cm⁻¹ immediately eliminates esters and ketones. The absence of signals at 1600 cm⁻¹ and 3100 cm⁻¹ rules out the presence of a benzene ring. The presence of an sp³ C-O stretch between 1000 and 1150 cm⁻¹ confirms the presence of an ether. Thus, the molecule is identified as an ether.
