IR Spectroscopy - Practice Problems

This paragraph introduces a series of practice problems related to infrared (IR) spectroscopy. It suggests watching a previous video for a basic introduction before attempting these problems. The focus is on identifying molecules from an IR spectrum, starting with the identification of a carbonyl group near 1700 cm⁻¹. The presence of a strong, broad O-H stretch between 2700 to 3300 cm⁻¹ helps to narrow down the options to a carboxylic acid. The absence of a C≡C triple bond signal at 2200 cm⁻¹ further eliminates certain options, leading to the conclusion that the correct molecule corresponds to answer choice A.

The second paragraph delves into distinguishing between different functional groups using IR spectroscopy. It starts with identifying the presence of a carbonyl group and an alkane CH stretch. A signal around 2700 cm⁻¹ indicates an aldehyde CH stretch, helping to eliminate certain options. The absence of a broad O-H signal further narrows down the choices. The presence of a C=C double bond stretch around 1600 cm⁻¹ and a CH stretch around 3100 cm⁻¹ indicates an alkene with a hydrogen on the double bond, leading to the conclusion that answer choice B is correct for this spectrum.

This paragraph discusses how to identify alkynes, specifically differentiating between terminal and internal alkynes, using IR spectroscopy. The presence of a C≡C triple bond stretch at 2200 cm⁻¹ is noted, followed by the identification of a signal at 3300 cm⁻¹, which corresponds to the alkyne CH stretch and is indicative of a terminal alkyne. The absence of a hydrogen attached to the triple-bonded carbon atoms in an internal alkyne helps to eliminate incorrect choices, leading to the conclusion that answer choice A is the correct identification for a terminal alkyne.

The fourth problem in the script focuses on identifying functional groups such as ketones, esters, aldehydes, and amides from an IR spectrum. The presence of an alkane CH stretch and a carbonyl CO stretch at 1700 cm⁻¹ are noted. The absence of an aldehyde CH stretch at 2700 cm⁻¹ and an amide NH2 signal around 3300-3500 cm⁻¹ helps to eliminate incorrect choices. The correct answer is determined by the presence of a ketone, which only requires the alkane CH stretch and the carbonyl CO stretch, leading to answer choice A.

The fifth problem addresses the identification of an amine and benzene rings in an IR spectrum. The presence of an NH2 group is confirmed by a double peak signal at 3300-3500 cm⁻¹. The absence of an alkane CH stretch at 2900 cm⁻¹ helps to distinguish between the remaining options. The presence of a C=C double bond stretch and the absence of the alkane CH stretch at 2900 cm⁻¹ leads to the conclusion that answer choice D is correct for a molecule with a benzene ring.

The final paragraph in the script discusses the identification of ethers and benzene rings in the absence of a carbonyl group. The absence of a carbonyl stretch at 1700 cm⁻¹ eliminates certain options. The presence of an SP3 C-O stretch between 1000 and 1150 cm⁻¹ helps to identify an ether. The absence of a signal at 1600 cm⁻¹ and a CH stretch at 3100 cm⁻¹ eliminates the possibility of a benzene ring. The correct answer is determined by the presence of an SP3 C-O stretch, leading to answer choice B.