Purpose of Analytical Techniques Practice Problems

Determine the structure of the compound using the following spectrums.

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True or False. The two signals seen in the ¹H NMR spectrum of [18]annulene are caused by the hydrogen atoms that protrude into the center of the compound's ring, accounting for the signal downfield of the TMS signal, and hydrogens that protrude out, accounting for the signal upfield of the TMS signal.

Which of the following corresponds to the DEPT ¹³C NMR spectrum of 1-bromopropan-1-ol.

A compound with the formula C₅H₈O has the following DEPT ¹³C NMR spectrum. Draw the structure of the compound.

When compound Y undergoes the following sequence of reactions, it yields a product with the ¹H NMR spectrum shown below. 

Identify the possible structure of Y.

True or false. Undiluted propanol exhibits an O−H stretch at a larger wavenumber than propanol dissolved in tetrachloroethylene.

Determine the compound with molecular weight is 116 and the mass spectrum as shown below. It shows a broad, strong absorption at 3600 cm⁻¹ in the infrared spectrum.

Write down how many signals are there and indicate their multiplicity in the ¹H NMR spectra of the given molecule.

(CH₃)₃CCH₂CH₂CH₂CH₂CH₂CH₂C(CH₃)₃

Determine the splitting patterns in the 1H NMR spectra for the molecule given below:

Determine the splitting patterns in the ¹H NMR spectra for the molecule given below:

Determine the product of the reaction when one equivalent of the reagent is employed. (Ignore stereoisomers)

For the following compound, predict the mass and structure of the fragment that forms as a result of McLafferty rearrangement.
2-methylhexan-3-one

When an acid is added to methoxymethanol, a higher boiling point fraction is obtained during redistillation. The mass spectrum of this unknown fraction showed its molecular mass as 106. Its IR spectrum showed an intense stretch near 1200 cm^-1 while ¹H NMR showed two peaks with the ratio 3:2. Both IR and NMR showed no OH peak. Propose the structure of this unknown compound, and suggest a mechanism for its formation.

The given spectrum shows the estimated ¹³C NMR peaks of phenyl butyrate. Show which of these carbons will appear in DEPT-90 and DEPT-135 spectra.

Different protons and carbons in alkanes may sometimes appear at close chemical shifts which makes it difficult to elucidate the structures. Show how ¹³C NMR along with DEPT experiments can be used to distinguish between the following isomers.

Based on the data provided, propose the structures of the structural isomers A and B. 

a. When the sample is shaken with D₂O (isomer A), the singlet at 1.5 ppm disappears from the NMR spectrum. Similarly, the singlet at 3.3 ppm in the B spectrum vanishes when shaken with D₂O (isomer B). The IR spectrum, on the other hand, revealed two peaks at 3300 cm^-1 for A and only one peak at 3300 cm^-1 for B. 

b. The mass spectrum shows a molecular ion peak at m/z 101 corresponds for both, as well as a prominent peak at m/z 72 for A and m/z 86 for B. Show which cleavage is responsible for these peaks.

For isomer A                                    For Isomer B

An unknown compound gives a weak molecular ion peak at m/z 99 in the mass spectrum. Its NMR spectrum is shown here. The IR spectrum has shown a weak peak at 2255 cm^-1, a strong peak at 1750 cm^-1, and a strong peak at 1205 cm^-1.
Determine its structure and propose a suitable fragmentation for the MS peak at m/z 68.

Fluency in MS and IR is when one can determine the structure of a moderately complex molecule using MS and IR spectra only. The IR and MS peaks of an unknown compound are given below. Determine the structure consistent with these peaks.
MS peaks: 107, 92 (base peak), 77.
IR peaks: 3400 cm⁻¹

Propose the structure of an unknown compound using the NMR spectrum and the additional data provided here. The elemental analysis confirmed the molecular formula of the compound to be C₈H₇OBr. The IR spectrum shows two stretched at 1692 cm⁻¹ and 1605 cm⁻¹. The mass spectrum shows a double molecular ion peak with equal intensities at m/z 198 and 200.

The MS and NMR spectra along with some prominent IR peaks of an unknown compound are given below. Propose a structure consistent with the given data. Also, show the fragments that give the prominent peaks at m/z 105 and 77.
IR peaks: 1685 cm⁻¹ and 1610 cm⁻¹

A scientist mixed 4-phenylbutanoic acid with concentrated sulfuric acid while maintaining the temperature at 170°C. After a few minutes, the reaction mixture was poured into water. The product was extracted from this solution using dichloromethane. The extraction solvent was then evaporated leaving behind a good amount of the product. The mass spectrum of this product shows a molecular ion peak at m/z 146 and its IR spectrum shows a prominent stretch at 1710 cm⁻¹ while the carboxylic stretch was missing. The ¹H NMR spectrum of this product is shown below. What is the product?

Carbonyl compounds often exhibit McLafferty rearrangement in mass spectrometry. Why doesn't acetone exhibit this rearrangement?

For the compound shown below:
(i) Write a splitting tree for proton H_c.
(ii) Determine the chemical shift of proton H_c.