Question
Which analytical technique is used to measure bond lengths in solid compounds?
B. Mass spectroscopy
C. NMR spectroscopy
D. X-ray crystallography
▶️Answer/Explanation
D
X-ray crystallography is a powerful technique used to determine the three-dimensional arrangement of atoms in a crystal structure, including bond lengths. It involves shining X-rays onto a crystalline sample and analyzing the resulting diffraction pattern.
Question
Which property explains why tetramethylsilane, Si(CH3)4, can be used as a reference standard in 1H NMR spectroscopy?
A. It has a high boiling point.
B. It is a reactive compound.
C. All its protons are in the same chemical environment.
D. It gives multiple signals.
▶️Answer/Explanation
C
Tetramethylsilane (TMS) is commonly used as a reference standard in 1H NMR spectroscopy for several reasons: TMS exhibits a single sharp peak in the 1H NMR spectrum at 0 ppm. This peak serves as a convenient reference point against which all other peaks in the spectrum are measured. Since TMS is non-reactive and does not participate in hydrogen bonding or other interactions, its chemical shift is relatively constant across a wide range of solvents and experimental conditions. Therefore, its position on the spectrum can be used as a reliable reference.
TMS is readily available, commercially stable, and relatively inexpensive. It is also highly volatile as it has very low boiling point, allowing it to be easily evaporated and added to the NMR sample. Its volatility ensures that it is evenly distributed throughout the sample, providing a uniform reference signal.
TMS is soluble in a wide range of common NMR solvents, such as deuterated chloroform (CDCl3), deuterated dimethyl sulfoxide (DMSO-d6), and deuterated methanol (CD3OD). Its compatibility with different solvents allows for flexibility in experimental conditions and enables the use of TMS as a reference in a variety of sample types.
Question
Which technique is used to determine the bond lengths and bond angles of a molecule?
A. X-ray crystallography
B. Infrared (IR) spectroscopy
C. Mass spectroscopy
D. 1H NMR spectroscopy
▶️Answer/Explanation
A
The technique commonly used to determine bond lengths and bond angles of a molecule is X-ray crystallography. X-ray crystallography is a powerful method for determining the three-dimensional arrangement of atoms within a crystal lattice.
Question
Which compound gives this 1H NMR spectrum?
A. CH3CH2OCH2CH3
B. CH3CH2OH
C. CH3CH2CH3
D. CH3CH2CH2OH
▶️Answer/Explanation
D
The 1H NMR spectrum of propyl alcohol (also known as n-propanol) typically consists of multiple peaks corresponding to the different hydrogen atoms present in the molecule.
Singlet at around 0.9 ppm: This peak corresponds to the methyl group (CH3) in propyl alcohol. It appears as a singlet because the methyl protons are not coupled to any neighboring protons.
Multiplet around 1.2-1.5 ppm: This region represents the protons on the methylene (CH2) group. The protons in this group experience some coupling with the adjacent protons, resulting in a complex multiplet.
- Singlet at around 3 ppm: This region represents the protons on the next methylene (CH2) group. It appears as a singlet because the methylene protons are not coupled to any neighboring protons.
Singlet at around 3.7 ppm: This peak corresponds to the hydroxyl (OH) proton. The hydroxyl group does not couple with any other protons in the molecule, resulting in a sharp singlet.
Question
Which technique can be used to identify bond length and bond angle?
A. 1H NMR spectroscopy
B. IR spectroscopy
C. Mass spectroscopy
D. X-ray crystallography
▶️Answer/Explanation
D
The technique commonly used to determine bond lengths and bond angles of a molecule is X-ray crystallography. X-ray crystallography is a powerful method for determining the three-dimensional arrangement of atoms within a crystal lattice.
Question
Which would be the most effective method to distinguish between liquid propan-1-ol and propan-2-ol?
A. Observation of colour change when warmed with acidified potassium dichromate
B. Determination of m/z value of molecular ion in the mass spectrum
C. Determination of percentage composition
D. 1H NMR spectroscopy
▶️Answer/Explanation
D
1H NMR spectroscopy can be an effective method to distinguish between propan-1-ol (also known as n-propanol) and propan-2-ol (also known as isopropanol). Although both compounds have similar molecular formulas (C3H8O), their chemical structures differ, resulting in distinct NMR spectra.
Coupling Patterns in Propan-1-ol, the methylene (CH2) protons adjacent to the hydroxyl group usually show complex coupling patterns (multiplets) due to their interactions with neighboring protons, whereas
In Propan-2-ol, the methylene (CH2) protons adjacent to the hydroxyl group in propan-2-ol typically show simpler coupling patterns (multiplets) compared to propan-1-ol.