We used the Bromine in DCM method to test for Unsaturation. We also took an IR spectrum of the compound. However, neither method could really give a direct answer for the number of CEC bonds present. What methods could we use to discern this information?
The correct answer and explanation is :
To determine the number of C≡C (alkyne) bonds in a compound, ¹H NMR (Proton Nuclear Magnetic Resonance) and ¹³C NMR (Carbon-13 Nuclear Magnetic Resonance) spectroscopy are the most reliable techniques.
Correct Answer:
Use ¹H NMR and ¹³C NMR spectroscopy to determine the number of C≡C bonds present.
Explanation (≈300 words):
While the Bromine in DCM test and IR spectroscopy can indicate the presence of unsaturation, such as C=C or C≡C bonds, they do not quantify the number of unsaturated sites. The bromine test detects unsaturation via decolorization, but it cannot differentiate between alkene and alkyne or determine how many unsaturations are present. Similarly, the IR spectrum can show a characteristic C≡C stretch (~2100–2260 cm⁻¹) or a C–H stretch (~3300 cm⁻¹) for terminal alkynes, but again, it doesn’t provide a count of C≡C bonds—just their presence.
To determine the number of C≡C bonds, ¹H and ¹³C NMR spectroscopy are far more powerful. In ¹H NMR, a terminal alkyne proton typically appears as a singlet between 1.8–3.1 ppm, and its presence indicates a terminal (vs. internal) triple bond. More importantly, ¹³C NMR will show distinct signals for sp-hybridized carbon atoms involved in C≡C bonds, generally appearing in the 65–90 ppm range. By counting the number of such signals (considering symmetry), you can estimate the number of C≡C bonds.
Additionally, advanced NMR techniques like DEPT (Distortionless Enhancement by Polarization Transfer) or 2D NMR (e.g., HSQC, HMBC) can help assign carbon types and confirm the structure.
For unambiguous structure determination, mass spectrometry (MS) and elemental analysis may also assist in deducing the degree of unsaturation (Double Bond Equivalents, DBE), which includes all π-bonds and rings.
Thus, combining ¹H and ¹³C NMR is the most effective way to discern both the presence and number of C≡C bonds in a compound.
