What is true about the IR spectrum of 2,3-dimethyl-2-butene? There will be no peak (C-=C) at 1600-1700 cm-1 There will be a peak (C-H; sp2-s bond) at 3100 cm 1 There will be a peak (C-H; sp3-s bond) above 3000 cm There will be a peak (C-H; sp2-s bond) just below 3000 cm-1 None of these
The Correct Answer and Explanation is:
Correct Answer:
There will be a peak (C–H; sp²–s bond) at 3100 cm⁻¹.
Explanation:
The IR (infrared) spectrum of 2,3-dimethyl-2-butene, a symmetrical alkene with the molecular formula C₆H₁₂, shows several characteristic peaks based on the functional groups present in the molecule. The key structural feature in this compound is the carbon–carbon double bond (C=C), and alkyl groups attached to it.
Let us analyze the given options based on infrared spectroscopy:
- “There will be no peak (C=C) at 1600–1700 cm⁻¹”:
This is false. Alkenes generally exhibit a weak to medium intensity peak for the C=C stretch in the region of 1600–1680 cm⁻¹. Even though 2,3-dimethyl-2-butene is symmetrical (reducing the dipole moment change and intensity of the signal), the C=C stretch may still appear as a very weak peak, not completely absent. - “There will be a peak (C–H; sp²–s bond) at 3100 cm⁻¹”:
This is true. Alkenes have vinylic hydrogen atoms (hydrogens attached to sp²-hybridized carbon atoms), which produce C–H stretching vibrations just above 3000 cm⁻¹, typically around 3020–3100 cm⁻¹. So, the IR spectrum of 2,3-dimethyl-2-butene will show a peak in this range. - “There will be a peak (C–H; sp³–s bond) above 3000 cm⁻¹”:
This is false. The C–H stretch from sp³-hybridized carbon atoms typically appears at 2850–2960 cm⁻¹, which is below 3000 cm⁻¹, not above. - “There will be a peak (C–H; sp²–s bond) just below 3000 cm⁻¹”:
This is false. The C–H stretch from sp²-hybridized carbon atoms appears just above 3000 cm⁻¹, not below.
Thus, the only correct statement about the IR spectrum of 2,3-dimethyl-2-butene is that it shows a C–H stretch for sp²–hybridized carbon at around 3100 cm⁻¹.
