Describe how the IR spectrum for the starting material (t-butyl alcohol) is similar to, or different from, the IR spectrum for the t-butyl chloride. What information, if any, does this comparison provide about the purity of the t-butyl chloride?
The correct answer and explanation is:
Correct Answer:
The IR spectrum of t-butyl alcohol and t-butyl chloride differ primarily in the presence or absence of the O–H stretch. T-butyl alcohol exhibits a broad O–H stretch around 3200–3550 cm⁻¹, which is absent in t-butyl chloride. Both compounds show C–H stretches near 2900 cm⁻¹ and similar C–C skeletal vibrations. The disappearance of the O–H stretch and the possible presence of a C–Cl stretch around 700 cm⁻¹ in the product indicate successful conversion. If the O–H stretch persists in the spectrum of the product, it suggests incomplete reaction or impurity due to residual alcohol.
Explanation (300 words):
Infrared (IR) spectroscopy is a valuable tool for identifying functional groups in organic molecules. When comparing the IR spectra of t-butyl alcohol (the starting material) and t-butyl chloride (the product), several important differences and similarities appear. The most significant difference is the presence of an O–H stretching vibration in t-butyl alcohol, which appears as a broad, strong band between 3200 and 3550 cm⁻¹. This band is characteristic of alcohols due to the hydrogen bonding in the O–H group.
In contrast, t-butyl chloride lacks this O–H functional group, so its IR spectrum does not show this broad absorption in the 3200–3550 cm⁻¹ region. Instead, the product may show a C–Cl stretching vibration, which typically appears as a medium to strong absorption in the 600–800 cm⁻¹ range, often near 700 cm⁻¹. Both spectra will show C–H stretching absorptions near 2850–3000 cm⁻¹ due to the methyl and methylene groups, which do not change significantly during the reaction.
This comparison provides insight into the purity of the t-butyl chloride product. If the O–H absorption is absent and a C–Cl absorption is visible, this suggests that the alcohol has been successfully converted to the alkyl halide and that the product is relatively pure. However, if the IR spectrum of the t-butyl chloride still shows a broad peak around 3400 cm⁻¹, it indicates that some t-butyl alcohol remains, meaning the reaction was incomplete or the product contains impurities.