R peaks and functional groups of each spectrum (cyclohexene, trans-bromocyclohexanol, and epoxycyclohexane)Which characteristics peaks (appearance/disappearance) are consistent with the conversion of each reactant into the product within the 2 step synthesis performed?
The Correct Answer and Explanation is:
IR Peaks and Functional Groups:
- Cyclohexene
Key IR Peaks:- C=C Stretch (alkene): ~1640–1680 cm⁻¹
- =C–H Stretch: ~3020–3100 cm⁻¹
- C–H Stretch (sp³): ~2850–2960 cm⁻¹
Functional Groups: Alkene (C=C), aliphatic hydrogens
- trans-Bromocyclohexanol
Key IR Peaks:- O–H Stretch (alcohol): ~3200–3550 cm⁻¹ (broad)
- C–Br Stretch: ~500–700 cm⁻¹
- C–H Stretch (sp³): ~2850–2960 cm⁻¹
Functional Groups: Alcohol (–OH), alkyl bromide (C–Br), no alkene present
- Epoxycyclohexane
Key IR Peaks:- C–O–C Stretch (epoxide): ~800–950 cm⁻¹ (ring deformation)
- C–H Stretch (sp³): ~2850–2960 cm⁻¹
- Absence of O–H peak
Functional Groups: Epoxide ring (cyclic ether), saturated ring
Explanation of Peak Changes During 2-Step Synthesis:
The 2-step synthesis from cyclohexene to epoxycyclohexane involves an electrophilic addition of bromine and water (forming trans-bromocyclohexanol), followed by base-promoted intramolecular substitution to form the epoxide.
In Step 1, cyclohexene’s alkene peak (~1640 cm⁻¹) disappears, indicating the double bond is consumed. A broad O–H stretch (~3200–3550 cm⁻¹) appears, confirming formation of an alcohol group. A new C–Br stretch (~500–700 cm⁻¹) supports the addition of bromine across the former double bond.
In Step 2, during cyclization, the O–H group reacts with the neighboring carbon bearing the Br, forming a three-membered epoxide ring. This causes the O–H stretch to disappear, indicating the alcohol is no longer free. The C–Br peak also disappears, showing bromine has left as a leaving group. A new epoxide-related C–O–C stretch (800–950 cm⁻¹) appears, confirming successful epoxide formation.
These IR spectral changes—disappearance of alkene and alcohol peaks, and appearance of epoxide-related stretches—provide strong evidence for the successful transformation of cyclohexene through trans-bromocyclohexanol into epoxycyclohexane.
