Four structural isomers having the molecular formula C4H10O are listed below.
• 1-Butanol • 2-Butanol • tert-Butyl alcohol (2-methyl-2-propanol) • Diethyl ether (CH3CH2OCH2CH3)
Average bond energies (kJ mol−1): C–H 413 C–C 348 C–O 358 O–H 463 O=O 498 C=O (in CO2) 799 O–H (in H2O) 463
Balanced combustion equation for each isomer (illustrated with 1-butanol):
C4H10O + 6 O2 → 4 CO2 + 5 H2O
(The same stoichiometry applies to all isomers.)
(a) Choose any two of the four isomers and, for each, enumerate all distinct bond types present. Indicate the total number of each bond type that must be broken during complete combustion.
(b) Using the bond counts from part (a) and the average bond energies provided, estimate ΔH (in kJ mol−1) for the combustion of each chosen isomer. Show all calculations.
(c) Compare your two ΔH values. Explain how differences in molecular structure (e.g., branching vs. straight chain, presence or absence of an O–H bond) influence the overall enthalpy change despite identical molecular formulas.
(d) A laboratory measurement gives ΔH = –2672 kJ mol−1 for the combustion of tert-butyl alcohol. Discuss two chemical reasons why this experimental value might differ from your bond-energy estimate.