SolutionsPhysical and Bond Properties
1. Heats of formation
Calculate the heats of formation of butane, cis- and trans-2-butene and 2-butyne from their heats of combustion (kcal/mol): −687.4, −647.7, −646.9 and −615.8, respectively. What conclusions can you draw?
Butane is much more stable than its elements; the butenes only slightly (the trans isomer somewhat more than the cis), and 2-butyne is much less stable than its elements.
In the scheme, −57.8 kcal/mol is the enthalpy of formation of gaseous water and −10.52 kcal/mol that of its condensation: together they give that of liquid water (−68.3 kcal/mol), which is the one that corresponds to the heats of combustion given.
2. Acidity
Order from most to least acidic: ethane, ethene, ethyne, ammonia and water. Which base would you use to deprotonate propyne completely?
Water (pKa 15.7) > ethyne (25) > ammonia (34) > ethene (44) > ethane (50). A base whose conjugate acid is weaker than propyne is needed, such as sodium amide (NaNH2) or butyllithium; hydroxide is not enough.
3. Synthesis with alkynides
Propose a synthesis of pent-2-yn-1-ol, CH3CH2C≡CCH2OH, from 1-butyne.
1) NaNH2 (or BuLi); 2) formaldehyde, HCHO; 3) H3O+. The alkynide adds to the carbonyl to give the alkoxide, which on hydrolysis gives the propargylic alcohol.