Question

`i.` Predict the more stable alkene of each pair.
`ii.` For which pairs could you use `DeltaH_(h)^(@)?`
`iii.` For which pairs could you use `DeltaH_(C)^(@)` to determine their relative stabilities ?
`I.` `a.` `2-` Methyl`-` pent `-2-` ene
`b.` `2,3-`Dimethyl`-` but`-2-`ene`DeltaH_(C)^(@)`
`II.` `c.` Hexane `d.` cis`-` Hex`-3-`ene
`III. e.` trans`-` Hex`-2-`ene `f.` `2-`Methyl`-` pent`-2-`ene
`IV.g.` cis`-` Hex `-3-`ene `b.` trans `-` Hex`-3-` ene

Answer 1

`1.`
`Delta H_(h)^(@)` cannot be used because the same alkane is not produced on hydrogenation. So, `Delta H_(C)^(@)` is used to determine the relative stabilities of the alkene pairs because ono complete combustions, the alkene requires same moles of `O_(2)` and produces same mole of `CO_(2)` and `H_(2)O` (same formula. `C_(6)H_(12)`.)
II. (d) (Mono - substituted)
`DeltaH_(h)^(@)` is used to determine the relative stabilities of the alkene pair because on hydrogenation two alkenes would yield the same alkane.
III. f
(Di-substituted)
`Delta H_(C)^(@)` is used, since it will not produce the same alkane on hydrogenation but on combustion alkene requires same moles of `O_(2)` and produces same moles of `CO_(2)` and `H_(2)O` (same formula : `C_(6)H_(12))`.
IV. (h) is more stable than `(g)`
Here, trans `-` alkene is more stable than cis `-` alkene .
`Delta H_(h)^(@)` is used, since it will produce the same alkane on hydrogenation.