From this topic on MOs of butadiene vs ethene, it is clear that when two ethene molecules are combined to extend the conjugate chain, the HOMO is raised in energy and the LUMO is lowered in energy. This implies that the resultant compound is more reactive towards nucleophiles(Lower LUMO more easily matched in energy with HOMO of Nu), and also towards electrophiles (Higher HOMO more easily matched in energy with LUMO of E).
In "Why chemical reactions happens" by Keeler and Wothers, "The conjugative effect" is mentioned as something that decreases reactivity (see p. 163-165): "The delocalization of a lone pair from X into the carbonyl π system decreases the reactivity of the carbonyl towards nucleophiles by raising the energy of the π LUMO" .
The second argument seems to contradict my original argument, so what am I missing?
- Perhaps extending conjugate system by pi bonds, empty p-orbitals or filled single orbital (LP) etc. have different effects on the reactivity?
I think an extensive answer explaining the effects of conjugation on reactivity(towards nucleophiles and electrophiles), perhaps also touching on the common statement that "presence of conjugation stabilizes a compound", would be very helpful for the site.
EDIT: Seeing as my original question is very broad, i would appreciate if someone could simply explain the apparant contradiction between the previous post and the Keeler&Wothers quote.