9
$\begingroup$

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.

$\endgroup$
  • 5
    $\begingroup$ My first thoughts are that this is very broad. The effect of conjugation on each compound is slightly different because the MOs are not the same (both the forms and the energies). Therefore one cannot really make generalisations about whether the energies of the HOMO/LUMO will increase/decrease. That’s just reactivity, but if you want to talk about stability as well then you need another nontrivial section. Maybe useful reading for you is Ian Fleming’s “Molecular Orbitals and Organic Chemical Reactions” $\endgroup$ – orthocresol Apr 1 '18 at 13:21
  • $\begingroup$ This dicotomy seems to stem from the fact that in your first argument you mention alkenes and in the second you talk about carbonyl compounds, which have very different reactivities and molecular orbitals. A computational treatent of the molecules in question, using quantum mechanics to calculate the orbitals, might help you understand this question. $\endgroup$ – Joao Brinco Apr 2 '18 at 7:02
3
+50
$\begingroup$

The effect of conjugation on electrophilicity and nucleophilicity of chemical species varies depending on the type of conjugation. There is no contradiction between the two cases you have presented in your question because in each case, the conjugation at work is different.

The conjugation which occurs in conjugated polyenes has been well elaborated upon by Philipp (and thus, I shall not elaborate further), in the answer you have linked to. This type of conjugation occurs as a result of interactions between parallel p orbitals in a chain of carbon atoms.

The other sort of conjugation you mentioned reading in the book by Keeler and Wothers is an interaction involving an interaction between the nonbonding molecular orbital, which holds the lone pair, localised on ${X}$ (e.g. $\ce {NH2,Cl, Br}$) and the antibonding $\pi$ molecular obital of the $\ce {C=O}$ bond, which is more localised on $\ce {C}$. This is a $\ce {HOMO-LUMO}$ interaction between the two orbitals and electron density is donated into the antibonding $\pi$ MO from the nonbonding MO on $X$. This interaction produces yet another set of bonding and antibonding molecular orbitals. Since the bonding MO produced is of a lower energy than the nonbonding MO on $X$, the lone pair is stabilised and its reactivity is decreased. Also, since the antibonding MO produced is of a higher energy than the previous antibonding MO of the $\ce {C=O}$ bond, the LUMO is raised in energy as a result, decreasing reactivity of the carbonyl group towards nucleophiles. One of the consequences of this conjugation is the lower reactivity of amides towards nucleophilic addition (Clayden et al., 2012).

Reference

Clayden, J., Greeves, N., & Warren, S. (2012). Organic Chemistry (2nd ed.). New York : Oxford University Press Inc.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.