Chemistry · Ch 15 — Hydrocarbons
Directive influence of a functional group in monosubstituted benzene
Directive influence of a functional group in monosubstituted benzene
Because all six hydrogens on unsubstituted benzene are chemically equivalent, a single electrophilic substitution on benzene itself can only ever give one product. Once that first substituent (S) is in place, though, a SECOND substitution on the resulting monosubstituted ring has five open ring positions to choose from, and these five positions fall into only three distinct types by symmetry: positions 2 and 6 (equivalent to each other) are called ortho (o-), positions 3 and 5 (equivalent to each other) are called meta (m-), and position 4 (unique) is called para (p-). All three constitutionally distinct disubstituted products -- ortho, meta and para -- are chemically possible in principle, but they are never formed in equal amounts; which one (or which pair) dominates depends almost entirely on the electronic character of the FIRST substituent S already on the ring, not on the identity of the incoming second electrophile E. Groups fall into two broad behavioural classes. Ortho/para-directing groups -- including -OH, -OCH3, -NH2, -NHR, the halogens (-Cl, -Br), and alkyl groups (-CH3, -C2H5, -R) -- push the incoming electrophile preferentially to the ortho and para positions. Resonance theory explains why: drawing out the resonance structures of, say, phenol (-OH on the ring) shows that oxygen's lone pair can delocalise specifically into the ring at the ortho and para carbons, building up extra electron density (and hence extra reactivity toward an electron-seeking electrophile) exactly at those two positions; every true ortho/para director other than alkyl groups shares this feature of having a non-bonding lone pair on the very atom attached to the ring. -OH's own inductive (-I) effect pulls electron density the opposite way, but its resonance (+R) donation is the stronger of the two effects, so -OH remains a net ring-activating, ortho/para-directing group; halogens, by contrast, have such a strong -I effect that they lower the ring's OVERALL electron density (making the ring deactivated, less reactive than benzene as a whole) even while their resonance donation still directs what substitution DOES occur to the ortho/para positions specifically. Alkyl groups are a genuine exception to the 'needs a lone pair' explanation: having no lone pair to donate, they are still ortho/para-directing, but through a different mechanism, hyperconjugation (also called no-bond resonance), rather than ordinary resonance donation. The second behavioural class is meta-directing groups -- including -NO2, -NH3+, -C-triple-bond-N, -CHO, -COR, -COOH and -SO3H -- which all share the feature of carrying a full or partial POSITIVE charge on the atom directly attached to the ring. Resonance structures of nitrobenzene show why: the electron-withdrawing -NO2 group pulls ring electron density toward …
Worked out. On a monosubstituted benzene ring numbered 1 (substituent) through 6, positions 2 and 6 are equivalent to each other and are called ortho (o-); positions 3 and 5 are equivalent to each other and are called meta (m-); and position 4 is unique, called para (p-). Because five ring positions remain open for a second electrophile E to attack, three constitutionally distinct disubstituted products (ortho, meta, para) are possible in principle, but they never form in equal amounts -- the group already on the ring biases the outcome strongly toward either (predominantly) ortho+para, or (predominantly) meta, depending only on that group's own electronic character, not on the identity of …
Worked out. Groups such as -OH, -OCH3, -NH2, -NHR, halogens (-Cl, -Br), and alkyl groups (-CH3, -C2H5, -R) direct an incoming electrophile to the ortho and para positions. For -OH on phenol, drawing out the resonance structures shows that a lone pair on oxygen can delocalise into the ring specifically onto the ortho and para carbons (giving those positions extra electron density and a formal negative-charge-bearing resonance contributor), so the ring is most reactive (activated) exactly at those positions. Every true ortho/para director (other than alkyl groups) has a non-bonding lone pair on the atom directly attached to the ring, which is what allows this donation; -OH's own -I (inductive electron-withdrawing) effect works in the opposite direction to its +R (resonance-donating) effect, but the resonance effect wins out, so -OH is still strongly o/p-directing and ring-activating overall. Alkyl/methyl groups are the one exception to the 'needs a lone pair' rule: they have no lone pair at all, yet are still o/p-directing, through a different mechanism called hyperconjugation (no-bond resonance) rather than ordinary lone-pair resonance. In aryl halides specifically, the halogen's strong -I effect lowers the ring's overall electron density (making the ring less reactive than benzene, i.e. deactivating), but its resonance donation stil …
Worked out. Groups such as -NO2, -NH3+, -C(triple bond)N, -CHO, -COR, -COOH and -SO3H direct an incoming electrophile to the meta position; every meta director carries a full or partial positive charge on the very atom attached to the ring. Drawing the resonance structures of nitrobenzene shows the electron-withdrawing -NO2 group pulling ring electron density toward itself specifically from the ortho and para positions (leaving those two positions comparatively electron-poor, i.e. deactivated relative to meta), so the incoming electrophile is forced to attack at the remaining, comparatively electron-richer meta position instead. Because this withdrawal lowers the ring's electron density everywhere, meta-directing groups are also always ring-deactivating (they make the ring less reacti …