16.5 Trisubstituted Benzenes: Additivity of Effects

Benzene rings with multiple substituents follow predictable patterns for electrophilic substitution. The combined effects of activating and deactivating groups determine where new substituents attach, with stronger groups having more influence.

Understanding these rules helps predict reaction outcomes for complex aromatic compounds. Factors like steric hindrance and electronic effects play key roles in determining substitution patterns and overall reactivity of trisubstituted benzenes.

Trisubstituted Benzenes: Additivity of Effects

Additivity of directing effects

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• Directing effects of substituents combine in an additive manner when multiple groups are present on a benzene ring
  • Activating groups ($\text{-NH}_2$, $\text{-NHR}$, $\text{-NR}_2$, $\text{-OH}$, $\text{-OR}$, $\text{-NHCOR}$) direct electrophilic substitution to occur at the ortho and para positions relative to themselves
  • Deactivating groups ($\text{-NO}_2$, $\text{-CN}$, $\text{-SO}_3\text{H}$, $\text{-COOH}$, $\text{-COOR}$, $\text{-CHO}$, $\text{-COR}$) direct substitution to take place at the meta position
  • Halogens ($\text{-F}$, $\text{-Cl}$, $\text{-Br}$, $\text{-I}$) exhibit ortho/para directing effects but are mildly deactivating
• The preferred position for electrophilic aromatic substitution is governed by the sum of the directing effects of all substituents
  • When all substituents are ortho/para directors, substitution favors the ortho or para position relative to the most strongly activating group (e.g. $\text{-NH}_2$ > $\text{-OH}$)
  • When all substituents are meta directors, substitution occurs meta to the least deactivating group (e.g. $\text{-CHO}$ < $\text{-NO}_2$)
  • When activating and deactivating groups are both present, the position of the strongest activator determines the major product (e.g. $\text{-OH}$ wins over $\text{-NO}_2$)
  • The substituent position on the benzene ring influences the overall electronic effects and reactivity

Rules for disubstituted benzenes

• Rule 1: A strongly activating group ($\text{-OH}$, $\text{-NH}_2$) will control the site of electrophilic attack when paired with a weaker activator or deactivator
  • Substitution occurs ortho/para to an $\text{-OH}$ group in the presence of a $\text{-COOH}$ substituent
• Rule 2: A strongly deactivating group ($\text{-NO}_2$, $\text{-CN}$) determines the position of substitution when no strong activators are present
  • Substitution takes place meta to an $\text{-NO}_2$ group when the other substituent is $\text{-Cl}$
• Rule 3: Halogens and alkyl groups exert minimal directing influence compared to other functional groups
  • An $\text{-NH}_2$ substituent will override the ortho/para directing effect of a $\text{-Br}$ group, leading to substitution ortho/para to the amine

Relative strength of opposing substituents

• Activating groups can be ranked from most activating to least: $\text{-NH}_2$ > $\text{-NHR}$ > $\text{-NR}_2$ > $\text{-OH}$ > $\text{-OR}$ > $\text{-NHCOR}$
  • Amines ($\text{-NH}_2$, $\text{-NHR}$, $\text{-NR}_2$) are the most powerful activators, with phenols ($\text{-OH}$) and ethers ($\text{-OR}$) being somewhat less activating
• Deactivating groups can be ranked from most deactivating to least: $\text{-NO}_2$ > $\text{-CN}$ > $\text{-SO}_3\text{H}$ > $\text{-COOH}$ > $\text{-COOR}$ > $\text{-CHO}$ > $\text{-COR}$
  • Nitro ($\text{-NO}_2$) and nitrile ($\text{-CN}$) groups are the strongest deactivators, followed by sulfonic acids ($\text{-SO}_3\text{H}$) and carboxylic acids ($\text{-COOH}$)
• Halogens are weak deactivators but still direct ortho/para, with deactivating ability decreasing down the periodic table: $\text{-F}$ > $\text{-Cl}$ > $\text{-Br}$ > $\text{-I}$
• Alkyl groups ($\text{-R}$) are weakly activating and exert a minor ortho/para directing effect

Factors affecting substitution patterns

• Steric hindrance can influence the preferred site of substitution, especially for bulky electrophiles
• Electronic effects of substituents determine the overall reactivity and directing influence
• Reaction kinetics are affected by the combined electronic and steric factors of substituents
• Aromatic stabilization plays a role in determining the stability of intermediates and products