Substituent Effects

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A substituent on a benzene ring can have 2 effects:

1. activate or deactivate the ring i.e. cause the substituted benzene to be more reactive or less reactive than benzene
2. direct the position of subsequent substitution i.e. 2,4-directing or 3-directing

 

	2,4-directing	3-directing
activating	alkyl groups –R   atom bonded directly to benzene has a lone pair (except halides) i.e. –OH, –OR, –NH2, –NHCOCH3
deactivating	halides –X	atom bonded directly to benzene has a + charge or partial + charge i.e. –C(=O)R, –COOH, –COOR, –CN, –NO2, –NH3+, –NR3+

• Recall: benzenes are e^– rich → undergo reaction with electrophiles
• Hence, substituents that are overall e^– donating will increase the e^– density on the ring, making them more susceptible to reaction with electrophiles (i.e. reactivity increases)
• Conversely, substituents that are overall e^– withdrawing will decrease the e^– density on the ring, making them less susceptible to reaction with electrophiles (i.e. reactivity decreases)