Groups that withdraw electron density from the benzene ring, such as the nitro group, -NO2, are 3-directing.Why
The -NO2 group pulls electron density out of the ring. This lowers the electron density most at positions 2, 4 and 6, so position 3 (and the equivalent position 5) is left relatively richer in electrons, and the electrophile substitutes there.Deactivation
Because the ring has less electron density overall, nitrobenzene reacts more slowly than benzene. Harsher conditions are needed, such as a higher temperature for a second nitration.Examples
• Nitration of nitrobenzene (concentrated HNO3 and H2SO4, heated above 50 °C) gives 1,3-dinitrobenzene.
• Nitrobenzene + Br2/FeBr3 gives 1-bromo-3-nitrobenzene.Groups containing a C=O bonded to the ring, such as -COOH and -CHO, are also electron-withdrawing and 3-directing.Worked example: planning the order of steps
Make 1-bromo-3-nitrobenzene from benzene.
• If benzene is brominated first, the Br atom is 2,4-directing, so nitration would give 1-bromo-2-nitrobenzene and 1-bromo-4-nitrobenzene, not the target.
• So nitrate first (concentrated HNO3/H2SO4, 50 °C) to form nitrobenzene, then brominate (Br2, FeBr3). The 3-directing -NO2 sends Br to position 3.
The first step must be nitration.
The -NO2 group pulls electron density out of the ring. This lowers the electron density most at positions 2, 4 and 6, so position 3 (and the equivalent position 5) is left relatively richer in electrons, and the electrophile substitutes there.Deactivation
Because the ring has less electron density overall, nitrobenzene reacts more slowly than benzene. Harsher conditions are needed, such as a higher temperature for a second nitration.Examples
• Nitration of nitrobenzene (concentrated HNO3 and H2SO4, heated above 50 °C) gives 1,3-dinitrobenzene.
• Nitrobenzene + Br2/FeBr3 gives 1-bromo-3-nitrobenzene.Groups containing a C=O bonded to the ring, such as -COOH and -CHO, are also electron-withdrawing and 3-directing.Worked example: planning the order of steps
Make 1-bromo-3-nitrobenzene from benzene.
• If benzene is brominated first, the Br atom is 2,4-directing, so nitration would give 1-bromo-2-nitrobenzene and 1-bromo-4-nitrobenzene, not the target.
• So nitrate first (concentrated HNO3/H2SO4, 50 °C) to form nitrobenzene, then brominate (Br2, FeBr3). The 3-directing -NO2 sends Br to position 3.
The first step must be nitration.