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Experiment 15:. SUBSTITUENT EFFECTS ON THE RATE OF ELECTROPHILIC AROMATIC SUBSTITUTION. Objectives:. To explore how different substituent groups on an aromatic ring affect the rate and orientation of electrophilic aromatic substitution using a qualitative bromine test.

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Experiment 15:


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experiment 15

Experiment 15:

SUBSTITUENT EFFECTS ON THE RATE OF ELECTROPHILIC AROMATIC SUBSTITUTION

objectives
Objectives:
  • To explore how different substituent groups on an aromatic ring affect the rate and orientation of electrophilic aromatic substitution using a qualitative bromine test.
  • To determine directing ability of acetamide group using TLC analysis.
chemical equation
CHEMICAL EQUATION
  • The substitution reaction that will be investigated isbrominationusing a solution of bromine in acetic acid, selected because the progress of the reaction can be followed easily by acolor change.
  • The more reactive the aromatic ring is, the fasterthe color will disappear.

Br2

red

reactivity of aromatic rings
REACTIVITY OF AROMATIC RINGS
  • Substituents affect thereactivityof the aromatic ring. Reactivity affects the rate of the reaction.
  • Substituents can also affect the orientation of the reaction (o-, m-, or p-).
  • The substituentALREADY ONthe aromaticring determines theposition and rate of substitutionof the second (INCOMING) electrophile.
  • We use the reactivity of BENZENE (no substituent) as a reference point.
reactivity of aromatic rings6
REACTIVITY OF AROMATIC RINGS
  • If we use the reactivity of benzene (substituent = H) as a reference point,activatingsubstituents are allelectrondonatinggroups, and their relative activation strengths are:

H < Phenyl < CH3 < NHCOCH3 < OCH3 < OH < NH2

  • Deactivating groupsareelectron withdrawinggroups and their activities relative to hydrogen are:

NO2 < COR < CHO < I < Br < Cl < F < H

generalizations
GENERALIZATIONS
  • Substituents in which the atom bonded to the ring has anunshared pair of electrons,with the exception of alkyl and phenyl groups, areortho-paradirecting. All other substituents are meta directing.
  • All ortho/para directors are activators, with the exception of halogens. Halogens are ortho-para deactivators.
  • Allmetadirectors aredeactivators.
  • Alkyl and Phenyl groups are also ortho-para directing.
hints
HINTS…
  • Be sure that your test tubes doNOTcontain anyacetone. It reacts very quickly with bromine and can give inaccurate test results.
  • If after the one hour period of heating in the water bath the solutions have not completely lost their color, estimate by the relative amount of color lost.
safety concerns
SAFETY CONCERNS
  • CAUTION: In this experimentyou are usingbromine, which is poisonous and can cause severe burns!
  • All aromatic solutions are prepared in glacial acetic acid which can cause severe burns!
waste management
WASTE MANAGEMENT
  • Place aqueous waste from extraction in bottle labeled “Aqueous Waste (EAS)”
  • Place all other waste from experiment in bottle labeled“Organic Waste (EAS)”
  • Place used TLC plates and filter papers in trash can.
  • Place used TLC spotters broken glass box.