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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:

SUBSTITUENT EFFECTS ON THE RATE OF ELECTROPHILIC AROMATIC SUBSTITUTION


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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.


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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



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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.


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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


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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.


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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.


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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!


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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.


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