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. • To determine directing ability of acetamide group using TLC analysis.
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 • 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 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 • 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… • 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 • 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 • 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.