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Experiment 15:

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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  1. Experiment 15: SUBSTITUENT EFFECTS ON THE RATE OF ELECTROPHILIC AROMATIC SUBSTITUTION

  2. 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.

  3. Before coming to lab… • Review these techniques: • TLC Analysis • Acid-base Extraction

  4. CHEMICAL EQUATION • The progress of the bromination of an aromatic ring can be followed easily by a color change. • The more reactive the aromatic ring is, the faster the color will disappear. Br2 red

  5. MECHANISM An electron pair from the aromatic ring attacks Br2, forming a new C-Br bond… …and leaving a nonaromatic, carbocation intermediate. The carbocation intermediate loses H+, and the neutral substitution product forms as two electrons from the C-H bond move to regenerate the aromatic ring. HBr forms as a byproduct.

  6. REACTIVITY OF AROMATIC RINGS • The substituent ALREADY ON the aromatic ring determines the positionand rate of substitution of the second (INCOMING) electrophile. • We use the reactivity of BENZENE (no substituent) as a reference point.

  7. REACTIVITY OF AROMATIC RINGS • If we use the reactivity of benzene (substituent = H) as a reference point, activating substituents are all electron donating groups, and their relative activation strengths are: H < Phenyl < CH3 < NHCOCH3 < OCH3 < OH < NH2 • Deactivating groups are electron withdrawing groups and their activities relative to hydrogen are: NO2 < COR < CHO < I < Br < Cl < F < H

  8. SUBSTITUENT EFFECTS

  9. GENERALIZATIONS • Substituents in which the atom bonded to the ring has an unshared pair of electrons, with the exception of alkyl and phenyl groups, are ortho-paradirecting. All other substituents are meta directing. • All ortho/para directors are activators, with the exception of halogens. Halogens are ortho-para deactivators. • Allmeta directors are deactivators. • Alkyl and Phenyl groups are also ortho-para directing.

  10. SYNTHESIS • Place small amount of monosubstituted aromatic compound in small test tube. • Place test tubes in water bath. • Add Bromine solution to each. • Record the approximate amount of time it takes for the solution to lose color. • Place in hot water bath if necessary to complete reaction. • After 1 hour, estimate the reaction order based on the relative amount of color lost.

  11. Table 15.1

  12. PURIFICATION/ISOLATION • Once pale yellow, remove acetanilide tube. • Add deionized water to tube. • Add 5 drops of NaOH. Test pH using glass rod. • Repeat until the solution is basic. • Add ethyl acetate. Place small cork in top of test tube, and shake to mix. • Allow layers to separate. I’m ACETANILIDE!

  13. ANALYSIS • Prepare TLC plate and chamber. • Apply provided standards to TLC plate. • Apply TOP layer from test tube to TLC plate ( sample solution). • Develop plate and visualize spots using UV lamp. • Calculate Rf value of all spots and identify product in sample solution.

  14. Tables 15.2 and 15.3

  15. 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.

  16. 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!

  17. WASTE MANAGEMENT • Place ALL liquid waste in container labeled “LIQUID ORGANIC WASTE”. • Place used TLC plates and filter papers in yellow trash can. • Place used TLC spotters broken glass box (NOT TRASHCAN!!!!!!!!!!!).

  18. CLEANING UP… • TEST TUBES: clean all test tubes with soap/water/brush and rinse with wash acetone. Leave inverted in test tube rack to dry. • TLC CHAMBER: remove filter paper and leave in drawer with cap off. • BEAKER: if only used for water, simply dry out with a paper towel. • GRADUATED CYLINDER: rinse any excess bromine solution into wash acetone container. Clean with soap/water/brush and rinse with wash acetone.

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