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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

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.


Before coming to lab
Before coming to lab…

  • Review these techniques:

    • TLC Analysis

    • Acid-base Extraction


Chemical equation
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


Mechanism
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.


Reactivity of aromatic rings
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.


Reactivity of aromatic rings1
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



Generalizations
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.


Synthesis
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.



Purification isolation
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!


Analysis
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.



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 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!!!!!!!!!!!).


Cleaning up
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.