Electrophilic Aromatic Substitution (Bromination of Toluene) Demonstration of the Effect of an...

Electrophilic Aromatic Substitution(Bromination of Toluene)

Demonstration of the Effect of an Electron Donar Group (ring activator) on a monosubstituted Benzene on subsequent substitution of other groups

References

Slayden Lab Manual – p. 75-76

Website: http://classweb.gmu.edu/jschorni/chem318 (Bromination of Toluene)

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Overview

Determine the effect of a monosubstituted Methyl group on a Benzene ring on the number of Bromine substitutions on the ring and the orientation of the substituted Bromine atoms (Ortho, Para, Meta) relative to the Methyl group

Approach

Electrophilic Aromatic Substitution reaction involving the halogenation of a mono-substituted Benzene ring (Toluene) with Bromine (Br2)

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Bromine (Br2) in the presence of FeBr3 forms Br+ ions which act as the electrophile in attacking the exposed electrons of the Benzene ring acting as the Nucleophile

Determine the Limiting Reagent and Theoretical Yield of the Bromotoluene reaction

Obtain IR Spectra of Bromotoluene isomers using IR Spectrophotometer in “Absorbance” mode

Use two methods (Triangulation Method and IR Spectrophotometer software) to compute the Mole Percents of the Bromotoluene isomers using the relationship between peak area and mole content of the reaction.

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Background Nucleophile - A Lewis Base with a pair of

unshared electrons that seeks a positive part of an atom.

Electrophile - A Lewis Acid seeking an electron pair

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Background Electrophilic Aromatic Substitution

Electrophiles (Electron deficient species, such as Br+ ions), attack the electron rich Nucleophile (Benzene Pi bond) replacing one of the Hydrogen atoms

Benzene and substituted Benzenes - as Aromatic Nucleophiles - can undergo Electrophilic Substitution

There are five (5) common Electrophilic Aromatic Substitutions used in organic chemistry synthesis Halogenation – X2

Nitration – HONO2/H2SO4

Sulfonation – SO3/H2SO4

Friedel-Crafts Alkylation – RCl/AlCl3 Friedel-Crafts Acylation – RC=OCl (acetyl

chloride), AlCl3.

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Background (Con’t) Today’s experiment involves Halogenation of a

substituted Benzene ring (Toluene) with Br2 in the presence of FeBr3, which produces the Br+ ions (Electrophilic reagent) that are substituted on the ring

Aromatic Hydrocarbons, e.g., Benzene and substituted benzenes, are referred to as Arenes - The symbol for an Arene is ArH

An Aryl group is derived from an Arene by removing one of the Hydrogen atoms - The symbol for an Aryl group is Ar-

Common Reactions of Arenes include Electrophilic Substitutions involving the Arene and an Electrophilic reagent, i.e., an electron seeking species with a positive charge (E+)

Arenes are susceptible to Electrophilic attack because of the exposed electrons

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Background (Con’t) When substituted benzenes undergo Electrophilic

attack, groups already on the ring affect both the rate of the reaction (reactivity) and the site of the attack (orientation)

If the initial substitution group (see 1st reaction) is an electron-releasing (donating) group relative to Hydrogen – an activator, such as a Methyl (CH3) group - the 2nd substitution occurs faster than substitution on an unsubstituted aromatic ring. The 2nd substitution tends to favor the Ortho/Para (o/p) positions

If the 1st substitution is an electron-withdrawing (accepting) group – a deactivator, such as the Nitronium Ion - the reaction occurs slower than substitution on an unsubstituted aromatic ring and favors meta substitution, i.e. (m) directing

04/18/23 7


Background (Con’t) In today’s experiment you will be performing an

Electrophilic Substitution on the Benzene ring of Toluene, i.e., Methyl Benzene

The Methyl group is an electron donor (activator) of the Benzene ring

Electron density from one of the carbon-hydrogen sigma bonds of the Methyl group flows to the vacant p orbital of the Carbocation (positively charged Carbon atom of the Benzene ring) because the orbitals can overlap – this is called Hyperconjugation

The electron density of the sp2-hybridized carbocation carbon on the Benzene ring becomes less dense; the Hydrogens of the Methyl group assuming some of this dispersed positive charge

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Background (Con’t) This dispersal (delocalization) of the charge stabilizes

the ring The resonance structures that maximize the positive

charge on the carbocation carbon attached to the Methyl group are the Ortho/Para positions (see slide 14)

Thus, substituted Methyl/Alkyl groups favor additional substitution at the Ortho and Para positions relative to the Methyl group

The relative reactivity of an activating group is a strong influence on the number of times a substitution will occur on the ring

One of the objectives of today’s experiment is to determine the effect of the Methyl group on the number of Bromine substitutions and the orientation of the substituted Bromine atoms

04/18/23 9


Background (Con’t) When Toluene (Methyl Benzene), an Aromatic

Hydrocarbon, is mixed with Bromine in the presence of Lewis Acid Catalyst, it undergoes an Electrophilic Aromatic Substitution

In this experiment the Catalyst is Ferric Bromide (FeBr3) produced by reaction of Bromine (Br2) with a “staple” as the source of Iron

Actually, the Ferric Bromide and some additional Bromine (Br2) form a complex that dissociates to produce the positive Br+ ions that act as the electrophilic agent seeking the exposed electrons of the Benzene ring activated by the electron donor Methyl group

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Background (Con’t) The products of the reaction are the Ortho,

Para, and Meta forms of Bromotoluene and HBr Substitution Positions on Benzene Ring:

If all sites on the Benzene ring are assumed to be equally reactive – they are not – the product mixture would produce a statistical 2:2:1 (40%, 40%, 20%) ratio of ortho, meta, and para isomers (there are two Ortho positions, 2 Meta positions, and 1 Para position)

Since the sites are not equally reactive, one of your tasks today is to determine the actual mole ratios in the product mixture

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Background (Con’t) You will do this by using the IR

Spectrophotometer in a new way Each of the 3 isomers of Bromotoluene in the

product mixture produces a unique absorption; and the relative peak areas for these absorptions are proportional to the mole ratios in the mixture

You will measure the areas by Triangulation as you did in the Gas Chromatography experiments last semester

You can also have the FT-IR instrument measure the areas

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Activators (Donate, Release Electrons to ring)

Available pair of unbonded electrons to donate to ring.

Protons on ring are more shielded Tendency for decreased Chemical

Shift

Deactivators (Withdraw, Accept Electrons)

No unbonded electron pairs Less Shielding of Protons on the ring Tendency for increase Chemical Shift

Methyl & Alkyl groups are activating because of the stabilizing effect of sp2 hybridization (hyperconjugation) of an unbonded electron in the Methyl radical.

Halogens are o,p directing because the electron donating resonance effect is more dominant than the withdrawal inductive (Dipole) effect of these electro-negative elements.

Electrophilic Substitution Directing Effects

on Aromatic Ring

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An electron releasing Electrophile, an Activator such as the Methyl group, stabilizes the ring resulting in a faster predominate Ortho/Para substitution reactionAn electron withdrawing Electrophile, a Deactivator, such as the Nitronium ion, destabilizes the ring, promoting a slower less predominate Meta substitution reaction


The Reactions

The Electrophiles attack the system to form a “Nonaromatic Carbocation” known as an Arenium ion

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The Reactions (Con’t)

Bromine Cations

In today’s experiment, Bromine (Br2) in the presence of

Fe (from an ordinary “staple”) will produce positive Bromine ions (Br+). The Bromine reacts with the Iron to form the Lewis Acid Ferric Bromide (FeBr3), which in turn reacts with excess Br2 to form a polarized complex that dissociates to form positive Bromine ion, Br+ and FeBr4

-

The Positive Br+ ion acts as the positively charged Electrophilic reagent (seeking electrons) which attacks the exposed electrons of the bond of the benzene ring

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The Reactions (Con’t) Substitution of Bromine

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Electrophilic attack on a Benzene ring containing an Electron donating group (Methyl group)

Substitution of Electrophile at the Ortho/Para positions is favored

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Procedure

Obtain a sample vial with cap containing a pre-treated “staple” (source of iron) from the instructor’s desk.

Note: The staple must be lying flat on the bottom of the vial

Put a small tape label on the outside with your name(s) and section number

In the hood, the instructor will use a pre-calibrated “autopipet” to deliver the following volumes of Toluene and Bromine to your vial:

115 l (0.115 mL) of Toluene (Mol Wgt – 92.139; Den – 0.867 g/mL)

65 l (0.065 mL) of Bromine (Br2)

(Mol Wgt – 159.81; Den – 3.119 g/mL)

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Procedure (Con’t)

When preparing your report:

1. Use the Volume and Density of Bromine to compute the Mass of Toluene and Bromine used

2. From the Mass and Molecular Weight calculate the Moles of Toluene and Bromine

3. Set up the stoichiometric equation

4. Determine the molar ratios involved.

Do you use mol wgt of Br2 (159.81) or

Br (79.90) to compute moles of bromine used in the reaction?

5. Show the reaction mechanism

6. Determine the Limiting Reagent

7. Compute the Theoretical Yield as mass (g)04/18/23 19


Procedure (Con’t)

Cap the vial loosely to allow the vapors to escape and set the vial in the hood to continue the reaction. Record any observations

Observe reaction for 10 minutes; then insert a “dry” piece ofLitmus paper into the reaction vapor in the upper portion of the vial – Do Not put litmus into the solution

Record observation – Does the Litmus paper change color; why?

Allow vial to sit in the hood for a total 45 minutes or until the deep reddish-brown Bromine color has faded to yellow

Record color change observations04/18/23 20


FT-IR Absorbance Mode (Setup will be done by Instructor) Press “T/A” to change the ordinate of the spectrum to

“Absorbance” mode. Change “Scan Range”

Press “Setup” Select “Scan” using 1st Soft Key from left Select “Range” using 3rd Soft Key from left Enter “high frequency” value using the numeric keypad

(Suggest 850 cm-1 to start) Press “Enter” Enter “Low frequency” value using the numeric keypad

(Suggest 650 cm-1 to start) Press ”Enter” Press “Execute” Soft Key – Last Soft Key on right

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FT-IR Absorbance Mode Change “Absorbance Range”

Press “Setup” Select “View” using 3rd Soft Key from left Select “Rescale” using 4th Soft Key from left Enter “Low Absorbance” value using the numeric

keypad. Suggest 0.0 to start

Press “Enter” Enter “High Absorbance” value using the numeric

keypad Suggest 2.0 to start

Press”Enter” Press “Execute” Soft Key – Last Soft Key on right. Range values may have to be changed depending on

actual peak ranges in spectrum04/18/23 22


IR Spectroscopy Place a drop of the reaction mixture between two IR salt

plate Obtain an IR spectrum in “Absorbance Mode” in the region

between 900 – 600 cm-1. The instrument will be preset for this spectrum determination.

Press Scan; “X”; 4; Execute As the instrument processes the sample, the status window

on right side of screen counts down from 4 to “ready.” Press “Plot” and label the principle peaks

Bromotoluene Reference Peaks

p-Bromotoluene m-Bromotoulene o-Bromotoluene Toluene 801 cm-1 770 cm-1 747 cm-1 726 cm-1

Note: The Toluene peak should be very small (missing!!)

The m-Bromotoluene peak should also be small04/18/23 23


Compute Peak Areas by Triangulation. The areas under the IR spectra reference peaks are

proportional to the mole ratio of the Bromotoluene isomer products in the reaction mixture.

The molar percentage composition of a mixture can be approximated by comparing the relative areas of the peaks in the spectra.

Define the “Baseline” by drawing a best-fit (eye-balled) straight line along the bottom of the spectra plot, similar to the way you did it in gas chromatography.

The area of the peak is determined by multiplying the height of the peak (in mm) above the baseline by the width of the peak (in mm) at half the height.

Add areas to get the total area. Divide each area by total area to get mole fraction. Multiply the mole fraction by 100 to get the mole percent.

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Peak Area by theTriangulation Method

Peak Area = h * w½ * 1.064

Where h = Peak Height w½ = width of peak at

½ the Peak Height

Note: The curve is assumed gaussian and the h * w1/2 value is about 6.4% less than the true value, thus the correction factor.

Total Peak Area(TA) = A + B

Mole Fraction(MF) = A/TA, B/TA

Mole Percent = MF x 100

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Measure Peak Area using FT-IR software

(To be done only if directed by Instructor) Press “T/A” to change the ordinate of the

spectrum to “Absorbance” mode Press “Area” (Shift 7) Press “X” region key, i.e. select the buffer

region for your spectrum Press “vcursor (Shift ←). The “vertical cursor”

appears Move the vertical cursor to a point representing

the upper limit (left side) of the frequency range of the peak you want to calculate

Type the wave number of the vertical cursor in the “From” field, then press “Enter”

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Move the vertical cursor to the lower frequency limit (right side) of the range of a peak you want to calculate

Type the wave number of the vertical cursor in the “To” field, then press “Enter”Note: The Baseline range determined below is

inclusive of the three peaks you are calculating; use the same baseline range for all three determinations

Move the vertical cursor to a point corresponding to a frequency slightly higher than the upper frequency limit of the peak with the highest frequency range. This first “Baseline Point” is the upper frequency (left side) of the Baseline range Type the wave number of the vertical cursor

in the “Basefrom” field, then press “Enter”04/18/23 27


Peak Area by FTR Software Move the vertical cursor to the second

“Baseline Point” at a frequency slightly lower than the lower frequency limit of the peak with the lowest frequency range

This second “Baseline Point” is the lower frequency (right side) of the Baseline range

Type the wave number of the vertical cursor in the “Baseto” field Press “Execute” to calculate the area. The

calculated result appears on the command line Record the result in your notebook Repeat steps 4 to 9 for the other peaks

04/18/23 28


Laboratory Report Synthesis Experiment

Mass, Moles, Molar Ratio, Limiting Reagent, Theoretical Yield

Procedures Title – Concise: Simple Distillation, Dry Sample,

IR Spectrum, etc. Materials & Equipment (2 Columns in list

(bullet) form) Note: include all reagents & principal

equipment used

Procedure Description:Use complete sentences in list (bullet) formConcise, but complete descriptionUse your own words – Don’t copy book!!

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Summary Paragraph summarizing the experimental

results, computed results, and the principal absorptions from IR spectra

Analysis / Conclusions Discuss the isomeric makeup of the product

relative to what you expected from theory No. of Isomers Orientation of the Substituted groups Molar Composition of the isomers Compare Triangulation vs. IR Software area

calculation

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