1. Friedel-Crafts Alkylation
Purpose
Preparation of 4,4-di-tert-butylbiphenyl using the Friedel-Crafts alkylation of Biphenyl through Electrophilic substitution of a Lewis Base (t-Butyl Chloride (Haloalkane)) in the presence of Ferric Chloride acting as a Lewis Acid
References:
Slayden, et. al. pp. 93 – 95
Website:
Solomon’s - pp
4/15/2017

2. Friedel-Crafts Alkylation
Background
Electrophile (Lewis Acid) – Electron Pair Acceptor
Electron deficient (positively charged) species seeking electron rich environment (electron pair donor)
e.g. – Protons, Aluminum, Zinc & Iron Halides
Nucleophile (Lewis Base) – Electron pair donor
e.g. – Ammonia, Alcohols, Alkyl Halides
Friedel–Crafts reactions are a set of reactions developed by Charles Friedel and James Crafts in 1877 to attach substituents to an aromatic ring through an electrophilic substitution process
4/15/2017

3. Friedel-Crafts Alkylation
Background (con’t)
There are two main types of Friedel–Crafts reactions:
Alkylation reactions
Substitution of Alkanes (From Haloalkanes)
Acylation reactions
Acyl Groups
Both proceeding by Electrophilic Aromatic Substitution 
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4. Friedel-Crafts Alkylation
Background (con’t)
Friedel–Crafts Alkylation involves the alkylation of an aromatic ring (Biphenyl) with an alkyl halide (t-Butyl Chloride) using a strong Lewis acid catalyst (FeCl3)
The catalyst, anhydrous Ferric Chloride, acting as a Lewis Acid, attracts the Halide from the Alkyl Halide forming FeCl4- and an Alkyl Carbocation
The Carbocation acts as an Electrophile seeking electrons and attacks the  electrons in the Biphenyl ring forming an intermediate Arenium ion
The Arenium ion, a Pentadienyl Cation) is referred to as a non-aromatic Carbocation (also called sigma  complex)
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5. Friedel-Crafts Alkylation
Background (con’t)
The Arenium ion loses 2 protons to FeCl4- (Lewis Acid) regenerating HCl and FeCl3
The ring  electrons are regenerated, thus reestablishing stable ring resonance and producing the product:
4,4-di-tert-butylbiphenyl
Mol Wgt g/mol
M.P – 129oC
CAS
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6. Friedel-Crafts Alkylation
The Reaction
The Mechanism
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7. Friedel-Crafts Alkylation
Today's Reaction
Using Dichloromethane (Methylene Chloride – BP 40oC) as the solvent, Tertiary Butyl Chloride will be reacted with the catalyst, Ferric Chloride (FeCl3), acting as a Lewis acid to form the t-Butyl Carbocation, an Electrophile
The Carbocation then reacts with Biphenyl, acting as a Nucleophile, to form the product:
4,4-di-tert-butylbiphenyl
The product is separated from the aqueous solution by liquid/liquid extraction with 10% HCl
The solvent is removed by evaporation
The product is then recrystallized from hot 95% Ethanol
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8. Freidel-Crafts Alkylation
Reaction Issues
Biphenyl is substituted for Benzene, which is a known carcinogen
Anhydrous Ferric Chloride (FeCl3) is substituted for Anhydrous Aluminum Chloride
Note: Instructor will assist student with addition of Anhydrous Ferric Chloride in the hood
It is most important that the reaction vessel and the Ferric Chloride be as dry as possible
Moisture will cause the Ferric Chloride to undergo hydrolysis forming Fe(OH)3 which competes with and diminishes the effectiveness of Ferric Chloride as a Lewis acid
Methylene Chloride (Dichloroethane) is also a known carcinogen – use caution and work in the hood
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9. Friedel-Crafts Alkylation
Procedure
Note: In the following procedure the beaker must be clean and very dry to avoid adding moisture to the Ferric Chloride, causing it to hydrolyze to Fe(OH)3; thus, reducing its effectiveness as a Lewis acid
Pre-weigh a 100 mL clean, dry beaker to nearest g
Transfer the Biphenyl (~1.25 g) from the vial to the beaker
Determine the mass of the Biphenyl by difference
Add about 2.5 ml Tertiary Butyl Chloride (2-Chloro-2- Methyl Propane) to the beaker containing the Biphenyl
Determine the mass of t-butyl Chloride by difference
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10. Friedel-Crafts Alkylation
Procedure (con’t)
Add about 10 ml (unweighed) of Dichloromethane (Methylene Chloride (BP – 40oC, Density 1.33 g/mL) to the beaker to act as a solvent for the reaction
In the hood, with instructors help, add about 0.1 g of anhydrous Ferric Chloride to the reaction mixture
Continuously swirl and heat the mixture gently on a hot plate for about 15 minutes while the HCl gas is being evolved
Be careful to not boil the Methylene Chloride solvent away
In the hood add about 10 mL cold 10% Hydrochloric acid to the reaction mixture with swirling
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11. Friedel-Crafts Alkylation
Procedure (con’t)
Transfer the mixture to a 125 mL separatory funnel
Mix the two phases well and vent any gas
Repeat mixing & venting until no more gas is released
Allow the two phases to separate
Drain the product and Methylene Chloride mixture (lower layer) into a clean beaker
Drain upper aqueous HCl layer (waste) into separate beaker and put aside
Return product mixture to the separatory funnel and repeat liquid/liquid extraction with more 10% HCl
Repeat process until most of the color is removed
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12. Friedel-Crafts Alkylation
Procedure (con’t)
Dry the product with anhydrous Sodium Sulfate and allow to settle
Carefully pour off (decant) liquid into a clean dry 50 mL beaker, leaving solid Na2SO4 particles behind
In the hood, remove the Methylene Chloride (BP – 40oC) solvent by gently evaporating the mixture until the bubbling action of the Methylene Chloride ceases – solution could go to dryness, but not likely
Note: If the liquid remains, cool to room temperature and then in an ice-water bath for a minute or two. This should precipitate the product
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13. Friedel-Crafts Alkylation
Procedure (con’t)
Recrystallize the product
Add 10 mL 95% Ethanol to the beaker
Use a spatula to scrape the product from the sides of the beaker into the Ethanol
Place the beaker containing the product and Ethanol in a 250 ml beaker containing just enough water to allow the sample beaker to sit on the bottom of the beaker
Heat the water bath until the solid particles dissolve completely
Note: water does not need to boil (85-90oC)
If all of the solid does not dissolve, add more EtOH in small increments until final dissolution
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14. Friedel-Crafts Alkylation
Procedure (con’t)
After the product is fully dissolved, allow solution to cool slowly to room temperature
Crystals of purified product should form as solution cools
Cool the mixture in a room temperature water bath
If the cooled product mixture looks like a non- filterable mass, put the beaker back on the hot plate, add 3-4 ml EtOH, and repeat the recrystallization
Repeat as necessary until mixture can be swirled
If the product does not recrystallize put the beaker back in the water bath and evaporate some of the alcohol
Allow mixture to cool again
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15. Friedel-Crafts Alkylation
Procedure (con’t)
Vacuum Filtration
Set up vacuum filtration unit and moisten filter paper in Buckner funnel with 95% Ethanol
Filter the product by vacuum filtration, washing the product with additional Ethanol
Product will usually be yellow due to trace amounts of hydrated Ferric Chloride
Transfer the product to a pre-weighed weighing tray and air-dry in the drawer until next lab session
Determine the mass of the dried product
Compute the percentage yield
Determine the Melting point
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