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Experiment 5 – Electrophilic Aromatic Substitution, Lab Reports of Chemistry

West Virginia University Institute of TechnologyChemistry

A Friedel-Crafts Acylation Reaction

Typology: Lab Reports

2020/2021

Uploaded on 05/11/2021

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Chemistry 2283g Experiment 5 – Electrophilic Aromatic Substitution
!
5-1!
EXPERIMENT 5: Electrophilic Aromatic Substitution – A Friedel-Crafts Acylation
Reaction
Relevant Sections in the text (Wade, 7th ed.)
• 17.1-17.2 (p. 751-755) Electrophilic aromatic substitution
• 17.6-17.8 (p. 761-770) Substituent effects in EAS
• 17.11 (p. 777-782) Friedel-Crafts acylation
General Concepts
A very important class of molecules in nature and in medicine is substituted benzene rings, also
known as aryl rings. Consequently, chemically synthesizing benzene with varying substituents is quite
common. Unlike alkyl compounds, which undergo nucleophilic substitution, aromatic systems involve
substitution reactions by an electrophilic pathway. Electrophilic Aromatic Substitution (EAS) reactions
involve the generation of an electrophile in one or more steps, leading to the two-step addition/elimination
process. In EAS reactions, a carbocation intermediate is formed when the nucleophilic π electrons of the
aromatic ring attack the electrophile reagent, E+. This carbocation intermediate then reforms the aromatic
system in the elimination step, with the loss of H+. The overall result is the substitution of an aromatic
hydrogen with the electrophile (E+), as shown in Scheme 1.
E A E++ A−
E++
H
E
H
E
H
E
H
E
A−
E
+ AH
Scheme 1. General Mechanism for electrophilic aromatic substitution
Pre-existing substituents on aromatic rings affect the reactivity and regioselectivity of the EAS reaction.
Electron-donating substituents cause an accelerated reaction where the strong electrophile preferentially
binds in the ortho- or para- positions. This preferred position is due to resonance (and inductive)
stabilization of the cation intermediate. Aromatic electron withdrawing groups however react less rapidly
and direct the external electrophile to the favored meta- position. Why?
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EXPERIMENT 5 : Electrophilic Aromatic Substitution – A Friedel-Crafts Acylation

Reaction

Relevant Sections in the text (Wade, 7th^ ed.)

  • 17.1-17.2 (p. 751 - 755 ) Electrophilic aromatic substitution
  • 17.6-17.8 (p. 761-770) Substituent effects in EAS
  • 17.11 (p. 77 7 - 782) Friedel-Crafts acylation General Concepts A very important class of molecules in nature and in medicine is substituted benzene rings, also known as aryl rings. Consequently, chemically synthesizing benzene with varying substituents is quite common. Unlike alkyl compounds, which undergo nucleophilic substitution, aromatic systems involve substitution reactions by an electrophilic pathway. Electrophilic Aromatic Substitution (EAS) reactions involve the generation of an electrophile in one or more steps, leading to the two-step addition/elimination process. In EAS reactions, a carbocation intermediate is formed when the nucleophilic π electrons of the aromatic ring attack the electrophile reagent, E+. This carbocation intermediate then reforms the aromatic system in the elimination step, with the loss of H+. The overall result is the substitution of an aromatic hydrogen with the electrophile (E+), as shown in Scheme 1. E A E+^ +^ A − E+^ + H E H E H E H E A − E
  • A H Scheme 1. General Mechanism for electrophilic aromatic substitution Pre-existing substituents on aromatic rings affect the reactivity and regioselectivity of the EAS reaction. Electron-donating substituents cause an accelerated reaction where the strong electrophile preferentially binds in the ortho- or para- positions. This preferred position is due to resonance (and inductive) stabilization of the cation intermediate. Aromatic electron withdrawing groups however react less rapidly and direct the external electrophile to the favored meta- position. Why?

The EAS reaction you will be performing is a Friedel-Crafts acylation, where the electrophile is an acylium ion (R-C≡O+). This reactive ion is formed by the reaction of an acid chloride (acetyl chloride, CH 3 CO-Cl) with aluminum chloride (AlCl 3 ) acting as the Lewis acid catalyst (see Scheme 2). R O Cl

  • AlCl 3 R O Cl AlCl 3 R O
  • AlCl 4 R C O acylium ion Scheme 2. Generation of the acylium ion electrophile You will be given an unknown mono-substituted aromatic compound which may yield any or all of the disubstituted benzenes (where the electrophile gets added ortho, meta or para to the substituent). After performing the reaction, the identity and structure of the product (and therefore the starting material) may be determined by IR and NMR spectroscopy. A proposed mechanism for the reaction will also be required, accounting for any regioselectivity. All the necessary background will be discussed in lecture, however you will need to read ahead in Chapter 17. In addition to the NMR analysis, disubstituted benzenes can be distinguished by the location of the out-of- plane (oop) C-H bending bands in the IR spectrum. The frequency decreases with the number of adjacent hydrogens (see Table 1) Table 1 : Frequencies of C-H (OOP) Bending Bands in the IR for Aromatic Rings Number of Adjacent Hydrogens Frequency Range/ cm-^1 1 900 – 860, weak 2 ( para ) 850 – 810 3 ( meta ) 810 – 750, 690 4 ( ortho ) 770 – 735 (usually one strong band near 750) 5 (monosubstituted) Strong band at 690 and 770 – 730 NOTE: Pre-lab calculations are required before beginning the lab. Read over the procedure on the next page, and make the necessary calculations!

The unknown is one of the following: A (MW = 92.1, density = 0.867 g/mL), B (MW = 108.2, density = 0.996 g/mL), C (MW = 106.17, density = 0.867 g/mL) or D (MW = 120.2, density = 0.864 g/mL).  Remove the ice bath after the addition is complete and bring the flask to room temperature. Stir for another 15 minutes and then slowly pour the contents (while stirring) into about 25 g of ice and 15 mL of conc. HCl in a large beaker. PART B: PURIFICATION AND ANALYSIS  After transferring the solution to a separatory flask, the organic layer is collected. The aqueous layer is extracted 2 x 20 mL of dichloromethane, CH 2 Cl 2. Combine the organic layers and wash with 2 x 50 mL of saturated aqueous sodium bicarbonate (NaHCO 3 ).  Dry the organic layer over anhydrous MgSO 4 and gravity filter the drying agent, collecting the product in a tared round bottomed flask. Remove the CH 2 Cl 2 by rotary evaporation.  Weigh the crude product. Obtain an IR spectrum of your product. Also prepare an NMR sample of your product using deuterated CDCl 3 as a solvent and give it to your TA properly labeled. Review the procedures from experiment 3 in Chem 2 2 73a if necessary.  If your product is a solid, make sure to obtain a melting point.  Run a TLC plate of your crude product directly against your unknown aromatic on the same TLC plate, using 2:1 petroleum ether: ethyl acetate as the eluent. Visualize with UV and iodine. If your sample is a solid, get a melting point. You will have to work up your 1 H and 13 C NMR data and analyze it. Obtain an accurate integration of your (^1) H NMR spectrum, including any “impurities”. If you suspect that your product is contaminated with starting material, carefully integrate the starting material peaks separately so you can calculate an approximate percent of each.

  • Calculate a percent yield of your crude product.
  • Analyze all spectra.

EXPERIMENT 5: Electrophilic Aromatic Substitution – A Friedel-Crafts Acylation

Reaction – DATA SHEET

DUE DATE: _____________________

UNKNOWN Aromatic (circle) A B C D

After determining the identity of your unknown aromatic draw the reaction equation showing the structures of starting material and possible products. Name the compounds using IUPAC rules: Calculate your yield: Acetyl chloride: Molecular Weight = _____________ Grams used = _______________ Volume used = _______________ Moles used = ________________ Unknown Aromatic (___): Molecular Weight = _____________ Grams used = _______________ Volume used = _______________ Moles used = ________________ Limiting reagent? ________________ Product: Molecular Weight = _______________ ( hint: MW of your unknown + 42 ) Moles Expected = _______________ Grams expected = _______________ ( theoretical yield ) Mass of round bottom flask and stopper = _______________ Mass of round bottom flask, stopper and products = _______________ Net Mass of products obtained = _______________ Percentage Yield: = _______________ ( Show calculations on reverse )

NAME:

LAB SECTION:

DEMONSTRATOR(S):

Analysis of NMR Spectrum of Product: Attach your NMR spectra to this Data Sheet. Assign a structure of your product by completely analyzing your 1 H and 13 C NMR spectra. In some cases it may be possible to see more than one of the disubstituted benzene products. Be sure to integrate all peaks and assign a relative proportion of the products.

Analysis of Infrared Spectrum: Attach and analyze the IR spectrum of your products, assigning appropriate peaks. Compare and comment on it compared to the IR spectrum of the starting material (provided in the laboratory). Also use Table 1 in experiment 4 to try to determine (confirm) the substitution pattern of your product ( ortho, meta or para ).