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Friedel-Crafts Alkylation Lab, Lab Reports of Chemistry

Minneapolis College of Art & Design (MCAD)Chemistry

Friedel-Crafts Alkylation of 1,4-Dimethoxybenzene, Jasperse Chem 365

Typology: Lab Reports

2020/2021

Uploaded on 05/12/2021

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Jasperse Chem 365
Friedel-Crafts Alkylation Lab
1
Friedel-Crafts Alkylation of 1,4-Dimethoxybenzene
OCH3
H2SO4
CH3CO2H
(acetic acid)
OCH3
1,4-Dimethoxybenzene
(Hydroquinone dimethyl ether)
MW = 138.16
mp = 57ºC
1,4-Di-t-butyl-2,5-dimethoxybenzene
MW = 250.37
mp 104-105˚C
OCH3OCH3
H3C
H3CCH3
CH3
CH3
CH3
CH3
H3C
CH3
OH
+2
t-Butyl Alcohl
(t-butanol, 2-methyl-2-propanol)
MW = 74.1, density = 0.79g/mL
mp = 26ºC
General Issues with Electrophilic Aromatic Substitution Reactions
1. Polysubstitution
Aromatic substitutions always involve the issue of how many substitutions will occur.
Will reaction stop after one substitution? Will it proceed to give a second substitution? Will it
proceed further to give a third substitution? In today’s experiment the reaction does not stop
after a single substitution, but proceeds on to a second. However, it does basically stop after the
second substitution and does not proceed appreciably to a third or fourth substitution. Influential
factors are electronic and steric:
• Electronics: are new substituents electron donating or electron withdrawing? Will they
stabilize or destabilize the cation involved in subsequent substitutions? As a result will they
activate or deactivate toward subsequent substitution?
• Steric: are new substituents large enough to obstruct further substitution?
2. Position of Substitution: Ortho, Meta, or Para To a Pre-existing Substituent?
Relative to a pre-existing substituent (or several of them…), will a new substitution occur
ortho, meta, or para? Electronic and steric factors are again influential. In today’s experiment,
for example, after the first t-butyl group is attached, why does the second t-butyl group go where
it does?
3. Generation of the Electrophile
All electrophilic aromatic substitution reaction mechanisms require a strong electrophile, usually
cationic. In today’s experiment, the active electrophile is the t-butyl cation. For a general
alkylation, in which an alkyl cation is required, there are several alternate precursors:
• Alkenes: Alkene + H+ Æ R+
• Alcohols: ROH + H+ Æ ROH2
+ Æ R++ H2O
• Alkyl Halides: R-X + AlX3 Æ R++ AlX4
-
pf3
pf4

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Friedel-Crafts Alkylation Lab

Friedel-Crafts Alkylation of 1,4-Dimethoxybenzene

OCH 3 H 2 SO 4 CH 3 CO 2 H (acetic acid) OCH 3 1,4-Dimethoxybenzene (Hydroquinone dimethyl ether) MW = 138. mp = 57ºC 1,4-Di-t-butyl-2,5-dimethoxybenzene MW = 250. mp 104-105˚C OCH 3 OCH 3 H 3 C H 3 C CH 3 CH 3 CH 3 CH 3 CH 3 H 3 C CH 3

  • (^2) OH t-Butyl Alcohl (t-butanol, 2-methyl-2-propanol) MW = 74.1, density = 0.79g/mL mp = 26ºC General Issues with Electrophilic Aromatic Substitution Reactions

1. Polysubstitution Aromatic substitutions always involve the issue of how many substitutions will occur. Will reaction stop after one substitution? Will it proceed to give a second substitution? Will it proceed further to give a third substitution? In today’s experiment the reaction does not stop after a single substitution, but proceeds on to a second. However, it does basically stop after the second substitution and does not proceed appreciably to a third or fourth substitution. Influential factors are electronic and steric:

  • Electronics: are new substituents electron donating or electron withdrawing? Will they stabilize or destabilize the cation involved in subsequent substitutions? As a result will they activate or deactivate toward subsequent substitution?
  • Steric: are new substituents large enough to obstruct further substitution? 2. Position of Substitution: Ortho, Meta, or Para To a Pre-existing Substituent? Relative to a pre-existing substituent (or several of them…), will a new substitution occur ortho, meta, or para? Electronic and steric factors are again influential. In today’s experiment, for example, after the first t-butyl group is attached, why does the second t-butyl group go where it does? 3. Generation of the Electrophile All electrophilic aromatic substitution reaction mechanisms require a strong electrophile, usually cationic. In today’s experiment, the active electrophile is the t-butyl cation. For a general alkylation, in which an alkyl cation is required, there are several alternate precursors:
    • Alkenes: Alkene + H+^ Æ R+
    • Alcohols: ROH + H+^ Æ ROH 2 +^ Æ R++ H 2 O
    • Alkyl Halides: R-X + AlX 3 Æ R++ AlX 4 -

Friedel-Crafts Alkylation Lab Reaction Procedure:

  1. Weigh out 1.50 g of 1,4-dimethoxybenzene and place in a 125-mL Erlenmeyer flask.
  2. Measure out 2.50 mL of warm t-butyl alcohol into a syringe, and inject into the Erlenmeyer flask. (Note: t-butanol freezes at 26ºC, so it’s best to handle it somewhat warm so it stays liquid. If it has much chance to cool off, it may solidify and complicate delivery.)
  3. Measure out 5 mL of acetic acid from a buret. You can deliver this directly into your Erlenmeyer, or else drain it into a small flask/beaker and pour it in. Acetic acid is smelly, so delivering it directly in the hood is a good way to reduce smells in the lab. If you do transfer via a small flask/beaker, rinse that out pretty quickly in the hood so that the lab doesn’t smell too much like vinegar.
  4. Cool the Erlenmeyer flask in an ice-water bath.
  5. Drain 10 mL of concentrated sulfuric acid from a buret into your separatory funnel. Note 1: Make sure the stopcock on your separatory funnel is not open! Note 2: Sulfuric acid is a very strong acid; you do not want any to touch your skin or clothes.
  6. Position your separatory funnel above your Erlenmeyer flask, and then drop in the sulfuric acid very slowly, drop by drop, over a period of 5-7 minutes. Swirl the Erlenmeyer flask frequently while doing so. Keep the Erlenmeyer flask in an ice-water bath throughout.
  7. After addition of the sulfuric acid is complete, remove the Erlenmeyer flask from the cold bath and let it stand at room temperature for 20 minutes to allow completion of the reaction. Swirl periodically.
  8. During this 20-minute wait, rinse the Erlenmeyer flask with the residual sulfurice acid with tap-water. You can drain into the sink. This is also a good time to write up your lab report, including stoichiometry calculations and procedure and observations. Isolation of the Crude Product:
  9. Add some ice to the Erlenmeyer flask to dilute the sulfuric acid, swirling the flask as you do so. (One of the functions of the ice is to absorb some of the heat that is produced when sulfuric acid and water mix.) Then add ice-cold water to a total volume of 75 mL or more, swirling vigorously as you add.
  10. Use a scoopula or micro-spatula to swirl and stir the mixture for at least two minutes.
  11. At this point, you should have a lot of crystal that formed. Because the desired product has so many carbons, it has very low solubility in water, so adding all the water basically crashes most of the the product from solution.
  12. Filter the solution using a Buchner funnel.
  13. Rinse thoroughly with another 70 mL of ice-cold water. (The function of the water is to make sure all the sulfuric acid, acetic acid, and t-butanol is washed away from the product.)
  14. Take the Buchner funnel off of the filter flask, and pour the water down the drain. Then reattach the Buchner funnel.
  15. Rinse with a 5-mL portion of ice-cold methanol
  16. Rinse again with a second 5-mL portion of ice-cold methanol. (The methanol washes away some organic impurity and also functions to remove much of the water.)
  17. Save a small portion of the crystals for a crude melting point.

Friedel-Crafts Alkylation Lab Questions:

  1. Draw a detailed mechanism for the formation of t-butyl-2,5-dimethoxybenzene. (In other words, for the first alkylation, but not the second…). OCH 3 OCH 3 CH 3 CH 3 OCH 3 CH 3 1,2-Di-t-butyl-3,6-dimethoxybenzene OCH 3 CH 3 CH 3 OCH 3 CH 3 1,4-Di-t-butyl-2,5-dimethoxybenzene OCH 3 H 3 C H 3 C CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 H 3 C CH 3 H 3 C 1,3-Di-t-butyl-3,6-dimethoxybenzene
  2. The actual dialkylation product is 1,4-di-t-butyl-2,5-dimethoxybenzene. Why is this isomer preferred rather than the alternative isomer 1,2-di-t-butyl-3,6-dimethoxybenzene (see above)?
  3. The actual dialkylation product is 1,4-di-t-butyl-2,5-dimethoxybenzene. Why is this isomer preferred rather than the alternative isomer 1,3-di-t-butyl-2,5-dimethoxybenzene (see above)?
  4. What was your molar ratio of t-butanol to dimethoxybenzene?
  5. Why do you think you did not stop after just a single alkylation? In other words, why were you able to add two t-butyls, not just one?
  6. Why do you think you did stop after two alkylations? In other words, why were you able to add two t-butyls, but did not continue on to add a third t-butyl group at least to some of your molecules?
  7. You used t-butanol and acid to generate the t-butyl cation used to form 1,4-di-t-butyl-2,5- dimethoxybenzene. Suggest two organic precursors other than t-butanol that could be used as precursors for t-butyl cation?