Carbocation Rearrangements and Product Distributions in Friedel-Crafts Alkylation of p-Xylene

Abstract:

This experiment examined the Friedel-Crafts alkylation of p-xylene with 1-chloropropane using AlCl₃ catalyst to study electrophilic aromatic substitution and carbocation rearrangements. The reaction yielded two isomeric products identified by GC/MS: 1,4-dimethyl-2-propylbenzene (119.1 amu) and 2-isopropyl-1,4-dimethylbenzene (133.1 amu). Minor peaks at 161.2 and 175.2 amu revealed disubstituted byproducts, which suggests continued ring reactivity due to methyl group activation. The products eluted in boiling point order, with the lower-boiling isopropyl isomer with a boiling point of 196.2°C preceding the linear propyl analog whose boiling point is 204°C. The major limitation was that quantitative ratios could not be determined without peak areas. Nonetheless, it was determined that there was successful monoalkylation with rearrangement, electronic effects governed regioselectivity, and over-alkylation was observed despite steric constraints. These results align with the mechanistic principles of Friedel-Crafts reactions, including carbocation stability and substituent-directed reactivity in aromatic systems.

Introduction

Figure 1

Figure 2 Reaction mechanism illustrating electrophilic attack on p-xylene by the primary propyl cation and stable isopropyl cation

Figure 3 The Reaction Products: 1,4-dimethyl-2-propylbenzene and 2-isopropyl-1,4-diimethylbenzene

Experimental

A 25-mL round-bottom flask with a stir bar was added 3.7 mL (3.19 g, 0.030 mol) of p-xylene. A Claisen adapter was attached to the flask, with a rubber septum fitted to the straight arm and a drying tube containing CaCl₂ connected to the curved arm. Afterward, 0.20g (0.0015 mol) of anhydrous AlCl₃ was quickly weighed and added to the flask to minimize exposure to atmospheric moisture. The mixture was stirred, and 0.14 mL (0.118 g, 0.0015 mol) of 1-chloropropane was added using a syringe through the septum. This was done for 2 hours, during which the solution transitioned from the initial yellow to orange.

GC/MS analysis was performed to identify the products, where the results showed early-eluting peaks at 91.1 amu, followed by 119.1 amu, then 133.1 amu, with later-eluting minor peaks at 161.2 amu and 175.2 amu. The tallest peak (91.1 amu) eluted first, while the rearranged product (133.1 amu) appeared after the non-rearranged isomer (119.1 amu). The highest-mass fragments (161.2 amu, 175.2 amu) eluted last. No significant peaks were observed beyond this range.

Results and Discussion

A minor later-eluting peak showed a molecular ion at m/z 175.2, as shown in appendix D below, indicating the presence of a disubstituted product, most likely dimethyldipropylbenzene, with a secondary fragment at m/z 161.2. However, no significant peaks were observed near m/z 232, suggesting negligible formation of tripropylated products. These result from further alkylation even after two new alkyl groups are added. This happens because alkyl groups are electron-donating substituents that activate the aromatic ring by increasing its electron density through both inductive and hyperconjugative effects. This enhanced nucleophilicity makes the ring more reactive toward subsequent electrophilic attacks.

The literature boiling points of 1,4-dimethyl-2-propylbenzene and 2-isopropyl-1,4-dimethylbenzene are 204 °C and 196.2 °C, respectively. Due to its low boiling point, 2-isopropyl-1,4-dimethylbenzene is expected to elute first due to its volatility.³ The observations matched this expectation since the rearranged product was eluted at an average of 6.677 to 6.908 min while the non-rearranged product was eluted at 7.067 to 7.213 min. 

The experiment successfully demonstrated the Friedel–Crafts alkylation mechanism, carbocation rearrangement, and the electronic and steric effects influencing aromatic substitution. The GC/MS data confirmed the formation of both isomeric monoalkylated products. However, the product ratio could not be determined quantitatively despite successfully identifying the characteristic fragments for the non-rearranged and rearranged products. This is because the peak area integration data was unavailable on the provided chromatogram. Nonetheless, the rearranged product is expected to be more abundant due to the preferential formation of the more stable secondary carbocation-derived isomer.

References

(1)  Mąkosza, M. Electrophilic and Nucleophilic Aromatic Substitutions are Mechanistically Similar with Opposite Polarity. Chemistry - a European Journal 2020, 26 (67), 15346–15353. https://doi.org/10.1002/chem.202003770.