Question

Suggest a reason why liquid hydrogen fluoride is preferable to concentrated, aqueous hydrobromic acid in inducing propene to react with benzene to produce isopropylbenzene.

Short answer

Liquid hydrogen fluoride is preferable to concentrated aqueous hydrobromic acid in inducing propene to react with benzene to produce isopropylbenzene because it is a stronger acid, has better nucleophilic properties due to the presence of fluoride ions, and does not involve water molecules that may interfere with the reaction.

Step-by-step solution

Recall the reaction between propene and benzene to produce isopropylbenzene

The reaction between propene and benzene to produce isopropylbenzene is an example of an electrophilic aromatic substitution. In this case, the electrophile is a protonated propene (propene-H+), and the benzene acts as a nucleophile. The overall reaction can be represented as: C6H6 + CH3-CH=CH2 -> C6H5-CH(CH3)2

Discuss the role of the acidic catalyst

For this reaction to occur, we need a strong acid to protonate propene, thus forming a positively charged electrophile that can attack the benzene ring. Hydrogen fluoride (HF) and hydrobromic acid (HBr) can both act as Bronsted-Lowry acids, donating a proton (H+).

Compare liquid hydrogen fluoride and aqueous hydrobromic acid

- Hydrogen fluoride (HF) is a relatively strong acid and a very good source of fluoride ions (F-), which are good nucleophiles. - Aqueous hydrobromic acid (HBr) is also a strong acid, but in its aqueous form, it is diluted, and the water molecules can interfere with the reaction. Additionally, its bromide ions (Br-) are less nucleophilic than fluoride ions (F-).

Indicate the advantages of using liquid hydrogen fluoride

The advantages of using liquid hydrogen fluoride as the catalyst for this reaction are: 1. As a stronger acid, hydrogen fluoride can more readily protonate propene, allowing the reaction to proceed more efficiently. 2. The fluoride ions (F-) in hydrogen fluoride are excellent nucleophiles. They can form a relatively stable complex with the carbocation formed during the reaction, allowing the propene molecule to more effectively attack the benzene ring. 3. The absence of water molecules prevents side reactions and hydrolysis, which can be a problem with aqueous hydrobromic acid.

Conclusion

Liquid hydrogen fluoride is a preferable catalyst for the reaction between propene and benzene to produce isopropylbenzene as it is a stronger acid, has better nucleophilic properties, and does not involve water molecules that may interfere with the reaction.

Key concepts

Hydrogen Fluoride as a Catalyst

In organic chemistry, certain reactions require a helping hand to proceed efficiently. This is where catalysts like hydrogen fluoride (HF) come into play. As a molecule, HF consists of a hydrogen atom bonded to a fluorine atom, and it stands out as a particularly strong acid when compared to others like hydrobromic acid (HBr).HF's unique properties make it an exceptional catalyst for electrophilic aromatic substitution reactions, such as the conversion of propene to isopropylbenzene in the presence of benzene. It's the high electronegativity of fluorine that enables HF to readily donate its proton to propene, forming a highly reactive electrophile that can interact with benzene. This reaction would be less efficient with a weaker acid or one that introduces interfering substances, like water in the case of aqueous hydrobromic acid.

Protonated Propene

Propene, which is an alkene, is a molecule itching to react, especially when it is turned into an even more reactive form. This process is facilitated by protonation—propene gains a positively charged hydrogen atom (H+), making it a 'protonated propene'. The reaction becomes possible when we introduce an acid like hydrogen fluoride.In its protonated state, the propene molecule is transformed into a strong electrophile that's ready to attack the stable π-electrons in a benzene ring. Protonated propene, or a propene molecule with an added hydrogen ion, is pivotal for the reaction because it initiates the process of electrophilic aromatic substitution by seeking out a pair of electrons to bond with, which it finds in the aromatic ring of benzene.

Nucleophilicity of Halide Ions

When it comes to electrophilic reactions like the formation of isopropylbenzene, not all players are aggressors. Some, like halide ions, are more like willing participants, reacting because they have something valuable to offer: spare electrons. Halide ions (like F-, Cl-, Br-, and I-) have varying degrees of nucleophilicity, which essentially measures their readiness to donate electrons.Among them, fluoride ions (F-) are particularly good at this role due to their small size and high charge density which allows them to approach positive charges closely, making them valuable in reactions requiring a strong nucleophile. Their keenness to form bonds makes them essential in the middle stages of many reactions, stabilizing intermediate structures that could otherwise be too reactive to control.

Role of Acidic Catalyst in Organic Reactions

Catalysts are the unsung heroes of the chemical world, and when it comes to organic reactions, acidic catalysts play a starring role. Their job is to kickstart reactions without being consumed in the process. In the case of electrophilic aromatic substitutions, the acidic catalyst, for example, hydrogen fluoride, donates a proton to the reactant, which in this case is propene. This proton donation is critical because it increases the electrophilicity of the reactant, creating what we call a 'protonated species'.This newly charged molecule can then interact effectively with other components of the reaction, such as benzene. Without this initial push by the acidic catalyst, reactions could proceed much slower or not at all. Acids like HF which have a strong proclivity for donating their protons, serve as excellent catalysts in this context, encouraging the formation of products with greater speed and selectivity.