Problem 37 - Chemistry The Central Science

Question

(a) When cyclopropane is treated with HI, 1-iodopropane is formed. A similar type of reaction does not occur with cyclopentane or cyclohexane. How do you account for the reactivity of cyclopropane? (b) Suggest a method of preparing ethylbenzene, starting with benzene and ethylene as the only organic reagents.

Step-by-Step Solution

Answer

(a) Cyclopropane is more reactive towards HI due to its significant ring strain, which weakens the C-H bonds and makes the carbon atoms more susceptible to nucleophilic attack. Cyclopentane and cyclohexane, with less ring strain, do not react this way with HI. (b) Ethylbenzene can be prepared from benzene and ethylene through a Friedel-Crafts alkylation reaction using a Lewis acid catalyst (AlCl₃). The electrophilic ethylene-AlCl₃ complex reacts with the nucleophilic benzene ring, forming ethylbenzene, and regenerating the AlCl₃ catalyst. Ethylbenzene can be separated and purified using solvent extraction and distillation.

Step 1: Part (a): Understanding the reactivity of cyclopropane with HI

Cyclopropane is a three-membered ring with carbon atoms bonded to each other, forming a triangular structure. Due to its small ring size and the bond angles being 60° (rather than the ideal angle of 109.5° for sp³ hybridized carbons), it experiences significant ring strain that makes its carbon-carbon bonds weaker and more reactive. Cyclopentane and cyclohexane, on the other hand, have larger rings with less strain, making them less reactive. 1-iodopropane is formed when a carbon-hydrogen (C-H) bond of cyclopropane is broken and replaced with a carbon-iodine (C-I) bond. This is a nucleophilic substitution reaction, where HI donates the iodine, which acts as a nucleophile. The high reactivity of cyclopropane is due to the ring strain, which weakens the C-H bonds and makes the carbon atoms more susceptible to nucleophilic attack. Cyclopentane and cyclohexane, having less ring strain, do not react this way with HI because their C-H bonds are more stable. Their carbon atoms are also less accessible to nucleophiles due to the larger ring size.

Step 2: Part (b): Preparing ethylbenzene from benzene and ethylene

To prepare ethylbenzene (C₆H₅CH₂CH₃) from benzene (C₆H₆) and ethylene (C₂H₄), we can use a Friedel-Crafts alkylation reaction. This involves the use of a Lewis acid catalyst, such as aluminum chloride (AlCl₃), which forms a complex with the ethylene molecule, making it a more reactive electrophile. Step 1: Generating the electrophilic ethylene-AlCl3 complex Add ethylene and AlCl3 together. The AlCl3 will coordinate to the double bond of the ethylene. This results in a positively charged complex with electrophilic properties. Step 2: Friedel-Crafts alkylation of benzene with the complex Now add benzene to the reaction mixture. The electrophilic ethylene-AlCl₃ complex will react with the nucleophilic benzene ring, forming ethylbenzene with the loss of a hydrogen atom and the regeneration of the AlCl₃ catalyst. Step 3: Separation of product Ethylbenzene is formed as a product. To separate it from the other components of the mixture, it can be extracted using a suitable solvent and then purified through distillation. In summary, the reaction between benzene and ethylene can be facilitated using a Friedel-Crafts alkylation reaction with the help of a Lewis acid catalyst like AlCl₃. This will result in the formation of ethylbenzene which can then be separated and purified.

Key Concepts

Cyclopropane Reactivity

Exploring the intriguing world of organic chemistry often leads to the topic of ring strains and their effect on reactivity. One example of this phenomenon can be seen in cyclopropane, a three-membered ring which, due to its petite size and severe angles, possesses significant ring strain. The bond angles in cyclopropane are only 60°, deviating from the more comfortable 109.5° angle found in sp³ hybridized carbons. This strain not only makes the ring itself less stable, but it also amplifies the reactivity of the carbon-hydrogen bonds within the ring.

When cyclopropane reacts with hydrogen iodide (HI), a substitution reaction occurs, leading to the formation of 1-iodopropane. The mechanism behind this involves the weakened C-H bonds which are more vulnerable to the attack by nucleophiles such as the iodide ion from HI. This increased susceptibility to reaction sets cyclopropane apart from its more stable relatives, cyclopentane and cyclohexane, which have larger rings and thus less ring strain. As a result, these larger cycles do not exhibit the same level of reactivity with nucleophiles.

Nucleophilic Substitution Reaction

Nucleophilic substitution reactions are a staple in the realm of organic chemistry, and understanding them is crucial for grasping the behavior of many compounds. A nucleophilic substitution is typically characterized by the replacement of an atom or group within a molecule by a nucleophile, which is an electron-rich species that seeks out positively charged or electron-deficient areas.

In the case of cyclopropane reacting with HI, the iodide ion serves as the nucleophile. The three-membered ring's significant ring strain leads to weakened C-H bonds, making the carbon atoms in cyclopropane more electrophilic and inviting for nucleophilic attack. The iodide ion seizes this opportunity, leading to the substitution of a hydrogen atom and forming 1-iodopropane. This process exemplifies the delicate dance between nucleophiles and electrophiles that drives many reactions in organic chemistry.

Friedel-Crafts Alkylation

Another powerful reaction utilized in organic synthesis is the Friedel-Crafts alkylation. This reaction allows chemists to add alkyl groups to aromatic rings, like benzene, thus modifying the ring's chemical properties and introducing versatility into its potential applications. The key to this reaction is an electrophilic intermediate that can readily form bonds with the electron-rich aromatic ring.

For example, the synthesis of ethylbenzene from benzene and ethylene involves the creation of an electrophilic ethylene-AlCl₃ complex. Here, aluminum chloride acts as a Lewis acid catalyst, forming a bond with the ethylene's double bond and making it positively charged. This complex then interacts with the benzene's electron-dense aromatic system, allowing the alkyl group to attach via the loss of hydrogen. The result is ethylbenzene, and, importantly, the regeneration of the AlCl₃ catalyst, which can be used again. Friedel-Crafts alkylation is a quintessential technique used to grow complex molecules in pharmaceuticals and material science.