Question

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

Short answer

(a) Cyclopropane's reactivity with HI is due to its high angle strain and ring strain, which result from its small ring size and forced 60° bond angles. Conversely, cyclopentane and cyclohexane have larger ring sizes with bond angles closer to the ideal value, resulting in less strain and lower reactivity with HI. (b) Ethylbenzene can be prepared from benzene and ethylene through a Friedel-Crafts alkylation reaction. First, a carbocation forms from ethylene and a Lewis acid catalyst like AlCl3. Next, an electrophilic aromatic substitution occurs between benzene and the carbocation, forming ethylbenzene. Finally, the catalyst is regenerated and can be reused in the reaction.

Step-by-step solution

1. Determine difference in structures of cyclopropane, cyclopentane, and cyclohexane

Cyclopropane is a three-membered ring, cyclopentane is a five-membered ring, and cyclohexane is a six-membered ring. The main difference between them is the size of the ring and the angle strain associated with it. Angle strain is the strain experienced by a molecule due to its bond angles being different from the ideal values.

2. Explain cyclopropane's reactivity due to angle strain and ring strain

Cyclopropane has a high amount of angle strain and ring strain due to its small ring size. The ideal bond angle for sp3 hybridized carbons is 109.5°. However, in cyclopropane, the bond angle is forced to be 60°, which causes the molecule to be highly strained. This high strain makes the molecule unstable and more reactive, hence it readily reacts with HI. On the other hand, cyclopentane and cyclohexane have less angle strain and ring strain due to larger ring sizes with bond angles closer to the ideal value. This makes these molecules more stable and less likely to undergo a similar reaction with HI.

3. Suggest a method for preparing ethylbenzene using benzene and ethylene

We'll use the Friedel-Crafts alkylation reaction to prepare ethylbenzene. Friedel-Crafts alkylation is an electrophilic aromatic substitution, in which a carbocation reacts with an aromatic ring, leading to the alkyl substitution.

4. List steps of Friedel-Crafts alkylation to form ethylbenzene

Step 1: Generate carbocation from ethylene and a strong Lewis acid catalyst, like AlCl3. The double bond of ethylene attacks the AlCl3, forming a carbocation on one of the carbons and releasing a chloride ion. \[C_2H_4 + AlCl_3 \rightarrow C_2H_5^+ + AlCl_4^-\] Step 2: Electrophilic aromatic substitution of benzene with the formed carbocation, resulting in the formation of ethylbenzene. \[C_6H_6 + C_2H_5^+ \rightarrow C_6H_5C_2H_5 + H^+\] Step 3: The catalyst is regenerated by combining the released chloride ion from step 1 and the released proton from step 2. The regenerated catalyst can be reused in the reaction. \[AlCl_4^- + H^+ \rightarrow AlCl_3 + HCl\] Thus, by using the Friedel-Crafts alkylation reaction, ethylbenzene can be prepared using benzene and ethylene as the only organic reagents.

Key concepts

Angle Strain in Cyclopropane

Angle strain plays a significant role in the reactivity of cyclic compounds, particularly in cyclopropane. Imagine the three carbon atoms in cyclopropane being forced into a triangle, rather than the natural tetrahedral shape they prefer. This deviation from the optimal sp3 hybridized carbon bonding angle of 109.5°, down to a harsh 60°, puts the molecule under significant tension. It is similar to a tightly coiled spring, where cyclopropane's bonds are under pressure, looking for a way to release that built-up strain.

When HI, a strong acid, is introduced, the strained bonds of cyclopropane are more susceptible to attack because they are energetically 'uphill', much like a stretched rubber band is easier to snap. The reaction with HI leads to the formation of 1-iodopropane as the bonds eagerly break, seeking stability. In contrast, cyclopentane and cyclohexane don't respond the same way because their larger ring sizes allow them to have bond angles closer to the ideal value, significantly reducing the strain and, consequently, the reactivity.

Friedel-Crafts Alkylation

The Friedel-Crafts alkylation reaction is like a master key for attaching carbon chains to aromatic rings such as benzene. The alkylation involves two main players: an alkyl halide and an aromatic compound, along with a strong Lewis acid catalyst, typically AlCl3. It’s a process akin to adding a side chain onto a stable ring, transforming it into a more complex molecule.

First, the catalyst snaps onto the alkyl halide, creating a highly reactive carbocation. This carbocation is an excellent electrophile, driven by its positive charge, raring to latch onto the electron-rich aromatic ring of benzene. It is the aromatic ring's pi electrons that do the heavy lifting, attacking the carbocation and bonding to it. Then, to stabilize the onslaught of charges, a hydrogen is donated back to complete the transformation, yielding a substituted benzene derivative like ethylbenzene. The beauty of this reaction lies in its regenerative power, with the Lewis acid catalyst being recycled, ready to initiate another round of alkylation.

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution is a prime method to modify benzene rings, a common theme in organic synthesis. In this dance of atoms, the benzene plays the role of an electron-rich floor, inviting electrophiles to step down onto its stable arena. These electrophiles are electron-seeking entities that yearn for the negative charge density of benzene.

Once the electrophile approaches, the aromatic pi electrons reach out and form a bond, kicking off a hydrogen atom in the process. This leads to a temporary loss of the aromaticity, but don't worry, it's all part of the plan. As the hydrogen departs, it takes with it a pair of electrons, which helps in re-establishing the aromatic nature of the compound. This ritual is pivotal in the preparation of diverse aromatic compounds, and it is beautifully highlighted in the synthesis of ethylbenzene from benzene and ethylene. The role of the electrophile is crucial, and in the case of Friedel-Crafts alkylation, this key player is a carbocation generated from a simpler alkene.