Question

In which of the following compounds resonance does not lead to stability of the compound? (a) C1CC1 (b) [C+]1C=C1 (c) c1ccccc1 (d) \(\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}_{2}\)

Short answer

Compound (a) Cyclopropane does not gain stability from resonance.

Step-by-step solution

Understanding Resonance

Resonance occurs when a molecule can be represented by two or more valid Lewis structures, called resonance structures. These structures contribute to the real structure by a sort of 'averaging', usually leading to increased stability. For resonance to occur, there must be delocalizable pi electrons or lone pairs across several atoms.

Analyze Compound (a) - Cyclopropane (C1CC1)

Cyclopropane (C1CC1) is a three-membered ring hydrocarbon with only single bonds. There are no pi bonds or lone pairs available that can be delocalized. Therefore, resonance does not occur in this structure.

Analyze Compound (b) - Cyclopropenyl Cation ([C+]1C=C1)

The cyclopropenyl cation has a positive charge on one of the carbons. It has access to resonance stabilization because the pi electrons can be delocalized across the three-carbon ring. The electron deficiency is offset by resonance structures that distribute the charge, stabilizing the compound.

Analyze Compound (c) - Benzene (c1ccccc1)

Benzene is a well-known aromatic compound with a six-membered carbon ring. The delocalization of pi electrons between the alternating double bonds is a classic example of resonance, leading to significant stabilization.

Analyze Compound (d) - Butadiene (CH=CH-CH=CH2)

Butadiene has a conjugated system of pi bonds. The electrons in the pi bonds can delocalize, creating resonance structures that help stabilize the molecule.

Determine the Compound without Resonance-Driven Stability

Considering the analysis above, compound (a) - Cyclopropane (C1CC1), lacks the delocalizable electrons needed for resonance stabilization. Thus, it does not benefit from resonance-led stability as the other compounds do.

Key concepts

Key Characteristics of Cyclopropane

Cyclopropane is a simple, three-membered ring compound composed entirely of carbon-hydrogen single bonds. This small ring structure is often described as a triangle, and its geometry subjects it to a considerable amount of angle strain. In fact, the typical bond angles in cyclopropane are only about 60 degrees, which is significantly less than the ideal 109.5 degrees found in a typical tetrahedral carbon atom.
Thus, cyclopropane experiences high internal strain.

Cyclopropane does not have pi bonds or lone pairs, which are necessary for resonance to occur. Resonance involves delocalization of electrons in pi bonds or lone pairs over multiple atoms, a phenomenon that increases stability. Since cyclopropane lacks any pi bond structure to facilitate this electron delocalization, it cannot be stabilized through resonance. This makes cyclopropane less stable compared to hydrocarbons like benzene, which benefit from resonance stabilization.
In summary, cyclopropane is characterized by its high strain and inability to achieve resonance-driven stability.

Unique Features of Cyclopropenyl Cation

The cyclopropenyl cation is an intriguing three-membered ring because it carries a positive charge on one of its carbons. Despite being a cation and, therefore, an electron-deficient species, it is stabilized by resonance. This cation has a single pi bond and a vacant p-orbital on the positively charged carbon. The pi electrons can be delocalized across the entire three-carbon system.

• The presence of a pi bond allows for electron distribution across all carbon atoms in the ring.
• This electron delocalization reduces the localization of charge, enhancing the overall stability of the molecule.

The delocalization in the cyclopropenyl cation provides it with a resonance stabilization that offsets the intrinsic instability of having a positive charge. Its capacity to resonate across the entire ring structure is what renders this otherwise highly strained molecule relatively stable in its category.

Understanding Benzene's Aromatic Stability

Benzene is a classic example of an aromatic compound where resonance plays a crucial role in its stability. This hexagonal molecule consists of six carbon atoms, each bonded through alternating single and double bonds. The concept of "aromaticity," goes hand-in-hand with benzene's structural integrity.
Benzene's resonance is not just about alternating double bonds; it's about true electron delocalization. In benzene, pi electrons are not localized between two specific carbons but rather shared equally across all six carbons, forming a continual pi-electron cloud above and below the plane of the carbon atoms. This delocalization:

• Enhances benzene’s structural and energy stability.
• Provides resistance against reactions that would break its aromatic system.

This results in benzene having an impressive level of chemical stability and provides insights into why it doesn't behave like typical alkenes, despite having multiple double bonds.

Resonance and Stability in Butadiene

Butadiene is an example of a conjugated diene - a molecule with two double bonds separated by a single bond. This arrangement allows the pi electrons from the double bonds to become delocalized over a greater section of the molecule than if they were isolated.
This conjugation results in resonance structures for butadiene, where:

• The electrons involved in the pi bonds can shift along the sequence of bonds, spreading the electron density across more atoms.
• The delocalization across the whole system gives butadiene increased stability compared to non-conjugated dienes.

This stability is thanks to the electrons being able to exist in lower energy states due to their delocalization. It underscores the power of conjugation and resonance in stabilizing molecules, making butadiene a fascinating compound for study in organic chemistry.