Stability of cycloalkanes | Organic chemistry | Khan Academy

Summary

The video script explores the concept of cycloalkanes, initially believed to be planar structures with varying degrees of angle strain. It delves into the heat of combustion as a measure of stability, revealing that cyclohexane is the most stable, contrary to earlier theories. The script also explains the different types of strain in cycloalkanes, such as angle and torsional strain, and how these affect their reactivity and structure. Finally, it uses the concept to compare the stability and heat of combustion of ethylcyclopropane and methylcyclobutane, concluding that the latter is more stable with a lower heat of combustion.

Takeaways

• 🔍 Initially, cycloalkanes were thought to be planar, with cyclopropane, cyclobutane, cyclopentane, and cyclohexane having bond angles deviating from the ideal tetrahedral angle of 109.5 degrees.
• 📏 Angle strain is the increase in energy due to bond angles deviating from the ideal, causing significant strain in small-ring cycloalkanes like cyclopropane and cyclobutane.
• 🔄 The stability of cycloalkanes was initially thought to be based on bond angles, with cyclopentane being considered the most stable due to its bond angle being closest to 109.5 degrees.
• 🔥 Heat of combustion was used to analyze the stability of cycloalkanes, revealing that cyclopropane has the highest heat of combustion per CH2 group, indicating it is the most unstable.
• ⚖️ To accurately compare cycloalkanes' stability, the heat of combustion should be divided by the number of CH2 groups present in the molecule.
• 📉 Cyclohexane was found to be strain-free and as stable as an open chain alkane, contrary to the earlier theory that suggested it was a flat hexagon.
• 🧬 The structure of cycloalkanes can be stabilized by adopting non-planar conformations, such as the puckered conformation for cyclobutane and the envelope conformation for cyclopentane.
• 🔑 The three-membered ring in cyclopropane is highly reactive due to significant angle and torsional strain, making it susceptible to ring-opening reactions.
• 🔄 The stability of cycloalkanes is inversely related to their heat of combustion; less stable compounds have higher heats of combustion.
• 🔑 Ethylcyclopropane, with a three-membered ring, is less stable than methylcyclobutane, which has a four-membered ring and less strain.
• 🔥 Ethylcyclopropane, being less stable, has a higher heat of combustion compared to the more stable methylcyclobutane.

Q & A

What was the initial belief about the shape of cycloalkanes?

Initially, it was thought that all cycloalkanes were planar, with cyclopropane being a flat triangle, cyclobutane a flat square, cyclopentane a flat pentagon, and cyclohexane a flat hexagon.

What is angle strain and how is it related to the ideal bond angle?

Angle strain is the increase in energy associated with a bond angle that deviates from the ideal bond angle of 109.5 degrees, which is characteristic of a carbon atom in tetrahedral geometry.

Why does cyclopropane have significant angle strain?

Cyclopropane has a bond angle of 60 degrees, which is significantly different from the ideal bond angle of 109.5 degrees, leading to a large amount of angle strain.

Why was cyclopentane initially considered the most stable cycloalkane?

Cyclopentane was considered the most stable because its bond angle of 108 degrees is closest to the ideal bond angle of 109.5 degrees among the cycloalkanes.

How does the heat of combustion relate to the stability of cycloalkanes?

The heat of combustion, when divided by the number of CH2 groups, gives a better indication of the stability of cycloalkanes. A lower value indicates more stability, as it means less energy is required to combust the compound.

Why can't the heats of combustion be used directly to compare cycloalkane stability?

Direct comparison is not possible because the number of carbons increases from cyclopropane to cyclohexane, which inherently increases the heat of combustion regardless of the ring strain.

Which cycloalkane is considered the most stable based on the heat of combustion per CH2 group?

Cyclohexane is considered the most stable cycloalkane based on the heat of combustion per CH2 group, as it is approximately the same as that of a straight chain alkane.

Why does cyclopropane have the highest heat of combustion per CH2 group?

Cyclopropane has the highest heat of combustion per CH2 group due to its significant angle strain, which increases the energy of the molecule, making it more reactive and susceptible to ring-opening reactions.

What is the non-planar confirmation of cyclobutane that helps relieve torsional strain?

Cyclobutane can adopt a puckered confirmation to relieve torsional strain, where the hydrogens in the front are no longer eclipsing the hydrogens in the back.

What is the envelope confirmation of cyclopentane and how does it help relieve torsional strain?

The envelope confirmation of cyclopentane is a non-planar structure where four carbons are in the same plane and the fifth carbon is out of the plane, relieving some of the torsional strain by reducing the eclipsing of hydrogens.

Which isomer, ethylcyclopropane or methylcyclobutane, is more stable and why?

Methylcyclobutane is more stable because it contains a four-membered ring which has less strain compared to the three-membered ring in ethylcyclopropane.

Which isomer has a higher heat of combustion, and what does this indicate about its stability?

Ethylcyclopropane has a higher heat of combustion, indicating that it is less stable compared to methylcyclobutane, which has a lower heat of combustion.