Cyclohexanes: Crash Course Organic Chemistry #7

Summary

In this Crash Course Organic Chemistry episode, Deboki Chakravarti dives into the fascinating world of cyclohexane. She explains how cycloalkanes like cyclohexane and cyclopentane have low ring strain due to their puckered structures, making them stable. The chair and boat conformations of cyclohexane are explored, with a focus on the chair conformation's stability. The episode also covers the concept of chair flips, where substituents switch positions, and the importance of substituents being in equatorial positions for stability. Cyclohexane's significance in organic molecules, such as carbohydrates and steroids, is emphasized, showcasing its role in chemistry.

Takeaways

• 😀 Hexagons are common in nature, from honeycombs to diamonds, and are a shape with scientific significance.
• 😀 Cycloalkanes are named similarly to alkanes, with the prefix 'cyclo-' and numbered to give substituents the lowest possible positions.
• 😀 Cycloalkanes have two faces, and when substituents are on the same face, the prefix 'cis-' is used; when on opposite faces, 'trans-' is used.
• 😀 Cyclohexane and cyclopentane are stable cycloalkanes with low ring strain due to their angular and torsional strain being minimized.
• 😀 Combustion reactions and bomb calorimetry helped organic chemists quantify the energy released from strained rings, like cyclopropane.
• 😀 Cyclopropane, a highly strained ring, releases extra energy upon combustion due to its ring strain.
• 😀 Cyclohexane's chair conformation is the most stable, with staggered hydrogens and bond angles of 109.5 degrees.
• 😀 Cyclohexane can switch between chair conformations in a process called a chair flip, which doesn’t involve breaking bonds.
• 😀 Methylcyclohexane has lower energy when the methyl group is in the equatorial position due to reduced steric strain.
• 😀 In molecules like cis-1,3-dimethylcyclohexane, the chair conformation where substituents are in equatorial positions is more stable to reduce steric hindrance.

Q & A

Why are hexagons commonly found in nature?

Hexagons are common in nature due to their space-efficient structure. For example, honeycombs, tortoise shells, and even the arrangement of carbon atoms in molecules like diamond and graphite often form hexagonal patterns. The shape minimizes space and energy, which is why it appears so frequently.

What are cycloalkanes and how are they named?

Cycloalkanes are hydrocarbons with carbon atoms arranged in a ring structure. They are named by adding the prefix 'cyclo-' to the name of the alkane with the same number of carbon atoms. For example, a five-carbon ring is called cyclopentane, and a six-carbon ring is cyclohexane.

What is ring strain, and how does it affect the stability of cycloalkanes?

Ring strain arises from angular strain, when bond angles deviate from the ideal 109.5 degrees for sp3 hybridized carbons, and torsional strain, when bonds are in an eclipsed conformation. High ring strain makes a molecule less stable, which is why cycloalkanes like cyclopropane with more strain are less stable than cyclohexane, which has low strain.

How did organic chemists measure ring strain using combustion?

Organic chemists measured ring strain by using a bomb calorimeter to burn cycloalkanes and other hydrocarbons, comparing the heat released during combustion. The difference in energy released provided a measure of the extra energy associated with ring strain.

What is the chair conformation of cyclohexane, and why is it important?

The chair conformation of cyclohexane is the most stable form of the molecule, where all carbon-carbon bonds are staggered, and the bond angles are close to the ideal 109.5 degrees. This conformation reduces ring strain and is energetically favorable compared to other forms like the boat conformation.

What happens during a chair flip in cyclohexane?

During a chair flip, the positions of axial and equatorial substituents on cyclohexane molecules switch. This process doesn't break bonds but rotates the molecule to shift the positions of groups around the ring. The flip maintains the stability of the molecule by balancing strain.

Why do substituents prefer to be in equatorial positions in cyclohexane?

Substituents prefer the equatorial position in cyclohexane because it minimizes steric hindrance and diaxial strain. When a bulky group is in the axial position, it experiences interactions with other axial hydrogens, creating unfavorable strain, whereas the equatorial position is more spacious and stable.

How does the size of a substituent affect its position in cyclohexane?

Larger substituents, such as tert-butyl groups, experience significant steric strain when in axial positions. As a result, these larger groups preferentially occupy equatorial positions in the cyclohexane ring, minimizing steric interactions and stabilizing the molecule.

What is the difference between cis- and trans- configurations in cyclohexanes?

In cyclohexane, 'cis-' refers to substituents on the same side of the ring, while 'trans-' means the substituents are on opposite sides. These configurations can affect the molecule's stability, with cis configurations sometimes leading to steric strain due to proximity of substituents.

How can you convert a flat skeletal structure of cyclohexane into a chair conformation?

To convert a flat skeletal structure of cyclohexane into a chair conformation, you first identify the positions of substituents, then draw the base chair structure, number the carbon atoms in the same sequence as the flat structure, and add the axial and equatorial bonds accordingly. This helps visualize the molecule’s three-dimensional shape and the position of substituents.