Cyclohexanes: Crash Course Organic Chemistry #7

CrashCourse
8 Jul 202014:09
EducationalLearning
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TLDRIn this episode of Crash Course Organic Chemistry, Deboki Chakravarti explores the prevalence of hexagons in nature and their significance in organic chemistry, particularly focusing on the cyclohexane molecule. The video delves into the naming conventions of cycloalkanes, the concept of ring strain, and how it is quantified using bomb calorimetry. It explains the structural differences between cyclohexane and cyclopentane, highlighting their stability due to low angular strain and the ability to pucker, which reduces torsional strain. The chair conformation of cyclohexane is introduced, along with the concept of chair flips and the preference for substituents to occupy equatorial positions to minimize steric hindrance. The episode emphasizes the importance of understanding cyclohexane conformations due to their role in a wide range of organic compounds, from carbohydrates to pesticides. It concludes with an encouragement to practice drawing cyclohexanes and a teaser for the next episode on stereochemistry.

Takeaways
  • 🔬 Hexagons are prevalent in nature and have practical applications, such as in pencil manufacturing, due to their space-efficient properties.
  • 📚 The arrangement of carbon atoms in hexagonal patterns contributes to the hardness of diamond and the unique properties of graphite.
  • 🧪 Cycloalkanes, including cyclohexanes, are named based on the number of carbons in the ring, with prefixes indicating the presence of substituents.
  • 🔍 Cycloalkanes exhibit different conformations due to restricted rotation around their bonds, leading to distinct cis and trans isomers.
  • ⚖️ Ring strain in cycloalkanes arises from angular strain (deviation from the ideal bond angle) and torsional strain (eclipsed conformations).
  • 🔥 Organic chemists use bomb calorimetry to measure the energy released during combustion, which helps quantify ring strain.
  • 📉 Cyclohexane and cyclopentane rings have low ring strain due to their bond angles being close to the ideal angle for sp3 hybridized carbons.
  • 🧬 The chair conformation of cyclohexane is particularly stable as it allows for all hydrogens to be staggered and bond angles to be at 109.5 degrees.
  • 🔄 Chair flips in cyclohexane involve the interconversion between axial and equatorial positions of substituents without breaking bonds.
  • 💊 The conformation of cyclohexane derivatives, such as methylcyclohexane, affects their energy levels and steric interactions, with equatorial positions often being more stable.
  • 📈 Bulky substituents on cyclohexane rings are favored in equatorial positions to minimize steric hindrance and diaxial strain.
  • 📝 Practice is essential for understanding the drawing and manipulation of cyclohexane conformations, which is crucial for comprehending their role in organic chemistry.
Q & A

  • What is the common feature shared by Saturn, a diamond, and Giant’s Causeway in Ireland?

    -The common feature is the presence of hexagonal shapes.

  • Why are hexagons considered a space-saving shape?

    -Hexagons are space-saving because they allow for more efficient packing, as demonstrated by the example of pencil companies being able to pack more hexagonal pencils in a box.

  • How does the arrangement of carbon atoms in diamond contribute to its hardness?

    -The close-packed hexagonal arrangement of carbon atoms in diamond contributes to its hardness due to the strong covalent bonds between the carbon atoms.

  • What are the two types of strain that contribute to ring strain in cycloalkanes?

    -The two types of strain are angular strain, which is the deviation from the ideal bond angle of 109.5 degrees for sp3 hybridized carbons, and torsional strain, which results from bonds being in an eclipsed conformation.

  • How did organic chemists initially figure out ring strain?

    -Organic chemists initially figured out ring strain by setting compounds on fire in combustion reactions and measuring the heat produced, a method known as bomb calorimetry.

  • What is the average energy released per CH2 unit when burning a very long acyclic alkane?

    -The average energy released per CH2 unit is 658.6 kiloJoules per mol.

  • Why is the chair conformation of cyclohexane considered more stable and free of ring strain?

    -The chair conformation is more stable because it allows all hydrogens to be staggered and bond angles to be at 109.5 degrees, minimizing both angular and torsional strain.

  • What is a chair flip in the context of cyclohexane molecules?

    -A chair flip is the process by which cyclohexane molecules switch between different chair conformations without breaking bonds, by rotating the carbon-carbon bonds to shift the positions of the hydrogens and substituents.

  • Why is the equatorial position of a methyl group in methylcyclohexane more energetically favorable?

    -The equatorial position is more energetically favorable due to less steric hindrance and no diaxial strain, as the methyl group does not experience the crowding effects that occur in the axial position.

  • What is the significance of understanding the different conformations of cyclohexane?

    -Understanding the different conformations of cyclohexane is important because it helps chemists predict the reactivity and stability of molecules, which is crucial for the design of drugs, the synthesis of complex organic compounds, and the study of biological molecules.

  • How can one convert a flat skeletal structure of cyclohexane to a chair drawing when there are substituents?

    -One should first identify the positions of the substituents in the flat structure, then number the carbons in the same orientation (clockwise or counterclockwise), and finally draw the chair structure with the substituents in their correct axial or equatorial positions based on the reference numbers.

Outlines
00:00
🔍 Introduction to Hexagons and Cyclohexane

The video begins with an introduction to the prevalence of hexagons in nature and their space-saving properties. Deboki Chakravarti, the presenter, explains the hexagonal structure of various natural and man-made objects, including Saturn's clouds, diamonds, and tortoise shells. The discussion then shifts to the scientific reasons behind the ubiquity of hexagons, which is still a topic of debate. The video delves into the arrangement of carbon atoms in diamonds and graphite, highlighting the role of hexagons in their respective properties. The focus then narrows to cyclohexane, a molecule of interest in organic chemistry, and how it is named and structured. The concept of ring strain in cycloalkanes is introduced, explaining how organic chemists have historically used combustion to study the energy stored in chemical bonds and to understand ring strain through the heat released during such reactions.

05:02
🧬 Conformations of Cyclohexane

This paragraph explores the dynamic nature of cyclohexane, a molecule that can exist in different conformations due to the lack of free rotation around its bonds. The presenter describes how cyclohexane achieves a stable, low-strain structure called the chair conformation, where all hydrogens are staggered, and bond angles are at the ideal 109.5 degrees. The video explains the concept of axial and equatorial positions for hydrogens in the chair conformation and how these positions affect the molecule's energy and stability. The difference between the chair and boat conformations is discussed, along with the process of a chair flip, which allows the molecule to switch between these conformations without breaking bonds. The influence of substituents, such as methyl groups, on the stability of cyclohexane conformations is also covered, emphasizing the preference for equatorial positions to minimize steric interactions and diaxial strain.

10:05
📚 Drawing Cyclohexanes and Substituent Effects

The final paragraph focuses on the practical aspect of drawing cyclohexane and its derivatives, particularly when there are substituents present. It emphasizes the importance of maintaining the correct spatial arrangement of substituents when transitioning from a flat skeletal structure to a three-dimensional chair representation. The process of numbering carbons, placing axial and equatorial bonds, and performing chair flips to achieve energetically favorable conformations is detailed. The video also touches on the impact of larger substituents, such as isopropyl and tert-butyl groups, on the stability of cyclohexane conformations, noting that equatorial positions are increasingly favored as the size of the substituents increases. The presenter concludes by encouraging practice and the use of molecular models to better understand cyclohexane's conformations and their significance in various organic compounds, including carbohydrates, steroids, and pesticides.

Mindmap
Keywords
💡Hexagons
Hexagons are six-sided polygons that are prevalent in nature and in various man-made structures. They are known for their space-efficient packing, which is why they are found in honeycombs, turtle shells, and even when bubbles are pushed together. In the context of the video, hexagons are highlighted as a common shape in organic chemistry, particularly in the arrangement of carbon atoms in diamonds and graphite, which contributes to their physical properties.
💡Cyclohexane
Cyclohexane is a cycloalkane, which is a type of hydrocarbon with a ring of carbon atoms. It is significant in organic chemistry due to its unique molecular structure that allows for different conformations, such as the chair conformation, which is crucial for understanding the molecule's stability and reactivity. The video focuses on cyclohexane's properties, its conformational flexibility, and how these conformations influence its role in organic chemistry.
💡Ring Strain
Ring strain refers to the energy stored in a cyclic molecule due to the deviation from the ideal bond angles and bond conformations. In the video, it is explained that cycloalkanes like cyclohexane and cyclopentane have low ring strain, which contributes to their stability. The concept is illustrated by discussing angular strain and torsional strain, and how they affect the molecule's energy and stability.
💡Chair Conformation
The chair conformation is a specific three-dimensional arrangement of cyclohexane where the carbon atoms are not in one plane, but rather in a 'puckered' structure that resembles a beach chair. This conformation is important because it minimizes ring strain and allows all hydrogen atoms to achieve an ideal tetrahedral angle. The video describes how to draw the chair conformation and emphasizes its stability compared to other conformations.
💡Axial and Equatorial Positions
In the context of cyclohexane's chair conformation, axial and equatorial positions describe the orientation of hydrogen atoms or substituents around the ring. Axial hydrogens are perpendicular to the plane of the ring, while equatorial hydrogens are roughly in the plane. The video explains that certain substituents, like methyl groups, prefer the equatorial position due to lower energy and less steric hindrance.
💡Diaxial Strain
Diaxial strain is a type of steric strain that occurs when two large groups in a cyclohexane ring are forced to occupy axial positions and are positioned directly across from each other. This leads to crowding and increased energy, making the molecule less stable. The video illustrates this concept by discussing how larger substituents, like tert-butyl groups, are energetically favored in equatorial positions to avoid diaxial strain.
💡Cis and Trans Isomers
Cis and trans isomers are stereoisomers that result from restricted rotation around the double bonds in cycloalkanes. The video explains that when two substituents are on the same side of the ring, the prefix 'cis-' is used, and when they are on opposite sides, 'trans-' is applied. This distinction is important for understanding the physical and chemical properties of the molecules.
💡Bomb Calorimetry
Bomb calorimetry is a scientific technique used to measure the heat of combustion of a substance in a closed, oxygen-filled vessel, known as a bomb. The video uses this concept to explain how organic chemists quantified ring strain by comparing the heat released during the combustion of different hydrocarbons. The more heat released by a cycloalkane compared to an acyclic alkane indicates higher ring strain.
💡Conformational Isomerism
Conformational isomerism refers to the different three-dimensional shapes that a molecule can take on while maintaining the same connectivity of atoms. In the video, this concept is central to understanding how cyclohexane can exist in various conformations, such as the chair, boat, and twisted forms, and how these conformations interconvert through processes like chair flips.
💡Steric Hindrance
Steric hindrance occurs when the size or position of a molecular group impedes the reaction or interaction of that molecule with other molecules. The video discusses how steric hindrance impacts the stability of cyclohexane derivatives, particularly when larger groups like methyl or tert-butyl groups are in axial positions, leading to unfavorable interactions and higher energy states.
💡Methylcyclohexane
Methylcyclohexane is a derivative of cyclohexane with a methyl group (CH3) as a substituent. The video explores how the position of this methyl group can affect the molecule's conformation and energy. It is shown that the equatorial position of the methyl group in methylcyclohexane is favored due to lower energy and less steric interaction compared to the axial position.
Highlights

Hexagons are found in various natural structures, such as Saturn's appearance, diamonds, and Giant's Causeway in Ireland.

Hexagonal shapes are space-efficient, as demonstrated by the pencil industry's use of hexagonal pencils to maximize storage.

The arrangement of carbon atoms in diamonds and graphite features hexagons, contributing to their unique properties.

Cyclohexane is a key molecule in organic chemistry, with its structure and properties being the focus of this episode.

Cycloalkanes are named with the prefix 'cyclo-' and are numbered to give substituents the lowest possible numbers.

Cycloalkanes have two distinct faces due to the lack of free rotation around their bonds, leading to cis and trans isomers.

Cyclopentanes and cyclohexanes are common due to their low ring strain, which results from less angular and torsional strain.

Organic chemists used combustion reactions and bomb calorimetry to study ring strain and the energy stored in chemical bonds.

Cyclohexane rings have no ring strain and can achieve a chair conformation with all bond angles at 109.5 degrees.

The chair conformation of cyclohexane allows for staggered hydrogens and reduced torsional strain.

Cyclohexane molecules constantly switch between chair conformations, a process known as a chair flip.

Substituents in cyclohexane prefer equatorial positions over axial positions due to lower energy and reduced steric hindrance.

Methylcyclohexane molecules predominantly exist in the chair conformation with the methyl group in the equatorial position.

The stability of cyclohexane conformations can be understood through Newman projections, which show hydrogen positions.

Diaconal strain occurs when axial substituents are forced into close proximity, increasing the molecule's energy.

Cyclohexane's conformational flexibility is crucial for understanding its role in various organic compounds, such as carbohydrates and steroids.

Practicing drawing cyclohexanes and using molecular models can help visualize and understand their conformational changes.

Transcripts

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Related Tags
Organic ChemistryCyclohexaneConformationsRing StrainSteric HindranceChemical StabilityMolecular DrawingChair ConformationAxial PositionEquatorial PositionDaiaxial StrainMolecular Function