Organic Chemistry 1, Chapter 4-3: Alkanes

Summary

This video script covers the concept of molecular conformations in alkanes, focusing on the different spatial arrangements that result from rotating around a single bond. It introduces **Newman projections** for visualizing these conformations, including **eclipsed**, **staggered**, **gauche**, and **anti** forms. The video emphasizes how torsional strain and steric hindrance influence the stability of these conformations, explaining the most stable configurations and how to calculate strain energies. Through examples like ethane, dichloroethane, and butane, it demonstrates how substituents affect molecular stability and the importance of understanding these concepts in organic chemistry.

Takeaways

• 😀 Conformation refers to the different spatial arrangements of atoms in a molecule, resulting from rotation around single bonds.
• 😀 Newman projection is a method for drawing conformation, where one carbon is represented as a dot (front carbon) and the other as a circle (back carbon).
• 😀 Eclipsed conformation is less stable due to high torsional strain between aligned atoms or groups on adjacent carbons.
• 😀 Staggered conformation is the most stable because atoms or groups are 60° apart, minimizing steric strain.
• 😀 The gauche conformation occurs when two non-hydrogen groups (like methyl and hydrogen) are 60° apart in a staggered arrangement.
• 😀 Anti conformation is the most stable staggered arrangement, where bulky groups are 180° apart from each other.
• 😀 For ethane, the most common conformation is staggered, while the eclipsed conformation is highly strained.
• 😀 Chlorine atoms, due to their size, contribute significantly to torsional strain when they are in eclipsed positions, but anti conformation is the most stable for molecules like dichloroethane.
• 😀 The torsional strain between eclipsed hydrogen-hydrogen pairs is 4 kJ/mol, while eclipsed hydrogen-CH3 pairs have a higher strain of 6 kJ/mol.
• 😀 When calculating the total strain energy, you add the energies for each pair of eclipsing atoms or groups (e.g., 3 hydrogen-CH3 pairs result in a total strain of 18 kJ/mol).
• 😀 In stability ranking, anti conformations are the most stable, followed by staggered, gauche, and finally eclipsed conformations, which are the least stable due to the highest torsional strain.

Q & A

What are the different types of conformations that can exist in alkanes?

The primary types of conformations in alkanes are eclipsed, staggered, gauche, and anti conformations. Eclipsed conformations involve atoms/groups directly aligned with each other, leading to higher torsional strain, while staggered conformations have atoms/groups spaced at 60° angles, minimizing torsional strain. Gauche and anti are specific types of staggered conformations.

Why can't methane have a conformation?

Methane cannot have a conformation because it only has one carbon atom. Conformations are variations in the spatial arrangement of atoms that occur due to rotation around a carbon-carbon bond, and methane, having no second carbon, does not exhibit such rotations.

How does rotating a molecule by 60° affect its conformation?

Rotating a molecule by 60° along the carbon-carbon bond changes the spatial arrangement of the atoms, resulting in a new conformation. In the case of ethane, a 60° rotation from an eclipsed conformation leads to a staggered conformation, where hydrogens or other substituents are positioned 60° apart, reducing steric strain.

What is a Newman projection, and why is it important?

A Newman projection is a method of representing the three-dimensional structure of a molecule by looking directly down the bond between two atoms, typically carbon atoms. It is important for visualizing and analyzing the different conformations of molecules, helping to identify strain and stability in alkanes.

How do you represent the front and back carbons in a Newman projection?

In a Newman projection, the front carbon is represented as a dot (•), while the back carbon is represented as a circle (○). This notation allows for a clear view of the spatial arrangement of atoms or groups attached to both carbons in the bond being analyzed.

What is the difference between eclipsed and staggered conformations?

In an eclipsed conformation, atoms or groups attached to the carbons are aligned directly with each other, leading to increased torsional strain. In a staggered conformation, atoms or groups are offset by 60° from each other, reducing torsional strain and making the conformation more stable.

What is torsional strain, and how does it relate to molecular stability?

Torsional strain is the resistance to rotation around a bond due to repulsion between atoms or groups that are in eclipsed positions. The greater the torsional strain, the less stable the molecule. Molecules with staggered conformations generally have lower torsional strain and are more stable than those in eclipsed conformations.

What is the anti conformation, and when does it occur?

The anti conformation is a specific type of staggered conformation where the largest groups (such as methyl or chlorine) are 180° apart. This arrangement minimizes steric interactions and torsional strain, making the anti conformation the most stable arrangement for bulky substituents.

How does the presence of large substituents, like chlorine, affect the stability of a molecule?

Large substituents, like chlorine, increase steric strain when in eclipsed conformations, making the molecule less stable. However, in staggered or anti conformations, where the large substituents are separated by 60° or 180°, the molecule becomes more stable as steric interactions are minimized.

How do you calculate the total steric strain energy in an eclipsed conformation?

To calculate the total steric strain energy in an eclipsed conformation, you need to add the strain values for each pair of eclipsing atoms or groups. For example, if the strain between hydrogen and methyl groups in an eclipsed conformation is 6 kilojoules per mole and there are three such pairs, the total strain energy would be 18 kilojoules per mole.