Chapter 4: Nomenclature and Conformations of Alkanes and Cycloalkanes

Question 1

What is the Index of Hydrogen Deficiency?

Answer 1

The index of hydrogen deficiency is used to determine the number of molecules of H2 needed to be added to a structure to obtain the corresponding saturated acyclic alkane
ie) A ring structure will correspond to 1 unit of IHD

Question 2

Nomenclature of Bridged and Fused Rings (Bicyclic Compounds)?

Answer 2

1. The ring is named according to the number of carbon atom it contains in the ring
2. Add bicyclo in front of the corresponding alkane. A structure with 8 carbons will be named bicyclooctane
   - Common carbon atoms in both rings are called bridgeheads, and the other carbon atoms are bridges
3. Use brackets in the name to indicate the number of carbon atoms in each bridge starting with the highest bridge length and then proceed to the lower bridge length
   - Fused rings do not have bridges

Question 3

What are Some Conformations of Alkanes?

Answer 3

Conformations are different forms of a certain molecule that is generated by rotation of a single bond

• Conformation Analysis is used to study the energy changes that happen when these rotations occur
  1. The Newman projection and Sawhorse are commonly used for these analysis

Question 4

What is a Staggered Newman Projection?

Answer 4

Staggered Conformation is more stable than the eclipsed conformation

• Due to complementary overlap of the sigma bond orbitals of the C-H bond, and the unoccupied antibonding sigma orbitals of the adjacent carbon
  1. Torsional barrier between the staggered and eclipsed form is 12kJ/mol

Question 5

Conformations of Butane?

Answer 5

1. In Butane there are no torsional strains in the anti conformation because the methyl groups are as far from each other. Most stable form
2. Gauche Conformations have the methyl groups that are close to each other, which will generate repulsion forces due to electron clouds of the methyl group
3. Eclipsed conformations will be the least stable structure. It has a higher energy in the potential energy diagram, because both methyl groups are eclipsed and this causes a large repulsive dispersion force

Question 6

What is Torsional Strain?

Answer 6

Arises when bonds are not ideally staggered

Question 7

What is Angle Strained?

Answer 7

Arises when the angle is not ideally tetrahedral geometry

Question 8

Conformations of Cyclohexanes?

Answer 8

1. The chair conformation is the most stable form among the cyclohexane (No torsional and angle strain, 109.5° )
2. A Boat conformation is less stable and has a higher energy than the chair, but it contains no angle strain, only torsional strain (This structure has two eclipsed bonds (torsional strain) and two hydrogen atoms)
3. The boat conformation can flex into the twist conformation to relieve some torsional strain
   - Twist boat is more stable than the boat, but less stable than the chair

Question 9

What happens when a cyclohexane undergoes a Ring flip?

Answer 9

A ring flip will make all the axial bonds become equatorial and vice versa

Question 10

What is the most stable chair conformation?

Answer 10

The most stable chair conformation is when the largest groups occupy the equatorial positions