Organic Chemistry Part 1: Conformations of Alkanes and Cycloalkanes

What are Newman projections?

In a Newman projection, molecules are viewed as if one was looking down the axis of a C-C bond.

What is the most stable conformation shown by a Newman projection?

It is the anti staggered where the groups of interest are at 180 degrees from each other in a staggered conformation.

What is the least stable conformation shown by a Newman projection?

It is the syn eclipsed where the groups of interest are at 0 degrees from each other in an eclipsed conformation.

Give the physical properties of cycloalkanes

Melting and boiling points generally increase as the number of carbons increases. Cycloalkanes are only soluble in non polar solvents. Cycloalkanes are less dense than water.

What are the two main directions of substituents for the chair conformation of cyclohexane?

Axial and equatorial.

In the chair conformation of cyclohexane, number the equatorial and axial positions.

Axial up for groups at carbons 1, 3, 5; Axial down for groups at carbons 2, 4, 6. Equatorial down for groups at carbons 1, 3, 5. Equatorial up for groups at carbons 2, 4, 6.

Line-wedge representation what is it?

Bonds pointing away from the viewer are hashed; points pointing towards the viewer are dark wedges and simples lines are in the same plane as the paper.

What the conformation sequence of cyclohexane?

Chair (0 kcal/mole), half-chair (+10.1 kcal/mole), twist-boat (+5.5 kcal/mole), boat (+6.5 kcal/mole), twist-boat, half-chair, chair.

What are the 4 conformations of cyclohexane?

Chair, half-chair, twist-boat, boat.

Name a cycloalkane with zero ring strain

Cyclohexane

Give two examples of cycloalkanes.

Cyclopentane and cyclohexane.

What is an eclipsed Newman projection?

Eclipsed Newman projection means that the dihedral angles are at 0, 120, and 240 degrees.

What is the most stable direction for a bulky substituent on cyclohexane?

Equatorial because it minimizes steric hindrance with other groups.

How do you name cycloalkanes?

First, you add the prefix cyclo- to the corresponding alkane name. Example: cyclopentane Second, substituents attached the the cycloalkane are assigned the lowest possible number. If there are more than one group, they are numbered alphabetically. Third, the term cis is used to describe two substituents located on the same side of the ring whereas trans refers to those positioned on opposite sides of the ring.

What is a staggered Newman projection?

Staggered Newman projection means that the dihedral angles are at 60, 180, 300 degrees.

Name 3 biologically important cycloalkanes

Steroids, terpenes, and prostaglandins.

Explain why anti staggered is the most stable conformation

The anti staggered conformation minimizes steric interactions and repulsion, which stabilizes the molecule.

Give the main chemical properties of cycloalkanes

The carbon atoms are sp3 hybridized and tetrahedral with an angle of 109.5 degrees. Cycloalkanes are mostly unreactive but the ones with increased ring strain due to angle strain are somewhat reactive.

Explain the ring flip of cyclohexane

The chair conformation can undergo a ring flip where substituent groups that were previously axial are equatorial after flipping and vice-versa.

What is the most stable conformation of cyclohexane?

The chair is the most stable, ground state E=0

Explain how to read an energy versus dihedral angle plot

The curve is sinusoidal and represents the alternation of the dihedral angles at which the alkane conformations are the most stable and least stable. Most stable means low energy and least stable means high energy. Staggered occurs at 60, 180, 300 degrees (most stable) and eclipsed occurs at 0, 120, 240, 360 degrees (least stable).

What is the least stable conformation of cyclohexane?

The half-chair, E= +10.1 kcal/mole.

How do I read a Newman projection?

The lines radiating from the center of the circle denote the bonds between the carbon closest to the viewer and its attached atoms, whereas those radiating from the outside of the circle indicate bonds between carbons furthest from the viewer and its attached atoms.

Explain why syn eclipsed is the least stable conformation

The syn eclipsed conformation increases steric interactions and repulsion, which are destabilizing.

What is a cycloalkane?

They are alicyclic compounds or aliphatic cyclic compounds or cyclic alkanes with the formula CnH2n.

How are conformational isomers produced?

They are produced by the rotation around a C-C bond.

How do we represent conformational isomers?

We use Newman projections.

What does zero ring strain mean?

Zero ring strain means that there is no angle, steric, or torsional strain.