chapter 2: isomers

What are cis-trans isomers?

A subtype of diastereomers where groups differ in position about an immovable bond.

What is the definition of a chiral center?

A central carbon with four different groups attached.

What does relative configuration indicate?

The stereochemistry of a compound compared to another molecule.

What does absolute configuration stand for?

The stereochemistry of a compound without comparison to others.

What rules are used for absolute configuration?

Cahn-Ingold-Prelog priority rules based on atomic numbers.

How is an alkene classified as (Z) or (E)?

(Z) if highest-priority substituents are on the same side; (E) if on opposite sides.

How is a stereocenter's configuration determined?

Lowest priority group in the back; circle drawn from 1 to 2 to 3.

What configuration is indicated if the circle drawn is clockwise?

(R) configuration.

What configuration is indicated if the circle drawn is counterclockwise?

(S) configuration.

What do vertical lines in Fischer diagrams represent?

Lines going into the plane of the page (dashes).

What do horizontal lines in Fischer diagrams signify?

Lines coming out of the plane of the page (wedges).

What happens when you switch one pair of substituents in a Fischer diagram?

Inverts the stereochemistry of the chiral center.

What is the effect of switching two pairs of substituents in a Fischer diagram?

Retains the stereochemistry.

What does rotating a Fischer diagram 90° do?

Inverts the stereochemistry of the chiral center.

What does rotating a Fischer diagram 180° do?

Retains the stereochemistry.

What do structural isomers share?

Only a molecular formula.

How do structural isomers differ?

They have different physical and chemical properties.

What characterizes conformational isomers?

Differ by rotation around a single bond.

What are staggered conformations?

Groups 60° apart in a Newman projection.

What are anti staggered molecules?

Two largest groups 180° apart.

What are gauche staggered molecules?

Two largest groups 60° apart.

Describe eclipsed conformations.

Groups directly in front of each other.

What are totally eclipsed conformations?

Two largest groups directly in front of each other.

What causes strain in cyclic molecules?

Angle strain, torsional strain, and nonbonded strain.

How do cyclohexane substituents differ?

Axial (up or down) vs. equatorial (in plane).

What affects strain in cyclohexane molecules?

Largest substituent usually takes equatorial position.

What are configurational isomers?

They can only be interchanged by breaking and reforming bonds.

Define enantiomers.

Nonsuperimposable mirror images with opposite stereochemistry at chiral carbons.

What is optical activity?

Ability to rotate plane-polarized light.

How do d- or (+) molecules interact with light?

Rotate light to the right.

How do l- or (−) molecules interact with light?

Rotate light to the left.

What is a racemic mixture?

Equal concentrations of two enantiomers that are not optically active.

What are meso compounds?

Compounds with an internal plane of symmetry, optically inactive.

What are diastereomers?

Non-mirror-image stereoisomers that differ at some but not all chiral centers.

What do diastereomers differ in?

Different chemical and physical properties.