Chapter 2: Isomers

Question 1

Structural isomers

Answer 1

• Compounds that share only a molecular formula.

- Have different chemical and physical properties

Question 2

Conformational isomers

Answer 2

Compounds that differ by the rotation of a single sigma bond.

Question 3

Types of conformational isomers

Answer 3

1. Staggered - have groups 60* apart. There are 2 types - anti, where the largest groups are 180* apart, and gauche, where they are 60*.
2. Eclipsed - have groups directly in front of each other. When the 2 largest groups are in front of each other they are totally eclipsed (max strain)

Question 4

Physical properties

Answer 4

Characteristics of processes that don’t change the composition of matter, such as boiling point, melting point, solubility, odor, color and density.
-Can be seen and measured

Question 5

Chemical properties

Answer 5

• Properties that become evident only during or after a chemical reaction; ie: any quality that can be established only by changing a substance’s chemical identity.
• the substance’s internal structure must be affected greatly for its chemical properties to be investigated

-Characteristics that have to do with the reactivity of the molecule with other molecules and results in changes in chemical composition.

reactivity with other chemicals.
toxicity.
coordination number.
flammability.
enthalpy of formation.
heat of combustion.
oxidation states.
chemical stability.

Question 6

Intensive property

Answer 6

Property that does not depend on the size or extent of the system, nor on the amount of matter in the object.

Question 7

Extensive property

Answer 7

Properties that show an additive relationship.

Question 8

Stereoisomers

Answer 8

Isomers that not only share the same formula, but also the same connectivity. They differ in their orientation in space.

Question 9

What are 2 types of stereoisomers and how do they differ?

Answer 9

• Conformational s. - differ around in rotation around a single sigma bond. They arise from the fact that varying degrees of rotation can cause different levels of strain.
• Configurational s. - can be interconverted only by breaking bonds.

Question 10

Angle strain

Answer 10

Results when bond angles deviate from their ideal values by being stretched or compressed.

Question 11

Torsional strain

Answer 11

Strain that occurs as a result of cyclic molecules having to assume eclipse or gauche interactions.

Question 12

Nonbonded strain (van der Waals repulsion)

Answer 12

results when non-adjacent atoms or groups compete for the same space. Nonbonded strain is the dominant source of steric strain in the flagpole interactions of the cyclohexane boat conformation.

Question 13

In cyclic conformations, which position is preferred by the largest substituent groups? Why?

Answer 13

• The equatorial position

- This position is preferred because it reduces nonbonded strain/flagpole interactions with axial position substituents.

Question 14

The wedge and dash system, as well as the axial and equatorial system refers to what positions, relative to each other, of substituent groups?

Answer 14

Cis and trans conformations

Question 15

What are the 2 subtypes of conformational isomers?

Answer 15

Diastereomers and enantiomers

Question 16

What defines chirality?

Answer 16

• A chiral molecule is that which cannot be superimposed upon its mirror image.
• The central C is bonded to 4 different substituents.
• The molecule lacks a plane of symmetry.
• Chirality = handedness

Question 17

Enantiomers

Answer 17

• Two molecules that are nonsuperimposable mirror images of each other.
• They have same connectivity but opposite configuration at every chiral center.

Question 18

Distereomers

Answer 18

Chiral molecules that share the same connectivity but are not mirror images of each other.

Question 19

What are 2 differences enantiomers share?

Answer 19

• Rotate plane-polarized light in opposite directions

- React differently in chiral environments

Question 20

What does it mean to be optically active?

Answer 20

A compound is optically active if it can rotate plane-polarized light

Question 21

Dextrorotatory

Answer 21

• Rotates light clockwise

- Labeled d- or (+)

Question 22

Levorotatory

Answer 22

• Rotates light counter clockwise

- Labeled l- or (-)

Question 23

Specific rotation formula

Answer 23

a = a (observed)/c *l

a = specific rotation
c = concentration in g/ml
l = path length in dm

Question 24

Racemic mixture

Answer 24

• Equal concentrations of the (+) and (-) enantiomers in a mixture.
• Rotations cancel each other out and no optical activity is observed.

Question 25

Formula for possible diastereomers

Answer 25

2^n

n = # of chiral centers

Question 26

Cis–trans isomers

Answer 26

A specific subtype of diastereomers in which substituents differ in their position around an immovable bond, such as a double bond, or around a ring structure, such as a cycloalkane in which the rotation of bonds is greatly restricted.

Question 27

What types of molecules contain chiral centers but are not optically active?

Answer 27

Meso compounds

Question 28

What does the relative configuration of a molecule refer to?

Answer 28

The configuration of a chiral molecule in relation to another chiral molecule.

Question 29

What types of molecules contain chiral centers but are not optically active?

Answer 29

Meso compounds

Question 30

What does the absolute configuration of a molecule refer to?

Answer 30

The exact spatial arrangement of atoms or groups in a chiral molecule, independent of other molecules.

Question 31

Cis / trans nomenclature

Answer 31

Nomenclature used to denote simply substituted double-bond containing compounds.

Question 32

In the Cahn/prelog/Ingold system, how are priorities for alkenes determined?

Answer 32

Look at the atoms bonded to the C=C. The atom with the highest atomic number gets highest priority. If they are equal, look to the next atom (and so on) until the tie is broken.

Question 33

What do Z and E nomenclature of alkenes refer to?

Answer 33

• Z = when the 2 highest priority substituents are on the same side (cis).
• E = when the 2 highest priority substituents are on opposite sides (trans).

Question 34

When is the R and S configuration used (for what kind of compounds)?

Answer 34

Used for compound with chiral centers

Question 35

What are the steps to determine absolute configuration?

Answer 35

1. Assign priority by atomic number.
2. Arrange the molecule with the lowest-priority substituent in the back (or invert the stereochemistry by switching two substituents)
3. Draw a circle around the molecule from highest to lowest priority (1 to 2 to 3).
4. Clockwise = (R); counterclockwise = (S)