2: Isomers: Concept Summary

Question 1

______ share only a molecular formula (and molecular weight)

Answer 1

Structural/Constitutional Isomers

Question 2

______ have different physical and chemical properties

Answer 2

Structural/Constitutional Isomers

Question 3

_______ differ by rotation around a single sigma bond

Answer 3

Conformational Isomers

Question 4

______ have groups 60 degrees apart, as seen in a Newman Projection

Answer 4

Staggered

Question 5

_________ have groups directly in front of each other as seen in a Newman Projection.

Answer 5

Eclipsed Conformations

Question 6

In _____________:

• two largest groups are 180 degrees apart (antiperiplanar)
• the strain is minimized
• lowest energy state
• most energetically favorable type

Answer 6

Anti Staggered

Question 7

In _______, the two largest groups are 60 degrees apart

Answer 7

gauche staggered

Question 8

In ________, the two largest groups are directly in front of each other and strain is maximized

Answer 8

totally eclipsed conformations

Question 9

The strain in cyclic molecules comes from

1) ______ (created by stretching or compressing angles from their normal size)
2) ______ (created from eclipsing conformations)
3) _______ (from interactions between substituents attached to nonadjacent carbons)
* Cyclic molecules will usually adopt nonplanar shapes to minimize this strain

Answer 9

1) angle strain
2) torsional strain
3) nonbonded strain

Question 10

Substituents attached to cyclohexane can be classified as ___ (sticking up or down from the plane of the molecule) or ______ (in the plane of the molecule).

Answer 10

Axial, equatorial

Question 11

_____ substituents create more nonbonded strain.

Answer 11

Axial

Question 12

Larger/bulkier substituents favor the _______ position.

Answer 12

equatorial

(In cyclohexane molecules with multiple substituents, the largest substituents will usually take the equatorial position to minimize the strain)

Question 13

________ can only be interchanged by breaking and reforming bonds.

Answer 13

Configurational isomers

Question 14

______ are non superimposable mirror images and thus have opposite stereochemistry at every chiral carbon. They have the same chemical and physical properties except for rotation of plane polarized light and reactions in a chiral environment.

Answer 14

Enantiomers

Question 15

______ refers to the ability of a molecule to roate plane-polarized light: d- or (+) molecules rotate light to the right;
l- or (-) molecules rotate light to the left

Answer 15

Optical activity

Question 16

_______, with equal concentrations of two enantiomers, will not be optically active the two enantiomers’ rotations cancel each other out

Answer 16

Racemic Mixtures

Question 17

_______, with an internal plane of symmetry, will also be optically inactive because the two sides of the molecule cancel each other out

Answer 17

Meso compounds

Question 18

_______ are non-mirror-image stereoisomers.
They differ at some, but not all, chiral centers.
They have different chemical and physical properties.

*May react similarly in particular reactions because they have the same functional groups

Answer 18

Diastereomers

Question 19

_____ isomers are a subtype of diastereomers in which groups differ in position about an immovable bond (such as a double bond or in a cycloalkane)

Answer 19

Cis-trans

Question 20

_____ have four different groups attached to the central carbon

Answer 20

Chiral Centers

Question 21

_______ gives the stereochemistry of a compound in comparison to another molecule

Answer 21

Relative Configuration

Question 22

________ gives the stereochemistry of a compound without having to compare to other molecules

Answer 22

Absolute Configuration

Absolute configuration uses the Cahn-Ingold-Prelog priority rules: priority is given by looking at the atoms connected to the chrial carbon or double-bonded carbons; whichever has the highest atomic number gets highest priority. If there is a tie, one moves outward from the chiral carbon or double-bonded carbon until the tie is broken

Question 23

An alkene is (__) if the highest-priority substituents are on the same side

Answer 23

Z

“Ze Zame Zide”

Question 24

An alkene is (__) if the highest-priority substituents are on opposite sides

Answer 24

E

“Epposite Sides”