Ch 2 - Isomers

Question 1

Isomers?

Answer 1

• compounds that have the same molecular formula but different structures

Question 2

What is the flow chart of isomer relationship?

Answer 2

isomers (same molecular formula, different structure)

• different connectivity: structural (constitutional)
• same connectivity: stereoisomers
• > no bond breaking to interconvert: conformational
• > bond breaking to interconvert: configurational
• -> nonsuperimposable images: enantiomers
• -> no nonsuperimposable images: diastereomers
• –> differ in arrangement around an immovable bond: cis/trans

Question 3

Structural isomers?

Answer 3

• constitutional isomers
• least similar of all isomers
• only thing they share is their molecular formula, meaning molecular weights are the same

Question 4

What is the difference between physical and chemical properties?

Answer 4

• physical: characteristics of processes that don’t change the composition of matter (mp, bp, solubility, odor, color, density
• chemical: reactivity of the molecule with other molecules resulting in composition change

Question 5

Stereoisomers?

Answer 5

• isomers that are not structural
• share the same atomic connectivity (same structural backbone)
• differ in how these atoms are arranged in space

Question 6

Difference between confirmational and configurational isomers?

Answer 6

• conformational: stereoisomers with the same molecular connectivity, but differ in rotation around a single sigma bond
• configurational: stereoisomers with different molecular connectivity that can be interconverted only by breaking bonds

Question 7

Staggered confirmations?

Answer 7

• have groups of 60 degrees apart

- no overlap of atoms along the line of sight

Question 8

Anti-staggered molecules?

Answer 8

• the 2 largest groups are 180 degrees apart and strain is minimized
• antiperiplanar (same plane, opposite sides)
• most energetically favorable type of staggered

Question 9

Gauche staggered molecules?

Answer 9

the 2 largest groups are 60 degrees apart

Question 10

Eclipse conformation?

Answer 10

• have groups directly in front of each other but 120 degrees apart
• to convert anti to gauche

Question 11

Totally eclipsed conformation?

Answer 11

the 2 largest groups are directly in front of each other with 0 degrees apart and strain is maximized
- highest energy, molecules spends least time in this state

Question 12

Difference between angle strain, torsional strain, and nonbonded strain?

Answer 12

• angle: created by stretching or compressing angles from their normal size
• torsional: from eclipsing or gauche conformations
• nonbonded: from interactions between substituents attached to nonadjacent carbons

Question 13

Axial v equatorial?

Answer 13

• substituents attached to cyclohexanes classified as such
• axial: sticking up or down from the plane of the molecule (create more non-bonded strain)
• equatorial: in the plane of the molecule

Question 14

What plane does the largest substituent take in cyclohexane molecules with multiple substituents?

Answer 14

usually will take the equatorial position to minimize strain

Question 15

Configurational isomers?

Answer 15

can only be interchanged by breaking and reforming bonds

Question 16

Enantiomers?

Answer 16

• 2 molecules with nonsuperimposable mirror images of each other and thus have opposite stereochemistry at every chiral carbon
• have the same chemical and physical properties except for rotation of plane-polarized light and reactions in a chiral environment

Question 17

Optical activity?

Answer 17

• the ability of a molecule to rotate plane-polarized light
• d- or (+) molecules rotate light to the right (clockwise)
• l- or (-) molecules rotates light to the left
• can only be determined experimentally
• molecule must have chiral center and lack plane of symmetry to be optically active

Question 18

Racemic mixtures?

Answer 18

• with equal concentrations of 2 enantiomers, will not be optically active because the 2 enantiomers’ rotations cancel each other out
• ambidextrous

Question 19

Meso compounds?

Answer 19

• contains chiral centers but also has an internal plane of symmetry (achiral, will not rotate plane polarized light)
• will also be optically inactive because the 2 sides of the molecule cancel each other out

Question 20

Diastereomers?

Answer 20

• non mirror image stereoisomers
• when a molecule has 2+ sterogenic centers and differ at some, but not all, chiral centers
• have different chemical/physical properties

Question 21

Cis/trans isomers?

Answer 21

• geometric isomers
• subtype of diastereomers in which groups differ in position about an immovable bond (such as double bond or in a cycloalkane)
• cis (same side), trans (different sides)

Question 22

Chiral centers?

Answer 22

have 4 different groups attached to central carbon

Question 23

Difference between Relative and Absolute configuration?

Answer 23

• relative: gives the stereochemistry of a compound in comparison to another molecule (determines enantiomers, diastereomers, or the same molecule)
• absolute: gives the stereochemistry of a compound without having to compare to other molecules

Question 24

What rules does the Absolute configuration follow?

Answer 24

• Cahn-Inglod-Prelog priority rules
• priority is given by looking at the atoms connected to the chiral 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
• Z: if the highest priority substituents are on the same side of the double bond
• E: if on opposite sides

Question 25

How are stereocenter configurations determined?

Answer 25

• by putting the lowest priority group in the back (towards page) and drawing a circle to form group 1 or 2 or 3 in descending priority
• if the circle (highest to lowest) is clockwise (R)
• if it is counterclockwise (S)

Question 26

How are Fischer diagrams oriented?

Answer 26

• vertical lines go into the plane of the page (dashes)
• horizontal lines come out of the plane of the page (wedges)
• switching one pair of substituents inverts the sterochemistry of the chiral center
• switching 2 pairs retains the stereochemistry
• rotation 90 degrees inverts the stereochemistry of the chiral center
• rotating 180 degrees retains stereochemistry

Question 27

What are the 3 main conformations of cyclohexane?

Answer 27

• chair (most stable)
• twist
• boat

Question 28

What flips can cyclohexanes undergo?

Answer 28

• chair flip: one chair form is converted to the other

- then all axial groups become equatorial and all equatorial groups become axial

Question 29

When is an object considered to be chiral?

Answer 29

• if its mirror image cannot be superimposed on the original object
• handness (right hand cannot fit in left hand glove)
• carbon with 4 different substituents

Question 30

When is an object achiral?

Answer 30

• have mirror images that can be superimposed (a fork is identical to its mirror image)
• carbon with 3 different substituents

Question 31

How can the amount of possible stereoisomers be determined from the number of chiral centers?

Answer 31

n chiral centers = 2^n possible steroisomers