Isomers

Question 1

What are isomers?

Answer 1

Molecules with the same formula but different chemical structure

Question 2

What is another name for structural isomers?

Answer 2

Constitutional isomers

Question 3

What are physical properties? Examples?

Answer 3

No change in composition or matter

Ex: boiling point, melting point, solubility, odor, color, density

Question 4

What are chemical properties? What are these generally attributable to?

Answer 4

Reactivity of molecule resulting in change in composition

To functional groups in the molecule

Question 5

What do stereoisomers share?

Answer 5

The same structural backbone

Question 6

By what do stereoisomers differ?

Answer 6

How their atoms are arranged in space

Question 7

What are the 2 types of stereoisomers?

Answer 7

1. Conformational

2. Configurational

Question 8

Out of all the isomers, which are the most similar?

Answer 8

Conformation stereoisomers

Question 9

What is another word for conformational isomers?

Answer 9

Conformers

Question 10

By what do conformers differ?

Answer 10

In rotation around single sigma bonds

Question 11

What is the antistaggered conformation of conformers?

Answer 11

The two largest groups are antiperiplanar (in the same plane but on opposite sidest

Question 12

What is the gauche conformation of conformers?

Answer 12

The two largest groups are 60 degrees apart

Question 13

What is the eclipsed conformation of conformers? What is the angle?

Answer 13

The two largest groups are eclipsed with other groups (120)

Question 14

What is the totally eclipsed conformation of conformers?

Answer 14

The two largest groups are eclipsed

Question 15

What is the highest energy state of conformers?

Answer 15

Totally eclipsed conformation

Question 16

What does the stability of cycloalkanes depend on?

Answer 16

Ring strain

Question 17

In cycloalkanes, what does ring strain arise from?

Answer 17

1. Angle strain
2. Torsional strain
3. Nonbonded strain

Question 18

In cycloalkanes, when does angle strain occur?

Answer 18

When bond angles deviate from their ideal values by being stretched or compressed

Question 19

In cycloalkanes, when does torsional strain occur?

Answer 19

When cyclic molecules must assume gauche or eclipsed conformations

Question 20

What is another name for nonbonded strain?

Answer 20

Van der Waals repulsion

Question 21

In cycloalkanes, when does nonbonded strain occur?

Answer 21

When nonadjacent groups compete for the same space

Question 22

What are the 3 main conformations of cyclohexane?

Answer 22

1. Chair
2. Boat
3. Twist/Skewboat

Question 23

What is the most stable conformation of cyclohexane?

Answer 23

Chair

Question 24

In the chair conformation of cyclohexane, what are the 2 types of H atoms?

Answer 24

1. Axial: perpendicular to the plane of the chair

2. Equatorial: parallel to the plane of the chair

Question 25

What happens during a chair flip of cyclohexane?

Answer 25

All axial groups become equatorial and vice versa

Question 26

On substituded rings, what is the preferred position of bulky groups?

Answer 26

The equatorial position

Question 27

What are the cis and trans forms of rings?

Answer 27

Cis: bulky groups on same side of the ring Trans: bulky groups on opposite sides of the ring

Question 28

What do configurational isomers require to interconvert?

Answer 28

Bond breaking

Question 29

What are the 2 categories of configurational isomers?

Answer 29

1. Enantiomers | 2. Diastereomers

Question 30

What are optical isomers? Why?

Answer 30

Enantiomers and diastereomers because the different spatial arrangement of groups affects the rotation of plane-polarized light

Question 31

When is an object considered chiral?

Answer 31

When its mirror image cannot be superimposed on the original object

Question 32

What is the best example of a chiral molecule?

Answer 32

A C with 4 different substituents

Question 33

What are enantiomers?

Answer 33

Nonsuperimposable mirror images with opposite stereochemistry at each chiral center

Question 34

What do enantiomers have in common? How do they differ?

Answer 34

Identical physical and chemical properties with two exceptions: 1. Optical activity 2. Reactivity in chiral environments

Question 35

When is a compound said to be optically active?

Answer 35

If it has the ability to rotate plane polarized light

Question 36

What is a levorotatory compound? How is it labelled?

Answer 36

Optically active compound that rotates plane polarized light counterclockwise Labelled - of l-

Question 37

What is a dextrorotatory compound? How is it labelled?

Answer 37

Optically active compounds that rotates plane polarized light clockwise Labelled + or d-

Question 38

How is the direction of the rotation of an optically active compound be determine?

Answer 38

Experimentally

Question 39

What is the equation to calculate the amount of rotation of polarized light?

Answer 39

``` alpha = alpha observed / (c . l) alpha = specific rotation in degrees c = concentration in g/ml l = path length in dm ```

Question 40

What is a racemic mixture?

Answer 40

A mixture in which + and - enantiomers are present in equal concentrations

Question 41

Does a racemic mixture display optical activity? Why?

Answer 41

It does not, because the rotations cancel each other out

Question 42

What are diastereomers? What does this mean they are required to have?

Answer 42

Non-mirror image configurational isomers, required to have more than one chiral center and they do not differ by all chiral centers

Question 43

What is the maximum number of optically active isomers/stereoisomers?

Answer 43

2^n | n = number of chiral centers

Question 44

What do diastereomers have in common? How do they differ?

Answer 44

They have different chemical properties, but might sometimes behave similarly in particular reactions because they have the same functional groups. They have different physical properties

Question 45

What were cis-trans isomers used to be called?

Answer 45

Geometric isomers

Question 46

What are cis-trans isomers?

Answer 46

A sub group of diastereomers in which substituents differ in their position around an immovable bond or around a ring

Question 47

What is a meso compound?

Answer 47

A molecule with chiral centers with a plane of symmetry

Question 48

Is a meso compound optically active? Why?

Answer 48

No, because it has a plane of symmetry

Question 49

What is the relative configuration of chiral molecule?

Answer 49

Its configuration in relation to another chiral molecule

Question 50

When is the (E) and (Z) nomenclature used?

Answer 50

Compounds with polysubstituted double bonds

Question 51

How do you determine if the molecule is (E) or (Z)?

Answer 51

1. Find the highest priority atom on each C of the double bond (highest atomic number) 2. If the two highest priority substituents of each C are on the same side of the double bond: Z 3. If the two highest priority substituents of each C are on different sides of the double bond: E

Question 52

How do you determine the absolute configuration of a chiral center?

Answer 52

1. Assign priority by atomic number 2. Arrange the molecule with the lowest priority substituent in the back 3. Draw a circle around the molecule from highest to lowest priority 4. Clockwise = R / Counterclockwise = S

Question 53

How do you identify a chiral center?

Answer 53

Attached to 4 different substituents

Question 54

How can you tell if a molecule retains absolute configuration?

Answer 54

The bonds on the stereocenters are not broken

Question 55

How can you tell if a molecule retains relative configuration?

Answer 55

The reactant and product are both R or both S