Ch. 4: Isomers

Question 1

What are the two types of isomers? Explain.

Answer 1

Isomers are compounds with the same molecular formula but different structures. The two kinds of isomers are constitutional isomers and stereoisomers. A constitutional isomer differs in the way their atoms are connected, like ethanol and dimethyl ether. The atoms in stereoisomers are connected in the same way but differ in their atoms arranged in space.

Question 2

What are the two kinds of stereoisomers? Explain.

Answer 2

Conformational isomers and configurational isomers. Conformational isomers are stereoisomers that rapidly interconvert at room temperature, and because they interconvert, they can’t be separated. Configurational isomers are stereoisomers that cannot interconvert unless covalent bonds are broken. Because they cannot interconvert, configurational isomers can be separated.

Question 3

What are the two types of conformational isomers? Explain.

Answer 3

1) conformers due to rotation about C–C single bonds: staggered, eclipsed, chair, boat, etc.

2) those due to amine inversion

Question 4

What are the two types of configurational isomers? Explain.

Answer 4

1) cis-trans isomers: result from restricted rotation

2) isomers that contain asymmetric centers

Question 5

What causes restricted rotation in cis-trans isomers?

Answer 5

cause by either a cyclic structure or a double bond

Question 6

Can compounds with C–C double bonds have cis and trans isomers?

Answer 6

Yes. But rotation about a double-bond can only occur if the pi bond breaks–that is ,only if the pi bonds are no longer parallel. Consequently, the energy barrier to rotation about a C–C double bond is much greater than the energy barrier to rotation about a C–C single bond: 62 kcal/mol vs. 2.9 kcal/mol

Question 7

What does “E” represent in the E,Z system?

Answer 7

the E isomer has the high-priority groups on opposite sides

Question 8

What does “Z” represent in the E,Z system?

Answer 8

the Z isomer has the high-priority groups on the same side

Question 9

What’s the first step in determining the relative properties of E,Z isomer groups?

Answer 9

1) the relative properties depend on the atomic numbers of the atoms bonded directly to the sp2 carbon. The greater the atomic number, the higher the priority.

Question 10

What’s the second step in determining the relative properties of E,Z isomer groups?

Answer 10

2) If the two atoms attached to an sp2 carbon are the same (there is a tie) then consider the atomic numbers of the atoms that are attached to the “tied” atoms.

Question 11

What’s the third step in determining the relative properties of E,Z isomer groups?

Answer 11

3) If an atom is doubly bonded to another atom, the priority system treats it as if it were singly bonded to two of those atoms. If an atom is triply bonded to another atom, it is treated as if it were singly bonded to three of those atoms

Question 12

What’s the fourth step in determining the relative properties of E,Z isomer groups?

Answer 12

4) If two isotopes (atoms with the same atomic number but different mass numbers) are being compared, the mass number is used to determine the relative properties

Question 13

What is the cause of chirality in a molecule? Explain.

Answer 13

An asymmetric center. An asymmetric center is an atom bonded to four different groups

Question 14

How many stereoisomers can exist for a compound with one asymmetric center?

Answer 14

two stereoisomers

Question 15

What is an enantiomer?

Answer 15

molecules that are non superimposable mirror images of each other, is chiral

Question 16

What is a stereocenter?

Answer 16

kind of like an asymmetric center, it is a atom at which the interchange of two groups produces a stereoisomer. It includes both (1) asymmetric centers, where the interchange of two groups produces an enantiomer, and (2) the sp2 carbons of an alkene or the sp3 carbons of a cyclic compound. (all asymmetric centers are stereocenters, but not all stereocenters are asymmetric centers)

stereocenters are three or more groups, and asymmetric centers are four

Question 17

How do you draw an enantiomer?

Answer 17

1) show two of the bonds to the asymmetric center in the plane of the paper (make sure the two bonds are adjacent to each other)
2) show one bond as a solid wedge protruding forward out of the paper
3) show the fourth bond as a hatched wedge extending behind the paper (the solid and hatched wedges must be adjacent to each other and the solid wedge must be below the hatched wedge.)

Question 18

What is a Fischer projection?

Answer 18

• horizontal lines represent the bonds that project out of the plane of the paper toward the viewer
• vertical lines represent the bonds that extend back from the plane of the paper away from the viewer
• the carbon chain is usually drawn vertically, with C-1 at the top

Question 19

What are the two letters used to name enantiomers?

Answer 19

R and S

Question 20

How do you name an enantiomer?

Answer 20

Question 21

How do you name a compound drawn as a Fischer Projection?

Answer 21

Question 22

How do you recognize a pair of enantiomers?

Answer 22

Question 23

What does it mean for a compound to be optically active?

Answer 23

a compound that rotates the plane of polarization of a plane-polarized light, a chiral compound

Question 24

What does it mean for a compound to be optically inactive?

Answer 24

a compound that does’t rotate the plane of polarization of a plane-polarized light, an achiral compound

Question 25

What does it mean for a compound to be dextrorotatory?

Answer 25

when an optically active compounds rotates the plane of polarization clockwise, is indicated by the prefix +

Question 26

What does it mean for a compound to be levorotatory?

Answer 26

when an optically active compound rotates the plane of polarization counterclockwise, indicated by the prefix -

Question 27

What is the only way to tell if a compound is dextrorotatory(+) or levorotatory(-)?

Answer 27

to put the compound in a polarimeter

Question 28

What is a polarimeter?

Answer 28

an instrument that measures the direction and amount an optically active compound rotates the plane of polarization of plane-polarized light

Question 29

How do you calculate the specific rotation of an optically active compound?

Answer 29

Question 30

What does it mean for a compound to enantiomerically pure and what is its observed specific rotation?

Answer 30

only one enantiomer is present, +23.1

Question 31

If a sample is a racemic mixture, what will its observed specific rotation be?

Answer 31

0

Question 32

What does it mean if a sample has an observed specific rotation that’s positive but less than +23.1?

Answer 32

it means the sample is a mixture of enantiomers but contains more S enantiomer than R enantiomer, because the S enantiomer is dextrorotatory

Question 33

What is enantiomeric excess and how do you calculate it?

Answer 33

also called optical purity, tells us how much of an excess of one enantiomer is in the mixture

Question 34

How do you calculate the number of stereoisomers a compound can have?

Answer 34

a compound can have a maximum number of 2^n stereoisomers, where n equals the number of asymmetric centers (provided it doesn’t also have stereoisomers that would cause it to have cis-trans isomers)

Question 35

Classify the stereoisomers of a compound with the same four atoms or groups bonded to two different asymmetric centers.

Answer 35

there will be three stereoisomers, one will be a meso compound, and the other two will be enantiomers

Question 36

How do you draw a perspective formula for a compound with two asymmetric centers?

Answer 36

Question 37

What is a receptor?

Answer 37

a protein that binds a particular molecule. and because proteins are chiral, a receptor binds one enantiomer better than the other.

Question 38

What is a racemic mixture?

Answer 38

a mixture of equal amounts of two enantiomers

Question 39

What are diastereomers?

Answer 39

Stereoisomers that aren’t enantiomers. They aren’t identical and are non-superimposable on each other. Diastereomers occur when two or more stereoisomers of a compound have different configurations at one or more of the equivalent stereocenters.

Question 40

What are erythro enantiomers?

Answer 40

When Fischer projections are drawn for stereoisomers with two adjacent asymmetric centers, the enantiomers with the hydrogens on the same side of the carbon chain are erythro enantiomers

Question 41

What are threo enantiomers?

Answer 41

When Fischer projections are drawn for stereoisomers with two adjacent asymmetric centers, the enantiomers with the hydrogens on opposite sides are called the threo enantiomers

Question 42

What is a mesocompound?

Answer 42

When a stereoisomer has two asymmetric centers but the same groups on each so is achiral.