October 29 - Isomers and Chirality

Question 1

Cis-trans isomers

Difference between cis and trans

Answer 1

Cis has substituents on same side of molecule, trans has substituents on opposite sides

Isomers that result from restricted rotation

Question 2

Difference between conformation and configuration

Answer 2

Conformation: same compound with different spatial arrangement that can’t be separated

Configuration: compounds with stereoisomers which are called different configurations. They can be separated from each other by breaking bonds

Imagine a dog standing vs. sitting. That’s a different conformation.
Now imagine a dog with its tail switched with its leg. That’s a different configuration
But both of these are still dogs

Question 3

E,Z system for naming isomers

What constitutes E and what constitutes Z?

Answer 3

E isomer has high priority groups on opposite sides of the double bond

Z isomer has high priority groups on same side of the double bond

4.2 page 121 in textbook

Question 4

How to determine priority for E,Z system

(for naming isomers)

Answer 4

1. For atoms bonded directly to the sp2 carbon, higher atomic number = higher priority
2. If the atoms bonded to the sp2 are the same, consider the atomic number of atoms bonded to the “tied” atoms
3. Double bonded atom treated as if it were single bonded to two of those atoms, triple bonded atom treated as if bonded to three

For 3, cancel atoms that are identical in the two groups, use the remaining atoms to determine the group with higher priority

Question 5

Enantiomer

Answer 5

A stereoisomer that has a non superimposable mirror image

Chiral

Question 6

Diastereomers

Answer 6

Stereoisomers that are not mirror images (not enantiomers)

Question 7

Achiral

Answer 7

Has a superimposable mirror image

Question 8

Asymmetric center

Also called a chiral center, stereogenic center, or a stereocenter

Answer 8

An atom bonded to four different groups

Question 9

Perspective formula

Answer 9

An enantiomer where two of the bonds are drawn with a solid and dashed line

The solid and dashed lines must be drawn next to each other

Question 10

Naming enantiomers using the R,S system

Answer 10

1. Rank the groups bonded to the asymmetric center in increasing order of priority till 3rd highest
2. If the group/atom w/ lowest priority is bonded by a hatched wedge, draw arrows from the highest to lowest groups
3. If lowest priority group (4) not bonded by hatched wedge, interchange with the group that has a hatched wedge

Priority also determined by atomic number like for the E,Z system

For 2, arrows pointing clockwise are R and arrows pointing counterclockwise are S

Steering wheel visualization trick: S is the direction you’d turn the wheel left, R is the direction you’d turn for right

Question 11

Difference between enantiomers and diastereomers

In terms of properties

Answer 11

Enantiomers have the same physical and chemical properties (except the way they interact w/ polarized light) and react at the same rate with achiral agents

Diastereomers have different physical and chemical properties and react at different rates with achiral agents

Question 12

Isomer

Answer 12

Each of two or more compounds with the same formula but that differ in spatial or structural arrangement

Question 13

Stereoisomers

Answer 13

isomers that differ in spatial arrangement of atoms, rather than order of atomic connectivity

Question 14

Constitutional isomers

Answer 14

Isomers with different structural formulas, or bonding arrangements

Question 15

Meso compound

Answer 15

Has two or more asymmetric centers and a plane of symmetry; achiral

Plane of symmetry can be horizontal or vertical

If a compound with two asymmetric centers has the same four groups bonded to each of the asymmetric centers, one of its stereoisomers will be meso

Question 16

Cyclic compounds with stereoisomers

What will the cis and trans isomers be? Meso or enantiomers?

Answer 16

The cis isomer will be meso
The trans isomer will be a pair of enantiomers

Question 17

Ways the same molecule can look different

Answer 17

1. Rotation around a single (NOT double) bond
2. Rotation of the entire molecule

An example of 1: a pentane will still have 5 carbons, but they don’t just have to be in a zigzag pattern

Question 18

Cis and trans alkenes

Where does the functional groups have to be to be considered cis/trans?

Answer 18

Cis alkene: Two functional groups are on the same side of the double bond
Trans alkene: Two functional groups are on the different sides of the double bond