Chapter 2: Isomers

Question 1

Isomers

Answer 1

compounds with the same molecular formula as another, but differ in arrangement or connectivity of their atoms

Question 2

Structural / constitutional isomers

Answer 2

share only their molecular formula (same atoms)

differ in their structures (connectivity of atoms)

different chemical and physical properties

Question 3

Physical properties and examples

Answer 3

aspects of a compound that do not play a role in changing chemical composition

ex. melting point, boiling point, solubility, odour, colour, density

Question 4

Chemical properties and examples

Answer 4

aspects of a compound that change chemical composition

have to do with the reactivity of the molecule with other molecules

dictated by the reactivity of functional groups

Question 5

in organic chem, the chemical properties of a compound are generally dictated by the ——- in the molecule

Answer 5

functional groups

Question 6

stereoisomers

Answer 6

same chemical formula AND same connectivity

differ in how these atoms are arranged in space (their wedge and dash pattern)

Question 7

2 categories of stereoisomers

Answer 7

conformational isomers

configurational isomers

Question 8

conformational isomers (conformers)

Answer 8

the same molecule, just at different points in their natural rotation around single (sigma) bonds

interconvert by simple bond rotation

recall: single bonds are free to rotate

Question 9

configurational isomers

Answer 9

have differing connectivity and can only be interconverted by breaking bonds

Question 10

Newman projections

Answer 10

a drawing that helps visualize the 3-dimensional structure of a molecule

there are drawn looking straight down a carbon-carbon bond

Question 11

name the 2 conformations of straight chain conformational isomers

Answer 11

staggered and eclipsed

Question 12

2 types of staggered conformation

Answer 12

anti conformation: the largest groups are opposite (180 degrees apart)

gauche conformation: the largest groups are close together (60 degrees apart)

Question 13

Eclipsed conformation

Answer 13

this conformation has groups directly in front of each other

totally eclipsed: highest energy state, 2 largest groups are in the same plane on the same side

Question 14

totally ecliped conformation

Answer 14

the two largest groups are directly in front of each other and strain is at a maximum

Question 15

explain the potential energy vs degree of rotation plot of butane

Answer 15

higher potential energy = less stable

Question 16

3 factors that can result in ring strain

Answer 16

angle strain, torsional strain, steric strain (non bonded strain)

Question 17

angle strain

Answer 17

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

Question 18

torsional strain

Answer 18

results when cyclic molecules must assume conformations that have eclipsed or gauche interactions

Question 19

steric strain (van der waals repulsion)

Answer 19

results when nonadjacent atoms or groups compete for the same space

ex. this is the main type of strain occuring in flagpole interactions

Question 20

what is the dominant source of strain in flagpole interactions

Answer 20

non bonded strain / steric strain

Question 21

what is the most stable conformation of cyclohexane

Answer 21

the chair conformation

Question 22

what are the 3 most common conformations of cyclohexane

Answer 22

chair

boat

twist (scew-boat)

Question 23

what occurs during a chair flip

Answer 23

the chair passes through the “half-chair” conformation

all equatorial groups become axial and vice versa

all dashes and wedges _remain the sam_e (components point UP stay pointing UP)

Question 24

axial vs equatorial

Answer 24

axial (black): group is perpendicular to the plane of the ring (stick up or down)

equatorial (red): parallel to the plane of the ring (stick out)

Question 25

which position do bulky groups favour in a boat conformation

Answer 25

equatorial

Question 26

cis vs trans rings

Answer 26

cis: both groups are on the same side of the ring (up/up or down/down)
trans: both groups are on opposite side of the ring (up/down)

Question 27

how to convert between ring and chair diagrams

Answer 27

wedges = UP

dashes = DOWN

ups alternate from equatorial to axial

Question 28

drawing bonds for chair conformations

Answer 28

each carbon has an “up” and a “down”
the “up”s and “downs” of each type alternate

Question 29

Configurational isomers

Answer 29

can only change from on form to another by breaking and reforming covalent bonds

Question 30

2 categories of configurational isomers

Answer 30

enantiomers & diastereomers

(both are “optical isomers”)

Question 31

optical isomers

Answer 31

compounds with different spatial arrangement of groups which affects the rotation of plane-polarized light

Question 32

chiral compounds

Answer 32

a compound whose mirror image cannot be superimposed on the original compound

lack an internal plane of symmetry

Question 33

achiral compounds

Answer 33

have mirror images that CAN be superimposed

Question 34

chiral centers

Answer 34

carbon atoms with 4 different constituents (these are chiral carbons)

Question 35

characteristic of ALL chiral carbons

Answer 35

have 4 DIFFERENT groups connected

Question 36

enantiomers (4)

Answer 36

2 molecules that are nonsuperimposable mirror images of each other

same connectivity but opposite configurations at EVERY chiral center

same physical and chemical properties (except optical activity and reactions in chiral environments)

rotate plane polarized light to the same magnitude but opposite directions

Question 37

optically active compounds

Answer 37

can rotate plane-polarized light

Question 38

optical activity

Answer 38

the rotation of plane polarized light by a chiral molecule

Question 39

dextrorotatory (d-) compounds

Answer 39

rotate plane-polarized light to the right (clockwise)

labelled (+)

Question 40

levorotatory (l-) compounds

Answer 40

rotate plane-polarized light to the left (counterclockwise)

labelled (-)

Question 41

how is d- vs l- determined

Answer 41

EXPERIMENTALLY!!!

it can NOT be determined from a molecules structure

it is NOT related to a molecules absolute configuration (R vs S)

Question 42

specific rotation formula

Answer 42

Question 43

standard concentration and path length for determining optical activity

Answer 43

concentration = 1 g/ml

path length = 1 dm (10 cm)

Question 44

racemic mixture

Answer 44

a mixture with equal concentrations of + and - enantiomers

no optical activity is observed (they “cancel” each other out)

will NOT rotate plane polarized light

Question 45

diastereomers

Answer 45

chiral molecules which have the same connectivity but are NOT mirror images of each other

must have multiple chiral centers (and differ at some but not all)

have different chemical properties

Question 46

number of possible stereoisomers

for a molecule with n chiral centers

Answer 46

a molecule with n chiral centers has 2^n stereoisomers

ex. I and II are enantiomers, III and IV are enantiomers, the other pairs are diastereomers of one another

Question 47

cis-trans isomers (geometric isomers)

Answer 47

a subtype of diastereomers

substituents differ in their position around an immovable bond (ex. double bond) or a ring

cis: substituents are on the same side of the bond/ring
trans: substituents are on the opposite side of the bond/ring

Question 48

nomenclature used for polysubstitued double bonds

Answer 48

E / Z

Question 49

meso compounds

Answer 49

molecules that have chiral centers but are NOT optically active due to a plane of symmetry within the molecule

it IS superimposable on its mirror image

Question 50

configuration

Answer 50

refers to the spatial arrangement of the atoms or groups in a molecule

Question 51

relative configuration

of a chiral molecule

Answer 51

a chiral molecules configuration in relation to another chiral molecule

used to determine whether molecules are enantiomers, diastereomers, or the same molecule

Question 52

absolute configuration

of a chiral molecul

Answer 52

describes the exact sptial arrangement of the atoms or groups in a chiral molecule, independent of other molecules

Question 53

when is cis-trans used, and when is E/Z used

Answer 53

cis-trans: when there are only 2 substituents other than H
E/Z: when therea re 3 or more substituents other than H

check…

Question 54

(E) and (Z) nomenclature is used for ….

Answer 54

compounds with polysubstituted double bonds (multiple substituents that are not just hydrogen)

Question 55

how to determine (E) vs (Z) form

Answer 55

1. find the groups with the highest priority on each side of the double bond
2. (Z) if both are on the same side; (E) if both are on oppsite sides

Question 56

absolute configuration: R and S forms (STEPS)

Answer 56

step 1: assign priority (by atomic number) to 4 substituents surrounding the chiral center

step 2: if the lowest priority group is on a dash, go directly to step 3; if the lowest priority group is on a wedge, the answer is the OPPOSITE of step 3

step 3: draw a circle connecting substituents 1→3 (clockwise = R, counterclockwise = S)

Question 57

Fischer projections

Answer 57

a method to represent 3D molecules

horizontal lines represent wedges (out of the page)

vertical lines represent dashes (into the page)