Ch 2 - Isomers Flashcards
1
Q
Isomers?
A
- compounds that have the same molecular formula but different structures
2
Q
What is the flow chart of isomer relationship?
A
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
3
Q
Structural isomers?
A
- constitutional isomers
- least similar of all isomers
- only thing they share is their molecular formula, meaning molecular weights are the same
4
Q
What is the difference between physical and chemical properties?
A
- 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
5
Q
Stereoisomers?
A
- isomers that are not structural
- share the same atomic connectivity (same structural backbone)
- differ in how these atoms are arranged in space
6
Q
Difference between confirmational and configurational isomers?
A
- 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
7
Q
Staggered confirmations?
A
- have groups of 60 degrees apart
- no overlap of atoms along the line of sight
8
Q
Anti-staggered molecules?
A
- the 2 largest groups are 180 degrees apart and strain is minimized
- antiperiplanar (same plane, opposite sides)
- most energetically favorable type of staggered
9
Q
Gauche staggered molecules?
A
the 2 largest groups are 60 degrees apart
10
Q
Eclipse conformation?
A
- have groups directly in front of each other but 120 degrees apart
- to convert anti to gauche
11
Q
Totally eclipsed conformation?
A
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
12
Q
Difference between angle strain, torsional strain, and nonbonded strain?
A
- 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
13
Q
Axial v equatorial?
A
- 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
14
Q
What plane does the largest substituent take in cyclohexane molecules with multiple substituents?
A
usually will take the equatorial position to minimize strain
15
Q
Configurational isomers?
A
can only be interchanged by breaking and reforming bonds
16
Q
Enantiomers?
A
- 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
17
Q
Optical activity?
A
- 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
18
Q
Racemic mixtures?
A
- with equal concentrations of 2 enantiomers, will not be optically active because the 2 enantiomers’ rotations cancel each other out
- ambidextrous
19
Q
Meso compounds?
A
- 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
20
Q
Diastereomers?
A
- 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
21
Q
Cis/trans isomers?
A
- 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)
22
Q
Chiral centers?
A
have 4 different groups attached to central carbon
23
Q
Difference between Relative and Absolute configuration?
A
- 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
24
Q
What rules does the Absolute configuration follow?
A
- 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
25
How are stereocenter configurations determined?
- 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)
26
How are Fischer diagrams oriented?
- 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
27
What are the 3 main conformations of cyclohexane?
- chair (most stable)
- twist
- boat
28
What flips can cyclohexanes undergo?
- chair flip: one chair form is converted to the other
| - then all axial groups become equatorial and all equatorial groups become axial
29
When is an object considered to be chiral?
- 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
30
When is an object achiral?
- have mirror images that can be superimposed (a fork is identical to its mirror image)
- carbon with 3 different substituents
31
How can the amount of possible stereoisomers be determined from the number of chiral centers?
n chiral centers = 2^n possible steroisomers
