Ch 2 - Isomers Flashcards
Isomers?
- compounds that have the same molecular formula but different structures
What is the flow chart of isomer relationship?
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
Structural isomers?
- constitutional isomers
- least similar of all isomers
- only thing they share is their molecular formula, meaning molecular weights are the same
What is the difference between physical and chemical properties?
- 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
Stereoisomers?
- isomers that are not structural
- share the same atomic connectivity (same structural backbone)
- differ in how these atoms are arranged in space
Difference between confirmational and configurational isomers?
- 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
Staggered confirmations?
- have groups of 60 degrees apart
- no overlap of atoms along the line of sight
Anti-staggered molecules?
- the 2 largest groups are 180 degrees apart and strain is minimized
- antiperiplanar (same plane, opposite sides)
- most energetically favorable type of staggered
Gauche staggered molecules?
the 2 largest groups are 60 degrees apart
Eclipse conformation?
- have groups directly in front of each other but 120 degrees apart
- to convert anti to gauche
Totally eclipsed conformation?
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
Difference between angle strain, torsional strain, and nonbonded strain?
- 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
Axial v equatorial?
- 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
What plane does the largest substituent take in cyclohexane molecules with multiple substituents?
usually will take the equatorial position to minimize strain
Configurational isomers?
can only be interchanged by breaking and reforming bonds
Enantiomers?
- 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
Optical activity?
- 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
Racemic mixtures?
- with equal concentrations of 2 enantiomers, will not be optically active because the 2 enantiomers’ rotations cancel each other out
- ambidextrous
Meso compounds?
- 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
Diastereomers?
- 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
Cis/trans isomers?
- 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)
Chiral centers?
have 4 different groups attached to central carbon
Difference between Relative and Absolute configuration?
- 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
What rules does the Absolute configuration follow?
- 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
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)
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
What are the 3 main conformations of cyclohexane?
- chair (most stable)
- twist
- boat
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
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
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
How can the amount of possible stereoisomers be determined from the number of chiral centers?
n chiral centers = 2^n possible steroisomers
