Isomers Flashcards
What are isomers?
Molecules with the same formula but different chemical structure
What is another name for structural isomers?
Constitutional isomers
What are physical properties? Examples?
No change in composition or matter
Ex: boiling point, melting point, solubility, odor, color, density
What are chemical properties? What are these generally attributable to?
Reactivity of molecule resulting in change in composition
To functional groups in the molecule
What do stereoisomers share?
The same structural backbone
By what do stereoisomers differ?
How their atoms are arranged in space
What are the 2 types of stereoisomers?
- Conformational
2. Configurational
Out of all the isomers, which are the most similar?
Conformation stereoisomers
What is another word for conformational isomers?
Conformers
By what do conformers differ?
In rotation around single sigma bonds
What is the antistaggered conformation of conformers?
The two largest groups are antiperiplanar (in the same plane but on opposite sidest
What is the gauche conformation of conformers?
The two largest groups are 60 degrees apart
What is the eclipsed conformation of conformers? What is the angle?
The two largest groups are eclipsed with other groups (120)
What is the totally eclipsed conformation of conformers?
The two largest groups are eclipsed
What is the highest energy state of conformers?
Totally eclipsed conformation
What does the stability of cycloalkanes depend on?
Ring strain
In cycloalkanes, what does ring strain arise from?
- Angle strain
- Torsional strain
- Nonbonded strain
In cycloalkanes, when does angle strain occur?
When bond angles deviate from their ideal values by being stretched or compressed
In cycloalkanes, when does torsional strain occur?
When cyclic molecules must assume gauche or eclipsed conformations
What is another name for nonbonded strain?
Van der Waals repulsion
In cycloalkanes, when does nonbonded strain occur?
When nonadjacent groups compete for the same space
What are the 3 main conformations of cyclohexane?
- Chair
- Boat
- Twist/Skewboat
What is the most stable conformation of cyclohexane?
Chair
In the chair conformation of cyclohexane, what are the 2 types of H atoms?
- Axial: perpendicular to the plane of the chair
2. Equatorial: parallel to the plane of the chair
What happens during a chair flip of cyclohexane?
All axial groups become equatorial and vice versa
On substituded rings, what is the preferred position of bulky groups?
The equatorial position
What are the cis and trans forms of rings?
Cis: bulky groups on same side of the ring
Trans: bulky groups on opposite sides of the ring
What do configurational isomers require to interconvert?
Bond breaking
What are the 2 categories of configurational isomers?
- Enantiomers
2. Diastereomers
What are optical isomers? Why?
Enantiomers and diastereomers because the different spatial arrangement of groups affects the rotation of plane-polarized light
When is an object considered chiral?
When its mirror image cannot be superimposed on the original object
What is the best example of a chiral molecule?
A C with 4 different substituents
What are enantiomers?
Nonsuperimposable mirror images with opposite stereochemistry at each chiral center
What do enantiomers have in common? How do they differ?
Identical physical and chemical properties with two exceptions:
- Optical activity
- Reactivity in chiral environments
When is a compound said to be optically active?
If it has the ability to rotate plane polarized light
What is a levorotatory compound? How is it labelled?
Optically active compound that rotates plane polarized light counterclockwise
Labelled - of l-
What is a dextrorotatory compound? How is it labelled?
Optically active compounds that rotates plane polarized light clockwise
Labelled + or d-
How is the direction of the rotation of an optically active compound be determine?
Experimentally
What is the equation to calculate the amount of rotation of polarized light?
alpha = alpha observed / (c . l) alpha = specific rotation in degrees c = concentration in g/ml l = path length in dm
What is a racemic mixture?
A mixture in which + and - enantiomers are present in equal concentrations
Does a racemic mixture display optical activity? Why?
It does not, because the rotations cancel each other out
What are diastereomers? What does this mean they are required to have?
Non-mirror image configurational isomers, required to have more than one chiral center and they do not differ by all chiral centers
What is the maximum number of optically active isomers/stereoisomers?
2^n
n = number of chiral centers
What do diastereomers have in common? How do they differ?
They have different chemical properties, but might sometimes behave similarly in particular reactions because they have the same functional groups.
They have different physical properties
What were cis-trans isomers used to be called?
Geometric isomers
What are cis-trans isomers?
A sub group of diastereomers in which substituents differ in their position around an immovable bond or around a ring
What is a meso compound?
A molecule with chiral centers with a plane of symmetry
Is a meso compound optically active? Why?
No, because it has a plane of symmetry
What is the relative configuration of chiral molecule?
Its configuration in relation to another chiral molecule
When is the (E) and (Z) nomenclature used?
Compounds with polysubstituted double bonds
How do you determine if the molecule is (E) or (Z)?
- Find the highest priority atom on each C of the double bond (highest atomic number)
- If the two highest priority substituents of each C are on the same side of the double bond: Z
- If the two highest priority substituents of each C are on different sides of the double bond: E
How do you determine the absolute configuration of a chiral center?
- Assign priority by atomic number
- Arrange the molecule with the lowest priority substituent in the back
- Draw a circle around the molecule from highest to lowest priority
- Clockwise = R / Counterclockwise = S
How do you identify a chiral center?
Attached to 4 different substituents
How can you tell if a molecule retains absolute configuration?
The bonds on the stereocenters are not broken
How can you tell if a molecule retains relative configuration?
The reactant and product are both R or both S
