Topic 3: isomers Flashcards
Define isomers
Chemical compounds = same chemical formula but different structure
Types of isomers
1) Constitutional/structural
2) Stereoisomers
> Configurational
- Geometric
- Optical
> Conformational
Explain constitutional isomers
- Same chemical formula
- Different connections
- E.g. butane = 2 isomers = 2-methylpropane
- E.g. pentane = 3 isomers = isopentane + neopentane
- E.g. C4H8O = aldehydes + ketones
- E.g. C4H10O = alcohols + ethers
Define tautomers
- Structural isomers form via internal rearrangement of atoms in molecule
- Equilibrium between forms
- E.g. Keto form of acetone where a CH3 loses a H = given to C=O > C-OH + CH2 > enol form
What is the difference between structural isomers + tautomers?
- Tautomers related to eachother through internal rearrangement
- Traditional structural isomers = don’t undergo internal rearrangement
Explain stereoisomers
- Same chemical formula + connections
-Different 3D structure
1) Conformational
2) Configurational
Define conformational isomers
- Differ in 3D arrangement = via rotation about single covalent bonds
1) Eclipsed
2) Staggered
Describe Newman projections
- Look across C-C bond in conformational isomers
STAGGERED: - Angle between C-X in front = 120
- Angle between X in front + X in back = 60
ECLIPSED: - Front + back bonds = close to each other = more repulsion
Describe torsional strain
- Interaction between front and behind atoms = torsional strain
- Difference in energy between staggered + eclipsed = 12kJ
- Eclipsed more repulsion = due to torsional strain
- H>H strain in eclipse = 4kJ
Describe the butane C2-C3 rotation
- CH3+CH3 = eclipsed
- CH3+CH3 opposite each other = anti
- CH3+CH3 next to each other = repulsive energy = steric strain = gauche
Define geometric isomers
- Alkenes + cycloalkanes
Describe alkene stereoisomers
- C=C each carbon connected to 2 different things
- Cis = 2 groups on the same side
- Trans = 2 groups opposite each other
- E.g. if 1 of the C= has 2 H it cannot have cis/trans isomers
Describe the labeling system for non-H group alkene stereoisomers
- If 2 higher priorities are on the same side = Z
- If 2 higher priorities are on opposite sides = E
Describe Cahn-Ingold-Prelog rules for sequencing
1) Look at 1st connective atom = higher atomic number = higher priority
2) If 1st is same go to next atom to get priority
3) Atoms with multiple bonds = same number of single bonded atoms e.g. -H-C=O > -H-C-O-O-C
- The more oxidized the C = higher priority
Explain the BP for cis/trans cycloalkanes
CIS:
- Can interact closely from the H side = can get close
- Higher LDF = higher BP
TRANS:
- Both sides have large -CH3 = difficult to get close between both molecules
- Low LDF = lower BP than cis
Define optical isomers
- Enantiomers = mirror images = cannot be superimposed
Define chiral center
- Atom bonded to 4 different groups
- This type of atom will have 2 optical isomers = mirror images
Explain the polarimeter
- Unpolarized light = 360
- Passes through polarizer = only 1 plane passes through = plane-polarized light
- Light enters sample = tube of single optical isomer
- Passes through solution = light angle rotates = clockwise/anticlockwise depending on optical isomer
- When light exits to find new angle use 2nd polarizer + rotate until light passes through
Describe how chiral centers are classified
- Clockwise = dextrorotatory/+
- Anticlockwise = levorotatory/-
- Can only find via polarimeter
Describe chiral center configurations
- In order to tell optical isomers apart
- R = right
- S = left
- Doesn’t define anti/clockwise rotation of light
Explain Cahn-Ingold-Prelog rules for chiral configuration
1) Assign priorities to the atoms attached directly to the chiral carbon = priorities in order of decreasing atomic number
2) If cannot base off 1st atom then see next
Describe Fischer projections
1) Assign priorities
2) Lowest priority placed on top by:
- Rotating 180
- 1 group held in place while other 3 rotated
3) Determine rotation from 1>2>3
- If clockwise R
- If anticlockwise S
Define diastereomer
Has 1 chiral configuration same and 1 different
Define meso compound
- Internal plane of symmetry between internal chiral carbons
- No net optical activity
