Organics 17 Flashcards
Optical isomerism
- a form of stereoisomerism
- non-superimposable mirror images
- occurs when a molecule contains a chiral atom
Chiral atom
- an atom which is attached to four different groups
- aka. asymmetric
- results in the formation of two enantiomers
Enantiomers
• two versions of an optical isomer
Racemic mixture
- equal number of each enantiomer
* does not affect polarised light
Polarimeter
• uses plane polarised light to detect racemic mixtures
Polarised light
- light passed through a sheet of Polaroid
- all the waves are vibrating in the same plane
- monochromatic
Chiral compounds rotate…
- the plane of polarisation (R-clockwise; S-anti-clockwise)
* in a racemic mixture, there is no effect; no net rotation of plane-polarised light
Optical àctivity
- affects polarised light
* provides evidence for reaction mechanisms
SN2
- 1° (and 2°) halogenoalkane hydrolysis
- this reaction inverts the stereochemistry
- if the reactant is optically active, so will the product be
- they will have the same asymmetric arrangements
SN1
- 3° (and 2°) halogenoalkane hydrolysis (inductive effect for alkyl group)
- nucleophile can attack trigonal planar carbocation intermediate from either side with equal probability
- generates a racemic mixture; optically inactive
Carbonyl groups are…
…polar, due to the electronegativity of the O
Carbonyl BP
- alcohol > carbonyl > alkane
- carbonyls higher than alkane because they have a dipole
- alcohol can hydrogen bond within the molecule
Carbonyl solubility
- alcohol > carbonyl > alkane
- alkanes can only form London forces
- carbonyls can hydrogen bond with water
- alcohols can form more hydrogen bonds per molecule
Carbonyl oxidation in alkaline conditions
• carboxyl ion + water
Carbonyl réduction
- carbonyl -> alcohol
- nucleophilic addition
- carbon must be fed electrons (in state H-, shown as [H] in eqns)
- reducing agent: LiAlH4 + dry ether
- OR NaBH4 + water
LiAlH4
- v. Strong reducing agent
- reacts dangerously with water
- produces LOADS of H2(g)
- must be dissolved in dry ether/dry ethoxyethane
NaBH4
- strong reducing agent
- doesn’t react with water if alkaline
- dissolved in (m)ethanol + water
Nucleophilic addition
• drawn to δ+ C
Hydrogen cyanide nucleophilic addition
- KCN/NaCN + dilute ethanoic acid
- provides both H+ and CN-, without combining them
- purposed to extend the carbon chain
- cyanide and alcohol groups add across the double bond
- room temperature
- pH 8: allows both HCN and CN- to be present
- if lower, HCN dissociation equilibrium would be too far left; too little CN- for step 1, or vice versa for step 2
Hydrogen cyanide
• colourless and extremely poisonous