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
Naming cyanide
- if you have the opportunity, ALWAYS use the suffix -nitrile, where the carbon in the cyanide counts in the chain
- if the prefix is cyano- , the carbon in the changed
HCN dissociation equilibrium
HCN H+ + CN-
Optical activity of the product of nucleophilic addition
- carbonyl compound is planar; attack can occur from either side with equal probability
- unless the carbonyl is a ketone with two identical R groups, the carbonyl compound will become chiral
- a racemic mixture will be formed
Testing for methyl ketones
- C=O on 2nd C
- produced from oxidising alcohols
- warm
- iodoform test
Iodoform test
- methyl ketones + iodine -> tri-iodomethane (pale yellow ppt.)
- CH3COR + 3I2 + 4NaOH -> CHI3 + 3NaI + RCOONa + 3H2O
Grignard Reagents
- réactive organic compounds containing magnesium
* used to extend the carbon chain
Production of Grignard Reagents:
- halogenoalkane + magnesium
- RBr + Mg -> RMgBr
- the resulting carbon is Cδ-, which acts as a nucleophile and à reducing agent, allowing C-C formation
- 100% atom economy
- low % yield: oxide layer on magnesium must be gently scratched/brown down with UV, otherwise it prevents reaction
Nucleophiles
• easily react with δ+
Grignard + dry ice (CO2)
-> carboxylic acid
Grignard + carbonyl ->
Alcohol
• because carbonyl is planar, racemic mixture is formed
Conditions for Grignard formation
- diethylether solvent -> non-acidic, no protonation
- dry, dehydrated
- 40°C, heat under reflux
- acidify (form Mg salt to remove) -> must be added after, to hydrolyse magnesium alkoxide (O-MgBr) bond
Carboxylic acid BP
• > alcohol because they have two electronegative oxygen atoms; Cδ+ allows for stronger hydrogen bonding
RCOOH solubility
- > alcohol
* up to C4H9COOH are miscible due to hydrogen bonding
RCOOH prep
- oxidation of 1° alcohols
* hydrolysis of nitriles
Nitrile hydrolysis in acid
- heat under reflux
* RCN + 2H2O + HCl -> RCOOH + NH4Cl
Nitrile hydrolysis in alkali
- heat under reflux
- RCN + H2O + NaOH -> RCOONa + NH3
- then add a strong acid to salt
- RCOONa + HCl -> RCOOH + NaCl
Carboxylic acid reduction
- -> 1° alcohol
- LiAlH4 in ether and water as reducing agent
- RCOOH + 4[H] -> RCH2OH + H2O
RCOOH + ROH
- low yield due to equilibrium
- -> Ester + water
- needs conc. H2SO4
- warmed
- RCOOH + ROH -> RCOOR + H2O
R* cannot be just hydrogen
Ester naming
- prefix -> alcohols
- suffix -> carboxylic acid
- -yl -oate
Ester uses
- flavourings
* solvents
Ester Prep
1) add reactants + H2SO4; heat under reflux
2) crude distillation
3) place distillate in separating funnel w/ Na2CO3(aq); removes acidic impurities. Release pressure.
4) acid removed (seen as bubbles)
5) discard Na2CO3 layer
6) add CaCl2 (aq) to remove remaining alcohol
7) discard CaCl2 (aq)
8) add anhydrous CaCl2 (s) to remove water
9) filter through mineral wool into pear-shaped flask
10) distill w/thermometer; collect product
Crude distillation
Without a thermometer
Ester hydrolysis
- boil w/ acid or alkali (saponification)
- RCOOR* + H2O -> RCOOH + R*OH
- RCOOR* + NaOH -> RCOONa + R*OH
Acyl chlorides production
- RCOOH + halogenating reagents -> -anoyl chloride
- room temperature
• RCOOH + PCl5 -> ROCl + POCl3 + HCl
Halogenating reagents
- PCl5
- PCl3
- SOCl2
Acyl chloride reactions
- highly reactive (good leading group)
- readily hydrolyse at room temperature
- fume in moist air
Acyl chloride hydrolysis
• RCOCl + H2O -> RCOOH + HCl
Acyl chloride + alcohols
• RCOCl + ROH -> RCOOR + HCl
Acyl chloride + conc. NH3
- RCOCl + NH3 -> RCONH2 + HCl
* RCONH2 -> acid amide
Acyl chloride + amine
- RCOCl + RNH2 -> RCO-NH-R + HCl
* RCO-NH-R* = 2° substituted amine
Acid strength
- strongest RCOOHs have electronegative atoms; withdraw electrons from COOH (allows H to be more easily released)
- weakest RCOOHs have longest chains (stabilising positive inductive effect on electronegativities)
Carboxylate ions
- charge sits between the oxygen atoms
* conjugate base of a carboxylic acid (lost H+)
