17 - Organic II Flashcards
why do we get optical isomerism
- chiral centre
- in a molecule with asymmetrical carbon atoms
what are optical isomers
enantiomers which are object and non-superimposable mirror images
how do you identify enantiomers
- pass plane polarised light through the beaker
- will rotate the plane by equal angles in opposite directions
what is a racemic mixture
a mixture with equal amounts of each enantiomer
optical isomers in SN1 reactions
- carbocation is planar
- nucleophile can attack from either side
- makes a racemic mixture
optical isomers in SN2 reactions
- nucelophile always attacks the carbon from the opposite side to the group that’s leaving
- always end up with the opposite optical isomer
solubility of aldehydes and ketones
- soluble
- the O can form hydrogen bonds with water
melting and boiling points of aldehydes and ketones
- low
- can’t form intermolecular hydrogen bonds
what can aldehydes and ketones be oxidised and reduced to
- ketones can be reduced to a secondary alcohol
- aldehydes can be reduced to a primary alcohol
- aldehydes can be oxidised to carboxylic acids
draw mechanism for reduction of aldehydes and ketones
- LiAlH4 dissolved in dry ether
- check mechanism
iodoform reaction
- reagent is iodine in the presence of alkali
- tests for methylketones, ethanal and secondary alcohols
- gives pale yellow precipitate with antiseptic smell
- check reaction
2,4-DNPH reaction
- tests for aldehydes and ketones
- forms yellow/orange precipitate
- can identify the compound by filtering the ppt, purify by recrystallisation, measure melting point, compare to known data.
draw the mechanism for nucleophilic addition of KCN
- check mechanism
- CN- acts as a catalyst
how can you make HCN for the nucleophilic addition reaction
- H2SO4 + KCN –> KHSO4 + HCN
Tollens’ reagent reaction
- tests for aldehydes
- is an alkaline solution of Ag+
- warm with R-CHO –> R-COOH –> R-COO-
- forms a silver mirror on the test tube due to formation of Ag(s)
how to make Tollens’ reagent
- NaOH + AgNO3 (aq) –> Ag2O (s)
- Ag2O + NH3 –> [Ag(NH3)2]+
Fehling’s/ Benedict’s Solution reaction
- tests for aldehydes
- is an alkaline solution of Cu2+
- warm with R-CHO –> R-COOH –> R-COO-
- Cu2+ –> Cu2O
- blue solution to red precipitate
oxidation of aldehydes back to carboxylic acids
- acidified potassium dichromate (VI) K2Cr2O7
- Cr2O7- –> Cr3+
- orange solution to green solution
physical properties of carboxylic acids
- good at hydrogen bonding due to two lone pairs on each oxygen
- small ones soluble, but not large as only functional group can hydrogen bond
- can form dimers (draw) resulting in twice the number of london forces, so high boiling points
two ways to make a carboxylic acid
- hydrolysis of a nitrile, heat under reflux with dilute HCl
- oxidise aldehydes or alcohols, add acidified potassium dichromate
- heat under reflux for alcohols
reduction of a carboxylic acid
- add LiAlH4 in dry ether
- goes from carboxylic acid –> aldehyde –> alcohol
carboxylic acid + PCl5
- makes an acyl chloride, replacing OH with Cl and POCl3
how to make an acyl chloride
- carboxylic acid + PCl5
- produces acyl chloride and POCl3
write out equation for ethanoic acid + NaOH
check
write out equation for ethanoic acid + CaO
check
slow way to make an ester
- carboxylic acid + alcohol –> ester + H2O
- requires concentrated H2SO4 catalyst
- reversible reaction so incomplete and low yield
what are esters used for
flavourings and perfumes
fast way to make an ester
- acyl chloride + alcohol –> ester + HCl
- fast and complete
acidic hydrolysis of an ester
- ester + H2O –> alcohol + carboxylic acid
- slow as it needs dilute H2SO4 catalyst
- incomplete so low yield
alkaline hydrolysis of an ester
- ester + NaOH –> salt + alcohol
- add HCl to turn salt into carboxylic acid
- fast and complete reaction
how to make a polyester
condensation reaction with an alcohol and carboxylic acid, each with 2 functional groups on each
