Optical isomerism Flashcards
When does optical isomerism occur?
when a carbon atom has 4 different atoms or groups of atoms attached to it, resulting in an asymmetrical carbon/ chiral centre
What are enantiomers?
2 molecules that are non-superimposable isomers that are mirror images of each other
What is the difference between enantiomers?
physical properties the same- but when exposed to plane polarised light, the enantiomers rotate the plane in opposite directions the same amount
Which enantiomer rotates the plane CLOCKWISE?
dextrorotary
Which enantiomer rotates the plane ANTICLOCKWISE?
laevorotary
How is rotation of light measured?
using a polarimeter
What is a 50:50 mixture of equal amounts of enantiomer called?
a racemic mixture/ a racemate
Why is a racemic mixture optically inactive?
contains equal amounts of both enantiomers , so is optically inactive because the optical effects of each enantiomer cancel each other out
What happens when optically active molecules are made in nature?
only one enantiomer is formed
Why is only one enantiomer formed in nature?
enzymes are stereospecific, as they themselves are optically active, so only react either one enantiomer and produce one enantiomer
What are the two ways aldehydes and ketones undergo nucleophilic addition?
a) hydride ion (:H-) from NaBH4 in reduction in aqueous solution
b) the cyanide ion (:CN-) provided by KCN, followed by dilute acid
What is the name of NaBH4?
sodium tetrahydridoborate (III)
What are the products or nucleophilic additon of NaBH4 and aldehydes/ketones
aldehyde- primary alcohol
ketone- secondary alcohol
What happens between aldehydes/ketones and KCN?
undergo nucleophilic addition with dilute acid to form nitriles
When are chiral centres formed from ketones?
when the starting ketone is asymmetrical
Why is potassium cyanide dangerous?
highly toxic, HCN is also extremely toxic, also highly flammable
Why is the -CN useful?
increases length of carbon chain, easily converted into other functional groups once added
What does the reaction of aldehydes/ketones with acidified KCN produce?
a racemic mixture because it has a chiral centre
What is the science behind a racemic mixture?
because the carbonyl group is planar, so susceptible to attack by the :CN- nucleophile is equally likely from either side of the plane
What is a non-specific way of reducing aldehydes/ketones?
reduced with hydrogen in the presence of a nickel catalyst (catalytic hydrogenation) at 150°C
Why is it non-specific?
molecules containing both C=O and C=C will become completely desaturated
Why is the reduction by NaBH4 specific?
nucleophiles cannot attack alkenes (electron rich double bond repels nucleophiles), so they can target aldehydes/ketones
Why are carboxylic aids soluble up to bromine (included)?
can form hydrogen bonds with water
Why do carboxylic acids have a higher boiling point than comparable alkenes?
they can form hydrogen bonds with each other
