Topic 17 - Organic Chemistry 2
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What are optical isomers?
stereoisomers where two molecules have the same molecular formula but different spatial arrangements due to a chiral centre, causing them to be non-superimposable on their mirror image -rotate the plane of polarised light in opposite centres
What is a chiral centre?
an atom surrounded by 4 different atoms/groups
How do properties differ between enantiomers?
• have same physical and chemical properties (eg. mp/bp, viscosity, way they react, etc) • have different biochemical properties (eg. interactions with enzymes and active sites)
How can enantiomers be separated?
by thin layer chromatography (TLC)
What is a racemic mixture?
a solution containing equal quantities of the two enantiomers -is optically inactive (as the enantiomers cancel eachother out)
What is a polarimeter?
an instrument which measures the rotations of monochromatic light to identify +/- enantiomers
Does an SN1 reaction produce optically active or optically inactive products? And why?
optically inactive products • 2 enantiomers are produced (a racemic mixture) • because nucleophile can attack from above or below the plane
Does an SN2 reaction produce optically active or optically inactive products? And why?
optically active products • only 1 enantiomers are produced • because nucleophile can only attack from the opposite side to the leaving group
How do the smells of aldehydes and ketones differ?
aldehydes - bitter/sour smell ketones - sweet smell
Are carbonyl compounds soluble in water?
yes -can H-bond with water -solubility decreases down homologous series as alky chains interfere with H-bonds so an increased chain length so London forces become stronger than H-bonds
What are the melting and boiling points of carbonyl compounds like in comparison to alkanes and alcohols?
higher than alkanes lower than alcohols -PD interactions
What is the test for carbonyl bonds?
Brady’s reagent aka 2,4DNP (2,4 dinitrophenyl hydrazine) +ve result = orange ppt (aldehyde/ketone 2,4 dinitrophenyl hydrazone derivative)
What can be used as a test to distinguish between aldehydes and ketones?
• Fehling’s/Benedict’s reagent (changes to brick red for aldehydes, remains blue for ketones) • Tollen’s reagent (silver mirror formed with aldehydes, not with ketones) • potassium dichromate soln (changes to green for aldehydes, remains orange for ketones)
What is produced when an aldehyde is oxidised?
carboxylic acid
aldehyde + Fehlings ->
carboxylic acid + water
aldehyde + Tollens ->
carboxylic acid + ammonia + silver + water
aldehyde + acidified potassium dichromate ->
carboxylic acid + water + chromium ion (CR3+)
What is produced when an aldehyde is reduced?
primary alcohol
aldehyde + lithium aluminium hydride ->
primary alcohol
aldehyde + hydrogen cyanide ->
hydroxy nitrile
What conditions are required for Fehling’s?
alkali conditions warming
What conditions are required for the reaction between a carbonyl compound and hydrogen cyanide?
pH 8 KCN present
What is produced when a ketone is reduced?
secondary alcohol
ketone + lithium aluminium hydride ->
secondary alcohol