Topic 17 - Organic Chemistry 2 Flashcards
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
What is produced when a ketone is oxidised?
nothing
ketone + hydrogen cyanide ->
hydroxy nitrile
How do you test for a methyl group adjacent to a carbonyl bond?
iodoform
-iodine in presence of an alkali
+ve result = yellow ppt (CHI3) and antiseptic smell
Which aldehydes give a positive result for iodoform?
only ethanal
Which ketones give a positive result for iodoform?
any ketone with its oxygen on its second carbon
What happens in an iodoform reaction?
- 3 hydrogen atoms on CH3 are substituted with 3 iodine atoms
- the bond between the CI3 and the rest of the molecule is broken
Which primary alcohols give a positive result for iodoform?
only ethanol
- iodine oxidises ethanol to ethanal
- ethanal then gives a positive result
Which secondary alcohols give a positive result for iodoform?
any secondary alcohol with its OH on the second carbon
- iodine oxidises it to a ketone with its oxygen on the second carbon
- this then gives a positive result
Which tertiary alcohols give a positive result for iodoform?
none
Why are HCN and KCN both needed for the reaction between a carbonyl compound and KCN?
- KCN provides CN- for the first step
- HCN provides H+ for the second step
What are the melting and boiling points of carboxylic acids like?
- higher than alkanes and alcohols as each carboxylic acid can form 2 H-bonds with another (alcohols can only form 1 H-bond and alkanes can only form London forces)
- increase down homologous series (as they have more electrons so form stronger London forces)
Are carboxylic acids soluble in water?
yes
- can H-bond with water
- solubility decreases down homologous series as alky chain length increases and interferes with the H-bonds
What is the acidity of carboxylic acids like?
pH 3-4
-weak acids (partially ionise)
How can carboxylic acids be prepared?
- oxidation of alcohols
- oxidation of aldehydes
- hydrolysis of nitriles
What conditions are required to prepare carboxylic acids by oxidising alcohols/aldehydes?
- heat under reflux and distillation
- sodium/potassium dichromate in excess
- conc sulfuric acid
What conditions are required to prepare carboxylic acids by the hydrolysis of nitriles?
- heat under reflux and distillation
- water and hydrochloric acid (for acid hydrolysis)
carboxylic acid + lithium aluminium hydride ->
primary alcohol
carboxylic acid + sodium carbonate ->
salt + carbon dioxide + water
effervescence
carboxylic acid + alcohol ->
ester + water
carboxylic acid + phosphorus pentachloride ->
acyl chloride + hydrogen chloride gas + phosphoryl chloride
steamy fumes
What is produced when a carboxylic acid is reduced?
primary alcohol
acyl chloride + water ->
carboxylic acid + hydrogen chloride
steamy fumes
acyl chloride + alcohol ->
ester + hydrogen chloride
steamy fumes
acyl chloride + conc ammonia ->
amide + ammonium chloride
dense white smoke
acyl chloride + amine ->
N-substituted amide + hydrogen chloride
steamy fumes
What reactions do acyl chlorides readily undergo?
addition-elimination reactions
What are the issues with using acyl chlorides?
- very exothermic rxns
- produce toxic HCl gas (∴ use fume cupboard)
What is a suitable alternative to using acyl chlorides and why?
acid anhydrides
- good yield
- quick rxns
- by-product (carboxylic acid) can be sold
Where does an ester’s carboxylate chain come from?
carboxylic acid
Where does an ester’s alkyl chain come from?
alcohol
Where does an ester’s bridging oxygen come from?
alcohol
What are the melting and boiling points of esters like?
relatively low compared to respective alcohols/carboxylic acids
-no H-bonds
What can esters be used for?
- flavours
- scents (in perfumes, etc)
What is the solubility of esters like?
not very soluble
-smaller esters are more soluble
What is produced in the acid hydrolysis of esters?
carboxylic acid and alcohol
What are the conditions of acid hydrolysis?
- reflux
- dilute acid catalyst (eg. H2SO4 or HCl)
What is produced in the base hydrolysis of esters?
carboxylate salt and alcohol
What are the conditions of base hydrolysis?
- reflux
- NaOH/KOH (base)
How can a carboxylate salt (produced by the base hydrolysis of an ester) be extracted?
by using a dilute strong acid (eg. HCl) in excess
-because it’s insoluble
Is the acid hydrolysis of an ester reversible or non-reversible?
reversible
∴ low yield
Is the base hydrolysis of an ester reversible or non-reversible?
non-reversible
∴ high yield
How can polyesters be produced by condensation polymerisation?
-condensation polymerisation of a diol and a dicarboxylic acid
or
-condensation polymerisation of a monomer containing both an alcohol and carboxylic acid group
