Topic 17: Organic II Flashcards
What is optical isomerism
Isomers that contain asymmetric centres which form stereoisomers that differ in their effect on plane polarised light
What is a chiral carbon
A carbon in a compound which has 4 different groups attached to
What are enantiomers
They are mirror images of each other and are non-superimposable
What are some general properties of enantiomers
- same boiling point, same solubility
-differ in how they react with other molecules
-differ in effect on plane-polarised light
How do enantiomers ‘change’ plane-polarised light
An enantiomer will rotate the plane-polarised light either clockwise or anti-clockwise
What is it called when there is no rotation of plane-polarised light
A racemic mixture/ racemate (50/50 mixture of two enantiomers)
What mechanism can form a racemic mixture
Sn1 reaction
Why can a racemic mixture be formed by an Sn1 reaction
-the C+ intermediate is Trigonal planar shape
-allows the Nu- to attack from either side
-this allows for the creation of 2 enantiomers in a racemic mixture
-test by seeing if there is a rotation of plane-polarised light
Why do large aldehydes and ketones (4+ C atoms) become more immiscible with water
-aldehydes and ketones have permanent dipole-dipole interactions between each other
-short aldehydes and ketones can form hydrogen bonds with H2O molecule, (oxygen from carbonyl and H from H2O) - these H bonds are similar in strength to the ones in water, so they will mix
-longer aldehydes + ketones will have more LFs, but smaller amount of H bonding, as chain increases. LFs are very weak than the H bonds in water, so they don’t mix
What are the conditions, reagents and product of the reduction of aldehydes + ketones
Mechanism: nucleophilic addition
Nucleophile: H-
Reagent: LiAlH4 (in dry ether) STRONG. Or NaBH4 MILDER.
Product: alcohol
What are the conditions, reagents and product of the cyanohydrin formation
Mechanism: Nucleophilic addition
Nucleophile: CN-
Reagent: KCN(aq), H+ cat.
Product: hydroxynitrile (OH and CN groups)
(Way of increasing carbon lengthening)
Draw the mechanism for the cyanohydrin formation reaction by nucleophilic addition
CN- attacks carbonyl carbon (on aldehyde or ketone), oxygen accepts electrons from one of the bonds in the double bond becoming negatively charged. electron takes a hydrogen ions (attacks) to form hydroxyl group.
What is the Grignard reaction and what do you need for it
-increases carbon chain length.
STEP 1: form Grignard reagent:
Halogenoalkane + magnesium (dry ether + heat) -> Grignard reagent
STEP 2: reaction with a carbonyl
What is needed for Iodoform reaction + what shows a positive test
-test for a methyl ketone
Reagent: I2 + NaOH
Product: Iodoform, if methyl ketone present (yellow precipitate and antiseptic smell)
What is needed for Tollen’s reaction + what shows a positive test
-test for aldehydes
Reagent: ammoniacal silver nitrate (tollen’s solution)
Observation: silver mirror as aldehyde is oxidised and Ag+ reduced to Ag(s)
What is needed for Fehling’s reaction + what shows a positive test
-test for aldehydes
Reagent: copper (II) sulphate + alkaline potassium sodium tartrate (FEHLING’S SOLUTION)
Observation: colour change from blue to brick red
(Cu2+ reduced to Cu+)
What is needed for 2, 4-DNP reaction + what shows a positive test
-test for aldehydes and ketones
Reagent: 2, 4-DNP
Observation: orange solution -> orange precipitate
-precipitate can then be filtered and recrystallised and melting point measured. Melting point of 2,4-DNP derivative can be used to identify certain aldehyde/ ketone.
How can you prepare a carboxylic acid
1) Oxidation of primary alcohol or aldehyde (K2Cr2O7 + H2SO4 (cat.)), heat under reflux, orange -> green
2) Hydrolysis of nitrile (C≡N)
Reagents: water + H+ catalyst (HCl), heat
What are some properties of carboxylic acids
-they are weak acids: only dissociate a small amount in solution
-high boiling point (can form 2 hydrogen bonds per molecule)
Why does ethanoic acid have the highest boiling point compared to butane, propanal and propan-1-ol
Ethanoic acid has London forces, permanent dipole-dipole interactions and can form 2 hydrogen bonds per molecule
-> it can create a dimer, which also increases the size of the London forces
What are the conditions and reagents for reduction of carboxylic acid to primary alcohol
LiAlH4 (dry ether), followed by acid work-up
What are the conditions and reagents for making esters from carboxylic acids
add alcohol, conc. H2SO4, heat under reflux
-this is a reversible reaction, low yield
What are the conditions and reagents for making an acyl chloride from a carboxylic acid
1) Using PCl5 (MUST BE DRY), also produces POCl3 and HCl
2) Using SOCl2 (BETTER), by-products of SO2 and HCl can be easily removed
What are the conditions and reagents for making a carboxylic acid from an acyl chloride
Add H2O
