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
What are the conditions and reagents for making an ester from an acyl chloride
Add alcohol, room temp (acyl chloride very reactive)
+ pyridine to neutralise the HCl formed
What are the conditions and reagents for making an amide from a acyl chloride
Concentrated NH3, and NaOH to neutralise the HCl formed
What are the conditions and reagents for making a substituted amide from an acyl chloride
Add an amine
What are the conditions and reagents for making a carboxylate salt from a carboxylic acid
1) + metal (MASH)
2) + base
What are the conditions, reagents and product for acid hydrolysis of esters
Hot aqueous acid (HCl)
Reversible - low yield, always in equilibrium
What are the conditions, reagents and product for base hydrolysis of esters
Hot aqueous NaOH/ KOH, heat
One way reaction - high yield
Why are acyl chlorides more reactive than carboxylic acids
In a carboxylic acid: very high electron density, carbon is less electrophilic and Nucleophiles don’t want to attack.
In acyl chlorides: C-Cl bond long enough and electrons density low enough around C atom, making it more electrophilic, so nucleophiles readily attack (only room temperature needed)
What process forms polymers of carboxylic acids
Condensation polymerisation
Properties of condensation polymers
Ester bond can be easily hydrolysed - biodegradable
What are the conditions for alkali hydrolysis of esters
Hot aqueous NaOH
Irriversible reaction (two steps: hydrolysis to form salt + alcohol, acidification of salt to make carboxylic acid)
What is the connection between optical activity and the following mechanisms:
SN1, SN2, Nucleophilic Addition
SN1; no optical activity as carbocation intermediate is trigonal planar: equal probability of attack from above or below the plane of the molecule
SN2; If an enantiopure reactant is used, an enantiopure product is formed due to steric hindrance preventing attack from both sides - i.e, must attack from opposite side of carbon to the halogen
NA; Carbonyl carbon is trigonal planar and therefore there is equal probability of attack from above or below the plane of the molecule
