Topic 17 - Organic 2 Flashcards
What is a chiral carbon
A carbon attached to 4 different groups in a compound
What is optical isomerism?
A form of isomerism where enantiomers rotate plane polarised light in opposite directions by the same amount
What are enantiomers?
A pair of molecules containing one chiral centre which are non-superimposable mirror images of each other
What name is given to an equal mixture of two enantiomers and what is the effect on plane polarised light?
A racemic mixture/ racemate
No overall effect on plane-polarised light
Why can a racemic mixture be formed by an Sn1 reaction?
- The C+ intermediate has a trigonal planar shape around the C+
- The OH- nucleophile equally likely to attack from the left or the right
- creates two enantiomers in a 50/50 mixture
Why can a racemic mixture be formed by a nucleophilic addition reaction?
- The aldehyde or ketone has a trigonal planar shape around the carbonyl carbon (C=O)
- The nucleophile equally likely to attack from the left or the right
- creates two enantiomers in a 50/50 mixture
Why are short chain miscible with water but long chains are not?
Before mixing:
- water forms hydrogen bonds between molecules
- aldehydes form permanent dipole-dipole forces
When mixed:
- short chains can use the carbonyl to form H-bonds with water that are similar in strength the the H-bonds in water, so do mix
- long chains mainly form London forces with the long non-polar alkyl chain and water. Much weaker than the H-bonds between water molecules, so they don’t mix
Reagents, conditions, products and mechanism:
Reduction of aldehydes and ketones
Reagent: LiAlH4
Conditions: dry ether. Follow with acid/water work up
Product: primary alcohol (aldehyde) or secondary alcohol (ketone)
Mechanism: nucleophilic addition
Reagents, conditions, products and mechanism:
Cyanohydrin formation (aldehydes and ketones reacting with CN-)
Reagent: KCN (aq)
Conditions: H+ cat. Follow with acid/water work up
Product: hydroxynitrile (has an OH and CN group)
Mechanism: nucleophilic addition
What is a Grignard reagent?
How do you make one?
Why is it useful?
R - Mg - X
(eg. CH3CH2 - Mg - Br)
React a halogenoalkane with Mg in dry ether and heat
Can be used to increase carbon chain length
Reagents, conditions, products and mechanism:
Reaction of aldehydes and ketones with a Grignard reagent
Reagent: Grignard reagent (haloalkane + Mg)
Conditions: dry ether. Follow with acid/water work up.
Product: longer chain alcohol
Mechanism: nucleophilic addition
Reagents, product and positive test:
Iodoform reaction
Tests for a methyl ketone (CH3-C=O)
Reagents: I2 + NaOH
Product: Iodoform
Positive test: Yellow ppt (iodoform), antiseptic smell
What chemical tests differentiate between an aldehyde and a ketone?
Tollen’s
- ammoniacal silver nitrate
- aldehyde gives silver mirror (Ag+ is reduced to Ag(s))
- ketone gives no change
Fehling’s
- Fehling’s solution
- aldehyde makes blue solution turn into brick red ppt
- ketone gives no change
What is a positive test with 2,4-DNP and what is it used for?
- used to test for aldehydes and ketones
- positive test: orange solution to orange ppt
- the melting point of the recrystallised 2,4-DNP derivative can be used to identify the specific aldehyde or ketone
Give two ways to make a carboxylic acid
1) Oxidation of a primary alcohol or aldehyde
Reagents + conditions: K2Cr2O7, H2SO4 cat.
Positive test: Orange to green
2) Hydrolysis of a nitrile
Reagents + conditions: water, H+ cat. (HCl), heat
Give 3 properties of carboxylic acids
- Weak acids
- High boiling points (can form two hydrogen bonds per molecule)
- Miscible with water up to 4 carbons (butanoic acid)
Explain the difference in boiling points of butane, propanal and propanoic acid
All have London forces
Propanal and propanoic acid can both form permanent dipole-dipole forces
Propanoic acid can form 2 H-bonds per molecule
Propanoic acid can also create a dimer which increases the size of London forces
Reagents, conditions and product:
Reduction of a carboxylic acid
Reagents: LiAlH4
Conditions: dry ether, followed by acid work up
Product: primary alcohol
Reagents, conditions and product:
Making an ester from carboxylic acids
Reagents: an alcohol, conc. H2SO4 cat.
Conditions: heat under reflux
Product: ester
Give two ways to make an acyl chloride
- PCl5 (must be dry conditions)
Products: acyl chloride, POCl3 + HCl - SOCl2
Products: acyl chloride, SO2 (g), HCl (g)
This way is better as other products are gases so easier to separate
How do you turn an acyl chloride back into a carboxylic acid?
Add H2O
Reagents, conditions and product:
Making an ester from an acyl chloride
Reagents: alcohol
Conditions: room temp, pyridine (to neutralise HCl produced)
Product: ester
Reagents, conditions and product:
Making an amide from an acyl chloride
Reagents: conc. ammonia
Conditions: room temp, NaOH (to neutralise HCl produced)
Product: amide
Reagents, conditions and product:
Making a substituted amide from an acyl chloride
Reagents: an amine
Conditions: room temp, NaOH (to neutralise HCl produced)
Product: N-substituted amide
Give the equations for 3 reactions which show carboxylic acids reacting as acids
- R-COOH + metal –> carboxylate salt + H2
- R-COOH + base –> carboxylate salt + H2O
- R-COOH + metal carbonate –> carboxylate salt + H2O + CO2
Reagents, conditions and products:
Acid hydrolysis of an ester
Reagent: aqueous HCl
Conditions: Hot
Products: carboxylic acid and alcohol
Reversible reaction so low yield!
Reagents, conditions and products:
Base hydrolysis of an ester
Reagent: aqueous NaOH/ KOH
Conditions: Hot
Products: carboxylate salt and alcohol
Not reversible reaction so high yield!
Why are acyl chlorides more reactive than carboxylic acids
Carbonyl carbon is more electrophilic in acyl chloride.
Electron density very high around C in carboxylic acid due to overlap of C=O pi bond and O p-orbital.
C-Cl bond in acyl chloride is longer so no overlap and so electron density around the carbonyl C is lower so easier for nucleophiles to attack.
What type of polymers are polyesters?
Condensation polymers
Properties of condensation polymers
Ester bond easily hydrolysed so biodegradable
