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
