Organic Chemistry II Flashcards
Chirality and Optical Isomerism
Chirality occurs in carbon compounds with 4 different groups attached to the carbon
Chiral molecules have similar physical and chemical properties but rotate plane polarised light in different directions
Optical isomerism results from chiral centres in a molecule - when the carbon has 4 different groups attached to the centre
The compounds have non-superimposable mirror images so they cannot match when directly on top of eachother
Optical Activity
An optically active substance will rotate plane-polarised light when a beam of monochromatic light is shone through a solution of an optically active substance
This is the only way to distinguish between two pure samples of enantiomers to find out the direction of rotation
Dextrorotatory (D) enantiomer rotates clockwise (+)
Laevorotatory (L) enantiomer rotates anticlockwise (-)
- Light source produces light vibrating in all directions
- Polarising filter only allows through light vibrating in one plane
- Plane polarised light passes through optically active sample
- Plane polarised light is rotated
- Analysing filter turned so that light reaches maximum
- Direction of rotation is measured coming toward observer
Racemic Mixture
A racemic mixture contains a 50:50 mixture of two enantiomers
Enantiomers are two molecules that are optical isomers of each other
A racemic mixture will not rotate plane-polarised light as the two opposing rotations will cancel each other
They are formed when a trigonal planar reactant or intermediate is attacked from both sides so there is an equal chance of forming each enantiomer
Reaction Mechanism
SN1
During hydrolysis by SN1 mechanism, a planar carbocation intermediate is formed and the OH- ions can approach from either side with equal probability
If the reactant is one isomer of a chiral halogenoalkane, the product will be a racemic mixture of the two optical isomers of the alcohol
SN2
During hydrolysis by SN2 mechanism, a chiral carbon atom undergoes inversion of its stereostructure so if the reactant is an optically active halogenoalkane, it will form an optically active alcohol
Halogenoalkane → alcohol
The different enantiomers of a chiral halogenoalkane, produce different enantiomers of the alcohol
- The nucleophile can only attack from one side
Aldehydes
Aldehydes are formed through the oxidation of primary alcohols
- Form permanent dipole interactions
- Soluble in water
- Polar due to the difference in electronegativity
- Attacked by nutrophiles
- Used in preservatives, flavourings and perfumes
- There is no hydrogen bonding between aldehydes themselves
- In large non-polar hydrocarbon groups, the collective london forces require lots of energy to overcome
- Short chain aldehydes and ketones are water soluble, whereas longer carbon chains are less soluble
- The oxygen on the carbonyl group can hydrogen bond with -OH group in water
- The carbonyl group causes dipole dipole interactions and london forces between molecules
Ketones
Ketones are formed through the oxidation of secondary alcohols
Reagents: hot acidified potassium dichromate
Not easily oxidised further
- Contains a carbonyl group
- Forms permanent dipole interactions
- Soluble in water
- Used in nail polish removers, embalming fluids, perfumes and pesticides
- There is no hydrogen bonding between ketones themselves
- In large non-polar hydrocarbon groups, the collective london forces require lots of energy to overcome
- Short chain aldehydes and ketones are water soluble, whereas longer carbon chains are less soluble
- The oxygen on the carbonyl group can hydrogen bond with -OH group in water
- The carbonyl group causes dipole dipole interactions and london forces between molecules
Reaction with Fehlings solution
Fehling’s solution
- Put 1cm3 Fehling’s solution A and then add Fehling’s solution 2 until the precipitate formed dissolves
- Add 7 drops of the aldehyde/ketone
- Shake gently and place in a beaker of boiling water until not further colour change
Aldehyde
- Blue ppt on heating
- Red ppt forms
- Oxidation occurs
(Cu2+ → Cu1+)
Cu2+ + e- → Cu1+
Ketone
- no change
- cannot be oxidised
Reaction with Potassium Dichromate
Potassium dichromate
- Put 5 drops of the aldehyde/ketone in a test tube
- Add 2 drops of 0.1M potassium dichromate (VI) solution
- Add 10 drops 1M sulphuric acid
- Shake gently and put in a beaker of warm water
Aldehyde
- Turns blue/green
- Can be oxidised
- Chromate is reduced to chromium
Ketone
- no change
- cannot be oxidised
Reaction with Tollens Reagent
Tollens reagent
- Put 1cm3 0.05M silver nitrate into a clean test tube
- Add 3-4 drops 2M sodium hydroxide solution
- Add 2M ammonia solution drop by drop until the precipitate nearly dissolves
- Add 1-2 drops of the aldehyde/ketone and place in a beaker of warm water
Aldehyde
- Silver mirror forms
- Silver ions reduced to silver atoms
Ketone
- no reaction when heated
Reaction with LiAlH4 in Dry Ether (Reduction)
Aldehyde/ketone → Alcohol
LiAlH4 is a powerful reducing agent - it releases H- ions which act as nucleophiles and attack the partially positive carbonyl carbon atom
Must be carried out in the absence of water as it would react very quickly with water
Reaction with Iodine in the presence of Alkali
Shows the presence of a methyl group next to a carbonyl group
- Place 5 drops aldehyde/ketone in a test tube
- Add 1cm3 iodine solution (10% in KI)
- Add 2M sodium hydroxide drop by drop until the colour of the iodine disappears
Aldehyde
- ethenal is the only aldehyde that reacts
Ketone
- straw coloured precipitate of triiodomethane
- 4
Reaction with 2,4-DNPH
Test to show presnce of a Carbonyl group
- Put 1-2 drops of aldehyde/ketone
- Add 2,4-DNPH
Aldehyde:
- Colourless → orange
- Ppt forms when H2SO4 is added
Ketone:
- Colourless → orange
- Orange ppt forms
Condensation/addition-elimination reaction
Orange crystalline solids formed have well defined melting points which are useful for identifying the carbonyl compound
Reaction with Cyanide Ions
Aldehyde/ketone → Hydroxynitrile
Reagents: HCN in the presence of KCN
Conditions: Room temperature and pressure
Mechanism: nucleophilic addition
