Topic 17 : Organic Chemistry II 2️⃣ Flashcards
(49 cards)
State what is meant by the term chiral
A chiral carbon has 4 different groups attached to it
Forms isomers which are non-superimposable
Give the steps in drawing optical isomers
- Identify the chiral centre
- Draw one enantiomer in a tetrahedral shape
- Draw its mirror image
A substance shows optical activity if…
It rotates the plane of polarisation of plane polarised light
What does a polarimeter measure ?
The amount of optical activity
How does a polarimeter work
A monochromatic light source passes through a polarising filter which converts unpolarised light into plane-polarised light
The plane-polarised light then passes through a sample tube containing some of the substance in solution
If the substance is optically active, the plane of polarisation is rotated
Describe the properties of enantiomers
Identical physical properties with 1 exception :
They both rotate plane-polarised light by equal angles but in opposite directions
Identical chemical properties with 1 exception:
The way in which they react with the enantiomers of other substances may be different for each enantiomer
What is a racemic mixture
Equimolar mixture of 2 enantiomers that has no optical activity as rotations cancel each other out
Discuss optical activity related to SN2 reaction mechanisms
- nucleophile always attacks from the opposite side of the leaving group
- means we only produce 1 product and this product will rotate plane polarised light differently to the reactant
Discuss optical activity related to SN1 reaction mechanisms
- Due to the planar nature of the carbocation, there is an even chance of the nucleophile attacking from the top and the bottom
- this means we are likely to get a 50/50mixtire of both enantiomers and hence produce a racemic mixture of products
- as with all racemic mixtures, they do not rotate plane polarised light (rotations cancel eachother out)
What is the carbonyl group
A carbon atoms joined by a double bond to an oxygen atom (C=O)
What are the only 2 types of compounds classified as carbonyls ?
Aldehydes and ketones
Describe and explain the bonding in the carbonyl group (C=O)
Composed of a combination of the sigma and pi bond like a C=C bond
C=O bond, however, is polar due to the differing electronegativities of carbon & oxygen - causes a shift in electron density towards the oxygen
Why are carbonyls prone to nucleophilic attack ?
Shift in electrondensity towards the oxygen in the C=O bond causes the carbon to be slightly electron deficient and therefore prone to nucelophilic attack
Describe and explain the boiling temperatures of carbonyl compounds
Contain the polar C=O group so have permanent dipole-dipole attractions
No hydrogen bonding
Boiling temps increase with increasing molecular mass as the extent of London forces increases
Describe and explain the solubility in water of carbonyl compounds
The smaller aldehydes and ketones are soluble in water as they can form hydrogen bonds with water molecules
The solubility decreases with increasing chain length as the hydrocarbon portion of the molecule becomes more significant
How are aldehydes and ketones converted to alcohols ?
Reduction using the reagent lithium tetrahydridoaluminate (LiAlH4)
Reaction is carried out with both the carbonyl compound and the reagent dissolved in dry ether
The reducing agent can be represent by a [H] in equations
What are the 4 different reagents that can be used to oxidise aldehydes to carboxylic acids
Acidified potassium dichromate (VI)
Fehling’s solution
Benedict’s solution
Tollens’ reagent
What colour change can be seen for the oxidation of aldehydes with Fehling’s or Benedict’s solution
Deep blue -> red precipitate
What colour change can be seen for the oxidation of aldehydes to carboxylic acids using Tollens’ reagent
Colourless solution —> silver mirror
Describe the reaction of a carbonyl with iodine
Redox reaction
Carbonyl compound is added to an alkaline solution iodine
Mixture is warmed and then cooled
Pale yellow precipitate forms = positive result for ethanal or a ketone
Describe the reaction of carbonyls with hydrogen cyanide
- carried out in an aqueous alkaline solution containing KCN
- reagent is HCl
- nucleophilic addition
- products are referred to as hydroxynitriles
Describe and explain the nucleophilic addition mechanism of the reaction of carbonyls and HCl
Step 1 : involves the nucleophilic attack by a :CN- on the carbon atom of the carbonyl compound
Step 2: involves the reaction between the intermediate and the H-CN molecule
Results in a product where the functional groups -OH and -CN are joined to a carbon atom forming hydroxynitriles
Describe the reaction of Brady’s reagent (2,4 dinitrophenylhydrazine) with carbonyl compounds
Reacts with most carbonyl compounds to form a bright orange precipitate
Describe and explain the boiling temperatures of carboxylic acids
Strong intermolecular forces (hydrogen bonding)
As carbon chain length increases so does boiling point due to increased London forces
Pure carboxylic acids can form hydrogen bonded dimers
