Topic 17 : Organic Chemistry II 2️⃣

A chiral carbon has 4 different groups attached to it Forms isomers which are non-superimposable

• Identify the chiral centre • Draw one enantiomer in a tetrahedral shape • Draw its mirror image

It rotates the plane of polarisation of plane polarised light

The amount of optical activity

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

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

Equimolar mixture of 2 enantiomers that has no optical activity as rotations cancel each other out

• 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

• 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)

A carbon atoms joined by a double bond to an oxygen atom (C=O)

Aldehydes and ketones

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

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

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

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

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

Acidified potassium dichromate (VI) Fehling’s solution Benedict’s solution Tollens’ reagent

Deep blue -> red precipitate

Colourless solution —> silver mirror

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

• carried out in an aqueous alkaline solution containing KCN • reagent is HCl • nucleophilic addition • products are referred to as hydroxynitriles

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

Reacts with most carbonyl compounds to form a bright orange precipitate

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