A-Level Chemistry Optical Isomerism & Carbonyls PDF

A LEVEL CHEMISTRY 3.3.7 OPTICAL ISOMERISM Optical isomerism is a form of stereoisomerism. Here’s a reminder…. Isomers Structural Isomers Stereoisomers These are molecules t...

A LEVEL CHEMISTRY 3.3.7 OPTICAL ISOMERISM Optical isomerism is a form of stereoisomerism. Here’s a reminder…. Isomers Structural Isomers Stereoisomers These are molecules that have These are molecules that have the same molecular formula but the same structural formula but a different structural formula the atoms are arranged in Chain Isomers different relative positions in space Position Isomers E / Z Isomers Functional Group Isomers Optical Isomers In order for an organic molecule to show optical isomerism it must have an asymmetrical carbon atom, AKA a chiral centre. This is a carbon atom that has 4 different groups bonded to it. It doesn’t matter what these groups are, so long as they are all different from each other. e.g. W Chiral Centre C* Z X Y H OH H H OH H C C* C C H C * H 3C C 2H 5 H H H H H butan-2-ol It’s easier to view the chiral centre when we focus on the tetrahedral shape around the asymmetrical C atom. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.7 OPTICAL ISOMERISM DRAWING OPTICAL ISOMERS Optical isomers are basically a pair mirror-images of each other. In other words, they are non-superimposable. These are also known as enantiomers. e.g. OH OH C C H 3C C 2H 5 H 5C 2 CH3 H H If you are asked to draw the optical isomers of an How To Identify & organic molecule: Draw Optical Isomers 1. Draw the molecule focusing on the tetrahedral arrangement around the chiral centre. The groups can be arranged in any order, so long as they are correct for the molecule. 2. Draw the mirror image. OPTICAL ACTIVITY These isomers are known as “optical” isomers as they rotate plan-polarised light. This is also known as polarimetry. One optical isomer rotates clockwise and the other rotates is anticlockwise to the same degree. e.g. separate samples of the optical isomers of butan-2-ol rotate it +/- 13.5o. However, if you have an equimolar mixture (same number of moles of each isomer) of optical isomers, this is known as a racemic mixture and the overall effect on plan- polarised light is 0o. In other words, they cancel each other out and the mixture is not optically active. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.8 ALDEHYDES & KETONES ALDEHYDES & KETONES OVERVIEW Aldehydes Ketones General CnH2nO Formula C=O is in a primary position C=O is in a secondary position (the end carbon in a chain) (a middle carbon in a chain) O O Description R C C H R R Bond angle around the group Bond angle around the group = 120o (Trigonal Planar) = 120o (Trigonal Planar) Polarity Polar C=O bond. Permanent Dipole IMF’s Greater than equivalent non-polar molecules, but lower than equivalent organic molecules that have hydrogen bonding (alcohols & C’ acids) Melting & Boiling Increase with increased chain length. Points Larger molecules have more electrons involved in the induced- dipole IMF’s making them stronger, so more energy is required to break them Solubility in Smaller molecules are water soluble. Water Molecules with larger, non-polar carbon chains are less soluble - Oxidation - Reduction Reactions - Reduction - Nucleophilic Addition - Nucleophilic Addition AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.8 ALDEHYDES & KETONES OXIDATION OF ALDEHYDES O H+ / Cr2O72- O R C + [O] R C Reflux OH H Aldehydes are oxidised to carboxylic acids by refluxing with acidified potassium dichromate. Notice the addition of one oxygen atom (oxidation). Ketones cannot be further oxidised. REDUCTION Reagent: NaBH4(aq) (a source of H- ions) followed by dil.H2SO4(aq) (a source of H+ ions) Mechanism: Nucleophilic Addition H- δ- H H+ H O R C δ+ R C O– R C OH H H H Aldehyde Product: 1o Alcohol NaBH4 Aldehyde + 2[H] 1o alcohol H+ O– δ- O OH H- C δ+ R C R R C R R R H H Ketone Product: 2o Alcohol NaBH4 Ketone + 2[H] 2o alcohol AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.8 ALDEHYDES & KETONES NUCLEOPHILIC ADDITION WITH KCN Aldehydes and Ketones undergo a nucleophilic addition reaction with KCN to produce hydroxynitriles. Overall, it is actually HCN (hydrogen cyanide) that reacts, however KCN(aq) is used as HCN is a highly toxic / deadly gas. It his safer to work with KCN in solution and supply the H+ ions using an acid. ALDEHYDES + KCN Reagent: KCN(aq) (a source of CN- ions) followed by dil.H2SO4(aq) (a source of H+ ions) Mechanism: Nucleophilic Addition CN- δ- CN H+ CN O R C δ+ R C O– R C OH H H H Aldehyde Product: hydroxynitrile H+ / KCN Aldehyde + HCN 2-hydroxy_______nitrile The hydroxynitrile produced from an aldehyde will always have the following name, as the C in the CN group is always carbon number 1 when naming these: 2-hydroxy_______nitrile Simply place the name of the carbon chain length in the blank space. However, be careful! Since we have added CN to the molecule, the carbon chain will be one carbon longer than the chain in the original aldehyde. e.g. propanal will produce 2-hydroxybutanenitrile. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.8 ALDEHYDES & KETONES KETONES + KCN Reagent: KCN(aq) (a source of CN- ions) followed by dil.H2SO4(aq) (a source of H+ ions) Mechanism: Nucleophilic Addition H+ δ- O O– OH CN- C δ+ R C R R C R R R CN CN Ketone Product: hydroxynitrile H+ / KCN Ketone + HCN 2-hydroxy-2-______-_____nitrile The hydroxynitrile produced from a ketone will have the following name: 2-hydroxy-2-______-_____nitrile Where the first blank is the name for the shortest R group (C chain). This is treated as a side chain. The second blank is for the longer R group which forms the main part of the molecule. e.g. butan-2-one will produce 2-hydroxy-2-methyl-butanenitrile. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.8 ALDEHYDES & KETONES NUCLEOPHILIC ADDITION + OPTICAL ISOMERS The nucleophilic addition of aldehydes and asymmetrical ketones produce hydroxynitriles that have optical isomers. The reason for this is that the shape of the molecule around the C=O is trigonal planar. This means that the CN- ion has an equal chance of bonding to Cδ+ from one of two sides. e.g. Aldehydes CN- δ- O R C δ+ H CN- CN CN C C R OH HO R H H Since the CN- ion has a 50:50 chance of attacking from either side, an equimolar mixture (racemic mixture) of the two enantiomers is produced. This means that, despite producing optical isomers of the hydroxynitrile, it will have no overall effect (rotation) on plane-polarised light. HINTS | TIPS | HACKS This is a common follow up question to the How To Tell If Optical mechanism for this reaction. Isomers Will Be Produced This is only true for aldehydes and asymmetrical ketones. The exceptions to this are methanal and symmetrical ketones. These do not produce optical isomers. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.8 ALDEHYDES & KETONES TESTING FOR ALDEHYDES & KETONES There are two reagents that can be used to distinguish between aldehydes and ketones. The chemistry of both tests relies on the fact that aldehydes can be further oxidised and ketones cannot. TOLLEN’S REAGENT Tollen’s Reagent: AgNO3(aq) + NH3(aq) (ammoniacal silver nitrate) This forms a linear complex ion [Ag(NH3)2]+ Add to test substance and heat gently. Positive Result: Formation of a silver mirror (Ag(s)) Aldehydes give a positive result. As the aldehyde is oxidised, the Ag+ is reduced to Ag. Ketones give a negative result. As they cannot be further oxidised, the Ag+ cannot be reduced to Ag. FEHLING’S SOLUTION Fehling’s Solution: Two solutions (A + B) are mixed. This is a blue solution containing Cu2+(aq). Add to test substance and heat gently. Positive Result: Formation of a red precipitate of Cu2O (Blue to Aldehydes give a positive result. As the aldehyde is oxidised, the Cu2+ is reduced to Cu+ (The oxidation state of Cu in Cu2O is +I) Ketones give a negative result. As they cannot be further oxidised, the Cu2+ cannot be reduced to Cu+. AQA www.chemistrycoach.co.uk © scidekick ltd 2024