AQA Chemistry A-Level 3.3.14: Organic Synthesis Notes PDF

AQA Chemistry A-level 3.3.14: Organic Synthesis Detailed notes This work by PMT Education is licensed under https://bit.ly/pmt-cc https://bit.ly/pmt-edu-cc CC BY-NC-ND 4.0...

AQA Chemistry A-level 3.3.14: Organic Synthesis Detailed notes This work by PMT Education is licensed under https://bit.ly/pmt-cc https://bit.ly/pmt-edu-cc CC BY-NC-ND 4.0 https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Why do we need organic synthesis? Synthesis pathways are needed to convert starting materials into a target compound. This is achieved through ​multistep pathways​ that, for example, lead to oxidation of functional groups, chain lengthening and saturation of double bonds. When designing a synthetic pathway a chemist must consider several things: Product yield - related to Le Chatelier’s principle Reaction conditions If a catalyst is needed Reagents The process involved - batch or continuous, (questions like - does the reaction mixture need to be purified before the next reactant can be added?) Hazards with the reagents or conditions Cost Formation of ​isomers​ - for instance, many drug targets are enzymes which are often stereospecific and will only react with one enantiomer, the synthetic pathway should be designed, if possible, to only produce this enantiomer and not a racemic mixture Reactivity of functional groups present - this includes other functional groups on the reactant itself, or on reagents and catalysts present. Maximising the yield It is important for chemists to yield their product, in order to maximise their profits.The best synthetic pathways are those that keep the number of steps as low as possible, to improve the yield of the final product. The ​yield​ of an individual step may be low for two reasons: 1. Product is left in the reaction mixture 2. The reaction is incomplete or irreversible Functional groups To best understand organic synthesis, you’ll need to know the structures of all the functional groups. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Key reagents Oxidising agent​ - Acidified potassium dichromate Oxidises primary alcohols to aldehydes and then to carboxylic acids. ○ If left in a reaction mixture an aldehyde will then be oxidised to carboxylic acids, to prevent this and stop at aldehyde an excess of alcohol can be used and distil the aldehyde off once formed. To form a carboxylic acid the reaction mixture is heated under reflux so the aldehyde remains in the mixture to be oxidised further. Oxidises secondary alcohols to ketones. Reducing agents NaBH​4​ - reduces a carbonyl group C=O to an alcohol group C-OH H​2​ and Nickel catalyst - reduces a C=C double bond Tin / HCl - reduces NO​2 to​ NH​2 Dehydrating agents A dehydrating agent is one that removes water (H​2​O) from a molecule Al​2​O​3 -​ pass vapour over it Acid-catalysed elimination by H​3​PO​4 https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Organic synthesis maps A good overview of this topic is through ​organic synthesis maps​. These tell you the conditions and reagents required for conversion from one functional group to another and shows the links between functional groups that require several steps to form others. It is also important to know the type of mechanism the reaction proceeds by. Aliphatic synthesis map COMPOUND INTEREST​, ​CC BY-NC-ND 4.0 Aromatic synthesis There is less to learn for ​aromatic synthesis.​ There are 4 key reactions: 1. Acylation​ of benzene to a phenylketone 2. Nitration​ of benzene to nitrobenzene 3. Reduction​ of nitrobenzene to an aromatic amine 4. Conversion of an aromatic amine to an aromatic amide using an acyl halide The conditions and reagents for these reactions are found in the topics 3.10, 3.11 and 3.09. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Answering exam questions Organic synthesis is a topic that requires you to draw together information from all of the other organic modules to be able to answer questions that require you to get from a starting material to an end product, with ​synthesis pathways​ of up to ​4 steps​. You will need to be able to: Suggest reagents and reaction conditions (such as temperature, pressure and a catalyst) Identify reaction intermediate structures from their chemical formula Draw mechanisms Suggest products https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc