A Level Chemistry Carboxylic Acids PDF

A LEVEL CHEMISTRY 3.3.9 CARBOXYLIC ACIDS & DERIVATIVES CARBOXYLIC ACIDS OVERVIEW DESCRIPTION General CnH2n+1COOH Formula The COOH gro...

A LEVEL CHEMISTRY 3.3.9 CARBOXYLIC ACIDS & DERIVATIVES CARBOXYLIC ACIDS OVERVIEW DESCRIPTION General CnH2n+1COOH Formula The COOH group is always found in a primary position i.e. carbon number 1 It consists of a C=O and -OH The H atoms are able to partially dissociate as a H+ ion, Description hence why they are considered weak acids. O O R C R C + H+ OH O– Contain a polar C=O bond as well as a highly polar O—H bond. δ- i.e O Polarity δ+ R C δ- δ+ O H - Neutralisation Reactions - Esterification Soluble. Since hydrogen is directly bonded to a highly electronegative element (oxygen), it can hydrogen bond Solubility in with water molecules. Water Solubility decreases with increased carbon chain length (i.e. a longer non-polar section of the molecule) Have very high melting and boiling points compared to their equivalents as they have permanent dipole and hydrogen bonding IMF’s between the molecules. Melting & Boiling Points Increase with increased chain length / Mr. Larger molecules have more electrons involved in the induced-dipole IMF’s making them stronger, so more energy is required to break them. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.9 CARBOXYLIC ACIDS & DERIVATIVES CARBOXYLIC ACID CHEMISTRY Carboxylic acids are weak acids as they partially dissociate in solution O O R C R C + H+ OH O– Carboxylic acid Carboxylate ion e.g. ethanoate ion They are the only acidic organic molecule that you will come across at A level, so are very easy to distinguish using a chemical test. Reagent: Na2CO3 (s)/(aq) Observation: Effervescence / Fizzing Neutralisation H+ + CO32- CO2(g) + H2O The gas can be collected and bubbled through limewater to prove the presence of CO2. It turns limewater milky / cloudy. Example overall equation: CH3COOH + Na2CO3 CH3COONa + CO2 + H2O ethanoic sodium sodium acid carbonate ethanoate AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.9 CARBOXYLIC ACIDS & DERIVATIVES PRODUCING ESTERS Carboxylic acids react with alcohols, in the presence of an acid catalyst, to form esters. Esterification H H O O conc. H2SO4 R C OH + C R R C O C R HO H H + H2O alcohol carboxylic acid ester This is the “ester” functional group The conc. H2SO4 provides H+ ions that act as a catalyst for this reaction. Since the reaction is reversible, it is beneficial to remove the ester from the reaction mixture as it is produced. Distillation is used to achieve this. Unlike the carboxylic acid & alcohol, esters do not have hydrogen bonding between the molecules, so boil at lower temperatures. Ester evaporates as it is produced and collected as a distillate. Reaction mixture heated carboxylic acid alcohol conc. H2SO4 AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.9 CARBOXYLIC ACIDS & DERIVATIVES Esters are essentially two separate carbon chains bonded together by the ester group. Their name reflects this. _______yl _____anoate Carbon chain Carbon chain from the from the alcohol carboxylic acid H H O H H e.g. H C C O C C C H H H H H ethyl propanoate COMMON EXAMPLES & USES OF ESTERS Plasticisers - A plasticiser makes a polymer more flexible. e.g. plasticised PVC is used as a covering for electrical wiring, whereas unplasticised PVC (uPVC) is used to make windows and doors, which are rigid. Perfumes & Flavourings - Short-chain esters are sweet smelling and non-toxic Triglycerides (Fats) - These are commonly found in biology and are used as an energy store. H O H C O C O H C O C O H C O C H The fatty acid chains can be Glycerol (propane-1,2,3-triol) forms saturated (as above) or unsaturated. three ester bonds with three “fatty This is the difference between acids” (long chain carboxylic acids) saturated and unsaturated fats. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.9 CARBOXYLIC ACIDS & DERIVATIVES Biodiesel - A mixture of methyl esters and long-chain carboxylic acids formed from the alkaline hydrolysis (breakdown) of the triglycerides in vegetable oil (commonly rapeseed oil). H O O H C O C C H HO H O H C O C H O H H C O C H HYDROLYSIS OF ESTERS Hydrolysis is the process of splitting a molecule (lysis) by the addition of water (hydro). It is essentially the opposite of esterification. H O H O R C O C R + H 2O R C OH + C R HO H H ester alcohol carboxylic acid However! There are two types of hydrolysis which produce slightly different products. Acid (H+) catalysed hydrolysis and Alkaline (OH-) catalysed hydrolysis. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.9 CARBOXYLIC ACIDS & DERIVATIVES ACID HYDROLYSIS This is nice and straight-forward. The ester is refluxed with dilute HCl(aq). H O H O H+ R C O C R + H 2O R C OH + C R HO H H Acid hydrolysis of an ester produces an alcohol and the undissociated carboxylic acid. Since acidic conditions are used, the H+ remains attached to the carboxylic acid molecule. ALKALINE HYDROLYSIS This is where the products change a little. The ester is refluxed with dilute KOH(aq). H O H - O OH R C O C R + H 2O R C OH + C R +K –O H H Alkaline hydrolysis of an ester produces an alcohol and the salt of the carboxylic acid. e.g. potassium ethanoate. Since alkaline conditions are used, the acid is neutralised by the KOH and forms a salt. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.9 CARBOXYLIC ACIDS & DERIVATIVES BIODIESEL (again) It is alkaline hydrolysis that is used to produce biodiesel. H O H C O C O H C O C O H C O C H OH- 1. Triglyceride is hydrolysed using dil. KOH(aq) forming glycerol and 3 separate carboxylic acid salts. O H +K –O C H C OH O H C OH +K –O C H C OH O H +K –O C H + H C OH 2. Carboxylic acid salts react with methanol to create new methyl esters. These, combined H with some unreacted long chain carboxylic acids make up biodiesel. H O H C O C H H O H C O C H H O H C O C H AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.3.9 CARBOXYLIC ACIDS & DERIVATIVES SOAP Soap is produced from triglycerides (animal and plant fats) using alkaline hydrolysis. Soap is partly made up of carboxylic acid salts. These enable hydrophobic oils etc to mix with water, making them very useful for cleaning. O –O C The polar end of the The non-polar end of the molecule is hydrophilic molecule is hydrophobic (water-loving). (water-hating) The two ends of the molecule act a bridge between water and oils enabling them to mix! AQA www.chemistrycoach.co.uk © scidekick ltd 2024