Alcohols PDF - AS Chemistry Past Paper Notes

AS CHEMISTRY 3.3.5 ALCOHOLS ALCOHOLS OVERVIEW DESCRIPTION General CnH2n+1OH Formula Have had 1 or more H atoms substituted for an -OH...

AS CHEMISTRY 3.3.5 ALCOHOLS ALCOHOLS OVERVIEW DESCRIPTION General CnH2n+1OH Formula Have had 1 or more H atoms substituted for an -OH group Description Contain a polar C—O bond as well as a highly polar O—H bond. i.e δ+ δ- Polarity C O Note the bond angle around δ+ the O atom. 107o / v-shaped H Nucleophiles are attracted to the Cδ+ Reactions - Oxidation - Elimination 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 alkane equivalents as they have hydrogen bonding IMF’s between the molecules. Melting & Increase with increased chain length / Mr. Boiling Points 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 AS CHEMISTRY 3.3.5 ALCOHOLS PRODUCING ETHANOL Ethanol is widely used as an antimicrobial (e.g. hand washes) and is found in alcoholic drinks. There are two main methods of producing ethanol on a large scale. 1. + H2O / dilute H3PO4 Catalyst + Reflux (AKA Hydration) H H H H C C + H2O H C C H H H H OH You need to know the mechanism for this. See earlier in this section. 2. Fermentation Where glucose (C6H12O6) from plants is converted into ethanol by yeast in anaerobic conditions at approx 35oC. H H C6H12O6 2 H C C H + 2 CO2 H OH Fermentation is a slower process than industrial hydration as it is carried out at a lower temperature. The reason for this is that the yeast’s enzymes are denatured at higher temperatures. Despite it being slower, it is more sustainable than the industrial hydration process as the glucose used comes from plants. Overall, compared to the industrial hydration, fermentation… is cheaper is slower (it’s a “batch” process) is more energy efficient has a lower percentage yield is more sustainable AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.5 ALCOHOLS BIOFUELS The ethanol produced by fermentation can be put into the fractional distillation process to purify it further and burned as a fuel. It is known as a “biofuel” as the source of the fuel is plant (or animal) material. The following equations suggest that the whole process is carbon neutral… Fermentation: C6H12O6 → 2C2H5OH + 2CO2 released Combustion: 2C2H5OH + 6O2 → 4CO2 + 6H2O released Photosynthesis: 6H2O + 6CO2 → C6H12O6 + 6O2 absorbed However, when you take into account the amount of energy required to harvest the plants, heat the fermentation process, purify the ethanol and transport materials, more CO2 released than absorbed overall. The ethical issue with the use of biofuels is the land use for growing the crops. The more land that is used for biofuel crops, the less there is for food production. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.5 ALCOHOLS OXIDATION OF ALCOHOLS Alcohols are classified as: (Where R = an alkyl carbon chain) Primary (1o) Secondary (2o) Tertiary (3o) OH OH OH R C H R C R R C R H H R 2 H’s bonded 1 H bonded 0 H’s bonded to the C-OH to the C-OH to the C-OH HINT: In organic chemistry, it is more useful to think of Oxidation & Reduction in these terms: Oxidation: Addition of Oxygen [O] or the removal of Hydrogen Reduction: Addition of Hydrogen [H] or the removal of Oxygen Oxidising Agent: Acidified Potassium Dichromate (VI) + Heat (H+ / Cr2O72-) If oxidation has taken place, colour change observed = Orange to Green This is caused by a change in oxidation state of Cr as it is reduced from +VI to +III. As the chromium is reduced, the alcohol is oxidised. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.5 ALCOHOLS PRIMARY ALCOHOLS 1o Alcohol Aldehyde Carboxylic Acid OH O O H+ / Cr2O72- H+ / Cr2O72- R C H R C R C Distillation Reflux OH H H 1o Alcohol to Aldehyde OH O H+ / Cr2O72- R C H + [O] R C + H2O H H Aldehyde to Carboxylic Acid O H+ / Cr2O72- O R C + [O] R C H OH How to Use & Explain How to Use & Explain Distillation Reflux AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.5 ALCOHOLS SECONDARY ALCOHOLS 2o Alcohol Ketone A ketone cannot be further oxidised! OH O H+ / Cr2O72- H+ / Cr2O72- R C R Reflux R C R ❌ Reflux No Reaction H 2o Alcohol to Ketone OH O H+ / Cr2O72- R C R + [O] R C R + H2O H TERTIARY ALCOHOLS 3o Alcohol OH H+ / Cr2O72- R C R Reflux ❌ No Reaction R A 3o alcohol cannot be oxidised! In the cases of 1o and 2o alcohols, there is a H atom bonded to the C-OH that can be removed as part of the oxidation process. A 3o alcohol does not have one of these! AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.5 ALCOHOLS TESTS FOR ALDEHYDES & KETONES There are two chemical tests you can carry out that distinguish between an aldehyde and a ketone. Both of these tests rely on the fact that an aldehyde can be oxidised and a ketone cannot. 1. Tollen’s Reagent (AgNO3(aq) / NH3(aq)) Add to test substance in a test tube and heat gently Positive Result = Silver mirror on the inside of the test tube (sometimes a black solid) Only aldehydes give a positive result. As they can be oxidised, the Ag+(aq) in the Tollen’s is reduced to Ag(s). 2. Fehling’s Solution (Fehling’s A + B: Blue solution containing Cu2+(aq)) Add to test substance in a test tube and heat gently Positive Result = Red Precipitate Only aldehydes give a positive result. As they can be oxidised, the Cu2+(aq) in the Fehling’s is reduced to Cu+. The red precipitate is Cu2O. TEST FOR A CARBOXYLIC ACID Carboxylic acids are acids! They react with a base / alkali just like any other. Sodium Carbonate Na2CO3(s) Add to test substance in a test tube Positive Result = Fizzing / Effervescence The neutralisation reaction produces CO2(g) AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.5 ALCOHOLS ELIMINATION / DEHYDRATION Alcohols can undergo an elimination reaction to form alkenes. It is also known as dehydration since 2 H atoms and 1 O atom are eliminated from the alcohol (H2O) conc. H2SO4 catalyst + Reflux H+ H H H H H H H H H H C C +O C C H C C O H C C H + H H H H H H H H H + H2O + H+ The H+ from the acid H2O is released from H+ is released / starts the reaction the molecule regenerated from the molecule and a C=C forms 1o alcohols must form the C=C at the end of a molecule, so only produce one product. In much the same way as “Asymmetrical Halogenoalkanes” can produce multiple isomers as products, so too can 2o & 3o alcohols when they dehydrate. e.g. butan-2-ol AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.5 ALCOHOLS Starting from the carbocation intermediate H H H H H C C C C H + H Br H H H H H H H H H H C C C C H H C C C C H H H H H H but-1-ene E-but-2-ene and Z-but-2-ene Alkenes produced from the elimination of alcohols can be used to produce addition polymers. Since alcohols can be produced from a renewable resource (fermentation), it means there is an alternative to to using crude oil for monomers when producing polymers / plastics. Glucose Alcohol Alkene Addition Polymer AQA www.chemistrycoach.co.uk © scidekick ltd 2024