Alcohols

• What is the functional group of an alcohol and define functional group

  OH

  Functional group is a group of atoms in an organic molecule that’s responsible for the characteristic reactions of that molecule
• Describe the shape of alcohols

  the oxygen atom has 2 bonding pairs and 2 lone pairs of electrons

  All H-C-H bonds and C-C-O bonds are 109.5° - 4 bonding pairs

  The H-O-C bond is 104.5° - 2 bonding pairs and 2 lone pairs of electrons, the lone pairs repel more than the bonding pairs so the bond angle is reduced
• Describe the boiling point of alcohols

  The OH functional group means that there is hydrogen bonding between molecules

  So alcohols have higher melting points than alkanes with a similar Mr

  The O-H bonds is polar because oxygen is more electronegative than hydrogen and causes higher electron density around the oxygen atom or causes slightly positive/negative charge
• describe the solubility of alcohols

  the OH functional group can hydrogen bond to water molecules but the non-polar hydrocarbon chain can’t

  so alcohols with short hydrocarbon chains are soluble in water, but longer ones are insoluble in water
• how can ethanol be formed?

  fermentation

  hydration of ethene
• describe the hydration of ethene

  formed from cracking/fractions from distillled crude oil

  hydration= addition of water to a molecule
• describe the conditions needed for hydration of ethene

  high temperatures (300°)

  high pressures (70 atm)

  strong acidic catalyst - concentrated phosphoric acid
• describe the advantages of hydration of ethene

  faster reaction

  product is more pure

  continuous process - no man power needed
• describe the disadvantages of hydration of ethene

  expensive initial costs - high technology equipment is needed

  ethene is non-renewable

  high energy costs for producing high pressures
• describe the conditions of fermentation

  yeast

  No air/aerobic conditions because presence of oxygen can cause oxidation of ethanol to ethanoic acid

  Temperature between 30° - 40°

  At low temperatures the reaction is too slow, but at high temperatures the yeast dies and enzymes denature
• advantages of fermentation

  Sugar is a renewable resource

  Production uses cheaper equipment
• disadvantages of fermentation

  the batch process is slow and gives high production costs

  ethanol made isn’t pure so wi=ould need to go through fractional distillation

  depots land used for growing crops
• define biofuel

  a fuel produced from plants
• define carbon neutral

  an activity that has no net annual argon emissions to the atmosphere
• Describe the equations to show no net contributions to carbon dioxide

  Removal of carbon dioxide by photosynthesis:

  6 carbon dioxide molecules are removed from the atmosphere by photosynthesis to produce 1 molecule of glucose

  Production of carbon dioxide by fermentation and glucose:

  When 1 molecule of glucose is fermended, 2 molecules of carbon dioxide is emitted

  the 2 ethanol molecules produced will produce 4 molecules of carbon dioxide when combusted
• Describe combustion of alcohols

  — burns completely in oxygen to produce carbon dioxide and water

  — if in a limited supply of oxygen, incomplete combustion occurs and carbon monoxide or soot and water forms
• describe an experiment to determine the enthalpy of combustion for an unknown alcohol (5)

  —weigh the spirit burner before and after combustion

  — put water in a beaker

  — measure the volume of water

  — burn alcohol to heat the water

  — measure the temperature rise in water

  — 2 major sources of error are incomplete combustion, evaporation of alcohol and inadequate stirring
• conditions and reagents for dehydration of alcohols

  — excess hot concentration sulfuric acid

  — or pass the vapours over heated aluminum oxide

  —an alkene is formed

  — can be formed sustainably if the alcohol has been made by fermentation to glucose
• describe oxidation of primary alcohols

  — partially oxidised to aldehydes

  — completely oxidised to carboxylic acids

  — oxidation in the reactions of alcohols means loss of hydrogen (breaking C-H bonds)
• describe oxidation of secondary alcohols

  — oxidised to ketones
• describe oxidation of tertiary alcohols

  — not easily oxidised

  — would need to break a C-C bond rather than a C-H bond

  — could oxidise to ketones
• Describe conditions to oxidise alcohols to aldehydes and ketones

  — a solution of acidified potassium dichromate - the oxidising agent, it reduces itself

  — the orange dichromate ions are reduced to green chromium ions

  Cr2O7(2-) +6e + 14H+ ———> 2Cr(2+) + 7H2O
• describe conditions and reagents of oxidising a primary alcohol to aldehyde

  reagents: limited amount of acidified potassium dichromate

  conditions: heat gently and then distil immediately
• describe the method of oxidising a primary alcohols to aldehydes

  — misuture is heated gently

  — receiver is cooiled in ice to reduce evaporation of product

  — the aldehyde vaporisises as soon as it forms and distills off immediately, this stops it being oxidised further to ethanoic acid

  — if the oxidising agents is in excess, the aldehyde may be further oxidised
• describe conditions and reagents of oxidising a primary alcohol to carboxylic acids

  conditions: heat under reflux

  reagents: excess acidified potassium dichromate
• describe reflux

  — vapour condenses and drips back into the reaction flask

  — until eventually all alcohol is oxidised to the carboxylic acids

  — after refluxing you can distill of the acid along with any water

  — reflux allows strong heating without losing volatile reactants and products

  — this technique will be used to completely oxidise a secondary alcohol to a ketone
• describe what happens with a reaction mixture is refluxed and why its necessary for complete oxidation to ethanoic acid

  — a mixture of liquids is heated to its boiling point for some time

  — the vapour is formed which escapes from the liquid mixture, then changes back into liquid and returns to the liquid mixture

  — Any ethanol/ethanal that initially evaporates can then be oxidised
• describe a reaction to produce ethanal from ethanol ?

  — mixture is heated in a flask

  — with a head containing a thermometer

  — water cooled condenser connected to the head

  — collect the sample at the boiling point of ethanal

  — a cooled collection vessel is necessary to reduce the evaporation of ethanal
• describe the dehydration of alcohols

  — done by distillation, making cyclohexene from cyclohexanol

  — separation of cyclohexene and water soluble impurities

  — purification of cyclohexene
• describe the first step of dehydrating alcohols - distillation

  — add concentrated sulfuric acid and cyclohexanol into a round bottom flask and add anti-bumping granules to allow a smooth boiling process

  — use a heating mantle to warm the reactants to the boiling point of cyclohexanol

  — chemicals with boiling point less than 83 degrees will evaporate, enter the condenser, cool down and condense back into a liquid

  — the produce is collected in a conical flask, but will still contain impurities like unreacted cyclohexanol and water
• describe the second step of dehydrating alcohols - separation

  — add the products from step 1 into a separating funnel

  — add water to dissolve soluble impurities and create an aqueous solution

  — 2 layers will form: top layer is cyclohexene, bottom layer is an aqueous layer containing water soluble impurities

  — drain the aqueous layer off
• describe the thrid step of dehydrating alcohols - purification

  — take impure cyclohexene and add to a round bottom flask

  —add anhydrous calcium chloride (a drying reagent), this will remove any aqueous substances remaining

  — invert flask and leave for 30 minutes
• describe Fehling‘s solution

  — an oxidising agent, oxidises aldehydes but NOT ketones

  — add warmed aldehyde/ketones to Fehling’s solution

  — aldehyde: solution goes from blue to a brick red precipitate (Cu2O)

  — ketone: solution remains blue

  — solution is originally blue due to the presence of copper 2+ ions
• describe Tollen’s reagent

  — an oxidising agent, oxidises aldehydes but NOT ketones

  — contains silver ammonia ions

  — add warmed aldehyde/ketones to Tollen’s Reagent

  — aldehyde: Tollens’ reduced to silver which coats the inside of the flask

  — ketones: no silver precipitate forms