Different types of alcohols C O H H H C H H C H H H C H H C H C H H H O H H C C H C H H H H H C H H O H H Propan-1-ol Primary Tertiary alcohols are alcohols where 3 carbon are attached to the carbon adjoining the oxygen Propan-2-ol Secondary methylpropan-2-ol Tertiary Primary alcohols are alcohols where 1 carbon is attached to the carbon adjoining the oxygen Secondary alcohols are alcohols where 2 carbon are attached to the carbon adjoining the oxygen. Oxidation reactions of the alcohols Potassium dichromate K2Cr2O7 is an oxidising agent that causes alcohols to oxidise. The exact reaction, however, depends on the type of alcohol, i.e. whether it is primary, secondary, or tertiary, and on the conditions. Partial Oxidation of Primary Alcohols Reaction: primary alcohol aldehyde Reagent: potassium dichromate (VI) solution and dilute sulphuric acid. Conditions: (use a limited amount of dichromate) warm gently and distil out the aldehyde as it forms: C O C H H H H An aldehyde’s name ends in –al It always has the C=O bond on the first carbon of the chain so it does not need Ethanal an extra number Observation: the orange dichromate ion (Cr2O7 2-) reduces to the green Cr 3+ ion Write the oxidation equations in a simplified form using [O] which represents O from the oxidising agent When writing the formulae of aldehydes in a condensed way write CHO and not COH e.g.CH3CH2CHO N Goalby chemrevise.org 2 propan-1-ol propanal + [O] + H2O CH3CH2CH2OH + [O] CH3CH2CHO + H2O OH + [O] O + H2O C O C H H C H H H H C O H H H C H H C H H H Distillation In general used as separation technique to separate an organic product from its reacting mixture. Need to collect the distillate of the approximate boiling point range of the desired liquid. Water in Water out Liebig condenser thermometer Heat Note the bulb of the thermometer should be at the T junction connecting to the condenser to measure the correct boiling point Note the water goes in the bottom of the condenser to go against gravity. This allows more efficient cooling and prevents back flow of water. It’s important to be able to draw and label this apparatus accurately. Don’t draw lines between flask, adaptor and condenser. Round bottomed flask N Goalby chemrevise.org 3 Full Oxidation of Primary Alcohols Reaction: primary alcohol carboxylic acid Reagent: potassium dichromate(VI) solution and dilute sulphuric acid Conditions: use an excess of dichromate, and heat under reflux: (distill off product after the reaction has finished) C C O O H H C H H H H Propanoic acid propan-1-ol Propanoic acid + 2 [O] + H2O Observation: the orange dichromate ion (Cr2O7 2-) reduces to the green Cr 3+ ion CH3CH2CH2OH + 2[O] CH3CH2COOH + H2O C O H H H C H H C H H H C C O H O H C H H H H OH + 2[O] + H2O O OH Reflux Reflux is used when heating organic reaction mixtures for long periods. The condenser prevents organic vapours from escaping by condensing them back to liquids. Never seal the end of the condenser as the build up of gas pressure could cause the apparatus to explode. This is true of any apparatus where volatile liquids are heated including the distillation set up Water in Water out Heat Anti-bumping granules are added to the flask in both distillation and reflux to prevent vigorous, uneven boiling by making small bubbles form instead of large bubbles It’s important to be able to draw and label this apparatus accurately. • Don’t draw lines between flask and condenser. • Don’t have top of condenser sealed • Condenser must have outer tube for water that is sealed at top and bottom • Condenser must have two openings for water in and out that are open Round bottomed flask condenser Oxidation of Secondary Alcohols Reaction: secondary alcohol ketone Reagent: potassium dichromate(VI) solution and dilute sulphuric acid. Conditions: heat under reflux C C C O H H H H H H Propanone Ketones end in -one When ketones have 5C’s or more in a chain then it needs a number to show the position of the double bond. E.g. pentan-2-one propan-2-ol Propanone C H H C H C H H H O H H C C C O H H H H H H + [O] + H2O There is no further oxidation of the ketone under these conditions. Observation: the orange dichromate ion (Cr2O7 2-) reduces to the green Cr 3+ ion Tertiary alcohols cannot be oxidised at all by potassium dichromate: This is because there is no hydrogen atom bonded to the carbon with the OH group Distinguishing between Aldehydes and Ketones The fact that aldehydes can be further oxidised to carboxylic acids whereas ketones cannot be further oxidised is the chemical basis for two tests that are commoly used to distinguish between aldehydes and ketones Tollen’s Reagent Reagent: Tollen’s Reagent formed by mixing aqueous ammonia and silver nitrate. The active substance is the complex ion of [Ag(NH3 )2 ]+ . Conditions: heat gently Reaction: aldehydes only are oxidised by Tollen’s reagent into a carboxylic acid and the silver(I) ions are reduced to silver atoms Observation: with aldehydes, a silver mirror forms coating the inside of the test tube. Ketones result in no visible change CH3CHO + 2Ag+ + H2O CH3COOH + 2Ag + 2H+ Reagent: Fehling’s Solution containing blue Cu 2+ ions. Conditions: heat gently Reaction: aldehydes only are oxidised by Fehling’s solution into a carboxylic acid and the copper ions are reduced to copper(I) oxide . Observation: Aldehydes :Blue Cu 2+ ions in solution change to a red precipitate of Cu2O. Ketones do not react Fehling’s solution CH3CHO + 2Cu2+ + 2H2O CH3COOH + Cu2O + 4H+ The presence of a carboxylic acid can be tested by addition of sodium carbonate. It will fizz and produce carbon dioxide
18.104.22.168 Oxidation of alcohols
Alcohols are classified as primary, secondary and tertiary.
Primary alcohols can be oxidised to aldehydes which can be further oxidised to carboxylic acids.
Secondary alcohols can be oxidised to ketones. Tertiary alcohols are not easily oxidised.
Acidified potassium dichromate(VI) is a suitable oxidising agent.
Students should be able to:
• write equations for these oxidation reactions (equations showing [O] as oxidant are acceptable)
• explain how the method used to oxidise a primary alcohol determines whether an aldehyde or carboxylic acid is obtained
• use chemical tests to distinguish between aldehydes and ketones including Fehling’s solution and Tollens’ reagent.
Reaction of Alcohols with Dehydrating Agents Dehydration Reaction: removal of a water molecule from a molecule Reaction: Alcohol Alkene Reagents: Concentrated Sulphuric or Phosphoric acids Conditions: warm (under reflux) Role of reagent: dehydrating agent/catalyst Type of reaction: acid catalysed elimination C C + H2O H H H C H H H propan-1-ol Propene Some 2o and 3o alcohols can give more than one product, when the double bond forms between different carbon atoms C H C H C H H H C H H H O H H C C C C H H H H H H H H C H H C C C H H H H H H Butan-2-ol can form both alkenes although more but-2-ene would be formed Butan-2-ol But-1-ene But-2-ene But-2-ene could also exist as E and Z isomers Producing alkenes from alcohols provides a possible route to polymers without using monomers derived from oil C O H H H C H H C H H H CH3CH2CH2OH CH2=CHCH3 + H2O : H3C C CH3 O H H H3C C CH3 O + H H H C C + CH3 H H H H H+ H+ C C CH3 H H H Acid catalysed elimination mechanism The H+ comes from the conc H2SO4 or conc H3PO4
Alkenes can be formed from alcohols by acid-catalysed elimination reactions.
Alkenes produced by this method can be used to produce addition polymers without using monomers derived from crude oil.
Students should be able to outline the mechanism for the elimination of water from alcohols
Forming ethanol Comparing two methods for producing ethanol: Fermentation or industrial formation from ethene Fermentation glucose ethanol + carbon dioxide C6H12O6 2 CH3CH2OH + 2 CO2 The conditions needed are: •Yeast •No air •temperatures 30 –40oC The optimum temperature for fermentation is around 38oC At lower temperatures the reaction is too slow. At higher temperatures the yeast dies and the enzymes denature. Fermentation is done in an absence of air because the presence of air can cause extra reactions to occur. It oxidises the ethanol produced to ethanoic acid (vinegar). Advantages •sugar is a renewable resource •production uses low level technology / cheap equipment Disadvantages •Batch process which is slow and gives high production costs •ethanol made is not pure and needs purifying by fractional distillation •Depletes land used for growing food crops CH2=CH2 (g) + H2O (g) CH3CH2OH (l) Essential Conditions high temperature 300 °C high pressure 70 atm strong acidic catalyst of conc H3PO4 Reagent: ETHENE – from cracking of fractions from distilled crude oil From ethene Advantages: •faster reaction •purer product •continuous process (which means cheaper manpower) Disadvantages: •High technology equipment needed (expensive initial costs) •ethene is non-renewable resource (will become more expensive when raw materials run out) •High energy costs for pumping to produce high pressures Type of reaction: Fermentation Type of reaction: Hydration/addition Definition: Hydration is the addition of water to a molecule N Goalby chemrevise.org 6 C C H H H H C C + H H H H H O H H C C O + H H H H H H H C C O H H H H H H H + H+ Acid catalysed addition mechanism for hydration of ethene The H+ comes from the conc H3PO4
22.214.171.124 Alcohol production
Alcohols are produced industrially by hydration of alkenes in the presence of an acid catalyst.
Ethanol is produced industrially by fermentation of glucose.
The conditions for this process.
Ethanol produced industrially by fermentation is separated by fractional distillation and can then be used as a biofuel.
Students should be able to:
• justify the conditions used in the production of ethanol by fermentation of glucose
• outline the mechanism for the formation of an alcohol by the reaction of an alkene with steam in the presence of an acid catalyst
Ethanol as biofuel
Ethanol as biofuel A biofuel is a fuel produced from plants Ethanol produced from fermentation is a biofuel. It can be argued that ethanol produced from this method is classed as carbon–neutral as any carbon dioxide given off when the biofuel is burnt would have been extracted from the air by photosynthesis when the plant grew. There would be no net CO2 emission into the atmosphere. This does not take into account any energy needed to irrigate plants, fractionally distil the ethanol from the reaction mixture or process the fuel. If the energy for this process comes from fossil fuels then the ethanol produced is not carbon neutral Removal of CO2 by photosynthesis 6CO2 + 6H2O C6H12O6 + 6O2 Production of CO2 by fermentation and combustion C6H12O6 2 CH3CH2OH + 2 CO2 2 CH3CH2OH + 6O2 4CO2 + 6H2O Equations to show no net contribution to CO2 6CO2 molecules are removed from the atmosphere when the plants grow by photosynthesis to produce one molecule of glucose When 1 molecule of glucose is fermented 2 molecules of CO2 is emitted. The two ethanol molecules produced will then produce 4 molecules of CO2 when they are combusted Overall for every 6 molecules of CO2 absorbed , 6 molecules of CO2 are emitted. There is no net contribution of CO2 to the atmosphere The term carbon neutral refers to “an activity that has no net annual carbon (greenhouse gas) emissions to the atmosphere”
126.96.36.199 Alcohol production
Students should be able to:
• explain the meaning of the term biofuel
• write equations to support the statement that ethanol produced by fermentation is a carbon-neutral fuel and give reasons why this statement is not valid
• discuss the environmental (including ethical) issues linked to decision making about biofuel use.
Required practical 5: Distillation
A-level Chemistry exemplar for required practical No. 5 – alternative a Distillation of a product from a reaction: To prepare cyclohexene by the dehydration of cyclohexanol and to distil the cyclohexene from the reaction mixture Student sheet Requirements You are provided with the following: semi-micro distillation apparatus OR Quickfit apparatus fitted with a thermometer and collection vessel concentrated phosphoric acid cyclohexanol protective gloves stand and clamp micro-burner 10 cm3 measuring cylinder 25 cm3 measuring cylinder anti-bumping granules separating funnel 250 cm3 beaker 100 cm3 conical flask fitted with a stopper saturated sodium chloride solution anhydrous calcium chloride (or molecular sieves) plastic graduated dropping pipette acidified potassium manganate(VII) solution sample container access to a digital mass balance (reading to 0.1 g). Suggested method The dehydration of cyclohexanol to form cyclohexene This experiment must be carried out in a fume cupboard. a) Pour about 20 cm3 of cyclohexanol into a weighed 50 cm 3 pear-shaped flask. Reweigh the flask and record the mass of cyclohexanol. b) Using a plastic graduated dropping pipette, carefully and with frequent shaking, add to the flask approximately 8 cm3 of concentrated phosphoric acid. c ) Add a few anti-bumping granules to the flask and assemble the semi-micro distillation apparatus, so that the contents of the flask may be distilled. Heat the flask gently, distilling over any liquid which boils below 100 °C. d) Pour the distillate into a separating funnel and add 50 cm 3 of saturated sodium chloride solution. Shake the mixture and allow the two layers to separate. e) Carefully run off the lower layer into a beaker (for later disposal) and then transfer the upper layer, which contains the crude cyclohexene, into a small conical flask. f) Add a few lumps of anhydrous calcium chloride or anhydrous sodium sulfate(VI) or anhydrous magnesium sulfate (or use molecular sieves (4A), if available) to the crude cyclohexene to remove water. Stopper the flask, shake the contents and allow this to stand until the liquid becomes clear. g) Decant the liquid into a clean, dry, weighed sample container. h) Reweigh the container, calculate the mass of dry cyclohexene produced and determine the percentage yield of your product. You should assume that the whole of the dry distillate is cyclohexene. i) Test the distillate as described below, to confirm that it contains an alkene. A test on the product to confirm the formation of an alkene a) To approximately 1 cm 3 of the distillate in a test tube, add an equal volume of acidified potassium manganate(VII) solution. Shake the contents of the test tube well. b) Record your observations. A-level Chemistry exemplar for required practical No. 5 – alternative b Distillation of a product from a reaction: To prepare ethanal by the oxidation of ethanol and to distil the ethanal from the reaction mixture Student sheet Requirements You are provided with the following: simple distillation apparatus OR Quickfit apparatus acidified sodium dichromate(VI) protective gloves stand and clamp 10 cm3 measuring cylinder 25 cm3 measuring cylinder anti-bumping granules test tube thermometer two 250 cm 3 beakers ethanol teat pipette 0.05 mol dm–3 silver nitrate solution 2 mol dm–3 dilute ammonia solution 2 mol dm–3 sodium hydroxide solution 1 mol dm–3 dilute sulfuric acid. Suggested method The oxidation of ethanol to ethanal a) Using a 25 cm3 measuring cylinder, carefully measure out 12 cm3 of the solution of acidified sodium dichromate(VI). Pour this oxidising agent into a boiling tube. You should wear protective gloves when handling the corrosive oxidising agent. b) Cool the boiling tube in cold water in a beaker. c) Using a 10 cm3 measuring cylinder, carefully measure out 2 cm 3 of ethanol. d) Using a teat pipette, slowly add the 2 cm3 of ethanol dropwise, to the oxidising agent in the cooled boiling tube (immersed in cold water in a beaker), shaking the tube gently to mix the contents. e) After the addition of ethanol, add a few anti-bumping granules to the boiling tube and attach to it a bung fitted with a right-angled glass delivery tube. f) Clamp the boiling tube at about 45° in a beaker of water. Heat this beaker of water gently and slowly distil off approximately 5 cm3 of liquid distillate into a test tube which is immersed in cold water in a beaker. Keep the test tube cool to avoid loss of the volatile ethanal. g) Carry out the test described below on the distillate to confirm that ethanal has been formed in this reaction. Test on the distillate to confirm the formation of ethanal Tollens’ silver mirror test: a) Prepare a sample of Tollens’ reagent by adding 5 drops of sodium hydroxide solution to 2 cm3 of silver nitrate solution in a test tube. b) To this test tube add just enough dilute ammonia solution to dissolve the brown precipitate completely. c) Using a beaker of hot water (50–60 °C), gently warm approximately 5 cm3 of this test reagent in a test tube. d) Add 10 drops of the distillate containing ethanal to the warmed Tollens’ reagent in the test tube. Wait a few minutes and note what happens. You should have produced a silver mirror on the walls of the tube. Make sure that you dispose of the Tollens’ reagent thoroughly by rinsing it away with plenty of water and then rinsing any glassware that has contained the reagent with a little dilute sulfuric acid when you are finished.Distillation
In general used as separation technique to
separate an organic product from its reacting
mixture. Need to collect the distillate of the
approximate boiling point range of the desired
Note the bulb of the thermometer should
be at the T junction connecting to the
condenser to measure the correct boiling
Note the water goes in the bottom of the
condenser to go against gravity. This allows
more efficient cooling and prevents back flow
It’s important to be able to
draw and label this
Don’t draw lines between
flask, adaptor and
Required practical 5
Distillation of a product from a reaction.
Credits: Neil Goalby