Presentation on theme: "Alcohols. Learning Objectives Candidates should be able to: recall the chemistry of alcohols, as exemplified by ethanol, including their oxidation to."— Presentation transcript:
Learning Objectives Candidates should be able to: recall the chemistry of alcohols, as exemplified by ethanol, including their oxidation to carbonyl compounds and carboxylic acids. classify hydroxy compounds into primary, secondary and tertiary alcohols. suggest characteristic distinguishing reactions, e.g. mild oxidation.
Starter activity Complete task 1 on your worksheet.
Production of alcohol MethodRate of Reaction Quality of product Raw material Type of process Finite resource? hydrationfastAlmost 100% ethenecontinuousYes fermentationslow~15%natural sugars batchNo
Classes of alcohols
Oxidation of alcohols Primary alcohols Secondary alcohols
Oxidation of alcohols Primary alcohol aldehyde carboxylic acid e.g. ethanol ethanal ethanoic acid Secondary alcohol ketone no reaction e.g. propan-2-ol propanone Tertiary alcohol no reaction
Oxidising agent Cr 2 O 7 2- Cr 3+ Orange Green
Heating under reflux Prevents evaporation of aldehyde
Physical properties and general chemistry
Learning Objectives Candidates should be able to: recall the chemistry of alcohols, exemplified by ethanol: combustion substitution to give halogenoalkanes reaction with sodium dehydration to alkenes
Starter activity Complete task 1 on your worksheet.
AlcoholMrMr Boiling point (K) ethanol46352 propan-1-ol60371 butan-1-ol74390 propane44231 butane58273 pentane72309 Physical properties – boiling point
The boiling point of an alcohol is always much higher than that of an alkane with similar Mr. This is due to hydrogen bonding between the alcohol molecules. In alkanes the only intermolecular forces are vdW.
Physical properties – solubility The small alcohols are completely soluble in water.
Physical properties – solubility In both pure water and pure ethanol the main intermolecular forces are hydrogen bonds. In order to mix them together you must supply energy to break these bonds.
Physical properties – solubility Fortunately, alcohols can form hydrogen bonds with water: H – OH R - OH
Physical properties – solubility This bond-making process releases energy which more or less compensates for the energy input. N.B. an alcohol only has one + H atom, unlike water which has 2.
Physical properties – solubility The hydrocarbon chains in an alcohol cannot form hydrogen bonds. As the chains get longer they force their way between water molecules preventing them from hydrogen bonding. Too little energy is released from bond-making to compensate for that used in bond-breaking.
Physical properties – boiling point The boiling point of an alcohol is always much higher than that of an alkane with similar Mr. This is due to hydrogen bonding between the alcohol molecules. In alkanes the only intermolecular forces are vdW.
Ethanol can be used as a fuel, mainly as a biofuel alternative to gasoline. Because it is easy to manufacture and process and can be made from renewable resources such as sugar cane and corn, it is an increasingly common alternative to gasoline in some parts of the world, e.g. Brazil. Anhydrous ethanol (ethanol with less than 1% water) can be blended with gasoline in varying quantities and most gasoline engines will operate well with mixtures of 10% ethanol (E10). General chemistry – combustion
Ethanol – CH 3 CH 2 OH Chemistry can involve breaking the C-O bond and the O-H bond. General Chemistry - electron density map
General chemistry – reaction with Sodium 2Na + 2HO-H 2NaOH + H 2 2Na + 2RO-H 2NaOR + H 2 NaOR is more usually written as RONa or RO - Na + 2Na + 2CH 3 CH 2 OH 2CH 3 CH 2 O - Na + + H 2 sodiumethanol sodium ethoxide hydrogen
General chemistry – substitution to form halogenoalkanes Consider the reaction below: CH 3 CH 2 Br + OH - CH 3 CH 2 OH + Br - You should remember that this hydrolysis reaction occurs quite readily in a warm, aqueous solution. The reverse reaction is more difficult, -OH is not a good leaving group. Presence of H + required to form water, a much better leaving group.
General equation The general reaction looks like this: ROH + HX RX + H 2 O
Rather than using hydrobromic acid, you usually treat the alcohol with a mixture of sodium or potassium bromide and concentrated sulphuric acid. This produces hydrogen bromide in situ which reacts with the alcohol. The mixture is warmed to distil off the bromoalkane (see page 338 of your textbook). Replacing –OH by bromine
Replacing –OH by iodine Iodoalkanes can be made by two different methods. In the first method the alcohol is reacted with a mixture of sodium or potassium iodide and concentrated phosphoric(V) acid, H 3 PO 4, and the iodoalkane is distilled off.
In the second method the iodoalkane can be made by warming the alcohol with a mixture of red phosphorus and iodine: This then reacts with the alcohol to give the corresponding halogenoalkane which can be distilled off. Replacing –OH by iodine
Replacing –OH by chlorine Chloroalkanes are more easily made by the second route: you can react an alcohol with phosphorus(III) chloride, PCl 3, phosphorus(V) chloride, PCl 5, or sulphur dichloride oxide (thionyl chloride, SOCl 2 ). CH 3 CH 2 OH + PCl 5 CH 3 CH 2 Cl + HCl + POCl 3 CH 3 CH 2 OH + SOCl 2 CH 3 CH 2 Cl + HCl + SO 2
Elimination Elimination (when a small molecule is removed from a larger molecule – converts a single bond to a double bond) C 2 H 5 OH (g) CH 2 =CH 2(g) + H 2 O (g)
Aldehydes and Ketones
Learning Objectives Candidates should be able to: describe the reduction of aldehydes and ketones e.g. using NaBH 4. describe the mechanism of the nucleophilic addition reactions of hydrogen cyanide with aldehydes and ketones. describe the use of 2,4-dinitrophenylhydrazine (2,4-DNPH) to detect the presence of carbonyl compounds. deduce the nature (aldehyde or ketone) of an unknown carbonyl compound from the result of simple tests (i.e. Fehling’s or Tollens’ reagents; ease of oxidation). describe the formation of carboxylic acids from nitriles.
Starter activity Can you write balanced equations for the synthesis of chloro-, bromo- and iodoethane from ethanol? Give the names of the reagents and the reaction conditions.
Using Tollens' reagent (the silver mirror test) Tollens' reagent contains the diamminesilver(I) ion, [Ag(NH 3 ) 2 ] +. To carry out the test, you add a few drops of the aldehyde or ketone to the freshly prepared reagent, and warm gently in a hot water bath for a few minutes. ketone No change in the colourless solution. aldehyde The colourless solution produces a grey precipitate of silver, or a silver mirror on the test tube. Aldehyde or Ketone
Aldehyde or Ketone A few drops of the aldehyde or ketone are added to the reagent, and the mixture is warmed gently in a hot water bath for a few minutes. ketone No change in the blue solution. aldehyde The blue solution produces a dark red precipitate of copper(I) oxide. Using Fehling's solution
Reduction of aldehydes and ketones – H 2 butanone butan-2-ol prop-2-enal propan-1-ol
Reduction of aldehydes and ketones – NaBH 4 ethanal ethanol propanonepropan-2-ol
Reduction of aldehydes and ketones – NaBH 4 prop-2-enal prop-2-en-1-ol
Formation of hydroxy nitriles (or cyanohydrins)
Reactions of nitriles Hydrolysis Reduction
Aldehyde or ketone? 2,4-DNPH Test: Add a solution of 2,4-dinitrophenylhydrazine (2,4-DNPH). Result: a deep yellow or orange precipitate
Learning Objectives Candidates should be able to: describe the reactions of carboxylic acids in the formation of salts.
Starter activity – Starter activity – can you complete task 1? methanoic acid2-methylbutanoic acid hexanedioic acid
Acidity of the carboxylic acids Reaction with metals
Neutralisation reactions Reaction with carbonates 2H + (aq) + CO 3 2- (aq)/(s) H 2 O (l) + CO 2(g) Reaction with hydrogencarbonates H + (aq) + HCO 3 - (aq)/(s) H 2 O (l) + CO 2(g) Reaction with alkalis CH 3 COOH (aq) + NaOH (aq) CH 3 COO - Na + (aq) + H 2 O (l)
Learning Objectives Candidates should be able to: describe the formation of esters from carboxylic acids using ethyl ethanoate as an example. Describe the acid and base hydrolysis of esters. State the commercial use of esters, e.g. solvents, perfumes, flavourings.
Naming esters Notice that the ester is named the opposite way around from the way the formula is written. The "ethanoate" bit comes from ethanoic acid. The "ethyl" bit comes from the ethyl group on the end.
Esterification and hydrolysis c. H 2 SO 4 warm dil. H 2 SO 4 reflux
Base hydrolysis dilute reflux This reaction is irreversible !! Note: These reaction is exactly the reverse of those used to make an ester from a carboxylic acid and an alcohol. The only difference in that case is that you use a concentrated acid as the catalyst. To get as much ester as possible, you wouldn't add any water otherwise you would favour the hydrolysis reaction
Alkaline hydrolysis of fats or oils Because of its relationship with soap making, the alkaline hydrolysis of fats and oils is sometimes known as saponification.
Uses of esters SolventsFlavourings Fragrances Adhesives