* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download ALCOHOLS
Survey
Document related concepts
Ring-closing metathesis wikipedia , lookup
Elias James Corey wikipedia , lookup
George S. Hammond wikipedia , lookup
Asymmetric induction wikipedia , lookup
Ene reaction wikipedia , lookup
Hofmann–Löffler reaction wikipedia , lookup
Physical organic chemistry wikipedia , lookup
Tiffeneau–Demjanov rearrangement wikipedia , lookup
Kinetic resolution wikipedia , lookup
Baylis–Hillman reaction wikipedia , lookup
Wolff–Kishner reduction wikipedia , lookup
Petasis reaction wikipedia , lookup
Strychnine total synthesis wikipedia , lookup
Transcript
ALCOHOLS Alcohols are compounds in which one of the hydrogen atoms of an alkane has been replaced by a hydroxyl group. They have the general formula C nH2n+1OH. The functional group (OH) gives alcohols their characteristic properties. The first members of the series are: CH3OH methanol CH3CH2OH ethanol CH3CH2CH2OH propan-1-ol CH3CH(OH)CH3 propan-2-ol Ethanol Production Ethanol is produced on an industrial scale by two separate and distinct processes: the fermentation of carbohydrates and the hydration of ethene. 1. Fermentation In fermentation, the enzyme zymase, which is present in the cells of yeast, converts sugars (normally glucose) into ethanol and carbon dioxide. C6H12O6 2C2H5OH + 2CO2 The reaction is sluggish at low temperatures, but if the temperature is raised too far, the enzyme is denatured. The normal temperature range is 25 to 35 oC. The reaction typically takes 3 to 5 days. Glucose is used in solution at a concentration of about 15-20% by mass; if this is exceeded, the ethanol produced in the reaction reaches a concentration sufficient to kill the yeast before the fermentation is complete. 2. Hydration of ethene This is the method currently favoured in the U.K. The hydration of ethene is carried out by the direct reaction of ethene and steam. The conditions used are: H H C H temperature: 300oC pressure: 65 atmospheres catalyst: phosphoric acid (H3PO4) adsorbed on celite + C H 2O H TOPIC 12.15: ORGANIC CHEMISTRY – ALCOHOLS 1 H H H C C H OH H Comparison of the two methods Fermentation Rate of reaction Type of process Raw material Purity of product Technology Labour Hydration of ethene slow batch renewable (sugars) impure (dilute solution) cheap, low-tech equipment unskilled; more manpower fast continuous non-renewable (ethene from oil) pure expensive, high-tech equipment semi-skilled; less manpower Ethanol as a biofuel Ethanol is now being used more and more a fuel. Ethanol produced by the process of fermentation can be mixed with petrol to produce a biofuel. A biofuel is a fuel that has been made from plant matter that is recently dead. One problem with this approach is that land previously used to grow sugar cane for the human food chain is now being used to grow sugar to make biofuels. However, ethanol produced in this way is considered to be carbon neutral since all of the CO2 released during its combustion was removed during photosynthesis by the growing crop. It can not be considered to be 100% carbon neutral, however, as making the fertilizer to grow the sugar cane and fuelling agricultural machinery to harvest and transport it, will invariably involve the burning of fossil fuels. Classification of alcohols Alcohols are usually classified as primary, secondary or tertiary. This classification has value since the three categories of alcohols often behave differently in chemical reactions. OH H C H R primary OH OH R C H secondary C R R R (1o) R (2o) TOPIC 12.15: ORGANIC CHEMISTRY – ALCOHOLS 2 tertiary (3o) Oxidation of alcohols When alcohols are heated with acidified potassium dichromate: 1. Primary alcohols are oxidised initially to aldehydes: CH3CH2OH + [O] ethanol CH3CHO + H2O ethanal The aldehyde can be distilled out of the reaction mixture as it is formed. (See practical: Experiment 12.15.1). If, however, the reaction is carried out under reflux, the aldehyde is oxidised further to a carboxylic acid: CH3CHO + [O] CH3COOH ethanoic acid The oxidising agent changes colour from orange to green. Cr2O72- + 14H+ + 6eorange 2Cr3+ + 7H2O green 2. Secondary alcohols are oxidised to ketones, which are not oxidised further. CH3CH(OH)CH3 propan-2-ol + [O] CH3COCH3 + H2O propanone The oxidising agent changes colour from orange to green. 3. Tertiary alcohols are not oxidised. The oxidising agent stays orange. The reaction with acidified potassium dichromate(VI) distinguishes tertiary alcohols from primary and secondary alcohols. Distinguishing between ketones & aldehydes It is not easy to distinguish between primary and secondary alcohols, but their oxidation products, aldehydes and ketones, can be differentiated. Aldehydes and ketones both contain a carbonyl group (>C=O). In aldehydes, the carbonyl carbon is at the end of a carbon chain, in ketones it is in the middle of a chain. 1. Tollen’s reagent Tollen’s reagent is prepared by adding concentrated aqueous ammonia dropwise to silver nitrate solution until the initial brown precipitate of silver oxide (Ag2O) just redissolves to give a clear, colourless solution. The solution contains the complex ion, diamminosilver(I), [Ag(NH3)2]+. When aldehydes are warmed with Tollen’s reagent, they reduce it to silver, which is deposited as a mirror on the walls of the test tube. There is no reaction with ketones. RCHO + 2[Ag(NH3)2]+ + 3OH- RCOO- + 2Ag + 4NH3 + 2H2O TOPIC 12.15: ORGANIC CHEMISTRY – ALCOHOLS 3 2. Fehling’s solution Fehling’s solution comprises two solutions, A & B, which are mixed in equal volume immediately prior to use. Solution A contains copper sulphate; solution B is a strongly alkaline solution of potassium sodium tartrate. When solutions A and B are mixed, a deep blue solution containing a complex copper(II) ion is formed. When aliphatic aldehydes are warmed with Fehling’s solution, they reduce it to copper(I) oxide, Cu2O, which appears as a brick red precipitate. There is no reaction with ketones. RCHO + 2Cu2+ + 5OH- RCOO- + Cu2O + 3H2O In both these reaction, the aldehyde is oxidised to the carboxylate anion. Elimination reaction of alcohols If alcohols contain a hydrogen atom on the carbon atom adjacent to the one to which the OH group is attached, i.e. the partial structure: H they can be dehydrated to alkenes by heating to about 180oC with a catalyst of concentrated sulphuric acid or concentrated phosphoric acid. (See experiment 12.15.2). H H H C C H OH C OH H H H C H C + C H 2O H The mechanism for this process is shown over the page. NB If the alcohol undergoing dehydration is unsymmetrical, more than one isomeric alkene can be produced. TOPIC 12.15: ORGANIC CHEMISTRY – ALCOHOLS 4 Mechanism: If a H is lost from the C atom to the left of the alcohol group (blue arrow) If a H is lost from the C atom to the right of the alcohol group (red arrow) + H2O + H+ but -1- ene but -2- ene NB don’t forget the other products of this reaction are water (this is dehydration) and H+ (which was the catalyst). TOPIC 12.15: ORGANIC CHEMISTRY – ALCOHOLS 5