Form 5 Chemistry Notes Ms. R. Buttigieg Pg. 20 Topic 2 Organic Chemistry Organic Chemistry is the chemistry of compounds, which are found in living things or are derived from living things. The basis of these molecules is carbon chains or rings. A Homologous series is a family of compounds. E.g. Alkanes, alkenes, alkynes, alcohols, fatty acids. Members of a series have: 1. Same general molecular formula, each member differing from the next by CH 2. 2. Similar chemical reactions 3. All possess the same functional group 4. Physical properties of members change gradually in the same direction along the series. e.g. melting points and boiling points and density rise as molecular mass increases. Alkanes (C n H 2n+1 ) See GCSE pg. 175, C4U pg. 159 1. Alkanes are hydrocarbons. 2. They have only single bonds between carbon atoms. 3. b.p and m.p. and density increase with chain length i. C 1 to C 4 are gases at room temperature ii. C 5 to C 15 are liquids iii. C 16 upwards are solids 4. They are volatile compared to ionic compounds. E.g. tar with RMM>500 melts on hot days While NaCl with RMM of 58.5 melts at 808 o C 5. Alkanes are found in natural gas and crude oil. 6. Insoluble in water. 7. Soluble in organic solvents like benzene and tetrachloromethane (CCl 4 ) 8. Very unreactive due to strong C-C and C-H bonds. 9. Do not react with metals, aqueous oxidising agents, acids or alkalis. 10. Burn. Combustion is very exothermic so used as fuels. CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2H 2 O (l) Make sure you can draw Methane to decane structures When there is little oxygen, C and CO form as well as CO 2. CO is poisonous so a danger.
Form 5 Chemistry Notes Ms. R. Buttigieg Pg. 21 11. Alkanes react with halogens by substitution reactions. This reaction is catalysed by light (U.V.) and is explosive in bright sunlight. The reaction is slower with bromine. Nothing happens with iodine. 12. Alkanes readily decompose when they are strongly heated, the higher members splitting more readily. This is the basis of cracking. Cracking helps to produce more petrol. High temperatures are used and a catalyst, finely divided aluminium oxide and silicon (IV) oxide speed up the reaction.
Form 5 Chemistry Notes Ms. R. Buttigieg Pg. 22 The products of crude oil, which are produced by a specific number of carbon atoms in a hydrogen chain at a particular temperature, are given in this table:
Form 5 Chemistry Notes Ms. R. Buttigieg Pg. 23 Answer the following: 1. Which one of the following compounds is not a hydrocarbon? C 2 H 4 C 2 H 6 O C 6 H 6 C 4 H 10 2. Which one of the compounds below isn t an alkane? C 6 H 12 C 7 H1 6 C 10 H 22 C 40 H 82 3. Write down a use for each of the following alkanes a. methane: b. propane: c. octane: 4. Write down the equations for burning of: a. methane: b. propane: Saturated Compounds contain only single bonds between carbon atoms. So alkanes are saturated hydrocarbons. Compounds which have double or triple bonds between carbon atoms are said to be unsaturated. E.g. alkenes and alkynes. They are called saturated as more atoms can be added to the molecules. Isomers compounds which have the same molecular formula but a different structural formula. E.g. Butane Isobutane
Form 5 Chemistry Notes Ms. R. Buttigieg Pg. 24 ETHENE A colourless gas Insoluble in water. It has a faint sweet smell, About the same density as air Burns with a yellow flame (luminous flame) Decolourises bromine water and acidified potassium permanganate. Produced industrially by cracking heavier fractions from crude oil. Laboratory preparation of ethene Ethene is produced by the dehydration of ethanol by hot, concentrated sulphuric acid. 100cm 3 of concentrated sulphuric acid (excess is used) are mixed with 50cm 3 of ethanol. This is done slowly and while cooling under running cold water as when the sulphuric acid is mixed with ethanol, a lot of heat is given out. Then some sand (acid washed) is placed in the flask so that the ethene forms on the sand and so avoid frothing. The contents are then heated on a sand-bath to about 180 o C. The ethene given off is passed through sodium hydroxide solution to remove impurities (sulphur dioxide and carbon dioxide) and collected over water. The liquid darkens in the flask due to carbon formed by charring. C 2 H 5 OH (aq) + H 2 SO 4 (aq) C 2 H 5 HSO 4 (aq) + H 2 O (l) C 2 H 5 OH - H 2 O C 2 H 4
Form 5 Chemistry Notes Ms. R. Buttigieg Pg. 25 JL Annual 2006 - Section 1
Form 5 Chemistry Notes Ms. R. Buttigieg Pg. 26 Properties of Alkenes (taking ethene as example) 1. More reactive than alkanes due to unstable Carbon-Carbon double bond. 2. Undergo addition reactions due to their double bond. a. With Hydrogen A nickel catalyst at 150 o C. Called Sabatier s Reaction Used in hardening oils to make margerine Oils are solidified by reacting them with hydrogen. As their RMM increases they solidify. This is also called a hydrogenation reaction as hydrogen is added. Ethene Hydrogen Ethane b. With water - e.g. Ethene With water ethanol is formed. This is the industrial preparation of ethanol. Takes place at 300 o C, 60 atm and over a catalysts of phosphoric acid. Ethene water ethanol 3. Ethene is oxidised with acidified or alkaline potassium permanganate C 2 H 4 + H 2 O + [O] CH 2 OH. CH 2 OH Ethan-1,2-diol is used as a coolant in aircraft engines, an antifreeze solution for motor car radiators and in the production of terylene. 4. Ethene burns. C 2 H 4 (g) + 3 O 2 (g) 2 CO 2 (g) + 2 H 2 O (g)
Form 5 Chemistry Notes Ms. R. Buttigieg Pg. 27 5. Ethene can undergo polymerisation. This happens when molecules of ethene add on to each other to form giant molecules of poly-ethene or polythene. Polythene is a white solid. High temperatures and pressure are needed and a catalyst (trace of oxygen) 6. Ethene is used: to make plastics (polythene, terylene, polystyrene, PVC), ethanol (in methylated spirits), and 1,2-dibromoethane (an important additive in petrol). JL2006 annual
Form 5 Chemistry Notes Ms. R. Buttigieg Pg. 28 Alkynes These are hydrocarbons which have a triple bond between 2 of the carbon atoms. Physical properties 1. They are insoluble in water. 2. They are quite soluble in the usual organic solvents of low polarity (e.g. ligroin, ether, benzene, carbon tetrachloride, etc.). 3. They are less dense than water. 4. Their boiling points show the usual increase with increasing carbon number. 5. They are very nearly the same as the boiling points of alkanes or alkenes with the same carbon skeletons. Chemical properties They burn in air to produce carbon dioxide and water. The ethynes are highly dangerously explosives when mixed with air or oxygen. They undergo addition reactions. Naming alkynes Number of Carbons Chemical formula Structural formula Ethyne Propyne Butyne The alkynes are said to be unsaturated because of the existence of a multiple bond in the molecule. The general structure of the alkyne series of hydrocarbons is C n H 2n-2.
Form 5 Chemistry Notes Ms. R. Buttigieg Pg. 29 1. Copy and complete the following on the HW copybook. Hydrocarbons are compounds containing the elements and. If they are saturated, the atoms of carbons are bound together by bonds. On the other hand, unsaturated hydrocarbons include at least one or bond. One example of unsaturated hydrocarbons is. This compound burns in a plentiful supply of air to produce and. It also reacts with bromine water changing the colour of the solution from to. (5 marks) 2. The statements below describe the chemistry of carbon and some of its compounds. Write brief notes to show that you understand the meaning of these statements with particular reference to the underlined terms. 3. This question concerns the laboratory preparation of ethene and its properties. a) Ethene can be prepared in the laboratory by dehydration of ethanol. Explain this statement, including essential conditions and an equation for the reaction. (A diagram is not required). (6 marks) b) Ethene undergoes a number of addition reactions such as halogenation, hydration and hydrogenation. For each of these addition reactions, (i) name the substance that adds on the ethene, (ii) give an equation for the reaction. (9 marks) c) Ethene burns with a smoky flame. Give a reason for this observation and write an equation for the reaction. (3 marks) d) Give two uses of ethene. (2 marks)
Form 5 Chemistry Notes Ms. R. Buttigieg Pg. 30 Alcohols (see Chemistry for You pg. 178 182, GCSE pg. 203) These contain the OH group. The OH group hydroxyl group does not form hydroxide ions in solution. Alcohols are NOT alkaline. Naming alcohols Number of Carbons Chemical formula Structural formula Methanol Ethanol Propanol Alcoholic drinks contain ethanol. That s what makes them alcoholic. So beer, wine and whisky all contain ethanol. If alcohol is taken in excess it: Slows down our reactions Makes us overconfident Damages the liver Is addictive Laboratory Preparation of Ethanol - by fermentation Sugar is dissolved in water. Some yeast is added. The mixture is put in a conical flask. A delivery tube leads to a test-tube containing calcium hydroxide solution (lime-water). The flask is left in a warm place for several days. The contents in the flask begin to froth. A gas is given out. When it is bubbled through the calcium hydroxide solution, this turns milky, indicating it is carbon dioxide. C 12 H 22 O 11 (s) + H 2 O (l) 2 C 6 H 12 O 6 (aq) sugar glucose C 6 H 12 O 6 (aq) 2 C 2 H 5 OH (aq) + 2 CO 2 (g)
Form 5 Chemistry Notes Ms. R. Buttigieg Pg. 31 These two reactions happen through the action of yeast. After several days the contents are found to contain between 5 and 10% ethanol. Pure ethanol can be obtained by fractional distillation. Ethanol can be tested by burning as it burns easily with a pale yellow flame. Industrial Preparation of Ethanol Ethene is reacted with steam to form ethanol. Temp: 300 o C Pressure: 65 atm Catalyst: Phosphoric Acid C 2 H 4 (g) + H 2 O (l) C 2 H 5 OH (aq) Properties of Ethanol 1. Miscible with water 2. Colourless liquid (b.p. 78 o C) 3. Very good solvent for molecular substances 4. Burns very easily with a pale yellow flame C 2 H 5 OH (l) + 3 O 2 (g) 2 CO 2 (g) + 3 H 2 O (l) 5. Ethanol reacts with sodium. Hydrogen gas is given off. 2 C 2 H 5 OH (l) + Na (s) 2 C 2 H 5 ONa (aq) + H 2 (g) If the excess ethanol is evaporated, sodium ethoxide is obtained as a white solid. Sodium ethoxide is ionic: C 2 H 5 O - Na + 6. Ethanol reacts with phosphorus pentachloride, PCl 5 C 2 H 5 OH (l) + PCl 5 (l) C 2 H 5 Cl (g) + POCl 3 (l) + HCl (g) It is a vigorous reaction in which steamy fumes of hydrogen chloride are formed. 7. Ethanol reacts with concentrated sulphuric acid that acts as a dehydrating agent. a. If it is in excess at a temperature of 180 o C C 2 H 5 OH - H 2 O C 2 H 4 b. If it is in excess at a temperature of 145 o C only 1 molecule of water is extracted from 2 molecules of ethanol. 2 C 2 H 5 OH - H 2 O C 2 H 5 OC 2 H 5 ethoxyethanoate Work these out on your copybook - Chemistry For You Pg. 185 numbers 1, 2 Pg. 254 numbers 11, 14, 15; Pg. 255 numbers 16, 19
Form 5 Chemistry Notes Ms. R. Buttigieg Pg. 32 8. Ethanol reacts with ethanoic acid to form an ESTER C 2 H 5 OH (l) + CH 3 COOH (l) CH 3 COOC 2 H 5 (l) + H 2 O (l) To make an ester, we use: 4 cm 3 of ethanol 2 cm 2 ethanoic acid, some concentrated sulphuric acid some antibumping granules. The contents are heated under reflux in a water bath for 25 minutes, then poured in a brine solution. Oily drops are seen on the surface smelling of pear drops. This is the ester ethyl ethanoate. 9. Ethanol can be oxidised to ethanoic acid, if we use acidified potassium dichromate as an oxidising agent. The ethanol is oxidised to ethanoic acid. C 2 H 5 OH (l) + 2 [O] CH 3 COOH (l) + H 2 O (l) The orange potassium dichromate turns to green. The experiment is performed in the following way: Mix ethanol with acidified potassium dichromate. Heat under reflux for 15 minutes. Fractionally distillate the contents to obtain ethanoic acid. Wine goes sour as the ethanol in it is oxidised to ethanoic acid (vinegar). The oxidation is catalysed by enzymes in the wine, using oxygen from the air Esters have a pleasant fruity smell. They are found in fruit, flowers and are used in perfumes. Fats are complex esters. See GCSE Chemistry, page 208, 209 Work out pg. 208 no. 2 Pg. 276 no. 76 Pg. 277 no. 78, 80
Form 5 Chemistry Notes Ms. R. Buttigieg Pg. 33 Number of Carbons Alkanes Alkenes Alkynes 1 2 3 4 Organic Acids An organic compound is an acid if it has the COOH group. This COOH is called a carboxyl group. Acid Chemical formula Chemical Structure Ethanoic acid Propanoic Acid. Properties 1. These acids are weak acids. i.e. they are not fully dissociated in solution. CH 3 COOH (aq) CH 3 COO - (aq) + H + (aq) 2. When pure they are not acidic, since it is in water that they break up into ions. 3. Ethanoic acid is a weak electrolyte. Like all acids, it reacts with metals, alkalis and carbonates. 2 CH 3 COOH (aq) + Ca (s) (CH 3 COO) 2 (aq) + H 2 (g) CH 3 COOH (aq) + NaOH (aq) CH 3 COONa (aq) + H 2 O (g) 2 CH 3 COOH (aq) + Na 2 CO 3 (s) 2CH 3 COONa (aq) + CO 2 (g) + H 2 O (g) Laboratory preparation of ethanoic Acid see previous page oxidation of ethanol
Form 5 Chemistry Notes Ms. R. Buttigieg Pg. 34 Synthetic Polymers These can be either addition or condensation polymers. Addition Polymers Polystyrene poly(phenyl ethene) Perspex Note: The above are addition polymers. Units are added together as the monomers are unsaturated. The polymer than gives a saturated product. Condensation Polymers Nylon Terylene Note: i. Formation of the above polymers requires two types of monomer molecules, each with two reactive groups. ii. Each time a bond is formed, a small molecule such as water is removed.
Form 5 Chemistry Notes Ms. R. Buttigieg Pg. 35 Answer the following questions: 1. The following represent two monomers, which react together to form a polymer. H 2 N - - NH 2 COOH - - COOH a. Name the type of polymerisation b. What is the substance lost during polymerisation c. Draw a diagram to show the structure of the polymer formed from, at least three monomers. d. Give the name of one example of this type of polymer. e. Another type of polymer, poly(chloroethene), is formed by the polymerisation of chloroethene. The following diagram represents part of the poly(chloroethene chain). - CH 2 CH CH 2 CH CH 2 CH CH 2 CH Cl Cl Cl Cl Draw a diagram to represent a single molecule of chloroethane JL Annual 2001 Past paper questions related to organic chemistry 2
Form 5 Chemistry Notes Ms. R. Buttigieg Pg. 36 JL Annual 2002 Past paper questions related to organic chemistry 3 JL Annual 2001 Past paper questions related to organic chemistry 4