Learning Objectives. Natural Gas. What is Organic Chemistry? 4/26/2010. Organic Compounds in Our Daily Life

Transcription

1 Learning Objectives State that petroleum (or crude oil) and natural gas (mainly methane) are important sources of energy. Describe petroleum as a mixture of hydrocarbons and it can be separated into its various fractions via fractional distillation. State that hydrocarbons are compounds that are made up of hydrogen and carbon. Describe the order and uses of the various fractions of petroleum. What is Organic Chemistry? Organic Compounds in Our Daily Life Organic chemistry refers to the study of carbon. compounds Carbon is able to form a large number of compounds because it forms stable covalent bonds with other atoms. Glucose (C 6 H 12 O 6 ) methane Organic compounds are made from petroleum (or crude oil) and. natural gas Petroleum and natural gas are hydrocarbons which are compounds made up only of carbon and. hydrogen Petroleum and natural gas are our main source of energy. Natural Gas Natural gas consists mainly of. methane It is used for heating and cooking. Combustion of Petroleum and natural gas: hydrocarbon O2 CO 2 water heat energy Main products 1

2 Petroleum Petroleum (or crude oil) is a dark brown, foul-smelling liquid. Petroleum (or crude oil) is a complex mixture of hydrocarbons. Petroleum can be separated into 7 fractions in oil refineries. The process to separate petroleum into various fractions is called. fractional distillation A fractionating column is used to separate petroleum into various fractions. What happens in an oil refinery? 3. Hot vapour rises up the column and begins to cool down. The smaller hydrocarbons are collected at the top of the fractionating column as gases. The bigger hydrocarbons are collected at the lower sections of the fractionating column. 2. The vapour is pumped into a huge fractionating column. 1. In the furnace at the bottom of the fractionating column, petroleum is heated into a vapour. What happens in an oil refinery? This fraction has the lowest range of boiling points and is distilled over first. Petroleum gas is a mixture of gases including methane, propane and butane. This fraction has the highest range of boiling points. It is the residue at the bottom of the fractionating column. Uses of Petroleum Fractions Fractions Boiling Range (C) # carbon atoms per molecule Uses Petroleum gas Below Fuel for cooking and heating Petrol (gasoline) Fuel for car engines Naphtha Feedstock (raw material) for petrolchemical industry (which produces plastics, detergents, etc) Kerosene (paraffin) Fuel for aircraft engines; for cooking using oil stoves; for heating purposes Diesel oil Fuel for diesel engines Lubricating oil For lubricating machines; for making waxes and polishes Bitumen (asphalt/residue) Above 350 More than 70 For paving road surfaces Alternative Fuels To conserve fuels, we should use these fuels carefully and consider using alternative sources of energy. One possible source of fuel comes from plants such as palm oil. Another important fuel is biogas. It is the gas produced when organic matter (waste material from plants or animals) is allowed to decay in the absence of air. Biogas contains about 50% methane. Alcohol produced from sugarcane mixed with petrol can be used to run vehicle engines. The Oil Refining Industry in Singapore Although Singapore is a country with limited natural resources, it is the world s third largest oil-refining centre and a major refining centre for Southeast Asia. The refineries here refine nearly twice the amount of petroleum consumed in Singapore. The rest of the petroleum is exported to different parts of the world. There are also two industrial plants on Jurong Island, which produce petrochemicals from naphtha fractions. 2

3 Learning Objectives General Introduction to Organic Compounds Describe homologous series as a family of compounds with similar chemical properties and functional group. Describe the general characteristics that will be observed for a homologous series. State the name of organic compounds by combining the prefix (number of carbon atoms) and the suffix (homologous series). Classification of Organic Compounds Homologous Series - Alkanes A homologous series is a family of organic compounds with similar chemical properties. Compounds in the same homologous series have the same functional groups. Functional group is an atom or a group of atoms that gives a molecules its characteristic properties. Propane Homologous Series - Alkenes General Characteristics of Homologous Series Organic compounds in the same homologous series have the following properties in common: a) They have the same functional group. b) They have similar chemical properties. c) There is a gradual change in their physical properties. Propene 3

4 Naming Organic Compounds Part 1 The PREFIX tells us the number of carbon atoms in each molecule. Naming Organic Compounds Part 2 The SUFFIX tells us the homologous series of the compound. First part of the name Number of carbon atoms in each molecule Meth- 1 Eth- 2 Prop- 3 But- 4 Name of ending -ane -ene -ol -oic acid Homologous Series Alkane Alkene Alcohol Carboxylic acid Naming Organic Compounds Propene is an alkene with 3 carbon atoms per molecule. Organic Chemistry: Alkanes Methane Learning Objectives Describe alkane as a homologous series of saturated hydrocarbons with the general formula of C n H 2n+2 Draw the full structure of straight chain alkanes C 1 to C 3 and name them. Describe the general trend in physical properties of alkanes. Describe the substitution reaction of alkane with halogen. Alkanes Alkanes are hydrocarbons that contain only single covalent bonds between the carbon atoms. They are known as saturated hydrocarbons because of the C-C single bond. Alkanes have general chemical formula of C n H 2n+2 where n represents the number of carbon atom. 4

5 Name First 5 members of alkane homologous series # carbon atom Molecular formula Melting Point Boiling point Physical state (r.t.p.) Methane 1 CH Gas Full Structural Formula of C1 to C3 Alkanes The full structural formula shows all the bonds between the atoms in a molecule. This allows us to see the arrangement of the atoms in a molecule. Ethane 2 C 2 H Gas Propane 3 C 3 H Gas Butane 4 C 4 H Gas Pentane 5 C 5 H Liquid Properties of Alkanes Property 1: Melting and boiling point The melting point and boiling point of alkanes increases as the molecular sizes increases. Properties of Alkanes Property 2: Density The density of alkanes increases as their molecular sizes increases. Explanation: As alkane molecules get bigger, the attractive force between the molecules become stronger. More energy is needed to overcome the attractive forces. Properties of Alkanes Property 3: Viscosity Definition of viscosity: the flowability of liquid. More viscous difficult to flow The viscosity of alkane increases (more difficult to flow) as their molecular size increases. Explanation: The intermolecular forces get stronger as the molecular sizes increases, thus, making it more difficult to flow. Properties of Alkanes Property 4: Flammability As the molecular size of alkane increases, there is an increases in the number of carbon in the molecules. This makes alkanes less flammable (more difficult to burn). As the molecular size of the alkane molecules increases, the percentage of carbon in the alkane molecules also increases. Produce a smokier flame, which is caused by incomplete combustion of alkane molecules. 5

6 Properties of Alkanes Property 5: Combustion Alkanes burn readily in air when ignited by a spark or flame to produce CO 2 (g) and H 2 O (g). Equation of combustion (for example Methane): Complete combustion Incomplete combustion Chemical Properties: Substitution Reaction Generally, alkanes are unreactive. Why? Alkanes are saturated hydrocarbons with single carbon-carbon (C C) bonds and carbon-hydrogen (C H) bonds. These bonds are strong and are difficult to break. However, alkanes reacts with halogens (group 7 elements) in the presence of UV (Ultraviolet) light. substitution reaction. Chemical Properties: Substitution Reactions In substitution reaction, the halogen atom will replace one of the hydrogen atom in the alkane. More hydrogen atoms can be replaced by halogen atom to form a mixture of products. The reaction continues until all the hydrogen atoms are replaced by chlorine atoms. Chemical Properties: Substitution Reactions Chemical Reaction: Cracking Alkanes with long carbon chains are usually not used in fuels because they are less flammable. So how can we use these alkanes? Solution: We can break alkanes with long carbon chains into smaller molecules through cracking process. 6

7 Learning Objectives Organic Chemistry: Alkenes Describe alkenes as a homologous series of unsaturated hydrocarbons with the general formula of C n H 2n Draw the full structure of straight chain alkanes C 2 and C 3 and name them. Describe the general trend in physical properties of alkanes. Alkenes Alkenes are hydrocarbons that contain one or more carbon-carbon double bonds between the carbon atoms. They are known as unsaturated hydrocarbons because of the C-C double bond. Functional group of alkenes First 3 members of Alkenes Name # carbon atom Molecular formula Boiling point Physical state (r.t.p.) Ethene 2 C 2 H Gas Propene 3 C 3 H 6-48 Gas Butene 4 C 4 H 8-6 Gas Alkanes have general chemical formula of C n H 2n where n represents the number of carbon atom. An alkene differs from the previous one by an additional CH 2 unit. Structure of Alkenes The first 3 members of the alkene homologous series... Structure of Alkenes The first member of alkene is ethene, not methene. Each alkene differs from the next by a CH 2 unit. 7

8 Structure of Alkenes Full Structural formula of Ethene and Propene Name: Ethene Molecular formula: C 2 H 4 Name: Propene Molecular formula: C 3 H 6 The first few alkenes are gases. Boiling point increases as the alkene size increases. Functional group: C-C double bond Please note that there must only be 4 bonds connecting to each carbon atom. Combustion Chemical Properties of Alkenes Similar to alkanes, Alkenes burn in oxygen to produce CO 2 and H 2 O. C 2 H 4 (g) + 3O 2 (g) 2CO 2 (g) + 2H 2 O(g) There is a higher percentage of carbon in alkenes compared to alkanes, alkenes burn with a smokier flame than alkanes with a similar number of carbon atoms. Addition Reactions The C-C double bonds in alkenes are very reactive, thus it readily combines with other substance to form a saturated organic compound. The C-C double bond will become single bonds in this reaction. General equation of addition reaction: + 3 Addition Reactions 1. Hydrogenation addition of hydrogen to alkenes 2. Bromination addition of bromine to alkenes 3. Hydration addition of steam to alkenes 8

9 Hydrogenation Addition of Hydrogen to Alkenes At 200 C and in the presence of a catalyst such as nickel, alkenes can react with hydrogen to form alkanes. For example when ethene reacts with hydrogen, + Application of Hydrogenation Hydrogenation is used in the production of margarine. Hydrogen is added to vegetable oil (in the presence of nickel catalyst and temperature of 200 C) to produce margarine. The greater the amount of hydrogen added, the more solid is the margarine. Bromination Addition of Bromine to Alkenes When alkene (unsaturated hydrocarbon) is added to bromine solution, it will immediately decolourises reddishbrown bromine to a colourless oil. Bromination Addition of Bromine to Alkenes We can use bromination to test for the presence of an alkene or unsaturation.. For example when ethene reacts with bromine, + If an alkene is present, bromine solution will be rapidly decolourised. Bromination Hydration Addition of Steam to Alkenes Ethene can react with steam to produce ethanol. Conditions: catalyst (phosphoric(v) acid), 300 C and 60 atm + 9

10 Manufacturing Alkenes by Cracking Alkenes are obtained by cracking petroleum (crude oil). Cracking is the breaking down of long-chain hydrocarbons into smaller molecules. Cracking Conditions and Products Cracking is done by passing the petroleum fraction (long chains of carbon atoms) over a catalyst (aluminium oxide or silicon(iv) oxide) at a high temperature (about 600 C). Here, hexane is cracked to produce butane and ethene. 3 possible products Cracking of Alkenes in School Lab Petroleum vapour is passedover a heated porous pot. The porous potcontains the catalysts. The product is a mixture of alkanes and alkenes especially ethene. Why is cracking important? Cracking is used to produce 1. petrol 2. short-chain alkenes 3. hydrogen 1. Cracking is used to produce petrol. Our need for petrol is greater than our need for diesel oil or lubricating oil. 2. Cracking is used to produce short-chain alkenes. Short-chain alkenes such as ethene and propene are used as starting materials for making ethanol and plastics. Through cracking, diesel oil or lubricating oil can be converted into petrol. 10

11 3. Cracking is used to produce hydrogen. Comparing Alkanes and Alkenes Hydrogen is a by-product in the cracking of alkanes. Similarities 1. both alkanes and alkenes are compounds that contains only hydrogen and carbon. Both alkanes and alkenes are flammable. On complete combustion, they produce CO2 and H2O. Comparing Alkanes and Alkenes Differences Properties Alkanes Alkenes Molecular structure Contains C-C single bonds Contains C-C double bonds Reactivity unreactive Very reactive (due to C- C double bonds) Types of reactions Reaction with aqueous bromine Combustion (of alkane and alkene containing same number of carbon atoms) Substitution reaction (with halogen) Do not react with bromine Produce less smoky flame Addition reaction Decolourise aqueous bromine immediately Produce smokier flame Quick Check Read through the following sentences and circle true or false. 1. The physical properties of a homologous series change gradually as the number of carbon atoms increase. True/False 2. The simplest alkene molecules has one carbon atom. True/False 3. Alkenes but not alkanes undergo addition reactions. True/False 4. Bromine solution can be used to distinguish alkanes from alkenes. 5. In the cracking of large alkane molecules, one product is always an alkene. True/False True/False 11