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There are loads of ways of representing organic compounds. General formula - an algebraic formula that can describe any member of a group of compounds. Empirical formula - simplest ratio of atoms of each element in a compound. Molecular formula - The actual number of atoms of each element in a molecule. Structural formula - Shows the atoms carbon by carbon, with attached hydrogens and functional groups. Displayed formula - Shows how all the atoms are arranged and the bonds between them. Skeletal formula - Shows the bonds of the carbon skeleton only, with any functional groups shown. Hydrogen and carbon atoms are not shown. Nomenclature is a fancy word for the naming of organic compounds. Rules of nomenclature: Count the carbon atoms in the longest continuous chain - this gives the stem. E.g 1 carbon - meth, 2 - eth, 3 prop, 4 but. And the other have the stems with the shape with the same number of sides. The main functional group of the molecule usually gives you the end of the name. E.g alkanes -ane, alkenes -ene. Number the carbons in the longest carbon chain so that the carbon with the functional group attached has the lowest possible number. If there's more than one, pick the one with the most side chains. Write the carbon number that the functional group is on before the suffix. Any side-chains or less important functional groups are added as prefixes at the start of the name. If there's more than one identical side-chain or functional group, use di-(2), tri-(3) or tetra-(4) before that part of the name - but ignore this when working out the alphabetical order. Homologous compounds have the same general formulas. Each member of a series has the same functional group but differs by -CH2- in its carbon chain. You need to know about alkanes, alkenes, alcohols and halogenoalkanes for AS organic chemistry. Note: If the double bond in an alkene could go in more than one place, you have to say which carbon it starts on.
Alkanes and Structural Isomerism
Alkanes are saturated hydrocarbons. Alkanes have the general formula CnH2n+2. They've only got carbon and hydrogen atoms, so they're hydrocarbons. Every carbon atom in an alkane has four single bonds with other atoms. It's impossible for carbon to make more than four bonds, so alkanes are saturated. Cycloalkanes have carbon atoms arranged in a ring with the general formula CnH2n, but they're still saturated. Structural isomers have different arrangements of the same atoms. In structural isomers the atoms are connected in different ways, but have the same molecular formula. Chain isomers have different arrangements of the carbon skeleton. Some are straight chains and others branched in different ways. To show the difference you write out their structural formulas, as their displayed formulas would be the same. Don't be fooled - what looks like an isomer might not be. Atoms can rotate as much as they like around C-C single bonds. Remember this when you work out structural isomers - sometimes what looks like an isomer, isn't. E.g there are only three chain isomers of C5H12, because everything else you draw will be the same as one of them, no matter how you draw it. Alkanes burn completely in oxygen. If you burn (oxidise) alkanes with oxygen, you'll get carbon dioxide and water - this is a combustion reaction. Combustion reactions only happen between gases, so liqiud alkanes have to be vaporised first. Smaller alkanes turn into gases more easily (they're more volatile) so they'll burn more easily too. Halogens react with alkanes, forming halogenoalkanes. A hydrogen aom is substituted by chlorine or bromine in a photochemical reaction (a reaction started by UV radiation). This is a free-radical substitution reaction. Free radicals are particles with unpaired electrons. For example, chlorine and methane react to make chloromethane: CH4 + Cl2 --> CH3Cl + HCl. The reaction mechanism has three stages. Initiation reactions - where free radicals are produced. Sunlight provides the energy to break the Cl.Cl bond (photodissociation). The bond splits equally and each atom keeps one electron - homolytic fission. They become highly reactive free radicals because of the unpaired electron. Propagation reactions - free radicals are used up and created in a chain reaction. Cl. attacks a methane molecule making CH3. and HCl. The new methyl free radical, CH3., can attack another Cl2 molecule making CH3Cl and a Cl free radical. The Cl free radical can attack another methane molecule, and so on, until all the chlorine or methane is used up. Termination reactions - free radicals are mopped up. If two free radicals join together they make a stable molecule. There are usually many of them.
Crude oil is a mixture of hyrdocarbons. Crude oil is mostly alkanes. They range from small alkanes like pentane to ones with more than 50 carbons. Crude oil can be seperated into useful fractions by fractional distillation. The crude oil is vaporised at about 350 degrees celsius. The vaporised crude oil goes into the fractionating column and rises up through the trays. The largest hydrocarbons are not vaporised at all, so they run to the bottom of the column. As the crude oil vapour goes up the fractionating column, it gets cooler. Because of the different chain lengths, each fraction condenses at a different temperature. The fractions are drawn off at different levels in the column. The small hydrocarbons don't condense. They're drawn off as gases at the top of the column. Heavy fractions can be 'cracked' to make smaller molecules. People want the lighter fractions, not so much the heavy ones. Cracking is breaking long-chain alkanes into smaller hydrocarbons (which can include alkenes). It involbes breaking the C-C bonds. Decane could be cracked to make ethene and octane for example. Thermal cracking is cracking that takes place at a high temperature and pressure (up to 1000 degrees celsius and 70 atm). It produces a lot of alkenes. These alkenes are then used to make more useful prodcuts like polymers. Catalytic cracking mostly makes motor fuels and aromatic hydrocarbons. It uses a zeolite catalyst, at a slight pressure and high temperature (about 450 degrees celsius). Using a catalyst cuts costs, because the reaction can be done more easily and cheaply. Fuels contain a mixture of types of alkane. Most people's cars run of petrol ro diesel, which contain a mixture of alkanes. Some of the alkanes in petrol are straight-chain alkanes like hexane. Petrol also contains some shorter, branched-chain alkanes like 2,3-dimethylbutane, cycloalkanes and aromatic hydrocarbons. These make the fuel burn more efficiently. Catalytic cracking produces aromatic hydrocarbons. Straight-chain alkanes can also be reformed to make cycloalkanes and more aromatic hydrocarbons. Alkanes can be reformed into cycloalkanes and aromatic hydrocarbons. It uses a catalyst e.g platinum stuck on aluminium oxide. E.g hexane can be reformed to make cyclohexane and a hydrogen molecule, which makes benzene and three hydrogen molecules.
Fuels and Climate Change
Alkanes are useful fuels. When you burn an alkane you get carbon dioxide and water. It's an exothermic reaction. Alkanes release energy when burned which we can use. But alkanes produce harmful emissions, like sulfur oxides, which are dangerous. They are poisonous and dangerous for asthma sufferers. They also dissolve in moisture in the air and make sulfuric acid, which causes acid rain that makes lakes and rivers acidic, and damages trees and buildings. It also releases carbon monoxide and hydrocarbon particles. This happens when there is incomplete combustion. Carbon monoxide is poisonous, and unburned hydrocarbon particles get in the air, leaving soot on buildings and cause health problems. Nitrogen oxides are also produced, which add to the acid rain problem and cause breathing problems(nitrogen dioxide). Nitrogen dioxide also makes ground-level ozone, an air pollutant. Greenhouse gases are produced. Greenhouse gases like carbon dioxide and water vapour work by absorbing reflected radiation from the Sun and re-emitting it back to the surface. This is the greenhouse effect which keeps our planet warm. But, global warming will probably cause significant climate change. Too much of the greenhouse effect causes global warming. Over the last two centuries, greenhouse gas concentration has been increasing, mainly because of us putting more carbon dioxide in the air. The extra carbon dioxide enhances the greenhouse effect, causing the Earth to warm up. Global warming is causing huge problems: The ice caps are melting, and sea levels are rising because of that and warming the water makes it expand. The world's climate is changing already - and climate change is likely to cause all sorts of problems, like severe shortages of food and water. Burning fossil fuels just isn't sustainable. Fossil fuels are non-renewable, and will run out some day. Because burning fossil fuels isn't sustainable, scientists are trying to develop alternative fuels, ones that are renewable and don't damage the climate.
Alkenes, Hazards and Risks
Alkenes are unsaturated hydrocarbons. Alkenes have the general formula CnH2n. They are hydrocarbons, with at least one carbon double covalent bond. They are unsaturated because carbon can make more bonds with extra atoms in addition reactions. Compounds with benzene ring structures are called arenes or aromatic compounds. Alkenes are much more reactive than alkanes. Each double bond in an alkene is made up of a sigma bond and a pi bond. Because there are two pairs of electrons in the bond, the carbon double bond has a high electron density that makes it pretty reactive. Another reason for the high reactivity is that the pi bond sticks out above and below the double bond, which makes it likely to be attacked by electrophiles. Adding hydrogen to carbon double bonds produces alkanes, such as ethene, which will react with hydrogen gas to produce ethane. Similar reactions happen with other alkenes. Electrophilic addition reactions happen to alkenes. Electrophilic addition reactions of alkenes work like this: The double bond opens up, and another atom is added to each of its carbons. Addition reactions happen because the double bond has got plenty of electrons and is easily attacked by an electrophile. The double bond is nucleophilic - it's attracted to places that don't have enough electrons. Electrophiles are electron-pair acceptors. They are attracted to electron-rich areas as they don't have enough. Electrophiles include positively charged ions and polar molecules. Using organic chemicals can be hazardous. Most organic chemicals are flammable - a flame symbol. Some are highly flammable, like ethanol and methane. An explosion symbol means that the gas is liable to explode if released into the air. A cross with an i next to it means irritant - it will irritate your skin, but wont cause permanent damage. A cross with a h next to it means harmful - harmful chemicals can damage your health, but most wont kill you. The ones that will are called toxic chemicals - a skull and crossbones sign. These chemicals can kill you if you swallow them, inhale them, or sometimes if they touch your skin. A symbol with a tree and dead fish means harmful to the environment. This appears on chemicals that cause serious environmental damage like hexane. A hazard is anything that can cause harm. A risk is the chance that what you're doing will cause harm. A risk assessment can help to make lab work safer. A risk assessment looks at the hazards of the reactants, products and procedures involved in an experiment and considers how to make the risks as small as possible. You can reduce risks by working on a smaller scale, taking appropiate precautions, and using different safer chemicals or lower concentrations if possible, but you can't get rid of all risk. A risk assessment minimises the risks.
Reactions of Alkenes
Alkenes undergo electrophilic addition with halogens. If you mix an alkene and bromine the bromine adds across the double bond to form a dibromoalkane. The double bond repels the electrons in Br2, polarising Br-Br. Heterolytic fission of Br2. The closer Br gives up the bonding electrons to the other Br and sticks to the C atom. You get a positively charged carbocation intermediate. The Br- now zooms over and bonds to the other C atom, forming 1,2-dibromoethane. If you add some Cl- ions to an ethene and bromine mixture, you'll also get some 1-bromo-2chloroethane. This is evidence for the mechanism - once the ethene has reacted with bromine to form the carbocation it can react with either another bromide ion or a chloride ion. Alkanes also undergo electrophilic addition reactions with hydrogen halides. For example ethene reacts with hydrogen bromide to form bromoethane. It is the same as before except with polar HBr instead of Br2. Adding hydrogen halides to unsymmetrical alkenes forms two products. Adding to a symmetrical molecule will mean only one product, but adding a hydrogen halide to an umsymmetrical alkene will mean there's at least two possible products. The amount of each product formed depends on how stable the carbocation formed in the middle of the reaction is. Carbocations with more alkyl groups are more stable, because the alkyl groups feed electrons towards the positive charge. E.g when hydrogen bromide reacts with propene, you get mostly 2-bromopropane, and not much of 1-bromopropane. Use bromine water to test for carbon double bonds. When you shake an alkene with orange bromine water, the solution decolourises. Bromine water is a dilute solution with more water than bromine. The carbocation is more likely to react with water than Br-, so an OH group adds to the second carbon rather than another Br. The proudct is mostly bromoalcohol. Alkenes are oxidised by acidified potassium manganate(VII). Shaking an alkene in a solution of acidified potassium manganate(VII), the purple solution is decolourised. You've oxidised the alkene and make a diol, an alcohol with two -OH groups. This reaction is another useful test for a carbon double bond.
Double bonds can't rotate. Carbon atoms in a carbon double bond and the atoms bonded to them are planar (in the same plane). Because of the way they're arranged they're actually said to be trigonal planar. Ethene is completely planar, but in larger alkenes only the carbon double bond unit is planar. Atoms can't rotate around the carbon double bond due to the pi bond formed by p-orbitals overlapping. In fact they don't bend much either. Things can still rotate around single bonds in the molecule. The restricted rotation causes E/Z isomerism. E/Z isomerism is a type of stereoisomerism. Stereoisomers have the same structural formula but a different arrangement of atoms. Some alkenes can have stereoisomers due to lack of rotation around the double bond. Stereoisomers occur when the two double-bonded carbon atoms each have different atoms or groups attached to them. then you get an E-isomer or a Z-isomer (also called trans- and cis- isomers respectively in some cases). For example, the double-bonded carbon atoms in but-2-ene each have an H and a CH3 group attached. When the same groups are across the double bond, it's called an E-isomer. If they are on opposite sides, then it's a Z-isomer. E/Z isomers can sometimes be called cis-trans isomers. E-but-2-ene can be called trans-but-2-ene, and Z-but-2-ene can be called cis but-2-ene. But if the carbon atoms both have totally different groups attached to them, the cis-trans system stops working. The E/Z system still works, because each group is given a priority. If the two carbon atoms have their 'higher priority group' on opposite sides, it's an E-isomer. If they are on the same side, it's a Z-isomer. In the E/Z naming system, Br has a higher priority than F, so the names depend on where the Br is in relation to the Ch3 group.
Alkenes join up to form addition polymers. The double bonds in alkenes can open up and join together to make long chains called polymers. The individual, small alkenes are called monomers. This is called addition polymerisation. Poly(ethene) is made by the addition polymerisation of ethene. The part in brackets is the repeating unit, where the n represents the number of repeat units. Because of the loss of the double bond, poly(alkenes) like alkanes, are unreactive. To find the monomer used to form an addition polymer, take the repeat unit and add a double bond. Waste plastics can be buried. The unreactive nature of most polymers leads to a problem. Most polymers aren't biodegradable. You can just bury it - take it to a landfill site, compact it, then cover it with soil. This method's used when the plastic is difficult to seperate from other waste, not in sufficient quantities to make seperation worth it, or too difficult technically to recycle. Waste plastics can be recycled, after the plastics have been sorted into different types. Some plastics like poly(propene) can be melted and remoulded. Some can be cracked into monomers, and these can be used to make more plastics or other chemicals. Waste plastics can be burned, if they can't be recycled. The process needs to be carefully controlled to reduce toxic gases. Polymers that contain chlorine produce HCl when burned, which has to be removed. Waste gases are passed through scrubbers which can neutralise gases like HCl by reacting with a base. Biodegradable polymers decompose in the right conditions. Biodegradable polymers decompose pretty quickly in certain conditions - because organisms can digest them. Biodegradable polymers can be made from materials like starch and from the hydrocarbon isoprene (2-methyl-1,3-butadiene). Therefore biodegradable polymers can be produced from renewable raw materials or from oil fractions. Using renewable raw material means they wont run out, they will absorb carbon dioxide produced when the polymer biodegrades (because the polymer comes from plants), and plant-based polymers save energy compared to oil-based plastics. Polymers still need the right conditions to decompose. You need to put them where there is moisture and oxygen available. But you still need to collect and seperate the biodegradable polymers from non-biodegradable plastics. There are various potential uses, e.g plastic sheeting used to protect plants from the frost can be made from poly(ethene) with starch grains embedded in it. The starch is broken down by microorganisms and the remaining poly(ethene) crumbles into dust.