Introductory Organic Chemistry

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Hazard and Risk in Organic Chemistry

  • A hazard is something with a definitive potential to cause harm
  • This is absolute- something either is or isn't hazardous.
  • Types of hazard are: flammable, corrosive, toxic and oxidising.
  • These hazards are all represented by different hazard symbols.
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Hazard and Risk in Organic Chemistry (cont)

  • Risk is the probability of a hazard causing active harm.
  • This is variable- the chance of something happening depends on the conditions.
  • A risk assesment identifies the hazards involved in an activity (does not reduce risk).
  • Once hazards are identified, steps are taken to reduce the risk of the hazards happening.
  • Risk can be reduced by using less reactive material, using lower concentrations, using electrical heating instead of a bunsen flame, wearing correct clothing
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Organic Compounds and Functional Groups

  • Carbon is unique in its ability to form 4 covalent bonds with either other carbon atoms or non-metals.
  • This is the main property that allows carbon to form so many organic covalent compounds.
  • Organic compounds are part of different homologous series, where all compounds have similar physical properties, but gradually change as number of carbon atoms increases.
  • They all also have the same general formula, and the same functional group(s)
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Naming and Drawing Organic Compounds

  • The systematic name of an organic compound gives: the number of carbon atoms, the structure (straight, branched or ring), its homologous series, and the name of any non-carbon atoms.
  • The prefix (fisrt part) of the name gives the number of carbon atoms in the longest chain.
  • The suffix (second part) of the name gives the homologous series which it belongs to.
  • -Ane = alkane, -oic = carboxylic acid, -al = aldehyde.
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Naming and Drawing Organic Compounds (cont)

  • We also have to include branches, funtional groups, and carbon=carbon double bonds in the name. 
  • These are numbered in respect to the longest carbon chain, starting from the side that gives the smallest numbers.
  • Multiple side chains or funtional groups are listed in alphabetical order.
  • Commas separate numbers, hyphans separate words from numbers. 
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Naming and Drawing Organic Compounds (cont)

Name this compound:

                                      (http://www.docbrown.info/page15/Image723.gif)

Answer: 1-bromo-1,2-dichlorobutane

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Alkanes and Alkenes

  • Alkanes are saturated hydrocarbons (contain maximum number of hydrogens), which means they only have single bonds.
  • Alkanes with the same molecular formula but different structures are called structural isomers

                                   (http://library.thinkquest.org/C006439/organic/images/alkanes.gif)

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Alkanes and Alkenes (cont)

  • Alkanes are used as fuels. They burn in excess oxygen to produce carbon dioxide and water.
  • Alkanes are produced by the fractional distillation of crude oil. Fractions are mixtures of hydrocarbons.
  • Long alkanes can be cracked to yield smaller, more useful molecules (alkane forms shorter alkane and a very short alkene).
  • Reforming is when long chain molecules are broken into smaller, branched or aromatic compounds (lower boiling point). 
  • Reforming and cracking are thermal decompositon.
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Alkanes and Alkenes (cont)

  • Crude oil is a non-sustainable resource- it will eventually run out.
  • Moreover, the increased use of fossil fuels is increasing CO2 emission, causing global warming.
  • Alternative fuels try to reduce greenhouse gas emissions and increase sustainability.
  • Hydrogen burns to make water only. It can also be combined with oxygen in a fuel cell to produce electricity.  Water is, however, a greenhouse gas.
  • Also, the energy used to produce the hydrogen is from burning hydrocarbons in power plants. 
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Alkanes and Alkenes (cont)

  • Biofuels are made from plants, which absorb carbon dioxide from the atmosphere as they grow, and then release it when the biofuel is burned.
  • Omitting the energy requirements for the manufacturing process, biofuels are more carbon neutral than coal, oil and gas.  
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Alkanes and Alkenes (cont)

  • Alkenes are unsaturated hydrocarbons, they contain C=C double bonds.
  • The C=C consists of a sigma and a pi bond.
  • In sigma bonds the electron cloud is concentrated between 2 nuclei (all single bonds).
  • In pi bonds, the electron cloud is above and below the plane of the molecule.
  • The pi bond does not allow rotation
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Alkanes and Alkenes (cont)

         (http://wiki.chemprime.chemeddl.org/images/8/88/Sigma-pi_double_bond.jpg)

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Naming Geometric Isomers

  • Geometric isomers occur due to the lack of rotation around a C=C double bond.
  • Different groups can therefore be arranged on different sides of the molecule.
  • E-Z system is used to name geometric isomers.
  • Each group is ranked by atomic number (higher atomic number, higher rank).
  • An 'E' isomer has the two highest ranked groups on opposite sides.
  • A 'Z' isomer has the two highest ranked groups on the same side
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Naming Geometric Isomers (cont)

                         (http://www.chemguide.co.uk/basicorg/isomerism/ez6.gif)

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Reactions of Alkanes

  • When burned in a limited supply of oxygen, alkanes undergo incomplete combustion and form carbon monoxide and water (oxygen is still O2).
  • Carbon monoxide is odourless, invisible and toxic- it can be fatal
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Reactions of Alkanes (cont)

  • In a substitution reaction, one atom/group is replaced with another atom/group.
  • Chlorine will replace hydrogen in methane to form chloromethane.
  • This is free-radical substitution.
  • A free-radical is a species with an unpaired electron, such as Cl(http://www.chemguide.co.uk/mechanisms/freerad/electron.GIF).
  • This is formed from the dissociation of chlorine at about 300'C or in UV light
  • The free-radical subsitution by chlorine is a 3 step mechanism.
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Reactions of Alkanes (cont)

  • Initiation- UV light provides energy to break Cl-Cl bond, generating 2 free-radicals. The electron pair in the covalent bond is split so each Cl takes an electron with it. This is called homolytic fission.
  • Propagation- a Cl(http://www.chemguide.co.uk/mechanisms/freerad/electron.GIF) reacts with a hydrogen in methane to form HCl and leaves a (http://www.chemguide.co.uk/mechanisms/freerad/electron.GIF)CH3 free-radical. The (http://www.chemguide.co.uk/mechanisms/freerad/electron.GIF)CH3 reacts with chlorine to produce chloromethane, leaving a Cl(http://www.chemguide.co.uk/mechanisms/freerad/electron.GIF) free radical.
  • Termination- the combination of any of the free-radicals with eachother or with themselves produces stable molecules, eliminating the free radicals. 
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Reactions of Alkenes

  • The C=C double bond in alkenes makes them very reactive, because they can react across the double bond- an addition reaction.
  • This forms a single, saturated product.
  • In catalytic hydrogenation, hydrogen is added to an alkene to produce an alkane at 200'C in the presence of a high surface area nickel catalyst.
  • When an alkene is reacted with acidified potassium manganate, the alkene is oxidised as as addition takes place, forming a diol.
  • potassium manganate changes purple to colourless.
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Reactions of Alkenes (cont)

  • When bromine approaches the C=C double bond, its electron cloud shifts due to the repulsion from the dense C=C; producing an instantaneous dipole.
  • The +ve part of the bromine acts as an electrophile and attracts an electron pair from the double bond, forming a C-Br bond on a carbocation, and leaving a bromide ion with an electron pair.
  • The bromide ion (nucleophile) attacks the carbocation, forming another C-Br bond.
  • The whole process is called electrophilic addition, because the first step is electrophilic. 
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Reactions of Alkenes (cont)

(http://alevelchem.com/img/electrophilic_addition_bromine.gif)

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Reactions of Alkenes (cont)

  • The electrophilic addition mechanism for the addition of HBr is the same as Br2, except no initial repulsion is needed, because H-Br already has a periminant dipole
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Reactions of Alkenes (cont)

  • In the addition of bromine to propene, a primary or a seconday carbocation can be formed, producing either 1-bromopropane or 2-bromopropane
  • The seconday carbocation contains more methyl groups on either side, which donate electron density and stabilise the carbocation more than in a primary carbocation.
  • The more stable carbocation is more likely to form, so the major product is 2-bromopropane and the minor product is 1-bromopropane. 
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Reactions of Alkenes (cont)

  • If a compound contains a C=C double bond, it will decolourise bromine water.
  • This mechanism is the same as the addition of bromine, however an OH- from the water can attack the carbocation instead of the bromide ion. 
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Polymers

  • A monomer is a small molecule.
  • A polymer is formed when a large number of monomers join together to form a chain.
  • Alkenes undergo addition polymerisation- addition occurs across the double bond.
  • A repeat unit is the monomer with the double bond replaced by a single bond and 2 side links drawn.

                  (http://www.chemistrydaily.com/chemistry/upload/2/27/Ethene_polymerization.png)

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Polymers (cont)

  • Polymers are derived from crude oil, so they have high-energy production costs. They also use up non-renewable resources.
  • Polymers are non-biodegradable so they forever occupy landfill space, and they burn to produce toxic gas.
  • Using renewable energy reduces the use of fossil fuels, and recycling solves the issue of disposal of polymers. 
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