Organic chemistry

  • Created by: larissa67
  • Created on: 26-08-18 06:52

Functional groups

  • Within a group of compounds, they all have the same atom or groups of atoms, known as a functional group.
  • The functional group in a group of compounds characterises how they behave chemically.
  • Alchohol functional group = O-H
  • Alkene functional group = C=C double bond
  • Alkane functional group = C-C single bond
  • Carboxylic acid functional group = -COOH
  • Ester functional group = -O- (alkyl group)
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Bonding in organic molecules

  • There are 2 types of bonding in organic molecules
    • 1. Sigma bonding
    • 2. Pi bonding
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Sigma bonding

  • Single covalent bonds in a compound are known as sigma bonds. 
  • The pair of electrons in a sigma bond is found between the nuclei of 2 atoms that are sharing the electrons.
  • Electrostatic attraction between negative electrons + positive nuclei bonds atoms to eachother.
  • Each carbon can form 4 pi bonds in a compound.
  • The 4 bonding pairs of electrons around each carbon atom repel eachother.
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Pi bonding

  • The pi bond is made up of double carbon bonds, such as the one in alkenes.
  • The carbon atoms involved in a pi bond each form 3 sigma bonds, leaving one carbon atom with a spare outer electron in a 2p orbital.
  • When 2 of these 2p orbitals overlap, it forms a pi bond.
  • The pi bonds are made up of 2 lobes that lie above and below a plane.
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Structural isomerism

  • Structual isomers are compounds with the same molecular formula, but a different structural formulae
  • There are 3 types of structural isomerism
    • 1. Positional isomerism
    • 2. Functional group isomerism
    • 3. Chain isomerism
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Structural isomerism; position

  • In postional isomerism, it is the position of the functional group that changes place in each isomer
  • There is free rotation around single C-C bonds, but NOT in double C=C bonds.
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Structural isomerism; functional group

  • In functional group isomerism, it is the functional group that changes in each isomer.
  • For example, for a compound such as C2H8O, we can draw both an alcohol and an ether.
  • The two isomers that can be drawn have different functional groups, so therfore have different chemical properties & chemically behave differently.
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Structural isomerism; chain

  • In chain isomerism, isomers differ in terms of their carbon skeleton and how it is arranged.
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Stereoisomerism

  • Stereoisomerism is when we have compounds that have the same molecules bonded to eachother but have different arrangements in space.
  • There are 2 types of stereoisomerism;
    • Cis-trans isomerism
    • Optical isomerism
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Stereoisomerism; cis-trans

  • Arrangements can result in cis-trans isomerism because there is restricted rotation about the double C=C bond
  • Cis isomerism means that both of the SAME type of atom are arranged on the same side of the molecule, imagine "cis" = "sis", and sisters stay together
  • Trans isomerism means that the same molecules are arranaged so that they are across from eachother, going in a diagonal line, imagine "trans" = "across", so they are diagonal
  • Whenever we have compounds with a double C=C bond, they can display cis-trans isomerism
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Stereoisomerism; optical

  • If a molecule contains a carbon atom that is bonded to 4 different atoms/groups, it can form 2 optical isomers
  • The 2 different molecules are mirror images of eachother 
  • The carbon atom with 4 different groups attached is called the chiral centre of the molecule
  • A pair of optical isomers will rotate the plane of polarised light by equal amounts, but in different directions 
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Organic reactions & mechanisms

  • There are 2 ways in which a covelant bond can be broken;
    • 1. Homolytic fission
    • 2. Heterolytic fission
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Homolytic fission

  • Type of bond breaking
  • This type of bond breaking is when an electron is taken by each of the atoms from the covalent bond
  • The species porduced when a bond breaks homolytically is called a free radical 
  • All free radicals have an upaired electron and are very reactive
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Heterolytic fission

  • This type of bond breaking involves the "uneven" breaking of the covalent bond between atoms
  • In heterolytic fission, the more electronegative atom takes both of the electrons
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Carbocations

  • Carbocations are formed when a carbon atom has 3 bonds and one positive charge
  • They are an alkyl atoms 
  • There are 3 types of carbocations
    • 1. Primary carbocations (least stable)
    • 2. Secondary carboctions
    • 3. Tertiary carbocations (most stable)
  • The carbocations become more stable when they have more alkyl groups
  • Alkyl groups, eg. CH3, C2H5 tend to push electrons away from themselves
  • This is called the positive inductive effect of alkyl groups
  • This means that the more alkyl groups in a carbocation, the more spread out the chrage density is on the carbocation, so they are more stable
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Electrophile & Nucleophiles

  • An electrophile is an acceptor of electrons
  • A nucleophile is a donator of a pair of electrons
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Types of organic reaction

  • Addition = involves the formation of a single product from 2 reactanct molecules
    • Eg. C2H4 + Br2 = C2H4Br2
  • Elimination = the removing of a small molecule from a larger one
    • Eg. C2H5OH = C2H4 + H2O
  • Substitution = the replacement of one atom by another
    • Eg. CH4 + Cl2 = CH3Cl + HCl
  • Hydrolysis = breakdown of a molecule using water 
    • Eg. C2H5Br + H2O = C2H5OH + NaBr
  • Oxidation = loss of electrons from a species 
  • Reduction = gain of electrons from a species
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Reactions of the alkanes; combustion

  • If an alkane is burnt in an excess of oxygen, it undergoes complete combustion
  • The carbon - gets fully oxidised to carbon dioxide
  • The hydrogen - gets fully oxidised to water
    • Alkane + oxygen = carbon dioxide + water
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Reactions of the alkanes; substitution

  • Alkanes undergo substitution reactions with halogen in sunlight, but will not take place in darkness (it is explained why later)
  • A hydrogen in the alkane molecule gets replaced by a halogen molecule
    • Eg. CH4 + Cl2 = CH3Cl + HCl
  • There are 3 steps to substitution reactions
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Substitution; initiation

  • The first step is called the initiation step. 
  • The bond between the two halogen atoms (we will use chlorine in this example) is broken by UV light from the sun
    • Cl2 = 2Cl*
    • Here is an example of homolytic fission, both of the chlorine atoms takes an electron from the covalent bond to form a free radical
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Substitution; propogation

  • Free radicals are very reactive, and will attck the alkanes.
    • CH4 + Cl* = CH3 + HCL
    • Here, a methyl free radical is produced
  • This can then attack a chlorine molecule
    • *CH3 + Cl2 = CH3Cl + Cl
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Substitution; termination

  • When 2 free radicals meet, they will react with eachother 
  • They only produce 1 single product
  • As no more free radicals are made, the chain reaction stops
    • CH3 + Cl = CH3Cl
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