AQA Chemistry Unit 2: 15 Alkenes

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15.1 Alkenes

Alkenes are unsaturated hydrocarbons

  • Made only of Carbon and Hydrogen
  • They have one or more Carbon Double Bonds
  • The double bond makes the molecule more reactive then alkanes
  • The general formula: CnH2n

Shape of Alkenes

  • Ethene is a planar molecule so its angles between each bond is 120 degree
  • There is no rotation around the double bond
  • There is no rotation due to the pi bond formed from the cloud of electron density above and below the single bond
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Alkenes Cont

Isomers

  • Alkenes with more than 3 carbons can form different isomers
  • Two types of isomers can be formed involving the double bond:
    • Position Isomers
    • Geometrical Isomers
  • Position isomers are where the double bond is positioned differently
  • Geometrical isomers are a form of stereoisomerism, they stereoisomers have the same structural formula but different bonds are arranged differently, it occurs around the double bond
  • When two types of the same group e.g. -CH3 are on the same side of a double bond this is called Z and when the groups are on opposite sides this called E
    • When atoms of higher atomic number are on the same side this is Z
    • When atoms of higher atomic number are opposite sides this is E
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Alkenes Cont

Physical Properties

  • Van der Waals are the only intermolecular forces acting between alkene molecules
  • Physical properties are similar to alkanes
  • The melting and boiling point increase with the number of carbon atom
  • Alkenes are not soluble in water as they are non-polar

How Alkenes React

  • The bond enthaply for a C=C bond is almost double of a C-C bond
  • Alkenes are more reactive than alkanes
  • C=C forms an electron rich area in a molecule which can be easily attacked by a positively charged reagent called an electrophile
  • Electrophiles are electron pair acceptors such as H+
  • The reaction is known as electrophilic addition
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15.2 Reactions of Alkenes

  • Alkenes can burn in air to form CO2 and H2O

Electrophilic Addition Reaction

  • The four electrons in the C=C bond make an alkene a centre of high electron density
  • Electrophiles are attracted to it and form a bond by using 2 of its 4 electrons in the C=C bond
  • The Mechanism:
    • Electrophile is attracted to the double bond
    • Electrophiles are positively charged and accept a pair of electrons from the double bond
    • A positive ion (carbocation) is formed
    • A negatively charged ion forms a bond with the carbocation
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Reaction with Hydrogen Halides

Reaction with Hydrogen Halides

  • Form a haloalkane
  • In a hydrogen halide the halogen is more electronegative
  • The electrophile is the H delta plus
  • The H is attracted to the C=C due to its high electron density
  • One of the Carbons bonds with the hydrogen to form a carbocation
  • The electrons in the H-Halogen bond are drawn towards the halogen
  • The bond in the H-Halogen breaks heterolytically so both electrons go to the halogen since it is more elctronegative
  • The Halogen ion then attaches to the positively charged carbon of the carbocation forming a bond with one of its electron pairs
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Reactions of Asymmetrical Alkenes

Asymmetrical Alkenes

  • When a double bond isn't exactly in the middle of the chain there are more than one possible product
  • Some alkyl groups have a tendency to release electrons this is positive inductive effect
  • The electron releasing effect tends to stabilise the positive charge of the carbocation
  • The more alkyl groups the more stable the carbocation
  • This means that a tertiary carbocation is more stable to a primary carbocation

 

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Reactions of Alkenes with Halogens

Reaction of Alkenes with Halogens

  • Alkenes react rapidly with chlorine gas, or solutions of bromine and iodine
  • A dihaloalkane is formed
  • The halogen acts as an electrophile as it has an instantaneous dipole
  • The delta positive end attracts to the carbon double bond of the alkene
  • The electrons in the double bond repel the electrons of the Br-Br bond which strengthens the dipole of the bromine molecule
  • One of the bromine ions joins with a carbon from the double bond leaving a carbocation
  • The Br- ion that is now formed then joins to the carbocation forming the dihaloalkane

The Addition takes place in two steps:

  • The formation of the carbocation by electrophilic substitution
  • Rapid reaction with a negative ion
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Reaction with Concentrated Sulfuric Acid

Test for a Double Bond

  • Add a few drops of bromine solution and with a double bond it will decolourise from red

Reaction with Concentrated Sulfuric Acid

  • Reaction occurs at room temperature and is exothermic
  • The electrophile is the partially positively charged hydrogen atom from the sulfuric acid
  • The sulfuric acid molecule: H-OSO3H
  • The carbocation formed then reacts rapidly with the negatively charged hydrogensulfate ion

Addition of Water

  • When water is added an alcohol is formed alongside sulfuric acid
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Reaction with Water

  • When an alkene reacts with steam it forms an alcohol
  • A suitable temperature and pressure is used
  • An acid catalyst such as phosphoric acid is used
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15.3 Polymerisation of Alkenes

Additional Polymerisation

  • Alkenes can polymerise, joining together to from a long chain with high relative molecular masses
  • Ethene is a monomer that can form Poly(ethene)
  • This reaction is additional polymerisation
  • The polymer is an alkane so is unreactive

The Repeating Unit

  • The smallest group of atoms that produce the polymer when repeated over and over
  • In polyethene the repeating unit is CH2CH2
  • Different forms of poly(ethene) can be made depending on the conditions of temperature, pressure and catalyst
  • The forms differ in chain length and branching
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