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1.10Unit 1. Structure, Bonding and Introduction to Organic Chemistry

Alkenes — IAL Edexcel Modular (Chemistry) Revision Notes & Topical Questions

5 min read 12 parts Lesson 13 of 20 2 quizzes

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1.10 Alkenes — IAL EDEXCEL MODULAR (CHEMISTRY) notes: Describe the bonding in alkenes, E/Z isomerism, electrophilic addition and its mechanism, addition polymerisation and the disposal of polymers.Alkenes — unsaturated hydrocarbons containing a C=C double bond, with general formu…

  • Position of the double bond
  • Sigma (σ) bond
  • Pi (π) bond
  • Reactivity
  • Restricted rotation
  • CIP tie-break

Section 1 of 12Structure and naming of alkenes

Structure and naming of alkenes
Alkenes
  • Alkenes unsaturated hydrocarbons containing a C=C double bond, with general formula CnH2n
  • The C=C bond is the functional group that lets alkenes react in ways that alkanes cannot.
  • They are named alk + ene.
Position of the double bond
  • Position of the double bond the carbons are numbered from the end nearest the double bond
  • The lowest number of the two carbons in the C=C is written before -ene, as in but-1-ene and but-2-ene.

Section 2 of 12Bonding in alkenes

1Sigma (σ) bond
  • Sigma (σ) bond formed by end-on overlap of orbitals, with the electron pair between the two nuclei
2Pi (π) bond
  • Pi (π) bond formed by sideways overlap of adjacent p orbitals, with the lobes above and below the plane of the atoms
  • The π bond holds two electrons.
3Ethene
  • Ethene each carbon uses three electrons in σ bonds and the fourth electron is in a p orbital
  • Ethene has five σ bonds (four C–H and one C–C) and one π bond.
  • The C=C bond is one σ and one π bond, and the bond angle is 120°, which is trigonal planar.
4Reactivity
  • Reactivity the π bond is weaker than the σ bond and its electron density is above and below the plane, so alkenes are more reactive than alkanes

Section 3 of 12Geometric (E/Z) isomerism

Restricted rotation
  • Restricted rotation the π bond stops rotation about the C=C bond, so groups stay fixed in position
Condition
  • Condition each carbon of the double bond must carry two different groups
  • If either carbon has two identical groups, as in but-1-ene and 2-methylpropene, there are no E/Z isomers.
Cis–trans
  • Cis–trans used when a common group is on each carbon
  • Cis has the two groups on the same side of the C=C and trans has them on opposite sides.
  • But-2-ene shows cis–trans isomerism.
E/Z
  • E/Z used when all four groups differ, based on Cahn–Ingold–Prelog (CIP) priority
  • The group whose first atom has the higher atomic number has the higher priority.
  • Z (zusammen) has the two higher-priority groups on the same side.
  • E (entgegen) has the two higher-priority groups on opposite sides.
CIP tie-break
  • CIP tie-break if the first atoms are the same, compare the next atoms outward
  • –CH2OH outranks –CH2CH3 because oxygen has a higher atomic number than carbon.
  • Cis-but-2-ene is Z-but-2-ene.

Section 4 of 12Electrophilic addition reactions

  • Electrophilic addition
    • an electrophile adds to the electron-rich C=C bond
    • The double bond breaks and a new single bond forms from each carbon.
ReagentConditionsProduct
HydrogenNickel catalyst (hydrogenation, used to harden vegetable oils into margarine)Alkane
Halogen, X2Room temperatureDihalogenoalkane
Hydrogen halide, HXRoom temperatureHalogenoalkane
SteamPhosphoric acid catalyst, high temperature and pressureAlcohol
  • Oxidation
    • cold dilute acidified potassium manganate(VII) turns from purple to colourless and forms a diol
    • It can be treated as oxidation followed by addition of two –OH groups across the C=C.
    • It can distinguish alkenes from alkanes.

Section 5 of 12Test for unsaturation

Test for unsaturation
Shake the compound with bromine water, an orange solution.
If the compound is unsaturated, bromine adds across the C=C bond.
The orange colour disappears, so the solution turns colourless.
Alkanes
  • Alkanes leave bromine water orange in the dark
Side product
  • Side product bromine water also gives some 2-bromoethanol from ethene, because water is in excess

Section 6 of 12Mechanism of electrophilic addition

Hydrogen bromide
  • Hydrogen bromide the H–Br bond is polar, with δ+ on hydrogen
  • The π electrons attack the δ+ hydrogen, drawn as a curly arrow from the C=C to H.
  • The H–Br bond breaks heterolytically and the bonding pair moves to Br, forming Br-.
  • A carbocation intermediate forms.
  • A lone pair on Br- attacks the C+ to give the bromoalkane.
Bromine
  • Bromine the non-polar Br–Br bond is polarised as it approaches the double bond
  • The high electron density of the C=C repels the electron pair in Br–Br, making the nearer Br δ+.
  • The nearer Br acts as an electrophile and the Br–Br bond breaks heterolytically to give Br-.
  • Br- attacks the carbocation to form the dibromoalkane.

Section 7 of 12Unsymmetrical alkenes and carbocation stability

1Stability of carbocations
  • Stability of carbocations tertiary > secondary > primary
  • Alkyl groups push electron density towards the positive carbon.
2Major and minor products
  • Major and minor products the more stable carbocation leads to the major product
  • Propene + HBr can form two carbocations.
  • The secondary carbocation is more stable, so 2-bromopropane is major and 1-bromopropane is minor.

Section 8 of 12Addition polymerisation

Addition polymerisation
  • Addition polymerisation many alkene monomers join into a long chain as the only product
  • The π bond in each monomer breaks and new C–C single bonds form.
  • Poly(ethene) forms from ethene and PVC from chloroethene.
Polymer
Polymer a long-chain molecule made of many repeating units; monomer = the small reactive molecule
Repeat unit
  • Repeat unit drawn in square brackets with extension bonds and the C=C changed to C–C
  • Propene gives –[CH2–CH(CH3)]–.
Finding the monomer
  • Finding the monomer restore the C=C between the two main-chain carbons of the repeat unit
  • The repeat unit in the main chain has only two carbons.
  • The monomer is the repeat unit with C=C instead of C–C.

Section 9 of 12Disposal of polymers

Poly(alkenes)
  • Poly(alkenes) saturated and non-polar, so chemically unreactive and non-biodegradable
Landfill
  • Landfill takes up valuable land, and plastics do not decompose for hundreds of years
Incineration
  • Incineration releases heat but produces carbon dioxide, a greenhouse gas
  • Burning PVC releases toxic hydrogen chloride.
  • HCl is removed by passing waste gas through a base or carbonate: CaO + 2HCl → CaCl2 + H2O.
Recycling
  • Recycling reduces landfill and incineration and conserves crude oil
  • Less energy is needed to recycle plastics than to make them.
Biodegradable polymers
  • Biodegradable polymers broken down by microorganisms into CO2, water and other small molecules
  • Polyesters and polyamides can be hydrolysed, unlike the non-polar backbone of poly(alkenes).
Compostable polymers
  • Compostable polymers usually plant-based, such as starch bin liners and sugar-cane fibre plates and cups
  • They break down in compost and leave no harmful residue.

Section 10 of 12More from the specification

  • Bromine water test as a diagnostic means
    • rapid electrophilic addition across carbon-carbon double bonds provides a simple chemical means to detect unsaturated alkenes.

Section 11 of 12Key terms

TermMeaning
AlkeneAn unsaturated hydrocarbon containing at least one C=C bond.
Sigma bondA bond from end-on overlap of orbitals along the line between two nuclei.
Pi bondA bond from sideways overlap of p orbitals above and below the bond axis.
E/Z isomerismStereoisomerism caused by restricted rotation about C=C when each carbon carries two different groups.
ElectrophileA species that accepts a pair of electrons to form a new covalent bond.
CarbocationA positive ion with a positively charged carbon atom.
Addition polymerisationJoining of many alkene monomers into a long chain with no other product.
Repeat unitThe smallest group of atoms that repeats along a polymer chain.

Section 12 of 12Exam technique

1
Saying cis–trans isomers form when a carbon has two identical groups — each carbon needs two different groups
2
Starting the electrophilic addition arrow at the H–Br bond — the first arrow comes from the C=C bond
3
Naming the major product of propene + HBr as 1-bromopropane — the more stable secondary carbocation gives 2-bromopropane
4
Drawing the monomer as the repeat unit — the monomer has C=C and the repeat unit has C–C with extension bonds
5
Saying addition polymers are biodegradable — their saturated non-polar chains are inert

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