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1.11Unit 1. Structure, Bonding and Introduction to Organic Chemistry

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

5 min read 12 parts Lesson 13 of 36 1 quiz

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Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
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Lesson summary

1.11 Alkenes — bonding, isomerism, addition reactions and polymers — IAL EDEXCEL LINEAR (CHEMISTRY) notes: Describe bonding in alkenes, deduce E/Z isomers, explain electrophilic addition and the bromine test, and work with addition polymers and their disposal.Alkenes are unsatura…

  • Sigma (σ) bond
  • Pi (π) bond
  • Addition polymerisation
  • Repeat unit
  • E/Z isomerism
  • Electrophile

Part 1 of 10Alkenes and the C=C bond

Alkenes and the C=C bond
Alkenes are unsaturated hydrocarbons with general formula CnH2n and a C=C functional group, which lets them react in ways alkanes cannot.
Named alk + ene.
  • For chains of four or more carbons the position of C=C is given by numbering from the end nearest the double bond.
  • The lowest-numbered carbon of the double bond is written before -ene, e.g. but-1-ene.

Part 2 of 10Sigma and pi bonds in ethene

1
Sigma (σ) bond
  • Sigma (σ) bond end-on overlap of orbitals, with electron density symmetrical about the line joining the nuclei.
  • s orbitals and p orbitals can both overlap this way.
2
Pi (π) bond
  • Pi (π) bond sideways overlap of adjacent p orbitals, with lobes above and below the plane of the σ bond.
  • The two clouds together form one bond of two electrons.
3
In ethene each carbon uses three of its four outer electrons for σ bonds.
  • Two σ bonds go to hydrogens and one σ bond goes to the other carbon.
  • The fourth electron is in a p orbital that overlaps sideways to form the π bond.
  • The C=C is therefore one σ bond and one π bond.

Part 3 of 10E/Z and cis/trans isomerism

E/Z isomerism is stereoisomerism in which different atoms or groups are attached to each carbon of the C=C.
The π bond prevents rotation about C=C, so the groups stay fixed in position.
  • In saturated compounds there is free rotation about C-C.
cis/trans is a special case in which two groups are the same.
Cis
  • Cis the two groups are on the same side of the double bond.
Trans
  • Trans the two groups are on opposite sides of the double bond.
Each carbon of the C=C must carry two different groups.
  • 2-methylpropene has no cis/trans isomers because both methyl groups are on one carbon.
  • But-2-ene and 1-chloroprop-1-ene do have them.
cis/trans fails when the groups on the double bond are all different, so E/Z must be used.

Part 4 of 10Naming E/Z isomers with CIP rules

1
Rank the two groups on each carbon by the atomic number of the first atom attached; the higher atomic number has priority.
2
If the first atoms are the same, compare the second atoms (ignore identical hydrogens) until a difference is found.
3
Z (zusammen = together) has the two higher-priority groups on the same side of C=C.
4
E (entgegen = opposite) has the two higher-priority groups on opposite sides.
Naming E/Z isomers with CIP rules
In 1-bromo-1-propen-2-ol, Br has priority over H and O has priority over C.
In CH2Br versus CH3CH2 the second atom Br beats C, and in CH2OH versus CH3 the second atom O beats H.

Part 5 of 10Reactions of alkenes

The C=C has high electron density (four electrons), so it is attacked by electrophiles.
Electrophilic addition
  • Electrophilic addition an electrophile adds across C=C, the π bond breaks and a new single bond forms on each carbon.
Addition reactions of alkenes
  • Hydrogen (hydrogenation) with Ni catalyst gives the alkane.
  • Steam with phosphoric acid catalyst gives an alcohol.
  • Hydrogen halides, HX, give halogenoalkanes.
  • Halogens give dihalogenoalkanes.
Oxidation
  • Oxidation cold dilute acidified KMnO4 turns from purple to colourless and forms a diol.
  • KMnO4 supplies one oxygen atom and water supplies another oxygen and two hydrogens, so two OH groups add across C=C.
  • Alkanes are not oxidised, so this also distinguishes alkenes from alkanes.

Part 6 of 10Saturation test

Saturation test
Shake the unknown compound with bromine water, Br2(aq), an orange or yellow solution.
An unsaturated compound reacts by addition and decolourises the bromine water.
A saturated compound such as an alkane leaves it orange in the dark.

Part 7 of 10Electrophilic addition mechanism

Electrophilic addition mechanism
HBr is polar, with H δ+ and Br δ−.
  • The π electrons attack the H atom, which acts as an electrophile.
  • The H-Br bond breaks heterolytically, forming Br− and a carbocation intermediate.
  • Br−, a nucleophile, attacks the carbocation to give the bromoalkane.
Br2 is non-polar but is polarised when it approaches the double bond.
  • The electron density of C=C repels the Br-Br electrons, so the nearer Br is δ+ and the further Br is δ−.
  • The nearer Br acts as the electrophile, the bond breaks heterolytically and Br− attacks the carbocation to give 1,2-dibromoethane.
Unsymmetrical alkenes give a major and a minor product.
  • Carbocation stability is tertiary > secondary > primary.
  • Propene and HBr give mainly 2-bromopropane (secondary carbocation) and a little 1-bromopropane.

Part 8 of 10Addition polymerisation

Addition polymerisation
  • Addition polymerisation many monomers containing at least one C=C join to form long chains, with the polymer as the only product.
The π bond of each monomer breaks and new C-C single bonds link the monomers.
Polymer
  • Polymer a long-chain molecule made of many repeating units.
Monomer
  • Monomer a small reactive molecule that joins with others to form a polymer.
Poly(ethene) and poly(chloroethene), PVC, are made from ethene and chloroethene.
Repeat unit
  • Repeat unit the smallest group of atoms that repeats along the chain, drawn in square brackets.
  • Its main chain is the monomer with C=C changed to C-C.
  • Ethenol, CH(OH)=CH2, gives -CH(OH)-CH2-.
  • To find the monomer, restore the C=C between the two main-chain carbons.

Part 9 of 10Polymer disposal

1
Addition polymers are non-biodegradable, so microorganisms cannot break them down.
2
Landfill
  • Takes up valuable land and plastics last for hundreds of years, so sites fill quickly.
3
Incineration
  • Releases a lot of heat energy but produces CO2, a greenhouse gas.
  • PVC releases toxic HCl, which can be removed by scrubbing the waste gas with a base such as CaO: CaO + 2HCl → CaCl2 + H2O.
4
Recycling
  • Reduces landfill and incineration and conserves crude oil, since recycling needs less energy than making new plastic.
5
Biodegradable polymers
  • Polyesters and polyamides can be broken down by hydrolysis and microorganisms into carbon dioxide, water and other organic compounds.
6
Compostable polymers
  • Plant-based, e.g. starch bin liners and sugar-cane-fibre plates, and degrade leaving no harmful residues.

Part 10 of 10More from the specification

  • Spectroscopic structural identification
    • analyzing infrared absorption frequencies provides an analytical means to detect carbonyl and hydroxyl functional groups.

Key terms

TermMeaning
AlkeneAn unsaturated hydrocarbon with a C=C bond, general formula CnH2n.
σ bondA bond formed by end-on overlap of orbitals along the line between the nuclei.
π bondA bond formed by sideways overlap of p orbitals above and below the plane of the nuclei.
E/Z isomerismStereoisomerism caused by restricted rotation about C=C with different groups on each carbon.
ElectrophileA species that accepts a pair of electrons to form a covalent bond.
CarbocationAn ion with a positively charged carbon, formed as an intermediate.
MonomerA small reactive molecule that joins with others to form a polymer.
Repeat unitThe smallest group of atoms that repeats along a polymer chain.

Avoid these mistakes

1
Thinking C=C is two σ bonds — it is one σ bond and one π bond.
2
Using cis/trans when all four groups on the double bond are different — use E/Z.
3
Assuming Z means the same as cis in every case — Z means the higher-priority groups are on the same side.
4
Ignoring carbocation stability in unsymmetrical additions — the major product comes from the more stable carbocation.
5
Giving the monomer as the repeat unit — the monomer has C=C where the repeat unit has C-C.

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