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3.3.1Organic Chemistry

Describing Alkenes — Edexcel AS Chemistry Revision Notes & Topical Questions

5 min read 4 parts Lesson 24 of 42

📚 Edexcel AS Chemistry Hub / Topical Revision
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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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Part 1 of 4Structure and Naming

General formula
  • General formula Alkenes have the general formula CₙH₂ₙ, so an alkene with four carbons has eight hydrogens.
Functional group
  • Functional group The functional group is the carbon-carbon double bond, C=C, which makes alkenes unsaturated.
Naming
  • Naming The ending –ene shows a double bond, and a number gives the position of the first carbon of the C=C.
First members
  • First members Ethene is C₂H₄, propene is C₃H₆ and but-1-ene is C₄H₈.
Isomers
  • Isomers From C₄ onward, alkenes have positional isomers such as but-1-ene and but-2-ene, and chain isomers such as 2-methylpropene.

Part 2 of 4Physical Properties and Source

1
Alkenes are non-polar or only very slightly polar, so only London forces act between their molecules.
2
Their boiling points increase as the chain length increases, just as for alkanes.
3
They are insoluble in water but dissolve in non-polar solvents.
4
Ethene and propene are gases at room temperature, while longer-chain alkenes are liquids.
5
Industry obtains alkenes by cracking longer alkanes from crude oil, as cracking makes alkenes as well as shorter alkanes.

Part 3 of 4Reactivity

Reactivity
Electron-rich bond
  • Electron-rich bond The C=C double bond has a region of high electron density, which attracts electron-seeking species.
Addition
  • Addition Alkenes undergo addition reactions in which the π bond breaks and two new bonds form.
Reaction types
  • Reaction types They add hydrogen, halogens, hydrogen halides and steam to give saturated products.
Polymerisation
  • Polymerisation Many alkene molecules add together to form long-chain addition polymers.
Combustion
  • Combustion Alkenes burn, but they tend to produce a smokier flame than alkanes because of their higher carbon content.
FeatureAlkaneAlkene
General formulaCₙH₂ₙ₊₂CₙH₂ₙ
BondingC–C onlyContains C=C
Reaction typeSubstitutionAddition
Bromine waterNo changeDecolourised

Part 4 of 4Exam Checklist & Tips

3.3.1 – Key Points for the Exam

  • Alkene An unsaturated hydrocarbon containing at least one C=C double bond.
  • Unsaturated Containing at least one carbon-carbon double or triple bond.
  • Addition reaction A reaction in which two molecules combine to form a single product.
  • Cracking Breaking long hydrocarbon chains into shorter ones, usually including alkenes.
  • Positional isomers Isomers with the same skeleton but the double bond in different places.

Common Mistakes

  • Writing the general formula of an alkene as CₙH₂ₙ₊₂.
  • Naming but-2-ene as but-3-ene by numbering from the wrong end of the chain.
  • Describing the reactions of alkenes as substitution, when they are addition reactions.
  • Explaining alkene reactivity by a weak double bond overall, rather than by the exposed, electron-rich π bond.

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