ما هي معادلة الأسترة وكيف يتكون الإستر؟ What is the esterification reaction equation?

تفاعل الأسترة هو تفاعل انعكاسي بين حمض كربوكسيلي وكحول في وجود حمض الكبريتيك المركز كعامل حفاز لنزع الماء: R-COOH + R2-OH ⇌ R-COO-R2 + H₂O، ونواتجه إستر وماء. [الصيغة الرمزية الموزونة: R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O]

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

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

5 min read 12 parts Lesson 12 of 36 1 quiz

📚 IAL Edexcel Linear (Chemistry) Hub / Topical Revision

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

1.10 Alkanes — structure, combustion, fuels, pollutants and free-radical substitution — IAL EDEXCEL LINEAR (CHEMISTRY) notes: Describe the structure and low reactivity of alkanes, their use and combustion as fuels, the resulting pollutants and alternative fuels, and the free-radi…

Hydrocarbon
High bond enthalpies
Very low polarity
Complete combustion
Carbon monoxide
Oxides of sulfur

Part 1 of 10Structure of alkanes

Structure of alkanes

Hydrocarbon

Hydrocarbon a compound containing hydrogen and carbon only;
- alkanes and alkenes are aliphatic, arenes are aromatic.

Alkanes are saturated with general formula C_nH_2n+2.

They contain only single bonds and no functional group.
They are named alk + ane, with stems meth-, eth-, prop-, but-, pent- and then hex-, hept- and so on.
Each carbon is tetrahedral, with bond angles of 109.5°.

Alkanes can be linear, branched or cyclic.

Cycloalkanes are named cyclo + alk + ane and have formula C_nH_2n, the same as alkenes, but are still saturated.

Part 2 of 10Low reactivity of alkanes

High bond enthalpies

C-C is 316 kJ mol⁻¹ and C-H is 411 kJ mol⁻¹.
C-H is stronger because hydrogen has only one shell, so the nuclei are closer and attract the shared pair more strongly.

Very low polarity

Very low polarity carbon and hydrogen have very similar electronegativities,
- so the σ bonds are almost non-polar and attract neither nucleophiles nor electrophiles.

Part 3 of 10Fractional distillation, cracking and reforming

Crude oil is a mixture of alkanes, cycloalkanes and arenes that is separated in an oil refinery by fractional distillation into fractions by boiling point.

Cracking

Cracking heavier fractions are broken into smaller, more useful alkanes and alkenes of lower M_r,
- because small fractions such as gasoline are in higher demand.
Example: decane gives octane and ethene.
Thermal cracking uses up to 1000 °C and up to 70 atm and gives alkanes and a lot of alkenes.
Catalytic cracking uses about 450 °C and slight pressure with a zeolite or aluminium oxide catalyst and gives mainly aromatic hydrocarbons.

Reforming

Reforming straight-chain alkanes are converted to branched alkanes and cycloalkanes, usually with a platinum catalyst.
Straight-chain alkanes tend to explode rather than burn smoothly in an engine, which is called knocking and lowers efficiency.
Octane can be reformed to 2,5-dimethylhexane.
Hexane can be reformed to cyclohexane, C₆H₁₂, and H₂.

Part 4 of 10Combustion of alkanes

Complete combustion

Complete combustion excess oxygen oxidises all carbon to CO₂ and all hydrogen to H₂O.
CH₄ + 2O₂ → CO₂ + 2H₂O.

Incomplete combustion

Incomplete combustion limited oxygen forms carbon monoxide, and with less oxygen still, carbon as soot.
2CH₄ + 3O₂ → 2CO + 4H₂O.
It often happens in a car engine where oxygen is limited.
Solid carbon particles (particulates) clump together as soot.

Part 5 of 10Pollutants from burning fuels

Carbon monoxide

Carbon monoxide a toxic, odourless gas that causes dizziness, loss of consciousness and death.
It binds strongly to haemoglobin, which then cannot carry oxygen and carbon dioxide.

Oxides of sulfur

Oxides of sulfur sulfur impurities form SO₂ and SO₃, which dissolve to form H₂SO₃ and H₂SO₄ and cause acid rain.
S + O₂ → SO₂ and 2SO₂ + O₂ → 2SO₃.
Acid rain kills aquatic life, damages crops and forests, corrodes metals and releases CO₂ from carbonate rocks and buildings.

Oxides of nitrogen

Oxides of nitrogen the high temperature and pressure in an engine oxidise nitrogen, N₂ + O₂ → 2NO and N₂ + 2O₂ → 2NO₂.
NO₂ dissolves to form HNO₂ and HNO₃, which cause acid rain.
NO_x reacts with unburnt hydrocarbons (VOCs) to form peroxyacetyl nitrate (PAN), the main pollutant of photochemical smog,
- which harms lungs, eyes and plants.

Particulates (soot) and unburnt hydrocarbons are also released in exhaust fumes.

Part 6 of 10Catalytic converters

Catalytic converters

A honeycomb coated with platinum, rhodium and palladium gives a large surface area for reaction.

Reactions in the converter

Oxidation of unburnt hydrocarbonsC_nH_2n+2 + (3n+1)[O] → nCO₂ + (n+1)H₂O.
Oxidation of CO2CO + O₂ → 2CO₂.
Reduction of NO by CO2CO + 2NO → 2CO₂ + N₂.

Nitrogen oxides are reduced on the hot catalyst surface to unreactive, harmless nitrogen gas.

Part 7 of 10Alternative fuels and carbon neutrality

Alternative fuels reduce pollution and global warming from fossil fuels and replace depleting non-renewable reserves.

Biofuels are renewable fuels from living matter.

Biodiesel is made by refining renewable fats and oils.
Bioethanol is made by fermentation of sugars with yeast.
Biogas is released when organic waste breaks down.

Benefits of biofuels

They are often considered carbon neutral.
Biodiesel and biogas reduce waste going to landfill.
Crop growing can bring income to less developed countries.

Drawbacks of biofuels

Engines and machinery are expensive to convert.
Land needed for food production limits how much can be grown.
Growing, harvesting, processing and transport use energy, so they are not fully carbon neutral.

Carbon neutral

Carbon neutral no net change in the amount of CO₂ in the atmosphere.
A growing plant absorbs CO₂ that is released when it is burnt.
Fossil fuels are not carbon neutral because the CO₂ they absorbed millions of years ago was at a higher atmospheric level.

Hydrogen

2H₂ + O₂ → 2H₂O is carbon neutral.
Its low density makes storage difficult and it is explosive.
Producing hydrogen is expensive, which limits its use.

Greenhouse effect

CO₂, methane and water vapour absorb the infrared radiation re-emitted by the Earth and trap heat.
Rising CO₂ causes the enhanced greenhouse effect.

Part 8 of 10Free radicals and bond fission

Heterolytic fission

Heterolytic fission the more electronegative atom takes both bonding electrons, forming a negative ion and a positive ion.
A double-headed arrow shows the movement of an electron pair.
The negative ion is a nucleophile; the positive ion is an electrophile.

Homolytic fission

Homolytic fission each atom takes one bonding electron, forming two free radicals.
A free radical is a species with an unpaired electron.
Single-headed (fish-hook) arrows show the movement of one electron.
Cl-Cl splits into two chlorine radicals, Cl•.

Part 9 of 10Free-radical substitution mechanism

Alkanes are unreactive, so UV light (sunlight) is needed for a halogen to substitute a hydrogen atom.

Initiation

UV energy breaks Cl-Cl or Br-Br homolytically.
Cl₂ → 2Cl•.

Propagation

Propagation a chain reaction in which radicals are used and regenerated.
Cl• + CH₄ → HCl + •CH₃.
•CH₃ + Cl₂ → CH₃Cl + Cl•.

Termination

Termination two radicals combine to form a single unreactive molecule.
Cl• + •CH₃ → CH₃Cl.
•CH₃ + •CH₃ → C₂H₆.

Bromine colour disappears only when mixed with an alkane in sunlight, which shows UV is essential.

Part 10 of 10Limitations of free-radical substitution

Further substitution gives a mixture of products.

Excess chlorine forms CH₂Cl₂, CHCl₃ and CCl₄, and with enough halogen all hydrogens are replaced (ethane gives C₂Cl₆).
Excess methane favours monosubstitution.

Termination can give impurities such as ethane from •CH₃ + •CH₃.

Longer alkanes can substitute at different carbon atoms.

For propane and bromine, a middle-carbon radical CH₃•CHCH₃ gives 2-bromopropane.

The reaction is therefore a poor way to prepare one pure halogenoalkane.

Key terms

Term	Meaning
Hydrocarbon	A compound containing hydrogen and carbon only.
Saturated	Containing only single carbon-carbon bonds.
Cracking	Breaking long-chain alkanes into shorter alkanes and alkenes.
Reforming	Converting straight-chain alkanes into branched alkanes and cycloalkanes.
Complete combustion	Burning in excess oxygen to give only CO₂ and H₂O.
Free radical	A species with an unpaired electron.
Homolytic fission	Breaking a bond so that each atom takes one electron.
Carbon neutral	Having no net change in atmospheric CO₂.
Propagation	A chain step in which a radical reacts and a new radical is formed.

Avoid these mistakes

Giving the wrong reason for alkane unreactivity — bonds are strong and almost non-polar.

Writing curly double-headed arrows for radical steps — radicals use single-headed arrows.

Forgetting UV light as a condition of free-radical substitution — without it the reaction does not start.

Calling biofuels completely carbon neutral — energy is used in growing, processing and transport.

Saying incomplete combustion produces only CO — carbon (soot) can also form.

Last updated: 18 October 2026