Alkanes
Conformations, free radical halogenation, combustion and pyrolysis. (Chemistry › Hydrocarbons, NEET UG syllabus.)
What is Alkanes?
Different spatial arrangements of atoms in a molecule that can be interconverted by rotation about single bonds.
Key formula / rule: General Combustion of Alkanes
Key points
- Draw and interpret Newman and Sawhorse projections for simple alkanes.
- Identify and compare the stability of different conformations of ethane and n-butane.
- Describe the mechanism of free radical halogenation (initiation, propagation, termination).
- Predict the major and minor products of free radical halogenation based on radical stability and selectivity.
Common exam trap
Confusing conformational isomers with structural isomers.
Definitions
- Term
Conformation
- Meaning
Different spatial arrangements of atoms in a molecule that can be interconverted by rotation about single bonds.
- Term
Free Radical
- Meaning
An atom or molecule with an unpaired electron, typically highly reactive and formed by homolytic bond cleavage.
- Term
Halogenation
- Meaning
A chemical reaction that incorporates one or more halogen atoms (F, Cl, Br, I) into an organic molecule.
- Term
Pyrolysis
- Meaning
The thermal decomposition of organic compounds at high temperatures in the absence of oxygen, also known as cracking.
- Term
Combustion
- Meaning
A high-temperature exothermic redox chemical reaction between a fuel (alkane) and an oxidant (usually oxygen).
Learning objectives
Draw and interpret Newman and Sawhorse projections for simple alkanes.
Identify and compare the stability of different conformations of ethane and n-butane.
Describe the mechanism of free radical halogenation (initiation, propagation, termination).
Predict the major and minor products of free radical halogenation based on radical stability and selectivity.
Write balanced chemical equations for complete and incomplete combustion of alkanes.
Explain the process and significance of pyrolysis (cracking) of alkanes.
Formulae
- Name
General Combustion of Alkanes
- Note
For complete combustion with excess oxygen.
- Expression
CnH2n+2 + (3n+1)/2 O₂ → nCO₂ + (n+1)H₂O
- Name
General Free Radical Halogenation
- Note
Where X = Cl or Br. R-H is an alkane.
- Expression
R-H + X₂ (UV light/heat) → R-X + HX
Prerequisites
Basic knowledge of organic chemistry nomenclature.
Understanding of bond types (single, double, triple).
Concept of isomerism (structural, stereoisomerism).
Basic understanding of reaction mechanisms (homolytic vs. heterolytic cleavage).
Knowledge of balancing chemical equations.
Common mistakes
Confusing conformational isomers with structural isomers.
Incorrectly drawing Newman or Sawhorse projections.
Forgetting the three steps of the free radical mechanism or their order.
Not understanding the difference in selectivity between chlorination and bromination.
Balancing combustion equations incorrectly, especially for incomplete combustion.
Confusing pyrolysis with combustion (pyrolysis is in absence of air).
Keywords
Alkanes
Conformations
Newman Projection
Sawhorse Projection
Staggered
Eclipsed
Anti
Gauche
Free Radical Halogenation
Initiation
Propagation
Termination
Alkyl Radical
Selectivity
Combustion
Pyrolysis
Cracking
Practice preview
Which of the following conformations of ethane is the most stable?…
easy
Which step initiates the free radical halogenation of alkanes?…
easy
Pyrolysis of alkanes is also known as cracking. What is its primary purpose in the petroleum industry?…
medium