13.1 Intro and Alkanes

• Chemistry
• Secondary Education
• Age 12,13

Introduction

Organic Chemistry

20 February 2024

Our Learning Map

Draw/write the general, displayed and skeletal formulae of hydrocarbons and other homologous series

Recognise the names of organic compounds by looking at their functional groups and structures

Explain that organic compounds have functional groups that dictate their chemical properties and the trend in physical properties

Introduction

Organic Chemistry

20 February 2024

What is Organic Chemistry?

• Study of carbon compounds (excluding simple compounds like CO, CO₂, CO₃²⁻, HCO₃⁻).

Why is Carbon so special?

Why is Carbon so special?

How do we present organic compounds or molecules?

1. Empirical formula
2. Molecular formula
3. Structural formula
4. Displayed formula
5. Skeletal formula

Study the formulae of this molecule.

Study the formulae of this molecule.

Study the formulae of these molecules.

PRACTICE.

1. Empirical formula
2. Molecular formula
3. Structural formula
4. Displayed formula
5. Skeletal formula

• Methane
• Ethane
• Ethene
• Butane
• Pentane
• 2-methyl butane

Introduction

Organic Chemistry

20 February 2024

Bonding in Organic Molecules

Tetrahedral shape (109.5^o bond angles, with sp³ hybridised carbon atoms)

Zigzag shape (109.5^o bond angles, for longer chains)

Trigonal Planar shape (120^o bond angles, with sp² hybridised carbon atoms)

Bonding in Organic Molecules

Mixture of trigonal planar and tetrahedral shape (with C-C double bond and a longer chain)

Isomerism

Organic Chemistry

20 February 2024

What is an isomer?

Two or more compounds with the same molecular formula but a different arrangement of atoms in space

Are these two isomers?

How can we identify isomers?

Are there any types of isomerism?

Structural Isomerism

Chain isomerism

Positional Isomerism

Functional isomerism

Stereoisomerism - 

Geometrical (cis-trans or E-Z): arises due to the rotation about a bond.

Geometrical (cis-trans or E-Z) isomerism

Properties

Cis-isomers 

Trans-isomers

• Higher boiling point
• Lower melting point

• Higher melting point
• Lower boiling point

Stereoisomerism - 

Optical: arises due to the ability of compounds to rotate the plane of polarisation of a plane-polarised light.

ALKANES

Organic Chemistry - Hydrocarbons

20 February 2024

What is a hydrocarbon?

• A compound containing carbon and hydrogen atoms only.
• Examples: alkanes, alkenes, alkynes, arenes

What are alkanes?

• saturated hydrocarbons
• carbon atoms are joined to each other via a single covalent bond only
• form the homologous series 
• general formula CnH2n+2

The Homologous Series of Alkanes

Physical Properties of Alkanes

• non-polar = soluble in non-polar solvents like CCl4
• insoluble and less dense than water
• boiling point of straight-chain alkanes increases with the size of the molecule
• branched alkane = more volatile + boiling point < unbranched alkane with the same number of carbon atoms
• density of liquid alkanes increases slightly

Cycloalkanes

• alkanes that joined up as a ring
• smallest possible cycloalkane is cyclopropane
• have two hydrogen less than the corresponding straight-chai alkanes

REACTIONS OF ALKANES

Organic Chemistry

20 February 2024

Nucleophiles and Electrophiles

Reactivity of Alkanes

• saturated and generally unreactive because they are non-polar; hence, they are unattractive towards nucleophiles and electrophiles.
• only react with non-polar reagents in the presence of heat or ultraviolet light

Complete Combustion of Alkanes (smaller)

in excess

• Alkanes only burn in the gaseous state.

Incomplete Combustion of Alkanes

limited

• Possible products are carbon monoxide, carbon and water.

REACTIONS OF ALKANES

Organic Chemistry

20 February 2024

Halogenation of alkanes

• via free-radical substitution
• hydrogen atoms in the molecule are gradually substituted by halogen atoms

• Reagent: Chlorine gas, Cl2 or bromine gas, Br2
• Condition: The presence of ultraviolet(UV) light or sunlight
• Product: Halogenoalkanes (alkanes with halogens)

Mechanisms of Free-radical substitution

• Initiation 
• Propagation
• Termination

• Reagent: Chlorine gas, Cl2 or bromine gas, Br2
• Condition: The presence of ultraviolet(UV) light or sunlight
• Product: Halogenoalkanes (alkanes with halogens)

Mechanisms of Free-radical substitution

• Initiation

• Reagent: Chlorine gas, Cl2 or bromine gas, Br2
• Condition: The presence of ultraviolet(UV) light or sunlight

Homolytic fission of chlorine molecule

Mechanisms of Free-radical substitution

• Propagation

Free radicals attack the methane molecules, leading to a chain reaction.

Mechanisms of Free-radical substitution

• Termination

All other alkanes react similarly, but the reactions are getting more complicated.