Reactions of Alkanes

• Free-Radical Substitution Reaction

• Combustion Reaction
  
  

Reactions of Alkanes

• free-radical substitution

• homolytic fission

• ultraviolet light

• initiation

• propagation

• termination

• complete combustion

• incomplete combustion

• carbon monoxide

Key Terms

• Free-radical Substitution

• Combustion of Alkanes

Topics

Lesson Outcomes

• Describe the free-radical substitution of alkanes by Cl₂ or Br₂ in the presence of ultraviolet light

• Describe the mechanism of a free-radical substitution with reference to the initiation, propagation and termination steps

• Describe the complete and incomplete combustion of methane

• Describe the effect of carbon monoxide gas to the human body system.

Free-Radical Substitution

a hydrogen atom gets substituted by a halogen (chlorine/bromine)

• Initiation

• Propagation

• Termination

Match

Match the following

• The reaction is terminated when two radicals collide with each other forming a single product

• The halogen bond (Cl-Cl or Br-Br) is broken by UV energy to form two radicals

• These radicals create further radicals in a chain reaction

termination step

initiation step

propagation step

• termination step

• initiation step

• propagation step

• In the initiation step the Cl-Cl or Br-Br is broken by energy from the UV light

• This produces two radicals in a homolytic fission reaction

• A radical is a species with an unpaired electron

Free-Radical Substitution

a hydrogen atom gets substituted by a halogen (chlorine/bromine)

Initiation Step

​Ultraviolet light (sunlight) is needed for this substitution reaction to occur

• Free radicals are very reactive and will attack the unreactive alkanes

• The propagation step refers to the progression (growing) of the substitution reaction in the chain type reaction

• a mixture of substitution products are formed

• If there is enough chlorine present, all the hydrogens in the alkane will eventually get substituted (eg. ethane will become C₂Cl₆)

• The termination step is when the chain reaction terminates (stops) due to two free radicals reacting together and forming a single unreactive molecule

• This mechanism will uses half headed arrows to show the movement of one electron

• A half headed arrow is known as a ‘fish hook’ arrow. 

Reactive Nature of Radicals and Spreading the Good News:

• Analogy: Radicals are highly reactive species, much like the eagerness to spread the good news of the gospel.

• Romans 1:16 (NIV): "For I am not ashamed of the gospel, because it is the power of God that brings salvation to everyone who believes..."

_{Chemistry and the Word}

Ultraviolet Light in Free-Radical Substitution and God's Wisdom and Energy:

• Ultraviolet light provides the necessary energy to initiate reactions, similar to how God's wisdom and energy empower us in our endeavors.

• Proverbs 2:6 (NIV): "For the Lord gives wisdom; from his mouth comes knowledge and understanding."

_{Chemistry and the Word}

Ultraviolet Light in Free-Radical Substitution and God's Wisdom and Energy:

• Ultraviolet light provides the necessary energy to initiate reactions, similar to how God's wisdom and energy empower us in our endeavors.

• Proverbs 2:6 (NIV): "For the Lord gives wisdom; from his mouth comes knowledge and understanding."

_{Chemistry and the Word}

• to generate electricity in power stations

• to heat our homes and cook our food

• to provide energy needed in industrial processes

• to provide power for ships, aeroplanes, trains, lorries, buses, cars and motorbikes.

Combustion of Alkanes

Complete Combustion

Complete Combustion

Incomplete Combustion

Incomplete Combustion

• Incomplete combustion often takes place inside a car engine due to a limited amount of oxygen present

• With a reduced supply of oxygen, carbon will be produced in the form of soot:

Carbon monoxide and its health effects

• CO is a toxic and odourless gas which can cause dizziness, loss of consciousness and eventually death

  • The CO binds well to haemoglobin which therefore cannot bind oxygen and carbon dioxide

  • Oxygen is transported to organs

  • Carbon dioxide is removed as waste material from organs

Carbon monoxide and its health effects

headache
dizziness
weakness
upset stomach
vomiting
chest pain
confusion

_{Common Symptoms of CO poisoning}

CO symptoms are often described as “flu-like.”

Complete Combustion - Filled with the Word of God and His Spirit:

• Just as enough oxygen is needed for the complete combustion of hydrocarbons, we need to be filled with the Word of God and His Holy Spirit for a fruitful and positive life.

• John 15:5 (NIV): "I am the vine; you are the branches. If you remain in me and I in you, you will bear much fruit; apart from me, you can do nothing."

_{Chemistry and the Word}

Complete Combustion - Filled with the Word of God and His Spirit:

• Just as enough oxygen is needed for the complete combustion of hydrocarbons, we need to be filled with the Word of God and His Holy Spirit for a fruitful and positive life.

• Galatians 5:22-23 (NIV): "But the fruit of the Spirit is love, joy, peace, forbearance, kindness, goodness, faithfulness, gentleness and self-control. Against such things there is no law."

_{Chemistry and the Word}

Incomplete Combustion - Compromised Lifestyle without God's Presence:

• Just as incomplete combustion with insufficient oxygen can lead to harmful products like carbon monoxide, a compromised lifestyle without the presence of the Lord can lead to harmful consequences.

• Proverbs 14:12 (NIV): "There is a way that appears to be right, but in the end, it leads to death."

• Psalm 16:11 (NIV): "You make known to me the path of life; you will fill me with joy in your presence, with eternal pleasures at your right hand."

_{Chemistry and the Word}