Chapter 12 - Alkanes Flashcards
What is the general formula of alkanes ?
CnH2n+2
What kind of covalent bonds are formed in alkanes ?
Sigma bonds
What is a sigma bond ?
The overlap of two orbitals
How are alkanes separated ?
Fractional distillation
How does chain length effect boiling point ?
Larger chain length raises boiling point
Why does chain length have that effect on boiling point ?
- Increased chain length
- Greater surface area of contact
- Stronger london forces
- More energy needed to overcome these forces
- Higher boiling points
What is the effect of branching on boiling point ?
Lowers the boiling point
Why does branching have that effect on boiling point ?
- More branching
- Less surface area of contact
- Weaker London forces
- Less energy required to overcome these forces
- Lower boiling point
What are the reasons for alkanes being fairly unreactive?
- C-C and C-H sigma bonds are strong
- C-C bonds are non polar and C-H bonds are fairly non polar too
What is complete combustion?
Alkanes burn completely to produce carbon dioxide and water.
What is the combustion reaction of an alkane in words?
Alkane + oxygen —> water + carbon dioxide
What are the two potential products from incomplete combustion of alkanes?
- Carbon monoxide
- Carbon or soot
What does carbon monoxide cause ?
Carbon monoxide is poisonous as it binds with haemoglobin in the blood and prevents oxygen binding
What does soot cause ?
Soot can cause breathing problems, make buildings dirty and clog up engines
How can these be removed ?
- These can be removed by using a catalytic converter
- A catalytic converter uses a rhodium catalyst to convert harmful products into more stable ones
What is the name of the mechanism for bromination of alkanes?
Free radical substitution
What are the three stages of radical substitution?
- Initiation
- Propagation
- Termination
What is initiation ?
- Radicals are produced normally using visible light or ultraviolet
- Called photodissociation reactions
- The bond breaks producing 2 radicals
Describe the initiation stage of radical substitution
- Covalent bond in a bromine molecule is broken by homolytic fission.
- Energy provided by UV radiation.
- Br-Br -> Br* + Br*
What is propagation ?
- When a radical reacts with a non radical molecule. New radicals are created which then go on to react with other non radicals.
- This is why its called a chain reaction
Describe the propagation stage of radical substitution
- Two propagation steps:
- Step 1 - CH4 + Br* -> *CH3 + HBr
- Step 2 - CH3 + Br2 -> CH3Br + Br
What is termination ?
- When 2 radicals react they form a non radical molecule
- This ends the chain reaction
- This is why its called termination
Describe the termination stage of radical substitution
- Two radicals collide forming a molecule with all electrons paired.
- Br* + Br* -> Br2
- CH3 + *CH3 -> C2H6
- CH3 + Br* -> CH3Br
What can occur after a radical substitution reaction?
Further substitution
