Alkanes Flashcards
Alkanes:
What are Alkanes?
Saturared HC
Alkanes: Boiling Point & Chain Length
What happens to the Bp as the chain length increases?
Increases
↳ increase in electrons : more LFs
Alkanes: Boiling Point & Branches
Do long-chain Alkanes have a higher Bp than branched ones?
Yes
↳ branched = more compact : easier to break
↳ decrease in intermolecular forces
Alkanes: Reaction w Halogens/ Free Radical Subsitution Reaction
What is needed for Alkanes to react w Halogens?
Sunlight (UV)
(3 step reaction)
Alkanes: Reaction w Halogens/ Free Radical Subsitution Reaction
Cl is broken into free Radicals in presence of UV radiation
(Step 1: Initiation)
- UV provides enough energy to break Cl-Cl bond (Photdissociation)
- Bonds break evenly, each atom keeps an electron (Homolytic Fission)
- Atom becomes a very reactive radical
Alkanes: Reaction w Halogens/ Free Radical Subsitution Reaction
Cl free Radicals attack Methane breaking the C-H bond
(Step 2: Propagation (1))
- Cl attacks Methane mol
Alkanes: Reaction w Halogens/ Free Radical Subsitution Reaction
Methyl free Radicals attack Cl mol
(Step 3: Propagation (2))
- Methyl radical attacks Cl mol
- New Cl radical can attack another methane mol
↳ until all Cl & CH4 mol are wiped out
Alkanes: Reaction w Halogens/ Free Radical Subsitution Reaction
When 2 Radicals combine, a single mol is formed & chain reaction stops
(Step 4: Termination)
Alkanes: Reaction w Halogens/ Free Radical Subsitution Reaction
What’s the Problem with this reaction?
You end up with a mixture of products
Alkanes: Crude Oil
What is Petroleum?
Crude oil
Alkanes: Crude Oil
What is Crude Oil made of?
Mixture of HC
* Mainly alkanes
Alkanes: Crude Oil
What is Fractional Distillation?
Breaking longer alkanes into smaller ones
Alkanes: Crude Oil
Why do we use Fractional Distillation?
Crude Oil isnt useful : so its separated into smaller units
Alkanes: Crude Oil
What happens as Crude Oil Vapour goes up the Fractionating Columns?
It cools down
Alkanes: Crude Oil
What happened to the Bp as you go down the Fractionating Columns?
It decreases
↳ longer alkanes are at the bottom
Alkanes: Crude Oil
What happens to HC with the lowest Bp?
They’re drawn off as gases at the top of the column
↳ they don’t condense
Alkanes: Cracking havier fractions into smaller ones
What is Cracking?
Breaking long-chained alkanes into smaller HC
Alkanes: Cracking havier fractions into smaller ones
Why do we use Cracking?
Smaller fractions are in high demand
↳ they’re more useful : valuable
Alkanes: Cracking havier fractions into smaller ones
Example of Cracking
Alkane (long) → Alkane (smaller) + Alkene
Alkanes: Types of Cracking
What is Thermal Cracking?
(heat)
Cracking using high temps & pressure
Alkanes: Types of Cracking
What is Produced in Thermal Cracking?
Lots of alkenes
Alkanes: Types of Cracking
What type of Alkanes is Thermal Cracking used for?
( 10 Carbons & above )
Longer
↳ have a high Bp
Alkanes: Types of Cracking
What is Catalytic Cracking?
(catalyst)
Cracking using Zeolite Catalyst at moderate temps & pressure
Alkanes: Types of Cracking
What is Produced in Catalytic Cracking?
- Aromatic HC
- Motor Fuels
Alkanes: Types of Cracking
What type of Alkanes is Catalytic Cracking used for?
( 10 Carbons & below )
Smaller
↳ broken quicker
Alkanes: Alkanes are reformed into Cycloalkanes & Aromatic HC
What is Knocking?
When Alkanes explode by themselves when the fuel is compressed in the engine
Alkanes: Alkanes are reformed into Cycloalkanes & Aromatic HC
Which HC are most likely to cause Knocking?
Straight chained Alkanes
Alkanes: Alkanes are reformed into Cycloalkanes & Aromatic HC
How could you decerease the likelyhood of Knocking?
Adding a branched chain & Cyclic HC to the petrol mixture
↳ combustion becomes more efficient
Alkanes: Alkanes are reformed into Cycloalkanes & Aromatic HC
What is needed for Reforming to occur?
A catalyst
