Organic 2 - Natural Gas, Crude Oil, Fractional Distillation, Petrol, Prep. of Ethene and Ethyne

crude oil (petroleum)

• fossil fuel formed by decomposition of dead bodies that died millions of years ago

• pumped from underground/sea

• thick, black, viscous, pungent liquid

composition of crude oil

mixture of hydrocarbons (mixture = not chemically combined)

fractionation of crude oil: fractional distillation + principle

used to separate the crude oil into its components

principle → oil is separated into “fractions” on the basis of different boiling points of compounds; these fractions are now more useful and can be used for various purposes

stages of fractionation of crude oil

1. crude oil is heated to high temperatures and is fed into large fractionating towers. towers 80m

2. temperature decreases as you ascend the tower

3. as crude oil rises up the tower, the fractions will reach a temp. that is just below their boiling point and turn into liquids which collect on the tray

4. heavier hydrocarbons will condense first, while lighter hydrocarbons will rise higher and condense higher up the tower

so…

1. gas fraction → C1 - C4 (refinery fuel, liquified to form propane or butane gas)

2. light gasoline fraction (incl. NAPTHA fraction) → C7-C10 (petrol)

3. kerosine → C10 - C14 (aviation fuel)

4. gas oil fraction → C15 - C25 (diesel)

5. heavy gas oil → C26 - C40 (diesel)

6. residue → C40+ (catalytic cracking & tar for road surfaces)

natural gas (C1 - C4) & what’s given to natural gas for safety?

1. mixture of hydrocarbons

2. usually made up of methane, ethane, propane, and butane

3. LPG - liquefied petroleum gas : propane and butane

4. as propane and butane are colourless, mercaptans (usually sulfur containing compounds) are added to the natural gas before distribution. they have a strong smell, so this alerts to a leakage

autoignition

the early explosion of a petrol-air mixture caused by increasing pressure in the engine (instead of being caused by a spark)

octane number

the measure of the tendency of a fuel to resist auto-ignition

structural features of alkanes that prevent autoignition

1. short chained

2. highly branched

3. cyclic/aromatic

Petrol - NAPTHA & Gasoline fraction

petrol and oxygen must ignite and explode evenly at the correct time to ensure the engine runs smoothly. opposite is autoignition

• autoignition (knocking) occurs if the petrol is rich in long straight-chained hydrocarbons

• petrol rich in branched-chained hydrocarbons burns more smoothly and efficiently e.g. 2, 2, 4-trimethylpentane (iso-octane)

solutions to autoignition

Additives:

1. add lead to petrol (in form of tetraethyl lead) so it can burn more smoothly. however it is toxic, harmful to environment and people’s health. also a catalytic poison

2. addition of oxygenate; either addition of alcohols and addition of ethers. they increase octane number & cause less pollution as they reduce the level of carbon monoxide in the exhaust fumes

Reformation:

1. Isomerisation → changing straight-chained hydrocarbons into branched-chained hydrocarbons

2. catalytic cracking → breaking down long chained hydrocarbons to short-chained

3. dehydrocyclisation → conversion of straight-chained hydrocharbons to form ring compounds, which are converted into aromatic compounds. done in the presence of a catalyst. also one of three commercial changes of creating h2 gas

reforming

involves changing straight-chained hydrocarbons into branched-chained hydrocarbons or cyclic hydrocarbons

cracking (catalytic cracking)

involves changing long chained hydrocarbons for which there is low demand, into short chained hydrocarbons for which there is high demand (can also be an economic reason for catalytic cracking)

catalytic cracking in cars

• catalytic converter in car exhausts to reduce pollution from exhaust fumes

• exhaust fumes contain carbon monoxide, nitrogen monoxide, nitrogen dioxide, lead compounds and unburned hydrocarbons

• catalytic converter changes these harmful gases into harmless gases

• e.g. CO + NO → CO2 + ½ N2 // CO → CO2 // NO → N2

• converters contain platinum, palladium and rhodium

• inside is like a honeycomb for a large surface area for increased removal of the harmless gases

elimination reaction → preparation of ethene

equation:

C2H5OH → C2H4 + H2O (w/ aluminum oxide & -H2O)

method:

1. some ethanol poured into a boiling tube

2. glass wool is pushed down into the tube to hold ethanol in place & soak up the ethanol

3. boiling tube clamped vertically using a retort stand

4. Al2O3 is placed halfway along the tube as it’s a dehydration agent & catalyst

5. the tube is stoppered by a rubber bund and glass tubing is attached to it into the water bath

6. test tube filled with water and placed in the water as shown in the diagram

7. bunsen burner placed directly under Al2O3 w/ blue flame

8. heat from bunsen burner vaporaises the ethanol → as it passes over Al2O3, ethanol is dehydrated to produce ethene

9. the first test tube will have a lower yield of ethene as it’ll mainly consist of displaced air from the boiling tube

10. collect 5 more and stopper them

elimination reaction

1. removal of a small molecule from a larger molecule

2. a double bond forms in a product molecule

tests for ethene gas & unsaturation

• it’s a colourless gas w/ a sweetish smell

• insoluble in water as ethene is non-polar; therefore can be collected by the displacement of water

• when lighted taper is placed in test tube, luminous flame observed

• if added to limewater, it goes milky, proving CO2 during combustion:

• C2H4 + 3O2 → 2CO2 + 2H2O

• unsaturation: bromine water: red-brown to colourless due to formation of 1,2 dibromoethane & KMnO4: purple to colourless, proving presence of C=C too

what to do before turning off bunsen burner in ethene prep

remove glass tubing from the waterbath to prevent suck back as cold water can cause the hot boiling tube to crack

preparation of ethyne

used for cutting and welding metals

equation:

CaC2 + 2H2O → C2H2 + Ca(OH)2

method:

1. set up

2. slowly add water from dropping funnel

3. note effervescence as H2O reacts w/ CaC2 due to release of ethyne gas

4. production of calcium hydroxide observed in buchner flask → white suspension (foamy)

5. first glass jar will have low yield of ethyne due to mainly displaced air from buchner flask; carefully discard

6. collect 4 tubes

CaC2; precautions & observations

• don’t touch as sweat from hands may start the reaction

• it’s dark, greyish solid

impurities:

(i) they are present in the calcium dicarbide. they are calcium phosphide, Ca3P2 & calcium sulfide CaS

(ii) these give rise to gaseous impurities in the ethyne gas collected; phosphine PH3 & hydrogen sulfide H2S

tests for ethyne gas & unsaturation

• colourless gas w/ sweetish smell if pure

• insoluble in water as it’s non-polar, so it can be collected by the displacement of water

• combustion is dangerous and burns w/ smoky luminous flame if in impure oxygen. luminous flame if in pure oxygen

• combustion: C2H2 + 2 ½ O2 → 2CO2 + H2O

• unsaturation: bromine water; red-brown to colourless & KMnO4; purple to colourless