Organic 2- Crude Oil, Fractional Distillation, Preparation of Ethene and Ethyne

Principal of fractional distillation

The principal involved is that the oil is separated in the basis of the different boiling points of the compounds

Stages of fractional distillation

1- Crude oil is heated to high temperatures and fed into a large fractionating tower, this may be 80m high.

2- The tower is kept at very high temperatures at the bottom

3- As you go up the tower there are trays present to collect the fractions

4- The temperature decreases as you ascend the tower

5- As the crude oil the fraction will reach a temperature just below their boiling point and turn into liquids which is collected in the tray

6- The heavier hydrocarbons will condense first while the lighter hydrocarbons will rise higher and condense higher into the tower

Natural gas uses- fraction (C1-C4)

Refinery fuel

Liquified to form butane or propane gas (LPG)

Light gasoline uses- Fraction (C5-C10)

Motor fuel/ Petrol

Naptha (subsection of gasoline) uses Fraction (C7-C10)

Petrol

Feedstock for petrochemical industry

Kerosene uses fraction (C10-C14)

Fuel for planes

Paraffin lamps/stoves

Paraffin oil

Gas oil and heavy gas oil uses fraction (C14-C40)

Diesel

Lubricating oil for engines

Residue uses fraction (C35 and upwards)

Heavy industrial fuels

used for catalytic breaking

Bitumen for road surfaces

LPG stands for

Liquified petroleum gas

(propane and butane gas)

As natural gas is odourless, what is added to them to alert leakage

Mercaptans (usually sulfur containing compounds) They have a strong smell

Octane number

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

Higher octane number is achieved by

-The shorter the alkane

-The greater number of branches

-Cyclic and Aromatic hydrocarbons

Structural features that prevent auto ignition

Short chained

Highly branched

Cyclic/Aromatic

Isomerisation

this involves changing straight-chained hydrocarbons into branched-chained isomers. The branched-chained isomers burn more smoothly and therefore have a higher octane number

Dehydrocyclisation

this process involves the conversion of straight-chained hydrocarbons to form ring compounds. The ring compounds are then converted into aromatic compounds. This is done in the presence of a catalyst. As hydrogen is a by-product of this reaction it is called ‘dehydro’. Aromatic compounds have a high octane number but are also carcinogenic

Catalytic cracking

this process involves breaking down long chained hydrocarbons for which there is low demand into short chained molecules for which there is high demand. These short-chained hydrocarbons have a higher octane number and also tend to be highly branched

Addition of oxygenates

this is the addition of oxygen compounds to the petrol. These tend to be of two types (a) addition of alcohols i.e. methanol (octane no. of 114) and (b) addition of ethers i.e. methyl tert-butyl ether (MTBE) (octane no. of 118). These compounds (a) increase the octane number of the petrol and (b) they also cause less pollution as they reduce the level of carbon monoxide in the exhaust fumes.

Reforming

Involves changing straight chained hydrocarbons into branched chained hydrocarbons or cyclic hydrocarbons

Cracking

Involves changing long chained hydrocarbons for which there is low demand to short chained hydrocarbons for which there is high demand.

Auto-ignition

The early explosion of the petrol-air mixture caused by increasing pressure in the engine

Catalytic converter function

changes harmful gases into harmless gases

Catalyst used into catalytic converter

Platinum, Palladium and Rhodium

Why is there a honeycomb arrangement in catalytic converter

Allows for larger surface area so increased removal of harmful gases into harmless gases such as carbon dioxide and nitrogen

Ethene is prepared by what reaction?

Dehydration reaction of ethanol

It’s an example of elimination reaction

Why is the glass wool pushed down a boiling tube

1- To soak up the ethanol

2- To hold the ethanol is place

What is the dehydrating agent/ catalyst in the preparation of Ethene

Aluminum oxide

Precautions in the preparation of Ethene

-Keep gas away from flames as gas is flammable and there is a risk of explosion

-Wear safety glasses

-Use twissers when handling glass wool as it can cause cuts

What precaution is taken before turning the Bunsen burner off in the preparation of Ethene

It is very important to remove the glass tubing from water bath to prevent ‘suck back’

If the tubing was left in the water bath, once the heat is removed the gases in the boiling tube would contract and this would cause water to be drawn back into the boiling tube to balance the drop in pressure which would cause the boiling tube to shatter.

The yield of ethene collected in the first tube

Low yield of ethene as it will mainly consist of displaced air from the boiling tube

Physical properties of Ethene

Colourless gas with sweetish smell

Solubility of Ethene

The gas is insoluble in water as water is a polar solvent and ethene is a non-polar therefore does not form hydrogen bonds with water. This is why ethene can be collected by displacement of water.

What is observed when ethene is combusted

A luminous flame

What is Ethyne used for

Cutting and welting metals as when it is burned in oxygen it can reach temperatures over 3000 C

Why do you not touch the calcium dicarbide

Because sweat from hands can start the reaction

Appearance of calcium dicarbide

Greyish solid

What is observed when calcium dicarbide reacts with water?

Effervescence

What is produced in the Buchner flask?

Calcium hydroxide which appears as a white suspension (foamy)

Why does the first gas jar collected have a low ethyne yield

Because it will contain mainly displaced air from the Buchner flask

What are the impurities present in calcium dicarbide

Calcium phosphide Ca3P2 and calcium sulfide CaS

The solid impurities present in calcium dicarbide give rise to what gaseous impurities

Phosphine PH3 and Hydrogen Sulfide H2S

How are the gaseous impurities removed from Ethyne gas

By bubbling the impure Ethyne gas through acidified copper (ll) sulfate solution

Ethyne physical properties

Colourless gas with a sweetish smell if pure

Solubility of Ethyne in water

Ethyne is insoluble in water as water is a polar solvent and ethyne being non polar does not form hydrogen bonds with water. This is why Ethyne can be collected by the displacement of water

When combusting Ethyne, why must it be done in a fume cupboard

Because it is very dangerous

In the combustion of Ethyne, what is observed

A smokey luminous flame - the smoke is due to the unburned carbon (very sooty)

How to ensure there is no soot when burning Ethyne

By using an excess of oxygen when burning