3.2 - Alkanes And Halogenoalkenes Flashcards
What is a hydrocarbon? What is saturated?
Only carbon and hydrogen atoms. Only single bonds.
Describe the process of fractional distillation?
Petroleum is a mixture of hydrocarbons, it’s mostly made up of alkanes. Firstly the crude oil is vaporised at 350°C. The vaporised crude oil goes into a fractionating column and rises up through the trays. The largest hydrocarbons don’t vaporise at all because their boiling points are too high, so they form a gooey residue. As the crude oil vapour goes up the fractionating column, it gets cooler. Because the alkane molecules have different chain lengths, they have different boiling points, so each fraction condenses at a different temperature. The fractions are drawn off at different levels in the column. The hydrocarbons with the lowest boiling points don’t condense. They’re drawn off as gases at the top of the column.
What is cracking? What are the two types?
Lighter fractions are more valuable than heavier fractions. Long-chain alkanes into smaller hydrocarbons (which can include alkenes). It involves breaking the C-C bonds.
Thermal cracking takes place at high temperature (1000°C) and high pressure (70 atm). It produces a lot of alkenes. These alkenes are used to make valuable products such as polymers, for example polyethene. Catalytic cracking uses a zeolite catalyst at a slight pressure and high pressure (450°C). It mostly produces aromatic hydrocarbons and motor fuels. Using a catalyst cuts costs, because the reaction can be done at a low pressure and a lower temperature, and the catalyst speeds up the reaction.
What is complete combustion?
Oxidise hydrocarbons with plenty of oxygen, you get carbon dioxide and water. Alkanes make great fuels because burning a small amount releases a lot of energy.
What is incomplete combustion?
Not enough oxygen, so you can get carbon monoxide instead of, or as well as carbon dioxide. Carbon monoxide can be removed from exhaust gases by catalytic converters. Carbon particles (soot) can cause breathing problems and build up in engines so they don’t work properly.
How does global warming occur?
Burning fossil fuels produces carbon dioxide, which is a greenhouse gas. They absorb infrared energy, and emit some of the energy they absorb back towards the earth - the greenhouse effect.
What can cause smog?
Unburnt hydrocarbons are produced as engines don’t burn all of the fuel molecules. Oxides of nitrogen are produced when the high pressure and temperature in a car engine cause the nitrogen and oxygen atoms from the air to react. Hydrocarbons and nitrogen oxides react in the presence of sunlight to form ground-level ozone which is a major component of smog. It irritates eyes, aggravates respiratory problems, and causes lung damage. Catalytic converters can remove unburnt hydrocarbons and oxides of nitrogen from the exhaust.
What causes acid rain?
Some fossil fuels contain sulfur. When they are burnt, the sulfur reacts to form sulfur dioxide gas. It dissolves into moisture and is converted into sulfuric acid, which causes acid rain. Acid rain destroys trees and vegetation, as well as corroding buildings and statues and killing fish in lakes. Sulfur dioxide can be removed from power station flue gases before it gets into the atmosphere. Powdered calcium carbonate or calcium oxide is mixed with water to make an alkaline slurry. When the flue gases mix with the alkaline slurry, the acidic sulfur dioxide gas reacts with the calcium compounds to form a harmless salt (calcium sulfite).
What is a free radical?
A particle with an unpaired electron. Free radicals form when a covalent bond splits equally, giving one electron to each atom. The unpaired electron makes them very reactive.
Explain free radical substitution.
Halogens react with alkanes in photochemical reactions - reactions that are started by ultraviolet light. For example, chlorine and methane react to form chloromethane.
1: initiation reaction - sunlight provides enough energy to break the Cl-Cl bond - this is photodissociation.
Cl2 -> 2Cl•
The bond splits equally and each atom keeps a free radical.
2: propagation reaction - Cl• attacks a methane molecule : Cl• + CH4 -> •CH3 + HCl
The new methyl free radical can attack another Cl2 molecule : •CH3 + Cl2 -> CH3Cl + Cl•
The new Cl• can attack another CH4 molecule until all the Cl2 or CH4 molecules are used up.
3: Termination - if two free radicals join together, they make a stable molecule.
If the chlorine’s in excess, Cl• free radicals will start attacking chloromethane, producing dichloromethane, trichloromethane and tetra. If the methanes in excess, then the product will mostly be chloromethane.
What are CFCs.
All of the hydrogen atoms have been replaced by chlorine and fluorine atoms.
How do chlorine atoms destroy the ozone layer?
Ozone acts a chemical sunscreen. It absorbs a lot of ultraviolet radiation from the sun which can cause sunburn or even skin cancer. Ozone’s formed naturally when an oxygen molecule is broken down into two free radicals by ultraviolet radiation. The free radicals attack other oxygen molecules forming ozone.
Chlorine free radicals are formed in the upper atmosphere when C-Cl bonds in CFCs are broken down by ultraviolet radiation. CCl3F -> •CCl2F + Cl•
Cl• + O3 -> O2 + •ClO
•ClO + O3 -> 2O2 + Cl•
These free radicals are catalysts. The chlorine free radicals is regenerated and can go straight on to attack another ozone molecule.
Overall reaction is 2O3 -> 3O2
Why are CFCs banned?
They are unreactive, non-flammable and non-toxic. The advantages of CFCs couldn’t outweigh the environmental problems they were causing, so they were banned. There are safer alternatives to CFCs which contain no chlorine such as HFCs and hydrocarbons.
Why is the carbon-halogen bond in halogenoalkanes polar?
Halogens are much more electronegative than carbon, so carbon-halogen bonds are polar. The delta positive charge on the carbon makes it prone to attacks from nucleophiles. A nucleophile is an electron-pair donor. OH-, CN- and NH3 are all nucleophiles that can react with halogenoalkanes.
What are some nucleophilic substitution reactions?
Halogenoalkanes read with hydroxides to form alcohols. You have to use warm aqueous sodium or potassium hydroxide. Partial charges on C and Br. Lone pair and - on OH. Arrow from lone pair to C, arrow from bond to Br.
Nitriles are formed by reacting halogenoalkanes with cyanide. Use warm ethanolic potassium cyanide under reflux.
