Hydrocarbons: Alkanes and Alkenes Flashcards
Are alkanes saturated or unsaturated hydrocarbons?
They are saturated hydrocarbons.
What does it mean if fuels are refined before use?
They have their sulphur compounds removed.
Describe the process of fractional distillation of crude oil.
Crude oil is vaporised. Evaporates. Rises. Cools. Condensates. Collected as fractions. Smaller molecules in fractions closer to the top of column where it is cooler.
Why do branched molecules have lower boiling points than straight chain molecules?
They have a reduced surface area leading to reduced magnitude of London forces.
The octane number of a fuel is a measure of its what?
Performance.
Higher octane numbers mean what?
Less straight chains and a smoother start when starting an engine (less knocking).
How can we make branched or cyclic chains?
By reforming or cracking.
What are the conditions for reforming?
A catalyst with a high temperature.
Conditions for thermal cracking?
Temperature and pressure (alkenes are a product).
Conditions for catalytic cracking?
Catalyst, temperature and pressure (forms aromatic compounds).
Alkanes react with halogens under what conditions?
UV light or 300C.
The process of halogenation of alkanes takes place in what three stages?
Initiation.
Propagation.
Termination.
A chlorine molecule can be split into two chlorine radicals by what process?
Homolytic fission.
Under what conditions can homolytic fission take place?
UV light or 300C.
Why does a large amount of homolytic fission take place in the upper atmosphere?
There is a large exposure to UV light in the upper atmosphere.
What is the popagation stage of the halogenation of alkanes?
When a halogen radical goes on to react with an alkane.
The propagation stage can be described as a _________ reaction.
A chain reaction because it is a rapid process that continues until no more reactant remains.
What is the step at the end of free radical substitution?
Termination.
What is the termination stage?
Where two radicals combine to form a molecule. There are number of different possibilities due to the variety of radicals that are formed.
If there is too much of a halogen what could happen in free radical substitution?
You could produce di, tri or tetra of that haloalkane.
How can we reduce the chance of forming di, tri or tetra haloalkanes?
By ensuring there is excess alkane.
If we have longer molecule alkanes for free radical substitution what could be an issue?
You may get a mixture of structural isomers.
Fractional distillation gives a large supply of heavier fractions but what are in high demand?
Smaller fractions for fuel.
Cracking converts long chain alkanes into…
…smaller more useful molecules in a mixture of branched alkanes, cycloalkanes, alkenes and branched alkenes.
How can a higher proportion of alkenes be produced from cracking?
When cracking is conducted at a high temperature with steam.
When conducted in the presence of a catalyst a higher proportion of what are produced from cracking?
Branched and cyclic alkanes.
What is the catalyst for cracking?
Synthetic sodium aluminium silicate which is a zeolite.
Zeolites have a three dimensional structure similar to what?
Clay.
Inside the zeolite the silicon, aluminium and oxygen atoms form what?
Tunnels into which small molecules can fit into.
Cracking takes place in the surface of the zeolite catalyst at what temperature?
500C.
Synthetic zeolites are excellent catalysts, why?
They can be developed to have an active site of specific shape to favour the desired products.
Reforming converts straight chain alkanes into…
…branched chain or cyclic alkanes or arenes such as benzene and cyclic alkanes into arenes.
What is a valuable by-product of reforming that can be used elsewhere in the refinery?
Hydrogen.
What is the catalyst for reforming?
Precious metals, such as platinum or rhodium supported on an inert material, such as aluminium oxide.
What temperature does reforming take place?
500C.
What causes incomplete combustion?
Insufficient air supply.
General formula of alkenes:
CxH2x
Are alkenes saturated or unsaturated?
Alkenes are unsaturated.
Test for saturation?
Bromine water decolourises in unsaturated substances.
Why don’t alkenes dissolve in water?
Hydrogen bonding in water is far stronger than the
London forces in between the alkenes.
How do you make an alkane from an alkene?
You hydrogenate the alkene at 150C with a nickel catalyst to saturate the alkene.
Making dihalogenalkanes from alkenes is an example of what type of reaction?
Electrophilic addition.
Hetrolytic fission forms what?
Ions.
Tertiary carbocations are attached to how many carbons?
3 carbon groups.
Secondary carbocations are attached to how many carbons?
2 carbon groups.
Primary carbocations are attached to how many carbons?
1 carbon group.
List the carbocations in order of stability.
Tertiary (most stable), secondary, primary (least stable).
In order to form halogenalkanes from alkenes we must react it with what?
A hydrogen halide.
How do we make alcohols from alkenes?
React alkenes with steam over an acid catalyst (not HCl).
Reacting alkenes with steam is a what type of reaction?
Hydration.
How can we make diols from alkenes?
React alkene with water and an oxidising agent to make a diol.
What is the oxidising agent for making a diol?
Dilute acidified potassium permanganate (VII).
Many monomers join together to form…
…a long chain polymer through polymerisation.
