15. Alkenes Flashcards
Which is more reactive? Alkenes or alkanes?
Alkenes are more reactive because they have a double bond
What type of hydrocarbons are alkenes?
They are unsaturated hydrocarbons
How are alkenes produced?
Alkenes are produced in large quantities when crude oil is thermally cracked
What is the general formula for alkenes?
CnH2n
Which is the simplest alkene?
Ethene
What type of molecule is ethene?
A planar molecule with bond angle 120 degrees
There is no rotation about the carbon to carbon double bond in alkenes. Why?
The formation of the pi orbital. This is restricted rotation.
Alkenes with more than how many carbons can form different types of isomers?
alkenes with more than three carbons.
What two types of isomers are there in alkenes?
Positional isomers
Geometrical isomers
What are geometrical isomers?
Same structural formula but bonds are differently arranged in space.
(Stereoisomerism)
What is a Z isomer?
When the chains with the greatest Ar are on the same side
What is an E isomer?
When the chains with the greatest Ar and on opposite sides.
What are the physical properties of alkenes?
The double bond does not effect the boiling point. van der Waals forces are the only intermolecular forces that act between the alkene molecules. They are not soluble in water as they are not polar.
How do alkenes react?
The double bond makes a big difference to the reactivity of alkenes as compared with alkanes. Alkenes are more reactive because although the double C-C bond has a high bond enthalpy, it forms an electron-rich area which can easily be attacked by positively charged reagents.
What are positively charged reagents?
These are called electrophiles. They are electron pair acceptors one example is the H+ ion.
What are most of the reactions of alkenes?
Electrophillic additions
Alkenes will burn in air to produce…
Carbine dioxide and water
What are alkenes not used as fuels?
Their reactivity makes them very useful for other purposes.
Describe the mechanism of an electrophilic addition
- The electrophile is attracted to the double bond
- Electrophiles are positively charged and accept a pair of electrons from the double bond. The electrophile may be a positively charged ion or have a positively charged area
- A positive ion (a carbocations) is formed
- A negatively charged ion forms a bond with the carbocation.
How do the arrows work in electrophillic addition?
Two stages
- Arrow from double bond to H or Br with positive partial charge
- Arrow from hydrogen halide/halogen bond to negatively charged component
- Negatively charged component now is a nucelophile which attacks the positive carbon (one with most hydrogen around it)
There are alkyl groups which release electrons to stabilise carbocations what is this electron release called?
Positive inductive effect
How do some halogens become electrophiles?
A halogen molecules is, at any moment, likely to have an instantaneous dipole which is attracted to the electron-rich double bond in the alkene.
What is the product formed from an electrophillic addition with sulfuric acid as the electrophile?
Ethyl hydrogensulfate.
If you add water to ethyl hydrogensulfate, what do you make?
Ethanol and sulfuric acid (catalyst for the process)
What is the test for a double bond?
Few drops of bromine water (reddish-brown)
Added to an alkene
Solution turns colourless
Alkenes can…
Polymerise and form long chains
What is the repeating unit of a polymer?
The smallest group of atoms that produce the polymer when repeated over and over.
Turning the ethene monomer into polythene is called…
Addition polymerisation
Is a polymer made from alkene monomers an alkene or an alkane?
Despite ending in -ene, the polymer is actually an alkene and is therefore unreactive.
What is a problem with plastics?
They are not biodegradable. This fills up landfill sites. Animals can ingest it.
How do we solve the problem of plastics not being biodegradable?
Reduce the amount of plastic we use or recycle it.