Alkenes
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What are alkenes?
Unsaturated hydrocarbons.
Describe the double bond in an alkene.
Region of high electron density. Electrophiles are attracted to this and can form a bond.
What is ethene used for?
Starting material for polymers such as polythene and PVC.
When are alkenes produced?
When crude oil is thermally cracked. Large quantities are made.
General formula of an alkene?
CnH2n
Describe the shape of ethene.
Planar molecule. 120 Bond angles.
Why is there restricted rotation around the C=C double bond?
As well as a normal C-C single bond, there is a p orbital which contains a single electron on each carbon. The two orbitals overlap to form an orbital with a cloud of electron density above and below the single bond. (This is a pi orbital)
What kind of isomerism can an alkene show?
E-Z isomerism Position isomerism Chain isomerism
Describe the physical properties of alkenes.
The double bond doesn’t greatly affect properties such as boiling and melting points. VDW forces are the only intermolecular forces that act between the alkene molecules. Thus the physical properties of alkenes are similar to that of alkanes. Melting and boiling points increase with the number of carbon atoms present.
Are alkenes soluble in water?
No
What is an electrophile?
Electron pair acceptor
What kind of reactions do alkenes undergo? Why?
Addition reactions They are unsaturated.
What mechanism do alkenes follow mostly?
Electrophilic addition
Describe electrophilic addition.
1) Electrophile attracted to double bond. 2) Electrophiles are positively charged and accept a pair of electrons from the double bond. 3) A positive ion (carbocation) is formed. 4) A negatively charged ion forms a bond with the carbocation.
How does the bond in HBr break?
Heterolytically. Both electrons in the shared pair go to the bromine atom because it is more electronegative than hydrogen leaving a :Br- ion.
Describe the effect of alkyl groups.
They have a tendency to release electrons. This is known as the positive inductive effect. This stabilises the positive charge on the carbocation. The more alkyl groups there are attached to the positively charged carbon, the more stable the carbocation is.
Describe the order of most to least stable carbocations.
Tertiary>Secondary>Primary The product comes from the more stable carbocation.
How do alkenes form dihalogenoalkanes?
By reacting with chlorine gas or bromine gas.
How do you test for a double bond?
Bromine solution Solution becomes decolourised if positive
What are polymers?
Large molecules made from monomers
What are addition polymers?
Made from monomers with a C=C double bond. Usually made from monomers based on ethene.
How can you modify the properties of polymers?
Plasticisers - makes the polymer flexible.
What are some common electrophiles?
HBr Br2 H2SO4
When are addition polymers produced?
When the double bond in alkenes is broken to form a repeating unit.