Alkenes Flashcards
What is the general formula of all Alkenes ?
CnH2n
Why do alkenes have limited movement ?
Less rotation due to the double bond.
What is the spatial arrangement around a double carbon bond ?
Planar
What is the bond angle around a double carbon bond ?
120 degrees
What are all single covalent bonds known as and explain this.
Sigma bonds
Orbitals overlap to form a sigma bond between two carbon atoms.
What are sigma bonds and how are they caused ?
A single Covalent bond between 2 carbon atoms which is cause by the overlapping of 2 orbitals.
What is the covalent bond between a double carbon bond called.
A ‘pie’ bobs.
Two 2p orbitals overlap above and below the internuclear axis. This causes a ‘pie’ bond.
How is a pie bond formed and what is it ?
A double covalent bond between two carbon atoms. Two 2p orbitals overlap above and below the internuclear axis.
What is a Z isomer ?
Atoms on the same side of the isomer.
What is an E isomer ?
Atoms on opposite sides of the atom.
Which isomers are Z or E depending on the Mr ?
If the atoms with the biggest Mr on the same size, it is the Z isomer.
What is optical isomerism ?
A molecule can exist as two isomers which are not superimosed and are mirror images of eachother.
How can optical isomerism be identified ?
Optical isomers can rotated polarised light in different directions.
What are the different properties of optical isomers ? What
Identical properties ( melting and boiling points, density) But different reactivity
Why is a double bond electron rich and what does this cause ?
It causes a high electron density and means a highly negative charge.
Why does an electrophile attack a double bond ?
Electophiles are electron deficient therefore they accept lone pairs of electrons as they have a positive or slight positive charge.
What are the charges of electrophiles ?
Positive or slightly positive
What are electrophiles (1 mark)
Electron pair acceptor
What reactions occur when alkenes are attacked by electrophiles ?
Electrophillic addition
Addition reaction
What are electrophillic addition reactions ?
Alkenes undergo electrophillic addition reactions when they are attacked by electrophiles
What causes the slight positive and slight negative charges in diative halogen molecules ?
There is an induced dipole.
What is the second step of electrophillic addition when using concentrated sulphuric acid ? And why is this done ?
Boil in water to produce an alcohol by hydrolysis reaction.
What are the 3 electrophiles ?
Hydrogen halides
Halogens
Concentrated sulphuric acid
Explain how you use bromine to test for alkenes and alkanes.
Add hydrocarbon to bromine water and shake.
If the bromine water remains organ/Brown, the hydrocarbon was a saturated alkane.
If the bromine water turns colourless, the hydrocarbon was an unsaturated alkene.
What type of hydrocarbon remains orange/ brown in bromine water.
A saturated alkane.
What type of hydrocarbon makes bromine water turn from brown/orange to colourless ?
An u saturated alkene
What is a co-ordinate covalent bond ?
Both pairs of electrons are donated by the same atom in the covalent bond.
What happens in step one of a electrophillic addition reaction ?
The ‘pie’ bond of the carbon to carbon double bond breaks forming a carbonation ion which has a positively charged carbon.
What happens in step two of an electrophillic addition reaction ?
The ion acts as a nucleophile and attacks the carbocation ion.
What are symmetrical alkenes ?
Alkenes which are unsymmetrical and therefore produce a mixture of products.
What are the products of unsymmetrical alkenes ?
Unsymmetrical alkenes produce a mixture of products which are minor and major products.
The minor major products and produced based on the relative st abilities of the carbocations which are produced.
What are minor products ?
Minor products are usually produced in primary carbocation and are less stable carbocation ions.
What are major products usually v
They are usually tertiary carbocations which are therefore highly stable.
What is a primary carbocation ?
A carbocation containing one carbon, 2 hydrogens and one R group.
What is a secondary carbocation ?
A carbocation containing one carbon, one hydrogen and 2 R groups.
What is a tertiary carbocation ?
A carbocation containing one carbon and 3 R groups.
Which types of carbocations are usually major products and why ?
Tertiary carbocations as they are more stable.
Which types of carbocations are usually minor products and why ?
Primary carbocations as they are not very stable.
How are poly alkenes produced ?
Via catalytic or thermal cracking of crude oil.
What are the issues of thermal/ catalytic cracking to obtain polyalkenes ?
High energy production costs
Use up non-renewable sources.
What are the issues with synthetic polymers ?
They are non-biodegradable as they are man made so are non-reactive to most chemicals and bacteria.
Why are synthetic polymers mostly non biodegradable?
They are man made so are unreactive to most chemicals and bacteria.
How are polyalkenes disposed of and what are the issues with this ?
They must be burnt but produce toxic.
Why is burning polyalkenes for disposal positive ?
The energy recovered from burning alkenes can be used to create electricity.
What are the advantages of recycling polymers ?
Consumption of finite resources is reduced.
Less disposal of plastics required.
Carbon footprint reduced.
Water consumption decreases.
Lost the disadvantages of recycling.
Transport of materials to a process plant is costly.
Energy consumed in melting of the products.
Transport of new products is costly.
What are low density polymers ?
Flexible, branches polymers which don’t back easily.
What are high density polymers ?
Long chain, unbranched rigid polymers which are packed easily.
What is life cycle analysis (of polymers?).
Quantities the energy and raw materials used and any environmental emissions produced during the extraction of raw materials, original manufacture, recycling, refuse or disposal of a polymer product.
