Topic 6 - OC Alkenes Flashcards
General formula of Alkenes
CnH2n
Are alkenes and cycloalkenes saturated
No because they both contain double bonds
The are more reactive than alkanes due to the double bond
Bonding in alkenes
sigma bond
A sigma bond is formed by the head on overlap of atomic oritals
- this can also be described as axial or end to end overlap
- - electron density is concentrated between the nuclei of the bonding atoms
- the bond is free to rotate
- single covalent bonds are always examples of sigma bonds because atoms will always try to arrange themselves to maximise the overlap of the orbitals
strongest type of covalent bond
Bonding in alkenes
pi bond
Formed by the sideways overlap of atomic orbitals above and below the atoms (can be described as lateral overlap)
- results in eklectron density above and below the plane of the nuclei of the bonding atoms
weaker
Electrophile
A species that is attracted tp a region of high electron density
Addition reaction of alkenes
Most reactions of alkenes involve the double bond becoming a single bond. The double bond contians a sigma and a pi bond. The sigma bond remains unchanged, but the pi bond electrons are used to form new bonds with an attacking molecule
addition reaction because two molecules join together to become one molecule
Test for alkene
Alkene + Bromine –>
colour of bromine will disappear and solution will become colourless if alkene is present
bromine is decolorised in presence of alkene
Hydrogenation
Alkene + Hydrogen = Alkane
(in the presence of a nickel catalyst)
Manufacture of margarine
Vegetable oils are unsaturated and contain a C=C double bonds - when these react with hydrogen some of the double bonds break to form single bonds - this process changes the properties of the vegetable oil and converts it into a solid, known as margarine
Halogenation
Alkene + Halogen = Dihalogenoalkane
Hydration
Steam and Alkene in the presence of an acid catalyst (phosphoric acid) to produce an alcohol
Hydrogen halides
Alkene + Hydrogen halide = halogenoalkanes
Oxidation to diols
oxidation then addition reaction
A diol is a compound containing two OH (alcohol) groups
Oxidising agent is potassium manganate in acid conditions - it provides an oxygen atom and the water in the solution provides another oxygen atom and two hydrogen atoms, so there is an addition of two OH groups
The colour of the potassium manganate solution changes from purple to colourless - the colour change eans that this reaction can be used like bromine to distinguish alkenes from alkanes
Heterolytic bond fission
Breaking of a covalnet bond so that both bonding electrons are taken by one atom - forms ions
what do curly arrows represent
the movement of a pair of electrons + they should always start from a bond or a lone pair of electrons
Electrophilic addition of hydrogen halides
- Heterolytic fission occurs where the hydrogen halide breaks and eleectrons are attracted to the halide (usually) because it is more electronegative than the hydrogen
- Carbocation occurs because the positive H+ ion is attracted to the double bond - causing it to break
- Carbon is left with a positive charge so bonds to the Br-
Electrophilic addition of halogens
Similar to the reaction with hydrogen halides
However the attacking molecule does not have a polar bond. As it approches the double bond, the electrons in the pi bond repel the electrons of the molecule and induce the molecule to become polar
Unsymmetrical molecules
two possible products
- also one product is formed in greater amounts than the other
Carbocation
Positive ion in which the charge is shown on the carbon atom
- when the positive charge can be spread over more atoms, it is more stable than when spread over fewer atoms
- alkykl groups are electron releasing so when there are two the positive charge is spread more than when there is 1
Alkenes form polymers through addition polymerisation
Polymer waste
recycling - convert into other materials - first stage is sorting - secod is processing which involves chopping waste into small bits and washing - material then used to make new materials such as melting or moulding
incineration - elements present are mostly hydrogen and carbon so can be used as fueln - incinerator takes uin polymer waste and converts it into heat energy - can cause pollution as other elements are found in polymer waste
feedstock for cracking - used to breakdown the polymer waste into gases (mainly hydrogen and carbon monoxide) - produces a feedstock that can be used in other chemical reactions, often to make new polymers
How do chemists limit the problems caused by polymer disposal
They develop biodegradable polymers - polymers that are able to be broken down by microbes in the environment
disadvantages: they are often made from plant material, so land is needed to grow plants + they are deisgned to break down in the enviornment so when they do, the ydrogen and carbon atoms they contain cannot be used - no recycling, incineration or use as chemical feedstock is possible
