Topic 6 - OC Alkenes

Question 1

General formula of Alkenes

Answer 1

CnH2n

Question 2

Are alkenes and cycloalkenes saturated

Answer 2

No because they both contain double bonds

The are more reactive than alkanes due to the double bond

Question 3

Bonding in alkenes
sigma bond

Answer 3

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

Question 4

Bonding in alkenes
pi bond

Answer 4

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

Question 5

Electrophile

Answer 5

A species that is attracted tp a region of high electron density

Question 6

Addition reaction of alkenes

Answer 6

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

Question 7

Test for alkene

Answer 7

Alkene + Bromine –>
colour of bromine will disappear and solution will become colourless if alkene is present

bromine is decolorised in presence of alkene

Question 8

Hydrogenation

Answer 8

Alkene + Hydrogen = Alkane
(in the presence of a nickel catalyst)

Question 9

Manufacture of margarine

Answer 9

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

Question 10

Halogenation

Answer 10

Alkene + Halogen = Dihalogenoalkane

Question 11

Hydration

Answer 11

Steam and Alkene in the presence of an acid catalyst (phosphoric acid) to produce an alcohol

Question 12

Hydrogen halides

Answer 12

Alkene + Hydrogen halide = halogenoalkanes

Question 13

Oxidation to diols

oxidation then addition reaction

A diol is a compound containing two OH (alcohol) groups

Answer 13

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

Question 14

Heterolytic bond fission

Answer 14

Breaking of a covalnet bond so that both bonding electrons are taken by one atom - forms ions

Question 15

what do curly arrows represent

Answer 15

the movement of a pair of electrons + they should always start from a bond or a lone pair of electrons

Question 16

Electrophilic addition of hydrogen halides

Answer 16

1. Heterolytic fission occurs where the hydrogen halide breaks and eleectrons are attracted to the halide (usually) because it is more electronegative than the hydrogen
2. Carbocation occurs because the positive H+ ion is attracted to the double bond - causing it to break
3. Carbon is left with a positive charge so bonds to the Br-

Question 17

Electrophilic addition of halogens

Answer 17

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

Question 18

Unsymmetrical molecules

Answer 18

two possible products
- also one product is formed in greater amounts than the other

Question 19

Carbocation

Answer 19

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

Question 20

Alkenes form polymers through addition polymerisation

Question 21

Polymer waste

Answer 21

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

Question 22

How do chemists limit the problems caused by polymer disposal

Answer 22

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