Alkenes

Question 1

what are alkenes

Answer 1

homologous series of unsaturated hydrocarbons, which have:

The same functional group - at least one carbon-carbon double bond / C=C
The same general formula - CnH2n
Similar chemical properties
Differ in their chemical formula by one -CH2- group
Have gradually changing physical properties

Question 2

how are alkenes produced

Answer 2

Two main methods to produce alkenes are:

Cracking of long-chain hydrocarbons
Chemical reactions such as the elimination reaction of a halogenoalkane to form an alkene

Question 3

what are the chemical and physical properties of alkenes

Answer 3

alkenes are not soluble in water because they are not polar and cannot overcome / disrupt the hydrogen bonding between water molecules.
As the alkene gets longer, the boiling point increases. This is because longer alkenes have increased intermolecular forces requiring more energy to overcome as the physical state changes from liquid to gas.

Question 4

how to the alkenes form a double covalent bond

Answer 4

Each carbon atom has four electrons in its outer shell (electronic configuration: 1s22s22p2)
Carbon atoms share these four electrons in four covalent bonds with other atoms to achieve a full outer shell configuration
These electrons are found in orbitals within the respective atoms
When carbon atoms use only three of their electron pairs to form a σ bond, each carbon atom will have a p orbital which contains one spare electron
When the p orbitals of two carbon atoms overlap with each other, a π bond is formed (the π bond contains two electrons)
The two orbitals that form the π bond lie above and below the plane of the two carbon atoms to maximise bond overlap
The three bonding pair of electrons are in the plane of the molecule and repel each other
The molecule adopts a planar arrangement with bond angles of 120

Question 5

what are the two covalent bonds in an alkene

Answer 5

Sigma bonds (σ)
Pi bonds (π)

Question 6

what is stereoisomerism

Answer 6

molecules that have the same molecular and structural formulas but with a different arrangement of the atoms in space

Question 7

what is E-Z stereoisomerism

Answer 7

it is caused by molecules with a c=c with two DIFFERENT groups attached to the C of the C=C

Question 8

why do alkenes only show this type of isomerism

Answer 8

the C=C cannot rotate therefore to rotate it you would have to break the bond

Question 9

what rule is used to determine priority

Answer 9

Cahn-Ingold-Prelog
E= highest priority opposite
Z= highest priority on same side

Question 10

how do you determine priority

Answer 10

higher atomic number is the higher in priotirty
H<CH3<CH2CH3

Question 11

what is an electrophile

Answer 11

electron pair acceptor

Question 12

what do electrophiles do

Answer 12

they attack the double bond in the alkene as are attacked to regions of negative charge.

Question 13

how are double bonds vulnerable to attack

Answer 13

it has a region of high electron density
so lots of -ve charge

Question 14

what are some comman electrophiles

Answer 14

+ve charged species eg H+
atoms with partial +ve charge eg HBr

Question 15

reaction of addition with hydrogen bromide

Answer 15

pair of e- from double bond/attack move to form a bond btw carbon and hydrogen
and at the same time the HBr bond breaks - H is partially +ve
then forms a carbocation- which is a +ve charged intermediate
the lone pair on the bromine attacks the now +ve carbon and forms a bond with the carbocation
forming bromoethane

Question 16

what are the conditions of HBr reaction

Answer 16

aqueous

Question 17

addition of Bromine

Answer 17

electron rich double bond repels bonding pair of electrons in the bromine causing a temporary dipole
this means that one brome is partially +ve and the other partially -ve
lone pair from bromine attacks ~+ve carbon and forms a bond
forms 1,2 -dibromoethane
this is the test for unsaturation

Question 18

addition of sulphuric acids

Answer 18

c=c bond attacks +ve hydrogen breaking the double bond - leaving a sigma bond
e- bwt H-O move to the O and bond breaks
leaves partial +ve charge on the hydrogen and O gets new lone pair
carbocation forms
the lone pair on O- is attracted to the +ve carbon and so attacks and forms bond with it
forms ethyl hydrogens sulphate

Question 19

how can sulphuric acid form from ethyl hydrogen sulphate

Answer 19

add water which forms ethanol and sulphuric acid

Question 20

what is the test for unsaturation

Answer 20

Halogens can be used to test if a molecule is unsaturated (i.e. contains a double bond)
Br2(aq) is an orange or yellow solution, called bromine water and this is the halogen most commonly used
The unknown compound is shaken with the bromine water
If the compound is unsaturated, an addition reaction will take place and the coloured solution will decolourise

Question 21

what is addition polymerisation

Answer 21

Addition polymerisation is the reaction in which many monomers containing at least one C=C double bond form long chains of polymers as the only product

Question 22

what is a polymer

Answer 22

long-chain molecule that is made up of many repeating units

Question 23

what is a repeating unit

Answer 23

is the smallest group of atoms that when connected one after the other make up the polymer chain
It is represented by square brackets in the displayed and general formula

Question 24

how do polymers cause pollution

Answer 24

Though poly(alkenes)s are extremely important in everyday such as their use as plastics, the disposal of these polymers is problematic
Poly(alkenes) are very large alkane molecules which are unreactive and therefore do not undergo any chemical reactions; they are resistant to chemical attack
Due to their unreactivity, polymers are non-biodegradable and take up to hundreds of years to decompose when dumped in landfill sites
Throwing away poly(alkenes) therefore cause the long-term pollution of the environment
Burning the polymers results in harmful combustion products which again cause the pollution of the environment

Question 25

what are some ways as a solution to plastic pollution

Answer 25

Feedstock Recycling
This is one possible solution to deal with the excess pollution that is caused by plastic
This process involves
Heating the plastic to a high enough temperature that the polymer bonds break, and monomers are formed
The monomers formed are then used to produce new plastics
Mechanical Recycling
This is a more straight forward type of recycling
The different types of plastic are separated and washed thoroughly
Then, they are ground up into small pellets
The pellets are heated up until they melt, and then remoulded so that they can be used again

Question 26

what are some of the issues with these types of plastic recycling

Answer 26

These types of recycling are not perfect solutions
Some plastics, like poly(propene), can only be heated and reused a number of times
Each time the plastic is heated, some of the key chains break
This means that over time, the properties of the plastic are degraded

Question 27

what are carbocations

Answer 27

Carbocations are positively charged carbon atoms with only three covalent bonds instead of four

Question 28

what are the 3 types of carbocation

Answer 28

primary
secondary
tertiary

Question 29

what is markovnikov’s rule

Answer 29

Electrophile will add to carbon to give most stable carbocation

Question 30

how many products does propene make

Answer 30

1-bromopropane
2-bromopropane

Question 31

why is 2-bromopropane the most commonly produced

Answer 31

its is more stable as it has 2 alkyl groups as they push electrons towards the +ve carbon making it less positive

Question 32

what is the alkyl groups tendancy to release e- known as

Answer 32

positive induction effect

Question 33

what is the positive induction effect

Answer 33

The alkyl groups push electrons away from themselves towards the positively charged carbon
This causes the carbocation to become less positively charged
As a result of this, the charge is spread around the carbocation which makes it energetically more stable

Question 34

what is the carbocation order of stability

Answer 34

tertiary-most
secondary
primary-least

Question 35

how are the alkyl groups represented

Answer 35

with an R and inductive effect is illustrated by the use of arrowheads on the bonds