Alkenes Flashcards
what are alkenes
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
how are alkenes produced
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
what are the chemical and physical properties of alkenes
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.
how to the alkenes form a double covalent bond
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
what are the two covalent bonds in an alkene
Sigma bonds (σ)
Pi bonds (π)
what is stereoisomerism
molecules that have the same molecular and structural formulas but with a different arrangement of the atoms in space
what is E-Z stereoisomerism
it is caused by molecules with a c=c with two DIFFERENT groups attached to the C of the C=C
why do alkenes only show this type of isomerism
the C=C cannot rotate therefore to rotate it you would have to break the bond
what rule is used to determine priority
Cahn-Ingold-Prelog
E= highest priority opposite
Z= highest priority on same side
how do you determine priority
higher atomic number is the higher in priotirty
H<CH3<CH2CH3
what is an electrophile
electron pair acceptor
what do electrophiles do
they attack the double bond in the alkene as are attacked to regions of negative charge.
how are double bonds vulnerable to attack
it has a region of high electron density
so lots of -ve charge
what are some comman electrophiles
+ve charged species eg H+
atoms with partial +ve charge eg HBr
reaction of addition with hydrogen bromide
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
what are the conditions of HBr reaction
aqueous
addition of Bromine
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
addition of sulphuric acids
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
how can sulphuric acid form from ethyl hydrogen sulphate
add water which forms ethanol and sulphuric acid
what is the test for unsaturation
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
what is addition polymerisation
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
what is a polymer
long-chain molecule that is made up of many repeating units
what is a repeating unit
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
how do polymers cause pollution
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
what are some ways as a solution to plastic pollution
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
what are some of the issues with these types of plastic recycling
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
what are carbocations
Carbocations are positively charged carbon atoms with only three covalent bonds instead of four
what are the 3 types of carbocation
primary
secondary
tertiary
what is markovnikov’s rule
Electrophile will add to carbon to give most stable carbocation
how many products does propene make
1-bromopropane
2-bromopropane
why is 2-bromopropane the most commonly produced
its is more stable as it has 2 alkyl groups as they push electrons towards the +ve carbon making it less positive
what is the alkyl groups tendancy to release e- known as
positive induction effect
what is the positive induction effect
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
what is the carbocation order of stability
tertiary-most
secondary
primary-least
how are the alkyl groups represented
with an R and inductive effect is illustrated by the use of arrowheads on the bonds
chemical properties of alkenes
Undergo hydrogenation when reacting with hydrogen
Form haloalkanes when reacting with halogens
Form long-chain polymers
Undergo addition reactions to the carbon-carbon double bond
physical properties of alkenes
State: Alkenes can exist in all three states of matter at room temperature: solid, liquid, and gas. For example, ethene, propene, and butene are gases at room temperature, while pentene, hexene, and heptene are liquids.
Color: Alkenes are colorless.
Polarity: Alkenes are nonpolar.
Combustibility: Alkenes are combustible.
Melting and boiling points: Alkenes have similar melting and boiling points to alkanes. However, cis alkenes have lower melting points than trans isomers.
Solubility: Alkenes are insoluble in water, but soluble in organic solvents like benzene or acetone.
Carbon-carbon double bond: Alkenes are unsaturated hydrocarbons that contain a carbon-carbon double bond. This double bond is an area of high electron density, making alkenes susceptible to attack from electrophiles.
Geometric isomerism: The carbon-carbon double bond doesn’t allow rotation, which means that the CH3 groups on either end of the molecule can be locked on one side or opposite each other. These are called cis- and trans- isomers
