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
