Alkenes Flashcards
what is the general formula for alkenes ?
CnH2n
how are alkenes hydrocarbons ?
they contain hydrogen and carbon only
how are alkenes unsaturated ?
there is at least 1 double covalent bond - undergo addition reactions
unsaturated compounds
one or more double or triple bonds between atoms in their molecules
what makes alkenes fairly reactive ?
double bonds having a high electron density
physical properties of alkenes
- melting and boiling temperatures increase as the number of carbon atoms in the molecules increase
- do not mix or dissolve in water
what does double bonds contain ?
sigma and pi bond
what are sigma bonds ?
when 2 s orbitals overlap
- align horizontally to give a single covalent bond
- free rotation is possible around single bonds
due to the high electron density between the nuclei ..
there is a strong electrostatic attraction between the nuclei and shared pair of electrons - high bond enthalpy and strongest type of covalent bond
what are pi bonds ?
the parallel overlap of 2 p orbitals
where is the pi bond found ?
molecules with double or triple bonds - rotation is restricted because of these regions of high electron density
what shape are the p orbitals ?
dumb-bell shaped and when they merge they form 2 oblong shaped - one on the top and one at the bottom
why are pi bonds weaker than stigma bonds ?
electron density is spread out above and below the nuclei
- the electrostatic attraction is weaker so pi bonds have a lower enthalpy
how many electron are shared in the double bond ?
4
how is pi bonds open to attack from electrophiles ?
pi bonds stick out a little and the whole double bond has a high electron density
electrophile
a chemical species with a vacant orbital that can affect a pair of electrons
- electrophile are positive or have a positive dipole
addition
a reaction in which two molecules add together to form a single product
nucleophile
a chemical species with a pair of electrons that it can use to form a new covalent bond
- nucleophiles are negative or have a high electron density
electrophilic addition
- alkenes are attacked by electrophiles due to their double bond
- double bond has a high density of electrons and is attacked by electrophiles - the electrophile adds to the molecule
what is an electrophile
electron pair acceptor - they are deficient in electrons and are attracted to the double bond
addition of hydrogen
reacting hydrogen gas with ethene at 150 degrees Celsius and a nickel catalyst will make ethane
CH3CH2 + H2 -> CH3CH3
- this reaction is called catalytic hydrogenation - used in the manufacture of margarine from unsaturated vegetable oils
addition of halogens
reacts with br2
to produce di halogenoalkanes
- alkene + halogen - > 1,2 dihaloalkane
addition of hydrogen halides
reacts with HBr
to produce halogenoalkanes
- alkene + hydrogen halide - > halogenoalkane
Bromine water test
- adding bromine water to a alkene causes a colour change from brown - orange to colourless
- bromine is the electrophile and adds to the alkene forming dibromo alkane
addition of steam
acid catalysed hydration of an alkene
alkene + steam - > alcohol
oxidation of potassium manganate ( VII )
- adding purple acidified potassium manganate (VII) solution to alkene will lead to the solution turning colourless
- potassium manganite (VII) is an oxidising agent and oxidises the alkene to make a diol
electrophilic addition
is the reaction mechanism that shows how electrophiles attack the double bond in alkenes
- when the double bond is broken, a carbocation forms
- this is a carbon atom with only three bonds, meaning it has a positive charge
electrophilic addition - mechanism
ethene + hydrogen bromide -> bromoethane
- bromine is more electronegative than hydrogen
- this means that the bromine pulls electron density toward it
- carbocation formed is now the electrophile is attacked by the nucleophile bromide ion to form the final product
heterolytic bond breaking definition
when a covalent bond is broken, one atom takes both of the electrons from the bond - ions are formed
carbocation definition
a reactive species which contains a carbon atom with a positive charge
electrophilic addition - selectivity
- reacting hydrogen halides with unsymmetrical alkanes produce 2 different products
how is the amount of the 2 products during electrophilic addition determined ?
by the stability of the carbocation intermediate
- the more alkyl groups bonded to the carbocation the more stable the intermediate is
Markovnikov’s rule
the hydrogen atom from HX adds mainly to the carbon with more hydrogen attached
- the two reaction pathways generate a different carbocation
In a primary carbocation the positive carbon is bonded..
to one carbon atom
In a secondary carbocation the positive carbon is bonded..
to two carbon atoms
In a tertiary carbocation the positive carbon is bonded..
to three carbon atoms
which carbocation is the most stable ?
tertiary
from primary to secondary to tertiary
the more alkyl chains,
the stronger the inductive effect and therefore the more stable the cation
inductive effect
the way in which electrons are either pushed toward or pulled by atoms or groups to which it is bonded
addition polymers
long chain organic molecules made by the addition polymerisation of alkenes
monomer
is a small molecule that can join with other monomer units to form a large polymer molecule
polymer
is a large molecule made from a chain of a monomer units bonded together
addition polymerisation - mechanism
it is a a free radical addition
what are the three stages in the polymer life cycle ?
birth - synthesis
life - use
death - disposal
green chemistry
- low carbon footprint
- high energy efficiency in manufacturing
- clean production process
- use of renewable resources and renewable energy
- high safety standards
birth of polymers
- when synthesising addition polymers, the regent is an alkene
what does current research focuses on ? - birth of polymers
- synthesising polymers based on naturally occurring monomers
- reducing the energy needed for the process to occur
- making synthesis, more selective
life of polymers
- when synthesising a polymer, a chemist must think about the properties of the polymer and how useful it is to be the consumer
poly ( ethene )
lightweight, cheap and strong for use in plastic bags
poly ( propene )
strong, durable, chemical and photo resistivity for use in the furniture and containers
poly ( ethanol )
flexible and water soluble for use in water soluble bags or capsules
death of polymers - name the methods to dispose of polymers
landfill
recycling
incineration
using as a cracking feedstock
landfill
positives: cheap and simple
negatives: lack of biodegradation - polymers break down and form tiny particulates which pollute the environment
biodegradable polymers
most addition polymers are not biodegradable
- addition polymers are effectively alkanes - highly unreactive
biodegradable ( condensation ) polymers
poly ( esters )
poly ( amides )
poly ( lactic acid ) - uses: disposable cutlery, plates, bags, hygiene products
recycling
positives: plastics do not get into the environment
negatives: labour intensive - sorting and takes energy to reprocess and mould the polymer
incineration
positives: plastic do not get into the environment
negatives: pollutants are released from burning the polymer
pyrolysis - polymer -> feedstock
positives: pollution into the environment and is lower
negatives: very energy intensive
