Alkenes Flashcards
What are alkenes
Alkanes are unsaturated hydrocarbons and contain at least one c=c bond
What’s the ending when more than one double bond is present
Diene , triene
What do we add to ene when we use diene
We add ‘a’ so it would be buta -1,2 diene
What is the shape of an alkene molecule
Three regions of electron density surround the central carbon atom . Pairs of electrons repel each other as far as possible to minimise repulsions
This gives a trigonal planar shape with approx 120bond angle
Ethene is a flat planar molecule
Why is it approx 120
Because the double bond has a higher electron density and repulsion is greater
How do sigma bonds form
Overlap of s orbitals which are spherical
How do pi bonds form
P orbitals are dumbbell shaped and can overlap in two ways , either end on forming sigma bonds or sideways overlap forming pi bonds
What is the double bond made up of
A sigma bond and a pi bond
Where is the sigma bond formed
Directly between the two carbon atoms
Where is the pi bond formed
Above and below the plane of the bonding carbon atoms
Draw the formation of a pi bond
Do double bonds rotate
No because the pi bond fixes the carbon atoms in position , at either end of the double bond this prevents any rotation
How many electrons are used for each bond in carbon
3 electrons used for 3x sigma bonds and 1 used for pi bond
What do the bond enthalpies show about double bonds and single bonds
Double bonds are stronger than single bonds however a pi bond is weaker than a sigma bond . When an alkene reacts the pi bond breaks and the sigma bond remains intact . As electron density is spread over the top and bottom the pi bond has a lower bond enthalpy .
What are the two types of isomerism
E/Z and optical isomerism
What is the definition for stereoisomerism
Stereoisomers are compounds with the same structural formula but with a different arrangement or atoms in space
What is E/Z isomerism
A type of stereoisomerism in which different groups attached to each carbon of a carbon =carbon double bond may be arranged differently in space because of the restricted rotation of the c=c double bond
What criteria is needed for an E/Z isomerism
- must have a carbon carbon double bond (because of the restricted rotation)
- each carbon in the double bond must be attached to two different groups
Which is the Z isomer and which is the E isomer
Z - groups / atoms are on the same side of the double bond
En- groups /atoms are on opposite sides across the double bond
What is cis /trans
Cis /trans nomenclature Can be applied to any such compounds in which two of the substituent groups (one on each carbon atom of the c=c group ) are the same .
The cis isomer is the ….
Same on both sides
The trans isomer is ….
Different on sides
What is cahn - Ingold prolog nomenclature
If the groups with higher priority ( higher atomic number ) are on the same side it is a Z isomer
If the groups with higher priority are on diferent sides it is an E isomer
the double bond of an alkene is an area of ……………
high electron density
the negative charge of the double bond may be attacked by electron- deficient species , which will accept ……
a pair of electrons
what is an electrophille
an electron pair acceptor
alkenes undergo addition reactions when attacked by electrophiles . this is called …………………………..
electrophillic addition
what is the definition for an electrophile
an electrophile is an atom or group of atoms attracted to an electron - rich centre or atom, where it accepts a pair of electrons to form a new covalent bond
explain electrophillic addition
1) when a non-polar molecule eg br2 approaches the alkene , the pi electrons in the carbon-carbon double bond interact with the electrons in the covalent bond. this interaction causes polarisation of the bond producing an induced dipole
2) the electron pair in the pi bond is atttracted to the delta positive end of the molecule, causing the double bond to break by heterolytic fission . a bond forms from one carbon atom from the double bond, and an atom from the approaching molecule
3) the bond in the approaching molecule breaks by heterolytic fission , with the electron pair going to the delta negative end of the molecule , forming a negative ion and a carbocation
4) the lone pair of electrons on the negative ion reacts with the positively charged carbon in the carbocation to form the addition product of the reaction
what is the definition for a carbocation
an organic ion in which a carbon atom has a positive charge
what are the examiners tips when drawing electrophillic mechanisms
- make sure arrow comes from middle of double bond to delta positive molecule
- make sure arrow comes from middle of bond to delta negative molecule .
-make sure arrow comes from lone pair to carbocation
what electrophillic addition reactions do alkenes participate in
- hydrogenation which is the reaction with hydrogen in the prescence of a nickel catalyst.
- halogenation - reaction with halogens
-reaction with hydrogen halides
-hydration-reaction with steam in the prescence of an acid catalyst
what are the conditions for the hydrogenation of alkenes
nickel catalyst and temperature of 423K
what is halogenation of alkenes
the reaction between alkenes and halogens
what is the test for unsaturation of the carbon chain
alkenes decolourise bromine water(orange to clear )
which shows the prescence of a c=c bond. if we add bromine water to a saturated compound there will be no addition reaction. additionally the more unsaturated the compound , the greater the volume of bromine water that can be decolourised
what are the conditions for alkenes with hydrogen halides
at roon temperature , mix gases together or bubble the gaseous halide through liquid alkene.
what happens when you have an unsymmetrical alkene and hydrogen halide
you make 2 different products ( as the hydrogen and halide can add onto differetn places on the chain
whats the conditions for the hydration of alkenes to form alcohols
320 c and high pressure in the prescence of an acid cataylst eg H3PO4 phosphoric acid
what happens when an unsymmetrical alkene is reacted with steam
2 possible products
why are unequal amounts of the products formed
due to the relative stabilities of the carbocation intermediates
what is markownikoffs rule
when a hydrogen halide reacts with an unsymmetrical alkene , the hydrogen of the halide attaches itself to the carbon atom of the alkene with the greater number of hydrogen atoms and the smaller number of carbon atoms
how do you know which alkene is the major product
the most stable carbocation immediate can be found by seeing where the most and least carbons and hydrogens are eg the halide will go where there is most carbon atoms and the hydrogen where there is least carbons and most hydrogens
what happens to the stability of the carbocations as the number of alkyl groups on the positively charged carbon increases
increases
primary , secondary , tertiary ect
What is an addition polymer
A giant structure comprised of a very long molecular chain with high molecular mass , formed by repeated addition reactions of many unsaturated alkene molecules
What is a monomer
A small molecule that combines with many other monomers to form a polymer
What is addition polymerisation
The process in which unsaturated alkene molecules add on to a growing polymer chain one at a time , by repeated addition reactions , to form a very long saturated chain
What would ethene become after polymerisation
Poly(ethene)
How can different polymers be made
Polymers with specific properties can be made by varying the monomer and the conditions /catalysts used .
What are the general conditions for making a polymer
High temperature and pressure in the presence of a catalyst
What is a repeat unit
A specific arrangement of atoms that occurs in the structure over and over again . They are included in brackets , outside of which Is the symbol n
What ways can waste polymers be processed
-Separation and recycling
- organic feedstock
-combustion
What is separation and recycling
Where the waste is sorted into each different type of polymer and then each type can be recycled by melting and remoulding
What does feedstock do
Can be used as a feedstock for cracking allowing for the new production of plastics and other chemicals
What happens with combustion
Can be incinerated to produce energy for electricity .
What’s a disadvantage of combustion of waste polymers
Formation of toxic , acidic waste products eg HCL , however this can be neutralised to neutralise acidic fumes
What polymers have chemists been developing
Biodegradable polymers. soluble polymers , compostable polymers and photodegradable polymers
What’s an advantage of combustion of waste polymers
Energy used to produce electricity
What’s a disadvantage of combustion of waste polymers
Products of combustion cause acid rain , formation of co2 causes global warming
Describe how HCl gas is removed from the waste gases produced
HCl gas is passed through alkali
why is a plasticisiser added to PVC
it makes it more flexible
during electrophillic addition using a haloalkane eg CL-I which would be the major product
the major product would be the one which forms the more stable carbocation intermediate . this would be the one in which the more electronegative (hungry for electrons) is bonded to the most R groups
