4.1.3 Alkenes Flashcards
why are alkenes unsaturated hydrocarbons
they have at least 1 C=C double bond
what is the formula for alkenes, and under what conditions is it valid
CnH2n
- if aliphatic
- if branched
- (not if cyclic or contain more than 1 C=C)
what is each C=C double bond made of
3 sigma bonds
1 pie bond
what is a pi bond
- the sideways overlap of 2 p-orbitals, one from each carbon atom of the C=C, and each contributing 1 electron, above and below the plane of C-atoms
where is electron density concentrated in a pi bond
concentrated above and below the line joining the nuclei of the bonded atoms
why can pi bonds not rotate
the double bond locks the atoms into position, so atoms cannot rotate
what shape forms around the C=C
trigonal planar
why is the C=C bond trignal planar
- 3 electron pair regions around C atom
- electron pair regions repel each other as far as possible
- 120 degree bond angle
- no lone pairs, so don’t have to worry
- all atoms in the same plane
what are stereoisomers, and how are they different to structural isomers
have the same molecular and structural formula, but a different arrangement of atoms in space
- structural = same molecular formula, different structural formula
why can E/Z isomers occur
the C=C double bond is fixed due to pi bond
what are the conditions of E/Z isomers
1) must have C=C
2) must have 2 different groups on each C atom in the double bond
what is difference between E and Z isomers
E= the groups are attached diagonally across double bond
Z= the groups are on the same side of the double bone (Zee Zame Zide)
what are cis/trans isomers
special type of E/Z isomer where one of the groups on each carbon atom in double bond must be a H
- trans = E
- cis = Z
what is the rule for assigning priority where there are more groups present
assign priority based upon highest atomic number, and if one group has more than one atom, read from left to write, comparing atoms
- do separate chains on each C=C separately
how do you determine if an isomer with 4 groups is an E/Z isomer
E= the groups with the highest priority are diagonally placed across the C=C
Z= the groups with highest priority are on the same side of the C=C
why are alkenes more reactive than alkanes
- due to presence of pi-bond
- means the electron density is concentrated above and below the plane of sigma bonds
- so outside of double bond, means that the pi-electrons are more exposed than sigma electrons
- pi bond readily breaks to undergo addition reactions
how can you determine the enthalpy of the pi bond alone
- subtract the enthalpy of the C=C to the equivalent C-C bond
what is an addition reaction in alkenes
- there is an addition of a small molecule across the double bond
- breaking the pi-bond and forming new bonds
what are the 4 addition reactions of alkenes
1) hydrogenation
2) halogenation
3) hydrogen halides
4) hydration
explain the hydrogenation of alkenes
- alkenes mix with hydrogen
- pass over NICKEL CATALYST at 423K
- breaks ALL C=C
- forms equivalent alkane
( consider balancing the H2 so ensure enough to attach to all C=C)
explain the halogenation of alkenes
- rapid addition reaction with
- CHLORINE/BROMINE
- at ROOM temperature
how can you test the saturation of alkenes
BROMINE WATER:
- bromine water is orange
- if bromine is added across double bond
- will turn clear
- if no C=C, will stay orange
explain the reaction between alkenes and hydrogen halides ( e.g. chloride and bromide)
- react with GASEOUS hydrogen halides
- or CONCENTRATED ACID equivalenT (solutions of halides in H2O)
- at ROOM TEMPERATURE
- form haloalkanes
- if liquid alkene, halide is bubble through
- if gaseous alkene, gases are mixed
why can the hydrogen halide reaction with alkenes form more than 1 molecule
- if unsymmetrical alkene reacts with asymmetrical molecule, will form 2 product
explain the hydration of alkenes
- react with STEAM (H2O (g))
- in presence of PHOSPHORIC ACID CATALYST (H3PO4)
- forms alcohol, as steam is added across double bond (OH to one carbon, H to other)
what is the addition reaction mechanism in alkenes
electrophilic addition
why does electrophilic addition occur in alkenes
- double bond in alkenes represents a region high in electron density
- due to pi-bond
- this attracts electrophiles to react
what are electrophiles
an atom or group of atoms that is attracted to an electron rich centre, and accepts an electron pair (usually a positive ion, or molecule containing atom with slightly positive charge)
explain how you would draw out electrophilic addition
- arrow from C=C BOND to atom facing it
- arrow from molecule BOND to other atom in molecule
- BREAKS VIA HETEROlYTIC FISSION, where electron PAIR goes to one atom)
- forms a carbocation, show on arrow
- line going from LONE PAIR on atom left behind (INCLUDE -ve charge too) to the + charge of unbonded C atom
- line going to end product
what is a carbocation
contains a positive carbon atom
how does electrophilic addition occur between alkenes and NON-POLAR atoms
- as molecule approaches alkene, the pi-bond electrons interact with the bond
- interaction causes the polarisation of the molecule, forming an induced dipole
AS: - usually electrons are evenly distributed
- but as they approach the pi-bond, the electrons in molecule are repelled, so induces polarity, as electrons move towards one atom more
REST IS SAME
what is Markinowoff’s rule
- the major product is made from the most stable carbocation ( the positive C is attached to the most alkyl groups)
- AS more of the positive charge can be spread out between the alkyl groups
explain the 3 types of carbocations
1) primary = only has 1 alkyl group attached to the positively charged C
2) secondary = has 2 alkyl groups attached to the positively charged C
3) tertiary = has 3 alkyl groups attached to the positively charged C
- stability increases as you go down, so major product is most likely to be made of the tertiary carbocation
what are polymers
large molecules made out of lots of small molecules called monomers joined together
what are the 2 types of polymers
- addition: no other products formed
- condensation: small molecule is released
how do you draw an equation for polymerisation
- put n in front of monomer unit
- draw arrow
- unjoin C=C, add brackets with elongated bonds at side, and n on the outside
- n= lots of repeating units
how do you find the monomer from a polymer
- just remove and replace the C=C in ONLY the chain joining the separate monomers together
why are polymers useful, and what are some of the drawbacks
- readily available, cheap to purchase, more convenient
- challenge to dispose, as has a lack of reactivity
how can you usefully recycle and process polymers
1) sort and recycle, and remould into new polymers
2) burn to release energy, which can be used to generate electricity
3) cracked into monomers, and used to make reform polymers (used as feedstock)
what are types of polymers which are now being usefully made
bioplastics, which protect environment and conserve valuable oil reserves
1) BIODEGRADABLE POLYMERS: which are plant based and can be broken down by microorganisms
2) PHOTODEGRADABLE POLYMERS: oil-based polymers, which contain bonds that can be weakened by absorbing light, and start degradation
what are impacts of burning polymers
- HCL formed, which causes acid rain
- can be neutralised with alkali, e.g. NaOH
- CO formed with incomplete, which is toxic to humans
- can be burned in excess O2, to prevent ever being made
- e.g. PVC= HCL produced, PTFE= HfF produced, which is also toxic and acidic
