13 - Alkenes Flashcards
bonding in C=C
- 3 e- in a sigma bond
- 1 electron left from a p-orbital on each carnbon in double bond
- a pi bond is formed
- the sideways overlap of two p orbitals
- e- density is above and below the joining line of the nuclei
- no rotation around bond
shape aroun C=C
trigonal planar
- 120
when can u use E/Z isomerism
- C=C
- different groups attatched to each atom of the double bond
when can u use cis/trnas
- C=C
- different groups attatched to each atom of the double bond
- one of the attatched groups on each carbon must be a hydrogen
usig cahn ingold
- if groups with higher priority on the same side of the double bone (Z)
- higher priority means higher atomic number
- or if they are identical, find the first point of difference. the one atom with the higher atomic number has the priority
conditions of hydrogenation of alkenes
- mixed with hydrogen and passed over a nickel catalyst at 423K
- forms an alkane
- hydrogen is added across the double bond
- all c=c bonds react like this
conditions of hyalogenation of alkenes
- a rapid addition reaction
- RTP
- react with X2
conditions of hydration of alkenes
alcohol is formed
- H20 gas adds across the double bond
- in presence of phosphoric acid H3PO4
- two possible products
electrophile
- electrophile is an atom/s that is attracted to an electron rich centre and accepts an electron pair, it is usually positive or delta positive
markownikoffs rule
hydrogen attaches to the carbon atom with the carbon with the greater number of hydrogens attached
carbocation stability
stability increases
primary->secondary->tertiary
more alkyl groups the more charge is spread so more stable carbocation
Each alkyl group donated and pushes electrons to the positive carbocation, the positive charge is spread over the alkyl groups, so it is more stable.
increased inductive effect
stereoisomers
have the same structural formula but a different arrangement of the atoms in space
-two types
-> E/Z isomerism
-> optical isomerism
addition reactions of alkenes
- Hydrogen in the presence of a nickel catalyst
- halogens
- hydrogen halides
- steam in the presence of an acid catalyst
reactivity of alkenes vs alkanes
- alkenes are more reactive because of the pi-bond they have
- the pi-electron density is concentrated above and below the plane of the sigma bond
- being on the outside of the double bond, pi-electrons are more exposed than electrons in the sigma bond.
- a pi-bond more readily reacts breaks and alkenes undergo addition reactions relatively easy
testing for unsaturation
- the reaction of alkenes with bromine is used
- if bromine water is added to an alkene, the addition reactions happens, and the solution goes from orange to colourless
- no colour change with a saturated compound
alkenes with HX
- react with HX at RTP to form haloalkanes
- if the alkene is liquid the HX is bubbled through
- if the alkene is gas, the reaction take place when the two gases mix
- alkenes also react with concentrated HCL of HBr (solutions of HX in water)
- there are two products
electrophilic addition reactions in alkenes
- alkenes undergo addition reactions, which can be called electrophilic addition
- the double bond in an alkene has high pi-electron density
- it attracts electrophiles
- electrophile is an atom/s that is attracted to an electron rich centre and accepts an electron pair, it is usually positive or delta positive
explain the mechanism of but-2-ene and Hbr
- draw mechanism
1) Br is more EN than H, so it forms a polar molecule
2) electron pair in pi-bond is attracted to delta + hydrogen, causing the double bond to break
3) a bond is formed between hydrogen and carbon in C=C
4)H-Br breaks by heterolytic fission, the electron pair goes to the Bromine
5) Br- and carbocation are formed
6) Br- reacts with carbocation to form a bond
what kind of polymerisation does alkenes undergo
addition
- produces molecule with no double bond
repeat unit
- the specific arrangement of atoms in he polymer that repeats over and over again
polyethene
- made by heating ethene molecules under high pressure and temperature
poly(chloroethene)
- or PVC
- monomer is chloroethene
environmental concerns of polymers
- disposing of waste polymers
- recycling
-> PVC recycling
-> using waste polymers as fuel
-> feedstock recycling
Biodegradable and photodegradable
disposing of waste polymers
- polymers are readily available, cheap and convenient
- the lack of reactivity means disposing is hard
Recycling - polymers
- reduces their environmental by conserving finite fossil fuels
- and decreases amount going to landfill
- sorted into types
- they are chopped washed dried and melted, and used to make new products
pvc recycling - polymers
-recycling PVC is hazardous because of high chlorine content
- putting it in landfills is not sustainable, and when burnt it releases HCl, a corrosive gas and other pollutants
- now, solvents are used to dissolve the polymer
using waste polymers as fuel - polymers
- some polymers cant be recycled
- can be incinerated to produce heat, generating steam, to drive a turbine producing electricity
feedstock recycling - polymers
- they chemical and thermal processes that can reclaim monomers, gases of oil from waste
- materials can be used as raw materials to produce new polymers
- similar to crude oil refineries
bioplastics
- produced from plant starch, cellulose and plant oils
- offer renewable and sustainable way
biodegradable polymers
- broken down by microorganism into water, CO2 and biological compounds
- usually made from starch or cellulose
- compostable polymers degrade and leave no toxic residue
photodegradable polymers
- contains bonds that are weakened by absorbing light to start the degradation
