Chapter 13 - Alkenes Flashcards
What makes up a double bond
sigma and pi bond
pi bond
sideways overlap of two p-orbitals
high electron density above and below the 2 bonding atoms
general formula
CnH2n
Bond angle and shape
120 & trigonal planar
Steroisomers
same structural formula but different arrangement of atoms in space
E/Z isomerism conditions
- double bond
- different groups attached to each C in double bond
E isomer
grps in different direction
Z isomer
grps in same direction
Cis/trans isomerism
special E/Z isomerism where 1 grp attached to the carbon is the same
Cahn-Ingold-Prelog rules
assigns priority based on atomic no.
higher the atomic no. higher the priority
why are alkenes more reactive then alkanes
- due to pi bond - above and below sigma bond
- as on outside pi bond more exposed allowing for more reactions
4 addition reactions
- Hydrogenation
- Halogenation
- Hydrogen halides
- Hydration
Hydrogenation
- nickel catalyst
- 423K
- forms alkane
Halogenation
- RTP
- Forms haloalkane
- can be used as test to see if double bond present
- decolourises from orange to clear
Hydrogen Halides
- RTP
- either as two gases or hydrogen halide bubbled through liquid alkene
- forms haloalkane
Hydration
- Steam
- phosphoric acid catalyst
- forms alcohol
Electrophile
- Electron pair acceptor
- atom/grp of atoms that are attracted to electron-rich centres and can attract electron pair
- positive ions or contains small partial positive charge
Nucleophile
- Electron pair donor
- atom/grp of atoms that are attracted to electron-deficient centres and can donate an electron pair
- negative ions or contains small partial negative charge
e.g. alkenes which contain pi bonds
mechanism
electrophilic addition
electrophilic addition
reaction
- hydrogen halide reaction
- halogenation
carbocation stability
- increases as go from primary to tertiary
- primary has one alkyl grp etc…
carbocation stability explanation
- each alkyl grp donates/ pushes electrons to positive charge of carbocation
- more groups more evenly spread out and stable
Markownikoff’s rule
when hydrogen halide reacts with unsymmetrical alkene, the H is attracted to the C atom with greater no. of H atoms and lower no. of C atoms
Addition polymerisation
- carried out by unsaturated alkenes to produce long saturated chains with no double bonds
- usually high molecular masses
addition polymerisation conditions
- high temp
- high pressure
- usually catalyst (tin/nickel)
environmental concerns
- disposing of waste polymers
- recycling
- PVC recycling
- polymers as waste fuels
- feedstock recycling
- bioplastics
- biodegradable polymers
- photodegradable polymers
disposing of waste polymers
- cheap and convenient to use
- unreactive - good for storing food etc.. but means usually not biodegradable
- harm marine life
- usually from crude oil - non-renewable
recycling
- conserves fossil fuels
- but have to be sorted by type (polymer content)
- chopped, washed, dried, melted for resue
PVC recycling
- hazardous - high Cl and other additives content
- releases toxic gases & dioxins when burnt
- use solvent to dissolve polymer then recover high grade PVC by precipitation from solvent
polymers as waste fuels
- hard to recycle as high stored energy value
- incinerated to produce heat - steam for electricity
feedstock recycling
- converting polymer chains back to monomers/resembling crude oil
- used as raw materials to make new polymers
bioplastics
- made from plant starch, cellulose, plant oils & proteins
- renewable & sustainable
- use protects environment & conserves oil reserves
biodegradable polymers
- broken by microorganisms into carbon dioxide, water & biological molecules
- compostable polymers degrade - leave no visible/toxic residue - based on poly(lactic) acid
- often condensation polymers
photodegradable polymers
- has bonds that are weakened by absorbing light to start degradation
- or light absorbing additives are used
