1.7 Alkenes
-
Upload
eyvavavaviviveveyvo -
Category
Documents
-
view
228 -
download
0
Transcript of 1.7 Alkenes
Alkenes
Alkenes & ethene
Occurs naturally in small quantity From crude oil – product of cracking & catalytic reforming Raw materials – polymers, detergents, solvents, etc. Unsaturated HC (has at least 1 C=C) For 1 C=C, less 2 H atoms General formula – CnH2n
Mp & bp increase with increase # of C
C=C C-C (σ bond) in ethane
e cloud symmetrical about central axis of molecule Rotate about the axis – 2 ends of ethane free to rotate
C=C (σ and π bonds) in ethene π bond above & below the plane of molecule Does not allow rotation around axis e density in π bond covers large area around the nuclei of the C
atoms C=C less than twice as strong as C-C (1 σ bond) in alkane Bond enthalpy for σ bond is 347, for π bond is 265
C=C Properties of molecules with C=C
e rich – positively polarised groups (electrophiles) more attracted to it
π bond provides e pair to form new bond with electrophile
Geometric isomerism – rotation around bond axis not possible
Naming & geometric isomerism Geometric isomers = occur when components of molecules
arranged on diff sides of molecule. Traditional way – cis-trans isomerism Example:
Diff physical props (mp, bp, density) Often same chemical reactivity
H
CH3 CH3
H H
CH3 H
CH3
cis-but-2-ene trans-but-2-ene
E-Z Isomerism Current IUPAC system Works for all geometric isomers Groups attached around C=C are ranked
Highest number, highest priority E = engegen = opposite Z = zussamen = together
Reaction – addition Energy to break π bond is lower Heterolytic fission Can be attacked by electrophiles & oxidising
agents
H
CH3 CH3
H
H
CH3 H
CH3
Z-but-2-ene
E-but-2-ene
E-Z Isomerism
Br
I Cl
F Br
I F
Cl
F < Cl < Br < I
2. Find the mainisomers of pentene & give them correct E-Z nomenclature
1. Give correct E-Z nomenclature
Electrophiles & addition rxn Electrophiles
Electron deficient species Most common is H+
More attracted to C atom (has e pair)
Rxn of alkenes w H2
condition – moderately high T (abt 200 ºC), Ni catalyst product - alkane
H
H H
H
+ H2 HH
H H
H HN i
Electrophiles & addition rxn Rxn w halogens
decolourise Br2 (orange) product – disubstituted halogenoalkane
rate of reaction decrease down halogen groupF > Cl > Br > I
H
H H
H
+ HH
H H
Br BrN i
Br Br
Addition rxn mechanism
HH
H H
Br BrC
+
H H
H
BrHBr
-
H
H
Br
Br
H
H
2 e in π bond form bond with H atom
H-Br bond break, 2 e goes to Br
Carbocation (carbonium ion)
Nucleophile
Testing for alkenes Bromine water (HBr) – test for alkene Orange bromine water turn to colourless Major product:
Minor product:
+ Br Br + OH2
H
H H
H
H
OH Br
H
HH + H Br
+ Br Br + OH2
H
H H
H
H
Br Br
H
HH
Rxns of alkenes with HX Product – monosubstituted haloalkane Rxn of HX with
Symmetrical alkenes (ethene) – 1 product Asymmetrical alkenes (propene) – 2 products
Asymmetrical alkene – Markovnikov’s rule: Major product form has H added to C with greater number of H
Rxns of symmetrical alkenes w HXHH
H H
H BrC
+
H H
H
HHBr
-
H
H
H
Br
H
H
Rxn of asymmetrical alkene w HXCH3
H H
H
H Br
C+
CH3
H H
H
H
Br-
CH3
H
Br
H
H
H
CH3
H H
H
H Br
C+
H
H H
HCH3
Br- H
H H
HCH3
Br
Major product
Minor product
Rxn with KMnO4/H+
Addition & oxidation Product – alkanediols Purple KMnO4 turn to colourless Can be used to differentiate btwn alkenes & alkanes
H
H H
H
H H
OH OH
HHKM nO 4/d il. H 2SO 4
Polymerisation Very large molecule made fr monomers (smaller units) Synthesis
Addition rxn btwn monomers with C=C or C≡C Ethene → polyethene Propene → polypropene Chloroethene → polycholoroethene (polyvinylchloride,
PVC)
H
H H
H H
H H
H H
H H
H
.
H
H
H
H
H
H
H
H
.
H
H
H
H
Polymerisation
LDPE• Highly branched• Soft & malleable• Low mp
HDPE• Few branched chains• Rigid & dense• High mp
Properties of polymer Depend on their monomers, various properties
•Tensile strength & mp ↑ when length ↑ until about 500 units
Avg chain length•Branched chains cannot pack closely – low
tensile strength, mp & density
Branching
•Strong forces – high mp•Due to side groups
Intermolecular forces
•Hold chains together – very rigid, hard, brittle, high mp
Cross links
Polymer problems & solutions
Energy
Fr fossil fuels
Energy to make polymers fr the
same source
Resources
Fossil fuel – limited supply,
costly
Need for new resource
Disposal
Waste problem, toxic gases
Endanger animals
C footprint of plastics C footprint = a measure of impact fr human activities in terms of
CO2
Purpose – to try to reduce C footprint as much as possible Source of CO2 – burning of fuels, waste plastics Solutions:
Use renewable energy Reduce use of polymer products, reuse Recycle – reduce diposal & demand to produce new ones Recycle thermoplastics – PET – carpets, textiles, waterproofing
materials
How are plastics recycled?
Mechanical• Melted, shredded, turned into granules• Sorting – done by hand & expensive
Chemical• aka feedstock recycling• Brkdown of waste to monomers – burning, thermal
cracking, & gasification• Expensive – use energy
Biodegradable polymers Development of renewable resources – biopolymers, bioplastics,
‘green plastics’ Modified starch or cellulose (fr plants)
Use CO2 during growth – reduce C footprint Bacterial fermentation – break down plant materials to monomers Broken down by microbes – biodegradable
Land use to grow plants for polymers – decrease land to grow food crops
Energy recovery Recovering some energy that was used into making polymers Example:
burn waste polymer to generate electricity – use incinerators at high T to prevent release of dioxin
Pyrolysis – break down polymer by heat in absence of O2
Syngas – gasification involves brkdown of solid HC in limited O2
Product – mainly mixture of H2 & CO Most process are not efficient & expensive
Life cycle analysis Use by industry – quantify effect to environment Product examined fr extraction of raw materials to cost of recycling
careful look at energy & material use & emission to environment to improve process & reduce impact to environment compare 2 diff polymers & decide which to be used based on their life cycle
End of alkenes