4.1.3 Alkenes Flashcards
What are alkenes examples of
Unsaturated hydrocarbons
General formula for alkenes
CnH2n
What does the C=C Bosnia consist of
One sigma bond
One pi bond
Extra information about pi bonds
Exposed and have a high electron density
Vulnerable to attack electrophiles
Formation of sigma bonds in alkenes
two sp2 orbitals one from each carbon overlap and form a single C-C bond called sigma bond
Formation of pi bond in alkenes
Formed by sideways overlap of two p orbitals on each carbon forming pi bond above and below the plane of the molecule
Is a pi or sigma bond stronger
Sigma
Shape of alkenes
Trigonal planar
Angle:120
What type of isomerism do alkenes show
E/Z stereoisomerism
When does E/Z isomerism occur
Restricted rotation around C=C bond
When two different groups attached to either side of the double bond
How can E/Z isomerism occur in alkenes
Restricted rotation around the C=C bond
Pi bond
When is it a Z isomer
When priority atom/group is on the same side
Z for zusammen
On Zee Zame Zide
When is it an E isomer
If priority atom /group is on opposite side
E for entgegen
Opposite side
Rules for Cahn-ingold-prelog
- Compare atomic number of atoms directly bonded to carbon double bond and higher atomic number is given priority
- If two atoms are the same then consider atoms at distance 2 from the double bond then the highest is given priority
What can Z isomers also be called
Cis
What can E isomers also be called
Trans
Effect of E/z isomerism on physical properties
Different melting or boiling points e.g. dichloroethene
Z isomer: molecule is polar but the chlorine is on same side, one side of the molecule is more negative, intermolecular forces are London and permanent dipole dipole
E isomer: non-polar as polar c-cl bonds are on opposite sides so the dipoles cancels out, only London forces so boiling point is lower
Reactivity of alkenes
Quite reactive because of the low bond enthalpy of the pi bond
Definition of addition reaction
A reaction where two molecules react together to produce one
Reaction of alkenes with hydrogen (hydrogenation)
Alkene-> alkane
Reagent: hydrogen
Conditions: nickel catalyst
Type of reaction: addition/reduction
Definition of an electrophile
Electron pair acceptor
Electrophilic addition of alkenes
Double bonds in alkenes are areas of high electron density
Attracts electrophiles and the alkenes undergo addition reactions
Reaction of alkenes with halogens (halogenation)
Alkene->dihaloalkane
Reagent:diatomic halogen
Conditions: room temperature (no UV light)
Mechanism: electrophilic addition
Type of reagent: electrophile, halogen with delta plus
Type of bond fission: heterolytic
Explanation of the mechanism of halogenation of an alkene
As halogen molecule approaches the alkene the pi bonds repel the electron pair in the halogen-halogen bond
Induces a dipole
Halogen is polarized and forms electrophile (delta positive halogen)
Nucleophile then attacks carbocation and adds onto the carbon
Reaction of hydrogen halide with alkenes
Alkene-> haloalkane Reagent: HCl or HBr Conditions: room temperature Mechanism: electrophilic addition Type of reagent: electrophile, hydrogen delta plus
Explain mechanism of hydrogen halide with alkene
Hydrogen halide is already polar so H delta plus is attracted to pi bond
Halogen nucleophile attack’s carbocation and adds across onto two carbons
Molecule is unsymmetrical so leads to formation of two products
Why does electrophilic addition with hydrogen halide and alkene lead to two products
As alkene is unsymmetrical
Markownikoffs rule
Atom will be added to the carbon with the least number of hydrogen attached to it
Explain why one is a major product and one is a minor product
Major is formed via a more stable carbocation intermediate
Stability of carbocations
Tertiary-> secondary -> primary
Reaction of alkenes with steam (hydration)
Alkenes -> alcohols
Reagent: steam/H2O
Conditions: high temp 300 to 600, high pressure 70 atm and concentrated acid catalyst e.g. conc H3PO4
Hydration reactions
Water is added to a molecule
Extra information hydration
High pressures means not done in labs
Preferred industrially as no waste products so has a high atom economy
Separation of products is easier and cheaper to carry out
Addition polymerisation
When addition polymers are formed from alkenes
Reactivity of poly alkenes
Unreactive due to the strong C-C and C-H bonds
What to remember when drawing addition polymerisation
n in monomer goes before
N in polymer goes after and outside brackets
Bonds in polymer must extend out of brackets
Must remove double bond to single bond
Industrial i,portamce of alkenes
Formation of polymers from ethane based monomers
Manufacture margarine by catalytic hydrogenation of unsaturated vegetable oils (uses hydrogen and nickel catalyst)
Liquid vegetable oils are generally polyunsaturated alkenes
Using hydrogen and catalyst converts double bonds to single bonds
Increases melting point of oil to make it harder and more solid
How can waste polymers be processed
Separation and recycling
Feedstock for cracking
Combustion for energy production
Separation and recycling
Waste is sorted into each type of polymer (PTFE,PVC,PET)
Each type can be recycled by melting and remolding
Feedstock for cracking
Waste polymers can be used as feedstock for cracking process allowing new production of plastics and other chemicals
Combustion for energy production
Waste polymers can be incinerated and heat released can be used to generate electricity
Combustion of halogenated plastics e.g. PVC can lead to formation of toxic acidic waste products such as HCl
Chemists can minimize environmental damage by removing HCl fumes formed from combustion process
Polymers formed from what are biodegradable
Isoprene (2-methyl-1,3-butadiene)
Maize
Starch
