Chapter 13 Alkenes Flashcards
Alkenes
- Unsaturated hydrocarbons (at least 1 C=C bond)
General formula of alkenes
- CnH2n
The nature of the C=C bond
- C has 4 electrons and 3 are used in sigma bonds (1 to the other C and the other 2 for other atoms)
- The remaining electron is a pi bond
pi bond formation
Formed by the sideways overlap of 2 electrons in the p-orbitals (one from each C of the C=C bond)
Where is the pi-electron density concentrated
above and below the line joining the nuclei of the bonding atoms
What does the pi bond do to the C atoms
Locks the C atoms in positions and prevents them from rotating around the C=C bond -> geometry of alkenes is different to that of alkanes
Alkenes in the natural world
- Carotene = makes flamingos pink
- Limonene = causes scent of oranges and lemon
Stereoisomers
Have the same structural formula but a different arrangement of the atoms in space
Types of stereoisomerism
- E/Z isomerism
- Optical isomerism
Why does stereoisomerism occur?
The rotation around the C=C bond is restricted so the groups attached to those Cs are therefore fixed relative to eachother
When will a molecule have E/Z isomerism
When the molecule has:
* A C=C bond (restricted rotation)
* 2 different groups attached to each C atom of C=C bond
Cis-trans isomerism
- Special case of E/Z isomerism
- Same requirements as E/Z isomerism but one of the attached groups on each C atom of the C=Cmust be the same
Relationship between cis/trans isomerism and E/Z isomerism
When there is a H on each C=C bond
* Cis isomer = Z isomer
* Trans isomer = E isomer
Cahn-Ingold-Prelog Rules
- Z isomer: If the groups of highest priority are on the same side of the C=C bond
- E isomer: If the groups of highest priority are diagonally places across the C=C bond
How to assign priority
- As atomic number increases, priority increases
- If it is a large atom, find the first point of difference and use this to compare priority
Why are alkenes much more reactive than alkanes
pi bond -> the electrons in the pi bond are more exposed than the electrons in the sigma bond. The pi bond readily breaks and alkenes undergo addition reactions relatively easily
Addition reactions of the alkenes
- hydrogen in the presence of a nickel catalyst
- halogens
- hydrogen halides
- steam in the presence of an acid catalyst
Hydrogenation of alkenes
Alkene + hydrogen -> alkane
What is the product of an addition reaction?
Always saturated
Halogenation of alkenes
Alkene + bromine -> dihaloalkane
Test for unsaturation (C=C bond)
Add bromine water
orange -> colourless
Hydrogen halides
Hydrogen + halogen
e.g. HCl
Alkenes + hydrogen halides
haloalkane
Hydration of alkenes
Alkene + Water -> alcohol
Electrophilic addition
Reaction mechanism for addition reactions of alkenes
Electrophile
- Electron pair acceptor (attracted to an electron-rich centre)
- Usually a positive ion, or a molecule containing an atom with a δ+ charge
Reaction between but-2-ene + hydrogen bromide
- Bromine is more electronegative than hydrogen, so hydrogen bromide is polar (contains dipole Hδ+ - Brδ-)
- The electron pair in the pi bond is attracted to the Hδ+ -> causes the double bond to break
- A bond forms between the H of the H-Br and a C that was part of the C=C bond
- The H-Br bond breaks by heterolytic fission, with the electron pair going to the bromine atom
- A Br- ion and carbocation are formed. These react together to form the addition product
How does polarisation of bromine occur (bromine is a non-polar molecule)
The pi electrons interact with the electrons in the Br-Br bond -> causes Brδ+ -Brδ-
Markownikoff’s rule
When a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen from the hydrogen halide attaches to the carbon with more hydrogen and less carbon
Steps of electrophilic addition
Sometimes, 3 different carbocations can be produced:
* Primary carbocation (has 1 alkyl group)
* Secondary carbocation (has 2 alkyl groups)
* Tertiary carbocation (has 3 alkyl groups)
R
Alkyl group
Polymers
Extremely large molecules from many repeat units (monomers)
What polymerisation do alkenes undergo?
Addition polymerisation -> produces long chains of saturated chains with no C=C bonds
Characteristics of addition polymers
- High molecular masses
- Have prefix poly-
Examples of synthetic addition polymers
- Poly(ethene)
- Poly(chloroethene)
- Poly(propene)
- Poly(styrene)
- Poly(tetrafluoroethene)
Environmental concerns of polymers
- Disposing of waste polymers
- Recycling
- PVC recycling
- Using waste polymers as fuel
- Feedstock recycling
Biodegradable and photodegradable polymers
- Biodegradable polymers
- Photodegradable polymers
Environmental concerns of polymers: Recycling
Reduces environmental impact by conserving finite fossil fuels and decreasing amount of waste going to landfill
Environmental concerns of polymers: PVC recycling
Disposal and recycling of PVC is hazardous due to high chlorine and additive content
- New technology uses solvents to dissolve the polymer
Environmental concerns of polymers: Using waste polymers as fuel
As polymers are derived from natural gas, they have a high stored energy value -> so can be incinerated to produce heat -> electricity
Environmental concerns of polymers: Feedstock recycling
Describes the chemical + thermal processes that can reclaim monomers, gases or oil from waste polymers
Advantage of feedstock recycling
Can handle unsorted and unwashed polymers
Bioplastics
- Produced from plant starch, cellulose, plant oils and proteins
- Offer a renewable + sustainable alternative to oil-based product, and protects our environment and conserves our valuable oil reserves
Biodegradable polymers
- Broken down by microorganisms into water, carbon dioxide and biological compounds
What are compostable polymers
Based on poly(lactic acid)
Photodegradable polymers
Contain bonds that are weakened by absorbing light to start the degradation
Catalyst for hydration of alkenes
Phosphoric acid
Differences between sigma bonds and pi bonds
- pi bond is weaker than sigma bonds
- pi bond is above and below the bonding atoms whereas sigma bond is between bonding atoms
When there is a possibility for different haloalkanes to be produced in a reaction, which one will be produced in greater quantity?
The tertiary haloalkane because it is more stable than the secondary and primary haloalkane
Advantages of combusting polymers
Produces electricity
Disadvantages of combusting polymers
Produces Hcl or chlorine (however this can be neutralised using an alkali)
Bonding angle around addition polymer
Tetrahedral
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