Chapter 13: Reactions of Alkenes Flashcards
Why are alkenes more reactive than alkanes?
- The pi bond has relatively low bond enthalpy (not much energy is required to break it) as the electrons in a pi bond are more exposed than those in a sigma bond
List 4 reactants alkenes can undergo addition reactions with.
- Hydrogen
- Halogens
- Hydrogen halides
- Steam
List the reactants, products and conditions of the hydrogenation of alkenes.
- Alkene and hydrogen
- Nickel catalyst
- Alkane
List the reactants, products and conditions of the halogenation of alkenes.
- Alkene and halogen
- Room temperature
- No catalyst
- Dihaloalkane
Why is the halogenation of alkenes helpful?
- It can be used as a test for saturation
- This is because when bromine water is added to an alkene, it goes from orange to colourless
- However, when this is done to an alkane, there is no addition reaction, and therefore no colour change, so the bromine water stays orange
List the products and conditions when hydrogen halides react with alkenes.
- Room temperature, no catalyst
- Concentrated hydrochloric or hydrobromic acid can be used
- Haloalkanes
What happens when a hydrogen halide reacts with an unsymmetrical alkene?
- They form a mixture of 2 different products (positional isomerism)
List the reactants, products and conditions of the hydration of alkenes.
- Alkene and steam
- Phosphoric acid (H3PO4) catalyst (or concentrated sulfuric acid)
- > 100°C (already indicated if you said steam/ wrote the gas state symbol)
- Alcohol (however, positional isomerism is possible again)
What is an electrophile? Required.
- An electron pair acceptor
- (They’re usually a positive ion or a molecule containing an atom with a partial positive charge)
What is electrophilic addition?
- An addition reaction in which the first step is attack by an electrophile on a region of high electron density
Describe why electrophiles take part in electrophilic addition.
- Electrophiles are atoms/ groups of atoms that are attracted to an electron rich-centre (in this case the high electron density of the pi-electrons) and accept an electron pair
Which reactions are examples of electrophilic addition?
- The reaction between alkenes and halogens/ hydrogen halides
Describe the mechanism for the reaction between an alkene and a hydrogen halide.
- The hydrogen halide molecule would be polar as halogens are more electronegative than hydrogen
- The electrons in the pi-bond are attracted to the H atom due to its partial positive charge, and shares its electrons with it, so the double bond breaks
- The H atom then bonds to one of the carbon atoms that was in the double bond
- The bond between the H atom and halogen atom breaks by heterolytic fission, and the electrons go to the halogen atom
- A halide ion and carbocation (C atom with a positive charge as it has only 3 bonds) are formed
- The halide ion reacts with the carbocation to form the final product
How should curly arrows be drawn?
- From the bond/ lone pair to the atom the electrons are being transferred to (always going form high electron density to low electron density)
What is the electrophile in this example of electrophilic addition?
- The hydrogen halide (more specifically the hydrogen atom)
Why do alkenes react with halogens?
- Halogen molecules aren’t polar, but as they approach the pi-bond, electrons are repelled, and a dipole is formed across the halogen molecule
When a hydrogen halide reacts with an unsymmetrical alkene, how do you know which product more is formed of?
- According to Markownikoff’s rule, most of the time the halide will attach itself to the carbon atom bonded to more carbon atoms than the other one
- The product formed in this situation is the major product, while the other is the minor product
Why does Markownikoff’s rule work?
- There are 3 types of carbocations; primary, secondary and tertiary
- Tertiary carbocations are bonded to 3 carbon atoms, secondary to 2 carbon atoms and primary carbocations to 1 carbon atom
- Tertiary carbocations are therefore the most stable, while primary carbocations are the least stable
- This is because alkyl groups can donate electrons
What is a monomer?
- A small molecule which combines with many other monomers to form a polymer
What is a polymer?
- A large molecule formed from many thousands of repeat units of smaller molecules (monomers)
What is a repeat unit?
- The simplest unit of a polymer that is repeated many times to form the full polymer (drawn with brackets and n)
How do alkenes form polymers?
- They undergo addition polymerisation to produce long saturated chains that contain no double bonds
How are general equations for addition polymerisation reactions written?
- The monomer is the reactant, and has an n next to it
- The product is the repeat unit drawn inside square brackets with bonds extending beyond the brackets, and with an n outside the bracket
How can the repeat unit be identified from a polymer chain?
- It consists of 2 carbons from the main chain, and the other atoms they are vertically bonded to (make sure to check to see which 2 carbons are being repeated)
How are polymers named?
- Poly(monomer)
- For example, ethene would become poly(ethene)
What are 2 sustainable ways that waste polymers can be processed?
- Combustion to produce energy
- Used as an organic feedstock to produce plastics and other organic chemicals
What are biodegradable and photodegradable polymers?
- Biodegradable polymers can be broken down by microorganisms into water, carbon dioxide and biological compounds
- Photodegradable polymers contain bonds that are weakened when they absorb light, which causes them to degrade
