Chapter 13 - Alkenes Flashcards
What is the general formula of alkenes?
CnH2n
Are alkenes unsaturated or saturated hydrocarbons?
Unsaturated
Contain at least one double carbon to carbon bond
Describe bonding in an alkene
In one C=C bond:
- 1 sigma bond formed direct overlap of orbitals. Head on overlap - giving highest possible electron density
- 1 Pi bond formed by sideways overlap of two p-orbitals.
Form two parts - one below and one above molecule’s axis.
Pi bonds cannot rotate
What kind of bond is a C-H bond?
Sigma
What kind of bond is a C-C bond?
Pi
Which one has lower enthalpy? Sigma or Pi bond?
Pi bonds have a lower enthalpy as electron density is spread out above and below the nuclei. Meaning electrostatic attraction between the nuclei and shared pair of electrons is weaker.
What is the shape around each carbon atom in a double bond (C=C)?
-3 regions of electron density around each of the carbon atoms.
repel each other as far apart as possible, so the bond bond angle = 120 degrees
giving trigonal planar shape
What are stereoisomers?
Stereoisomers have the same structural formula but a different arrangement of atoms in space.
What is an E isomer?
When the 2 highest priority groups are on OPPOSITE sides of a C=C bond
What is a Z isomer?
When the 2 highest priority groups are on the SAME side of the C=C bond
What are the requirements for a molecule to have E/Z isomerism?
- A C=C bond - as this cannot rotate
- 2 different groups attached to each carbon atom of the double bond
What is cis/trans isomerism?
Special case of E/Z isomerism where one of the attached groups on each carbon atom of the double bond must be the same.
Which one of cis and trans isomers are E and Z isomers?
Cis - Z isomer
trans - E isomer
What are the CIP rules used to find?
Priority rules used to find E and Z stereoisomers
How are the CIP rules used to assign priority?
- ) look at the atoms directly attached to carbon atoms in the double bond and decide which 2 atoms have the highest priority - the higher atomic number = higher priority.
- ) If 2 atoms assessed are the same then find the first point of difference - and the higher the atomic number = higher priority.
What is the name - CIP rules short for?
Cahn - Ingold - Prelog rules
Why are alkenes more reactive than alkanes?
Pi bond electrons are more exposed than the electrons in the sigma bond.
Give examples of what alkenes undergo addition reactions with.
- Hydrogen
- Halogens
- Hydrogen halides
- Steam in the presence of an acid catalyst
What is hydrogenation of an alkene?
Alkene mixed with hydrogen and passed over nickel catalyst at 423K. Addition reaction where hydrogen is added across a double bond
Draw hydrogenation of buta-1,3-diene
Look at paper flashcards
What is halogenation of alkenes?
Rapid addition reaction with halogen. The halogen adds across the double bond.
What test is used to detect C=C bond (unsaturation)?
Bromine test
bromine adds across C=C bond and orange colour disappears.
What addition reaction occurs with alkenes that forms a haloalkane?
(gaseous) hydrogen halide addition
What does hydration of alkenes with steam in presence of a phosphoric acid catalyst form?
Alcohol
Draw hydration of propene
Look at paper flashcards
What is electrophilic addition?
Double bond in alkenes open up and atoms are added to the carbon atoms
What is an electrophile?
Atom or group of atoms that is attracted to an electron-rich centre and accepts an electron pair
Usually a positive ion or has slight positive charge
Describe electrophilic addition mechanism of but-2-ene and hydrogen bromide.
- )Bromine is more electronegative than hydrogen, so hydrogen bromide is polar and contains the dipole H + - Br -
- ) The electron pair in the pi bond is attracted to the partially positive hydrogen atom, causing the double bond to break.
- ) A bond forms between the hydrogen atom of H-Br molecule and a carbon atom that was part of the double bond.
- ) The H-Br bond breaks by hetrolytic fission, with the electron pair going to the bromine atom
- )Bromide ion (Br-) and a carbocation are formed. A carbocation contains positively charged carbon atoms.
- ) The br- ion reacts with the carbocation to form the addition product.
Draw mechanism of electrophillic addition of hydrogen bromide to but-2-ene
Look at paper flashcards
Whats the difference when a hydrogen halide is added to a symmetrical alkene to when it’s added to an unsymmetrical alkene?
Two possible products when added to a unsymmetrical one.
What does the amount of product formed in a electrophilic addition reaction depend on?
How stable the carbocation formed in the middle of the reaction is
Are carbocations more stable with more or less alkyl groups? And why?
More alkyl groups
Alkyl groups feed electrons towards the positive charge
State Markownikoff’s rule.
The major product from addition of a hydrogen halide (HX) to an unsymmetrical alkene is one where hydrogen adds to the carbon with the most hydrogens already attached.
What is addition polymerisation?
Joining of monomers to form chain - called polymers
How do you write polyethene?
Monomer in brackets. With ‘n’ outside representing the repeating unit.
How can we reduce environmental impacts of waste polymers?
- Recycling
- Using waste polymers as fuel
- Feedstock recycling
How does recycling decrease environmental impact of waste polymers?
Conserves finite fossil fuels and decreases amount of waste going into landfill
What are the downsides to recycling waste polymers?
Pointless if polymers are not sorted by type and are mixed.
How does feedstock recycling reduce environmental impacts?
- Reassemble products produced from crude oil. Can be used as raw materials for production of new polymers
- Able to handle unsorted or unwashed polymers.
How can waste polymers be used as a fuel?
Incinerated to produce heat, generating steam to drive a turbine producing electricity.
What are 3 developments that can be made to benefit the environment in polymers?
- Bioplastics
- Biodegradable polymers
- Photodegradable polymers
