Chapter 13: Alkenes Flashcards
Learn properties of Alkenes, stereoisomerism, reactions, electrophilic addition, polymerisation
Define the alkene homologous series?
A molecule containing at least One carbon-carbon double bond.
Define the nature of the Carbon-carbon double bond?
A π-bond. The sideways overlap of p-orbitals as each carbon atom contributes one electron to the bond. High electron density occurs above and below the joining nuclei.
What is the shape around each carbon atom in a double bonded pair?
Trigonal Planar; 120° bond angle.
Why is the shape around a double bonded carbon atom trigonal planar?
- Three regions of high electron density
- Three regions equally repel
- All bonded atoms are on the same plane
Why does stereoisomerism occur in alkenes?
The π-bond restricts rotation enabling variation across a chain due to a high electron density above and below the σ-bond.
When are cis/trans definitions of stereoisomerism applicable?
When one of the attached groups to each carbon atom is hydrogen.
When is the stereoisomer Cis?
When hydrogen is on the same side of the c=c bond. (z)
When is the stereoisomer Trans?
When hydrogen is on different sides of the c=c bond. (E)
What are the Cahn-Ingold-Prelog rules?
- assign a priority to each branch
- if Highest priority is on the same side (z) if on alternate (E)
How do you assign Priority according to Cahn-Ingold-Prelog rules?
Assign each atom a value equal to the atomic number across one carbon in a c=c bond.continue down each chain to the first point of difference. The Branch with the highest priority at this point is the highest priority.
What is the Hydrogenation of Alkenes?
The addition of hydrogen to an alkene to form an alkane. (Addition reaction)
What are the conditions for the Hydrogenation of Alkenes?
Presence of a Nickel catalyst at 423K
What is the Halogenation of Alkenes?
The addition of halogens to an alkene to form an haloalkane. (Addition reaction)
What are the conditions for the Halogenation of Alkenes?
Room Temperature
How do you test for a saturated hydrocarbon(aq)?
Addition of a halogen (Br) if colour remains then saturated; If colourless returns unsaturated.
What are the reacting conditions for the addition of hydrogen halides to alkenes?
- Gaseous (RTP) reaction when mixed
- Hydrogen Halide bubbled through when alkene is a liquid.
What is the Hydration of Alkenes?
Addition of water to an alkene to produce an alcohol (addition)
What are the conditions for the Hydration of Alkenes?
Passing steam across the alkene in the presence of a phosphoric acid catalyst.
What is the mechanism which usually takes place when an alkene is saturated?
Electrophilic addition.
What is an Electrophile?
An atom or group which is strongly attracted to an area of electron-rich density.
What should you include in a diagram for electrophilic addition?
ẟ+,ẟ-, Curly arrows (electron movement), Electrons, carbocations, charges.
What is Markownikoff’s rule?
The hydrogen in a hydrogen halide will tend towards the carbocation with the greatest number of hydrogen atoms and fewest carbon atoms.
What are primary, secondary…. carbocations?
Cations given an order directly proportional to the number of bonded carbon atoms.
In the reaction of hydrogen halides with alkenes where will the halide tend towards?
The cation of highest order
How and why does the stability of carbocations vary with order?
Greater order greater stability as thethe charge in the cation can be shared to the surrounding carbon atoms. More atoms, less charge difference, greater stability.
What is addition polymerisation?
The addition of monomers over a break in c=c bond to form a chain
What are the problems with PVC recycling?
Release of chlorine in hydrogen chloride, corrosive gasses and toxins.
What are bioplastics?
Plastics formed from plant starch, cellulose, oils and proteins
What are biodegradable polymers?
Polymers which can be digested by bacteria to leave no toxin
What are photodegradable polymers?
Polymers which breakdown in response to light.
