Chapter 13 - Alkenes Flashcards
What kind of hydrocarbons are alkenes ?
Unsaturated
What is the general formula of alkenes ?
CnH2n
How is a double bond formed ?
- Three sigma bonds are formed to other atoms from the carbon atom
- One electron is left in a p-orbital
- A pi bond forms from the sideways overlap of two p orbitals, one from each carbon atom of the double bond
Where is the electron density focused ?
- The pi electron density is concentrated above and below the line joining the nuclei of the two bonding atoms
- The double bond has a high electron density
What prohibits rotation in alkenes?
The pi bond as it locks the two carbon atoms in position and prevents them from rotating
What is the shape around the carbon atoms in the double bond ?
- Trigonal planar
- 120 degree bond angle
What is stereoisomerism ?
The same structural formula but a different arrangement of the atoms in space
What are the two types of stereoisomerism ?
- E/Z isomerism
- Cis/trans isomerism
When is cis-trans isomerism applicable ?
- When molecules have a C=C bond
- Each carbon in the double bond is attached to two different groups.
What is the cis isomer ?
Hydrogen atoms on each carbon on the same side.
What is the trans isomer ?
Hydrogen atoms diagonally opposite each other.
What are the Cahn-Ingold-Prelog rules for deciding E/Z isomerism?
Higher atomic number is higher priority.
E isomer
Groups of higher priority on different sides of the double bond, then E isomer.
Z isomer
Groups of higher priority on the same side of the double bond, then Z isomer.
How can you remember whether it is a Z or E isomer ?
- For a Z isomer you can remember it as, Zee Zame Zide, the groups of higher priority are on the same side
- If they arent then it is an E isomer
Why are alkenes more reactive than alkanes ?
- They are more reactive due to the fact that they have a pi bond
- The electrons in the pi bond are more exposed, as they are on the outside
- The fact that the electrons are exposed leads to their higher reactivity
How are alkenes tested for ?
When bromine water (orange) is added to the sample of an alkene, the bromine reacts with the alkene and the colour disappears.
Describe the hydrogenation of alkenes
An alkene is mixed with hydrogen and passed over a nickel catalyst at 423K forming an alkane.
What does hydrogenation form ?
An alkane
Describe the halogenation of alkenes
Addition reaction between an alkene and either chlorine or bromine at room temperature to form a halogenoalkane.
What does the halogenation of alkenes form ?
Halogenoalkane
Describe the hydration of alkenes
An alkene is reacted with steam in the presence of a phosphoric acid catalyst
What does the hydration of alkenes form ?
An alcohol
How many products can be formed in the hydration of alkenes ?
Two possible products can be formed
How is the hydration of alkenes used in industry ?
This addition reaction is used widely in industry to produce ethanol from ethene
Describe the hydrohalogenation of alkenes
Alkenes react with gaseous hydrogen halides at room temperature to form haloalkanes
What does the hydrohalogenation of alkenes form ?
Haloalkanes
How many products can be formed in the hydrohalogenation of alkenes ?
Two products can be formed
How do you test for unsaturation ?
The reaction of alkenes with bromine water can be used to identify if there is a C=C bond present and the organic compound is unsaturated
What would the observation be when adding bromine water to an alkene ?
If it is an alkene, the orange colour of the bromine water would disappear, indicating the presence of a C=C bond
What does the high electron density of the pi bond attract ?
Electrophiles
What are electrophiles ?
An electrophile is an atom or group of atoms that is attracted to an electron rich centre and accepts an electron pair
What are electrophiles said to be ?
Electron pair acceptors
What is the mechanism for addition reactions ?
Electrophilic addition
Outline an electrophilic addition reaction between an alkene and hydrogen bromide
- H-Br is a polar molecule
- Electron pair in pi bond attracted to H
- Double bond breaks
- Bond forms between H (in HBr) and C atom
- H-Br bond breaks by heterolytic fission
- Bromide ion and carbocation formed
- Br- reacts with carbocation to form the addition product
What is a carbocation ?
A compound that contains a positively charged carbon ion
How does electrophilic addition take place with molecules that are non polar ?
When the molecule approaches the pi bond, the electrons in the bond are repelled and this induces polarity
Outline an electrophilic addition reaction between an alkene and bromine
- Br-Br is now an induced polar molecule
- Electron pair in pi bond attracted to Br
- Double bond breaks
- Bond forms between B (in Br2) and C atom
- Br-Br bond breaks by heterolytic fission
- Bromide ion and carbocation formed
- Br- reacts with carbocation to form the addition product
What is Markownikoff’s rule ?
- In electrophilic addition reactions, when the carbocation is formed it can be primary, secondary or tertiary
- The major product is the one in which the carbocation is attached to the carbon atom with the most alkyl groups
How are carbocations classified ?
They are classified by the number of alkyl groups attached to the positively charged carbon
Which carbocation is the most stable ?
The tertiary carbocation is the most stable so is most likely to be the product
Why is the tertiary carbocation the most stable ?
- The connecting alkyl groups donate and push electrons towards the positive charge of the carbocation.
- Therefore more alkyl groups means a more stable ion
What is the name of the reaction for a polymers formation from alkenes ?
Addition polymerisation
How is the monomer unit displayed ?
Displayed formula with an ‘n’ before it to signify many units
How is the repeat unit of the polymer displayed ?
- Displayed formula
- In square brackets
- An n placed after to show a large number of repeats
What are the conditions for a polymerisation reaction ?
High temperature and pressure
What are the uses of polyethene ?
Supermarket bags, shampoo bottles, childrens toys
What are the uses of polychloroethene aka polyvinylchloride ?
Pipes, films and sheeting, insulation, bottles and flooring
What are the uses of polypropene ?
Children toys, packing crates, guttering, uPVC windows, rope fibre
What are the uses of polyphenylethene aka polysterene ?
Packaging material, food trays and cups
What are the uses of polytetrafluoroethene aka PTFE or Teflon ?
Coating for non stick pans, permeable membrane for clothing and shoes, cable insulation.
What are the uses of phenylethene aka styrene ?
Used in packing material and in food trays and cups due to its thermal properties
What is the downside of current disposal of waste polymers ?
Waste polymers are very unreactive so they are non biodegradable
How are polymers recycled ?
They are chopped into flakes, washed, dried, melted and then cut into pellets for use
What are the benefits of recycling polymers ?
It reduces environmental impact by conserving fossil fuels
What are the weaknesses of recycling polymers ?
- Polymers have to be sorted by type
- Process is undermined if polymers are mixed as this renders the product unusable
What is the method of recycling PVC ?
- Dissolved by solvents
- High grade PVC is recovered by precipitation
- Solvent can be used again
What are the dangers of PVC disposal ?
High chlorine content makes it hazardous
How can polymers be used as fuel ?
Can be incinerated to form heat
What is feedstock recycling ?
Chemical and thermal processes that can reclaim monomers, gases and oils from waste polymers
What is a major advantage of feedstock recycling ?
Able to handle unsorted and unwashed polymers
What are biodegradable polymers ?
- Produced from renewable sources and can be broken down by microorganisms
- Made from starch or cellulose
What are photodegradable polymers ?
Polymers that contain bonds that are weakened by absorbing light to start degradation, or light absorbing additives are used
