Alkenes Flashcards
What do curly arrows represent?
The movement of electrons
What does the double bond of alkenes have?
A high electron density
What happens due to the high electron density of carbon carbon double bonds in alkenes?
Makes the bond more susceptible to attack by electrophiles
What are electrophiles?
Electron loving species
What is electrophilic addition?
The addition of an electrophile to a double bond
What does electrophilic addition include?
Hydrogen (hydrogenation reaction)
Steam
Hydrogen halide
Halogens
Which catalyst does hydrogenation use and what does it produce?
A nickel or platinum catalyst and it produces an alkane
Occurs under heat
What catalyst does reaction with steam use and what does it produce?
Phosphoric acid catalyst and it produces an alcohol
Occurs under heat
What do electrophilic additions with hydrogen halides and halogens produce and what conditions do they occur under?
Halogenoalkanes
Happens at room temperature
What is hydrogenisation extensively used for in industry?
To manufacture margarine
How is margarine produced?
- Naturally occurring vegetable oils are unsaturated
- Reacted with H2 and become C-C
- Process changes properties of vegetable oils and converts it to solid
What can alkenes be oxidised and acidified by?
Potassium Manganate (VII) (KMnO4) - very powerful oxidising agent
What happens when alkenes are shaken with cold, dilute KMnO4)?
Pale purple solution turns colourless and product is a diol ( colour change can be used to distinguish between alkenes and alkanes as alkanes don’t change colour)
How can we think of the reaction between an alkene and an oxidising agent?
As an oxidation reaction followed by an addition
What are the steps in reacting an alkene with oxidising agent?
- Potassium manganate solution provides an oxygen atom
- Water in solution provides another oxygen and 2 hydrogen atoms, so addition of 2 OH groups across the double bond
What is heterolytic fission?
Breaking a covalent bond so that the more electronegative atom takes both the electrons from the bond to form a negative ion and leave behind a positive ion
What is the charge of hydrogen halides?
They are polar
e.g. HBr = H is positive, Br is negative
Which atom in HBr would be the electrophile?
H as it is electron deficient
What happens in an addition reaction of an alkene and a hydrogen halide?
The H atom acts as an electrophile and accepts a pair of electrons from the C=C in the alkene
What happens to the H-Br bond during electrophilic addition?
Breaks heterolytically, forming a Br- ion
What happens after the H has received the electron pair from the C=C in electrophilic addition?
Results in formation of a highly reactive carbocation intermediate which reacts with the Br- (nucleophile), to form halogenoalkanes
What is the charge of halogen molecules?
Non-polar
What happens when a Br molecule gets closer to the double bond of an alkene?
The high electron density in the double bond repels the electron pair in the Br-Br away from the closest Br atom
What happens in terms of charge on each Br atom as it gets closer to the C=C in an alkene?
Br closest becomes an electrophile and Br furthest away gets a negative charge (there is an induced dipole)
What happens in the addition reaction of a halogen with an alkene?
Closes Br atoms acts as an electrophile and accepts a pair of electrons from C=C (breaks heterolytically, forming Br- ion)
What is formed in the addition reaction of a halogen with an alkene?
Results in formation of a highly reactive carbocation intermediate which reacts with Br- to form a halogenoalkane
What is the general formula for cycloalkenes?
CnH2n-2
How does the double bond in an alkene affect its name?
Wherever the double bond is, you add that in the name
e.g. double bond on carbon 2 in pentene - would be called pent-2-ene
What 2 types of bonding are there in the double bond of an alkene?
Sigma and pi
How does a sigma bond form?
One sp2 orbitals from each C overlap to form a single C-C bond
Can rotation occur around a sigma bond?
Yes
How does a pi bond form?
By the sideways overlap of 2 p orbitals on each C atom forming a pi bond above and below the plane of molecule
Can rotation occur around a pi bond?
There is restricted rotation
Is the sigma bond or pi bond stronger?
Sigma
Are pi bonds exposed and if so what does this mean?
Yes - they have a high electron density
What is caused by the fact that pi bonds have high electron densities?
They are vulnerable to attack by electrophiles
What can be used to test if a molecule is unsaturated?
Halogens
What is the halogen most commonly used for saturation tests?
Bromine water
How do you test if a compound is unsaturated?
Shake it with bromine water
What will happen when an unsaturated compound reacts with bromine water?
An addition reaction will take place and the solution will decolourise
What is addition polymerisation?
It is the reaction in which many monomers containing at least one C=C form long chains
What are the products of addition polymerisation?
Only the polymer
Which bond breaks in addition polymerisation?
The pi bond in each C-C
What happens after the pi bond breaks in addition polymerisation?
The monomers link together to form new C-C bonds
What is a polymer?
A long chain molecule that is made up of many repeating units
What can a polymerisation molecule be reprsented by?
The general formula or the displayed formula
What is a repeat unit?
The smallest group of atoms that when connected one after the other, make up the polymer chain
What do polymers provide for everyday use?
A readily available cheap alternative to many metal, glass, paper and cardboard materials
What makes polymers unreactive?
They are saturated and usually non-polar
What does the low reactivity of polymers make them useful for?
Certain uses e.g. food packaging
What is the bad aspect of polymers having a low reactivity?
Many of them are non-biodegradable
How do polymers affect sealife?
Waste plastic kills marine animals
What is one method of polymer disposal?
Use of landfill sites
What does polymer recycling do?
Reduces amount of waste going to landfill - newer landill sites have points where new waste is brought before going into actual landfill
What can recycling of polymers reduce the use of?
Finite resources (given that many polymers are made from products of cracking crude oil)
Why is recycling polymers a time-consuming process?
They have to be sorted into the different categories
What happens after polymers are sorted in recycling?
They are chopped, washed, dried, melted and then cast into pellets ready for use
What happens if the polymers are mixed when trying to recycle them?
When they are processed, it is wasted as it produces an unusable mix of polymers
Why can certain polymers cause problems when recycling?
Due to their composition (e.g. PVC contains chlorine)
How are modern techniques overcoming the PVC problem when recycling polymers (also other polymers)?
By dissolving the polymer and precipitating out the recycled material
Which polymers are still difficult to recycle?
Petroleum/natural gas derived polymers
How else can we dispose of petroleum/natural gas derived polymers and why?
Since they have a large amount of energy stored within the polymer chains, they can be incinerated
What can the process of incineration be used for?
- Can use the enrgy to boil water
- The water vapour used to turn turbines in a power station
Why does the process of incineration cause environmental pollution?
Carbon in polymer can be released as CO2, contributing to global warming
What are some toxic waste products produced from the combustion of PVC?
Hydrogen chloride and other chlorinated molecules
What is PVC?
Poly(chloroethene)
What is feedstock recycling?
Waste polymers are broken down, by chemical and thermal processes - into monomers, gases and oils
What are the products from feedstock recycling used for?
As the raw materials in the production of new polymers and other organic chemicals
What is the major benefit of feedstock recycling?
It works with unsorted and unwashed polymers
What will chemists use when designing a sustainable polymer manufacturing process?
Principles of green chemistry
What kind of chemicals will scientists aim to use?
Ones that are as safe and environmentally friendly as possible
What will scientists try and do with the amount of chemicals they use?
Reduce it, physically as well as actual number of chemicals
What will scientists use where possible in terms of chemicals?
Renewable feedstock chemicals
What will scientists try and do in terms of energy in reactions?
Reduce energy requirement as well as increase energy efficiency (has both environmental and financial bonus)
Will scientists consides the lifespan of the polymer?
Yes - to see if its suitable for use
What will scientists try to do to atom economy?
Improve it (and reduce amount of waste by products)
How have chemists designed ways to remove toxic waste products before they are emitted into the atmosphere?
The waste gases from the incinerator are scrubbed or reacted with a base or carbonate
Why are waste gases scrubbed or reacted with bases or carbonates?
These neutralise the acids
e.g. Calcium oxide neutralises HCl
Can biodegradable polymers be broken down?
Yes - over time by microorganisms
What are common products of the breakdown of biodegradable polymers?
CO2, water and other organic compounds
Which polymers are considered to be biodegradable and why?
Polyester and polyamide condensation polymers - can be broken down using hydrolysis
(major advantage over products produced using alkene monomers)
What happens when polyester and polyamide condensation polymers are taken to landfill sites?
Broken down more easily so products used for other applications
What are compostable polymers made of?
Commonly plant based
What is being used in the production of biodegradable bin liners?
Plant starch
How do compostable polymers biodegrade?
Naturally - leaves no harmful residue
