Chapter 13 - Alkenes Flashcards
What type of hydrocarbons are Alkenes?
Unsaturated hydrocarbons as they contain at least one double bond
What is the general formula of aliphatic Alkenes with one double bond?
CnH2n
Why do some Alkenes not obey the formula CnH2n?
They can contain more than one double bond or be cyclic
Do branched Alkenes obey the general formula CnH2n?
Yes
What are the electrons from the carbon atoms of the double bond used in?
- three of the four electrons are used in three sigma bonds, one to the other carbon atom of the double bond and the other two electrons to two other atoms
- this leaves one electron on each carbon atom not involved in sigma bonds
- this electron is in a p-orbital
- a pi bond is formed by the sideways overlap of two p-orbitals
How many electrons does each atom contribute to the pi bond?
Each carbon atom contributes one electron to the electron pair in the pi bond
Where is the pi electron density concentrated?
Above and below the line joining the nuclei of the bonding atoms
Can atoms in a pi bond rotate?
The pi bond locks the two carbon atoms in position and prevents them from rotating around the double bond
This means rotation is not possible around every atom in an Alkene
What is the shape around a double bond and why?
- the shape is trigonal planar
- there are 3 regions of electron density around each of the carbon atoms
- the three regions repel each other as far apart as possible so the bond angle is 120 degrees
What are stereoisomers?
Stereoisomers have the same structural formula but a different arrangement of the atoms in space
What are the two types of stereoisomerism?
- E/Z isomerism
- optical isomerism
Where does E/Z isomerism occur?
E/Z isomerism only occurs in compounds with a C=C double bond, whereas optical isomerism occurs in a wider range of compounds
Why does stereoisomerism occur around double bonds?
Rotation about the double bond is restricted and the groups attached to each carbon atom are fixed relative to each other
What are the two conditions needed for E/Z isomerism?
- a C=C double bond
- different groups attached to each carbon atom of the double bond (different on the left and different on the right)
What is cis-trans isomerism?
A special case of E/Z isomerism
All of the conditions for E/Z isomerism need to be met and one of the attached groups on each carbon atom of the double bond must be hydrogen
What is the cis isomer in cis-trans isomerism?
The cis isomer has the hydrogen atoms on each carbon in the double bond on the same side of the molecule
What is the trans isomer in cis-trans isomerism?
The trans isomer has the hydrogen atoms diagonally opposite each other
What type of isomer (E/Z) is the cis isomer?
The cis isomer is the Z isomer
What type of isomer (E/Z) is the trans isomer?
The trans isomer is the E isomer
How do you decide which is the E isomer and which is the Z isomer?
- Assign priority to each atoms attached directly to the carbon atoms of the double bond. The higher the atomic number, the higher the priority
- If the two atoms attached to a carbon atom in the double bond are the same, then you need to find the first point of difference. The group which has the higher atomic number at the first point of difference is given the higher priority
- If the groups of higher priority are on the same side of the double bond, the compound is the Z isomer. If the groups of higher priority are diagonally placed across the double bond, the compound is the E isomer.
How does the reactivity of alkanes and Alkenes compare?
Alkenes are much more reactive than alkanes because of the presence of the pi bond
The C=C double bond is made up of a sigma bond and a pi bond, and the pi electron density is concentrated above and below the plane of the sigma bond
Because the pi electrons are outside the double bond, they are more easily exposed than the electrons in the sigma bond, so they break more easily.
What are examples of addition reactions Alkenes undergo?
- hydrogen (nickel catalyst)
- halogens
- hydrogen halides
- steam (acid catalyst)
What do addition reactions of the Alkenes involve?
The addition of a small molecule across the double bond, causing the pi bond to break and new bonds to form
What is hydrogenation?
Where hydrogen is added across a double bond
What happens during hydrogenation of Alkenes?
- when an alkene is mixed with hydrogen and passed over a nickel catalyst, an addition reaction takes place to form an alkane
- all C=C bonds react with hydrogen in this way
What happens during halogenation of Alkenes?
Alkenes undergo a rapid addition reactions with the halogens chlorine or bromine at room temperature
How do you test for unsaturation?
- when bromine water is added to a sample of an alkene, bromine adds across the double bond
- the orange colour disappears, indicating the presence of a C=C bond
- if the same test is carried out with a saturated compound there is no addition reaction and no colour change
What happens when Alkenes react with hydrogen halides?
- Alkenes react with gaseous hydrogen halides at room temperature to form haloalkanes
- if the alkene is a gas, then the reaction takes place when the two gases are mixed
- if the alkene is a liquid, then the hydrogen halide is bubbled through it
- Alkenes also react with concentrated hydrochloric acid and concentrated hydrobromic acid
What happens during hydration reactions of Alkenes?
- Alkenes react with steam in the prescence of a phosphoric acid catalyst
- steam adds across the double bond
- this addition reaction is used in industry to produce ethanol from ethene
What is a reaction mechanism?
A series of steps that shows how a reaction takes place
What happens during electrophilic addition?
- the double bond in an alkene represents a religion of high electron density because of the presence of the pi-electrons
- the high electron density attracts electrophiles
- the electrophiles accept an electron pair and the product is formed
What is an electrophile?
An atom or group of atoms that is attracted to an electron-rich centre and accepts an electron pair
An electrophile is usually a positive ion or a molecule containing an atom with a partial positive (delta +) charge
What happens during the reaction between but-2-ene and hydrogen bromide?
Hydrogen bromide adds to but-2-ene to form a single addition product
What are curly arrows used to show in reaction mechanisms?
The movement of electron pairs
How do you draw curly arrows precisely?
- start the arrow at the bond or lone pair
- draw the head of the arrow at the atom to which the electron pair transfers
What is the mechanism for the reaction between but-2-ene and hydrogen bromide?
- bromine is more electronegative than hydrogen, so hydrogen bromide is polar and contains dipoles (H is delta plus and Br is delta minus)
- 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 the H-Br molecule and a carbon atom that was part of the double bond
- the H-Br bond breaks by heterolytic fission, with the electron pair going to the bromine atom
- a bromide ion and a carbocation are formed
- in the final step the bromide ion reacts with the carbocation to form the addition product
What is the electrophile in the reaction between but-2-ene and hydrogen bromide?
Hydrogen bromide
What happens during the reaction between propene and bromine?
Bromine adds to the product to form a single addition product
What is the mechanism for the reaction between propene and bromine?
- when bromine approaches an alkene, the pi-electrons interact with the electrons in the Br-Br bond
- this interaction causes polarisation of the Br-Br bond, with one end becoming delta positive and the other end becoming delta negative. This is known as an induced dipole
- the electron pair in the pi-bond is attracted to the delta positive end of the molecule, causing the double bond to break
- a bond has now been formed between one of the carbon atoms from the double bond and a bromine atom
- the Br-Br bond breaks by heterolytic fission, with the electron pair going the the delta negative end of the molecule
- a bromide ion and a carbocation are formed
- in the final stage the bromide ion reacts with the carbocation to form the addition product
How many possible products are there when propene reacts with hydrogen bromide?
2, 2-bromopropane and 1-bromopropane
How do you predict which of the two possible isomers is formed during electrophilic addition with a hydrogen halide?
The hydrogen of the hydrogen halide attaches itself to the carbon atom of the alkene with the greater number of hydrogen atoms
How many carbocations is it possible to form during electrophilic addition?
Two - a primary and a secondary
Where is the primary carbocation?
The positive charge is on a carbon atom at the end of a chain
Where is the secondary carbocation?
The positive charge is on a carbon atom with two carbon chains attached
How are carbocations classified?
They are classified by the number of alkyl groups attached to the positive charged carbon atom
What is an alkyl group normally represented by?
The symbol -R
Which carbocations are the most stable?
Tertiary carbocations (with three R groups) are the most stable
Which carbocations are the least stable?
Primary carbocations are the least stable
What is carbocation stability linked to?
- carbocation stability is linked to the electro-donating ability of alkyl groups
- each alkyl group donates and pushed electrons towards the positive charge of the carbocation
- the positive charge is spread over the alkyl groups
- the more alkyl groups attached, the more the charge is spread out, making the ion more stable
How do you find out which product is the major product of electrophilic addition?
The major product is formed via the most stable carbocation
What are polymers?
Extremely large molecules formed from many thousands of repeat units of smaller molecules known as monomers
What is addition polymerisation?
Unsaturated alkene molecules undergo addition polymerisation to produce long saturated chains containing no double bonds
What conditions is industrial polymerisation carried out at?
High temperature, high pressure and using catalysts
What is the trend in molecular mass for addition polymers?
Addition polymers have high molecular mass
What are synthetic polymers named after?
They are usually named after the monomer that reacts to form their giant molecules and prefixed by ‘poly’
What is a repeat unit?
The specific arrangement of atoms in the polymer molecule that repeats over and over again
How is the repeat unit written?
- it is always written in square brackets
- after the bracket you place a letter n to show that there is a large number of repeats
How do you balance polymerisation equations?
Make sure the letter n is before the monomer and outside the bracket after the repeat unit in the polymer
How is poly(ethene) made?
By heating a large number of ethene monomers are high pressure
What is poly(ethene) used in?
Supermarket bags, shampoo bottles and children’s toys
How is poly(chloroethene) made?
From the polymerisation of chloroethene
What are the common uses of poly(chloroethene)?
- pipes
- films and sheeting
- ducts and profiles
- insulation cable
- bottles
What are some uses of poly(styrene)?
- packing material
- food trays and cups
What are uses for poly(tetrafluoroethane)?
- coating for non-stick pans
- permeable membrane for clothing and shoes
- cable insulation
How do you identify monomers from polymer chains?
- identify the repeat unit
- change the single C-C bond into a double bond
What are the advantages of polymers?
- readily available
- cheap to purchase
- lack of reactive so can safely store food and chemicals
What is a disadvantage of polymers?
They are challenging to dispose of
How do you recycle polymers?
- discarded polymers have to be sorted by type
- the recycling process is useless if polymers are mixed as the product would be unusable
- once sorted, polymers are chopped into flakes, washed, dried and melted
How do you dispose of PVC?
- recycling of PVC is hazardous due to the high chlorine content and the range of additives present in the polymer
- dumping in a landfill isn’t sustainable
- when burnt, PVC releases hydrogen chloride and other pollutants
- you can grind PVC and reuse to manufacture new products or use solvents to dissolve the polymer
How do you use waste polymers as fuel?
- some polymers have a high stored energy value
- waste polymers can be incinerated to produce heat, generating steam to drive a turbine producing electronic
How do you use waste polymers as feedstock recycling?
- feedstock recycling is the chemical and thermal processes that can reclaim monomers, gases or oil from waste polymers
- the products from feedstock recycling resemble those produced from crude oil in refineries
- these materials can be used as raw materials for the production of new polymers
What are the ways to dispose of waste polymers(non-biodegradable)?
- recycling
- using as fuel
- feedstock recycling
What are bioplastics?
Bioplastics are produced from plant starch, cellulose, plant oils and proteins
They offer a renewable and sustainable alternative to oil-based products
How are biodegradable polymers broken down?
They are broken down by microorganisms into water, carbon dioxide and biological compounds
Why can biodegradable polymers be broken down by microorganisms?
They are made from starch or cellulose, or contain additives that alter the structure of traditional polymers so microorganisms can break them down
How are compostable polymers disposed of?
They degrade and leave no visible or toxic residues
What can compostable polymers be used as?
- supermarket bags made from plant starch
- plates, cups and food trays made from sugar cane fibre
What are photodegradable polymers?
Polymers that contain bonds that are weakened by absorbing light or light-absorbing additives so they degrade in light
