Alkenes Flashcards
1
Q
What are alkenes?
A
- Unsaturated hydrocarbons containing a C=C comprising a pi bond and a sigma bond
- Restricted rotation of the pi bond
2
Q
What is a pi bond?
A
- A pi bond is formed by the sideways overlap of two p-orbitals, one from each carbon atom of the double bond
- Each carbon atom contributes one electron to the electron pair in the pi bond
- The pi electron density is concentrated above and below the line joining the nuclei of the bonding atoms
3
Q
What is the shape of an alkene?
A
- The shape around each of the carbon atoms in the double bond is trigonal planar because:
1. There are three regions of electron density around each of the carbon atoms
2. The three regions repel each other as far apart as possible, so the bond angle around each carbon atom is 120 degrees
3. All of the atoms are in the same plane
4
Q
What are stereoisomers?
A
-Compounds with the same structural formula but with a different arrangement in space
5
Q
What is E/Z isomerism? Why does it happen?
A
- An example of stereoisomerism, in terms of restricted rotation about a double bond and the requirement for two different groups to be attached to each carbon atom of the C=C group
- Only occurs in compounds with a C=C double bond
- Arises because rotation about the double bond is restricted and the groups attached to each carbon atom are therefore fixed relative to each other
- The reason for the rigidity is the position of the pi bond’s electron density above and below the plane of the sigma bond
6
Q
What is cis-trans isomerism?
A
- A special case of E/Z isomerism in which two of the substituent groups attached to each carbon atom of the C=C group are the same
- In cis-trans isomers one of the attached groups one aha carbon atoms of the double bond must be hydrogen
7
Q
When is it E/Z isomerism?
A
- A C=C double bond
2. Different groups attached to each carbon atoms of the double bond
8
Q
When is it cis-trans isomerism?
A
- The cis isomer has the hydrogen atoms on each carbon in the double bond on the same side of the molecule (Z)
- The trans isomer has the hydrogen atoms diagonally opposite each other (E)
- BUT the use of E equivalent to trans and Z as equivalent to cis is only consistently correct when there is an H on each carbon atoms of the C=C bond
9
Q
How do you use the Cahn-Ingold-Prelog rules?
A
- In this system the atoms attached to each carbon atom in a double bond are given a priority based Upton their atomic number
1. If the groups of higher priority are on the same side of the double bond, the compound is the Z isomer
2. If the groups of higher priority are diagonally placed across the double bond, the compound is the E isomer
10
Q
How do you assign priority?
A
- Examine the atoms attached directly to the carbon atoms of the double bond and decide which of the two atoms has the highest priority. 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 will need to find the first point of difference. The group which has the higher atomic number at the firta point of difference is given the higher priority
11
Q
Are alkenes more reactive than alkanes?
A
- Yes because of the relatively low bond enthalpy of the pi bond, the pi bond is weaker than the sigma bond and is therefore broken more readily
- The C=C bond is made up of a sigma bond and a pi bond, and the electron density is concentrated above and below the plane of the sigma bond
- Being on the outside of the double bond , the pi electrons are more exposed than the electrons in the sigma bond
- A pi bond reality breaks and alkenes undergo addition reactions relatively easily
12
Q
What sort of reaction is alkene and hydrogen? What are the conditions?
A
- Hydrogenation of alkene (ADDITION)
- Ni catalyst
- Makes alkanes
13
Q
What sort of reaction is alkene and halogen? What are the conditions?
A
- Halogenation of alkene (ADDITION)
- Chlorine and bromine at room temperature
- Forms dihaloalkanes
14
Q
How do you test for unsaturation?
A
- Alkene and bromine water
- When bromine water (orange solution) is added drowse to a sample of an alkene, bromine adds across the double bond
- The orange colour disappears (C=C bond decoloruises bromine water)
- Addition reaction
15
Q
What sort of reaction is alkene and hydrogen halide? What are the conditions?
A
- Addition
- React with gaseous hydrogen halides at room temperature to from haloalkanes
- If alkene is a gas, like ethene then the reaction takes place when the two gases are mixed
- If the alkene is a liquid, then the hydrogen halide is bubbled through
- Two possible products
16
Q
What sort of reaction is alkene and steam? What are the conditions?
A
- Hydration reaction with alkenes (ADDITION)
- Alcohols are formed when alkenes react with steam in the presence of phosphoric acid catalyst H3PO4
- Steam adds across the double bond, and this addition reaction is sued widely in industry to produce ethanol from ethene
- Two possible products
17
Q
What is an electrophile?
A
An atom (or group of atoms) which is started to an electron-rich centre or atoms, where it accepts a pair of electrons to form a new covalent bond (an electron pair accepter
18
Q
What is a polymer?
A
A large molecule formed from many thousands of repeat units of smaller molecules known as monomers
19
Q
What is a monomer?
A
A small molecule that combines with many other monomers to from a polymer
20
Q
What is a repeat unit?
A
A specific arrangement of atoms that occurs in the structure over and over again. Repeat units are included in brackets outside of which is the symbol n
21
Q
What are the environmental concerns of polymers (readily available and cheap)?
A
- Their formation uses up valuable oil supplies. Thus much is waster if it ends up in a landfill
- The polymers are non-biodegradable (lack of reactivity as strong C-C bond, bacteria can’t break down) and so takes thousands of years to break down. This leads to visual pollution as city councils struggle to deal with the increasing volume of household waste
- Polymers can be burned to reduce their volume but this often leads to the formation of toxic fumes, particularly CO, HCL, CL2 and dioxins
22
Q
How can recycling polymers help?
A
- Reduces environmental impact by conserving finite fossil fuels and decreases amount going to landfill
- Discarded polymers must be sorted by type; then chopped into flakes, washed, dried and melted. The recycled polymer is cut into pellets and used by manufacturers to make new products
23
Q
What is PVC recycling and how can it help?
A
- The disposal and recycling of PVC is hazardous due to the high chlorine content and the range of additives present in the polymer
- Dumping it in a landfill is unsustainable and burning it relates HCL ( a corrosive gas) and other pollutants like toxic dioxins
- Previously recycling involved grinding PVC and reusing it to manufacture new products. New technology involves using solvent to dissolve the polymer. High grade PVC is then recovered by precipitation from the solved and the solvent is used again
24
Q
How does using waste polymers as fuel help?
A
- Combustion, pyrolysis (for energy)
1. They have high stored energy value as derived from petroleum or natural gas
2. Waster polymers can be incinerated to produce heat, generating steam to drive a turbine, generating electricity
25
How does feedstock recycling help?
1. The chemical and thermal process that can reclaim monomers, gases or oil from waste polymers
2. The products from feedstock recycling resemble those produced from crude oil in refineries
3. These materials can be used as raw materials for the production of new polymers
4. A major advantage of feedstock recycling is that it is able to handle unsorted and unwashed polymers
26
Why are bioplastics good?
1. Bioplastics produced for, plant starch, cellulose, plant oils and proteins offer a renewable, sustainable alternative to oil based products
2. The use of bioplastics not only protects our environment but also conserves valuable oil reserves
27
What are biodegradable polymers?
1. They are broken down by microorganisms into water and carbon dioxide and biological compounds
2. These polymers are usually made from starch or cellulose or contain additive that alter the structure of traditional polymers so that microorganism can break them down
3. Compostable polymers degrade and leave no visible or toxic residues
4. Supermarket bags made from plant starch can be used as bin liners for food waste so that the waste and bag can be composted together
5. Compostable plates, cups and food trays made from sugar cane fibre are replacing expanded polystyrene
28
What are photodegradable polymers?
1. Where the use of plant-based polymers is not possible, photodegradable oil based polymers are being developed
2. These polymers contain bonds that a re weakened by absorbing light tot start the degradation. Alternatively, light absorbing additive are used
29
Why do alkenes undergo addition reactions?
1. The double bond in an alkene represents a region of high electron denser because of the presence of the pi electrons
2. The high electron density of the pi electrons attracts electrophiles
30
What is the mechanism for the reaction between but-2-ene and hydrogen bromide?
1. Bromine is more electronegative than hydrogen, so hydrogen bromide is polar and donations the dipole Hdelta+ and Brdelta-
2. The electron pair in the pi bond is attracted to the partially positive hydrogen atom, causing the double bond to break
3. A bond forms between the hydrogen atom of the H-Br molecule and a carbon atom that was part of the double bond
4. The H-Br bond breaks by heterolytic fission, with the electron pair going to the bromine atom
5. The Bromide ion (Br-) and a carbocation are formed. A carbonation contains a positively charged carbon atom
6. In the final step the Br- ion reacts with the carbocation to from the addition product
31
What is the mechanism for the reaction between propene and bromine?
- Bromine is a non-polar molecule, when bromine approaches an alkene, the pi electrons interact with he electrons in the Br-Br bond
1. This interaction causes polarisation of the Br-Br bond, with one end of the molecule becoming Brdelta+ and the other end of the molecule becoming Brdelta-. This is known as an induced dipole
2. The electron pair in the pi bond is attracted to the Brdelta+ end of the molecule, causing the double bond to break
3. A bond has now been formed between one of the carbon atoms fro the double bond and a bromine atom
4. The Br-Br bond breaks by heterolytic fission, with the electron pair going to the Br delta- end of the molecule
5. A bromide ion and a carbocation are formed
6. In the final stage of the reaction mechanism the br- ion reacts with the carbocation to from the addition product of the reaction
32
What is Markownikoff's rule?
When a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen of the hydrogen halide attaches itself to the carbon atom of the alkene with the greater number of hydrogen atoms and the smaller number of carbon atoms
33
What are the steps of electrophilic addition?
1. In the first step a carbocation is formed (has a positive charge on a carbon atom)
- In the reaction between propene and hydrogen halide, two carbocations are possible a primary and a secondary carbocation
- In the primary carbocation the positive charge is on a carbon atom at the end of the chain
- In the secondary carbocation the positive charge is on a carbon atom with two carbon chains attached
2. 2-bromopropane, formed from the secondary carbocation is the major product and the yield of 1-bromopropane is much smaller
34
What is the stability of carbocations?
1. Carbocations are classified by the number of alkyl groups attached to the positively changed carbon atom
2. An alkyl group is normally represented by the symbol-R
3. Tertiary carbocations (with three R groups) are the most stable and primary carbocations are the least stable
35
What affects the stability of carbocations?
1. Carbocation stability is linked to the electron-doanting ability of alkyl groups
2. Each alkyl group donated and pushes electrons towards the positive charge of the carbocation
3. The positive charge is spread over the alkyl groups
4. The more alkyl groups attached to the positively charged carbon toms, the more the charge is spread out, making the ion more stable
5. Therefore tertiary carbocations are more stable than secondary carbocations, which are more stable than primary carbocations
- Addition of hydrogen halide to an unsymmetrical alkene from the major product via the most stable carbocation
36
What is an unsaturated Hydrocarbon?
- An unsaturated compound contains only single covalent bonds
- A hydrocarbon is a compound of carbon and hydrogen only
37
What would be a bond angle in But-1-ene?
-109.5
38
Explain the shape of an ethene molecule? What is the bond angle H-C-H?
- 3 areas of electron density surround each carbon atom and repel one another as far apart as possible
- Bond angle: 120 degrees
39
What is meant by structural isomer?
- Structural isomers are compounds with the same molecular formula
- but different structural formula
40
What is a stereoisomers?
- Stereoisomers have the same structural formula
| - But different arrangements in space
41
Explain why some alkenes have E/Z isomerism?
- The double C=C bond which does not rotate
| - The alkene needs to have two different groups attached to each carbon atom of C=C bond
42
What won't react with a nucleophile?
If a double bond as nucleophile is negative and negative repel negative
43
What is true about recrystallisation?
1. The organic compound is more soluble in hot solvent
| 2. The hot solution is cooled before the purified organic compound is collected
44
What does unsaturated mean?
Contains at least one carbon-carbon double bond
45
What is the problem of radical substitution?
More than one hydrogen is substituted / multisubsition by chlorine, or further substitution occurs
46
Why is something suitable as a nucleophile?
It contains a lone pair that can be donated
47
What are two ways that monomers are processed usefully?
1. Separation into types and recycling or sort plastics, melt and remould
2. Combustion for energy generation
3. Used for cracking or feedstock for plastics or chemicals
