Alkenes

Question 1

What are alkenes?

Answer 1

• Unsaturated hydrocarbons containing a C=C comprising a pi bond and a sigma bond
• Restricted rotation of the pi bond

Question 2

What is a pi bond?

Answer 2

1. A pi bond is formed by the sideways overlap of two p-orbitals, one from each carbon atom of the double bond
2. Each carbon atom contributes one electron to the electron pair in the pi bond
3. The pi electron density is concentrated above and below the line joining the nuclei of the bonding atoms

Question 3

What is the shape of an alkene?

Answer 3

• 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

Question 4

What are stereoisomers?

Answer 4

-Compounds with the same structural formula but with a different arrangement in space

Question 5

What is E/Z isomerism? Why does it happen?

Answer 5

• 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

Question 6

What is cis-trans isomerism?

Answer 6

• 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

Question 7

When is it E/Z isomerism?

Answer 7

1. A C=C double bond

2. Different groups attached to each carbon atoms of the double bond

Question 8

When is it cis-trans isomerism?

Answer 8

1. The cis isomer has the hydrogen atoms on each carbon in the double bond on the same side of the molecule (Z)
2. 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

Question 9

How do you use the Cahn-Ingold-Prelog rules?

Answer 9

• 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

Question 10

How do you assign priority?

Answer 10

1. 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
2. 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

Question 11

Are alkenes more reactive than alkanes?

Answer 11

1. 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
2. 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
3. Being on the outside of the double bond , the pi electrons are more exposed than the electrons in the sigma bond
4. A pi bond reality breaks and alkenes undergo addition reactions relatively easily

Question 12

What sort of reaction is alkene and hydrogen? What are the conditions?

Answer 12

1. Hydrogenation of alkene (ADDITION)
2. Ni catalyst
3. Makes alkanes

Question 13

What sort of reaction is alkene and halogen? What are the conditions?

Answer 13

1. Halogenation of alkene (ADDITION)
2. Chlorine and bromine at room temperature
3. Forms dihaloalkanes

Question 14

How do you test for unsaturation?

Answer 14

1. Alkene and bromine water
2. When bromine water (orange solution) is added drowse to a sample of an alkene, bromine adds across the double bond
3. The orange colour disappears (C=C bond decoloruises bromine water)
   - Addition reaction

Question 15

What sort of reaction is alkene and hydrogen halide? What are the conditions?

Answer 15

1. Addition
2. 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

Question 16

What sort of reaction is alkene and steam? What are the conditions?

Answer 16

1. Hydration reaction with alkenes (ADDITION)
2. Alcohols are formed when alkenes react with steam in the presence of phosphoric acid catalyst H3PO4
3. Steam adds across the double bond, and this addition reaction is sued widely in industry to produce ethanol from ethene
   - Two possible products

Question 17

What is an electrophile?

Answer 17

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

Question 18

What is a polymer?

Answer 18

A large molecule formed from many thousands of repeat units of smaller molecules known as monomers

Question 19

What is a monomer?

Answer 19

A small molecule that combines with many other monomers to from a polymer

Question 20

What is a repeat unit?

Answer 20

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

Question 21

What are the environmental concerns of polymers (readily available and cheap)?

Answer 21

1. Their formation uses up valuable oil supplies. Thus much is waster if it ends up in a landfill
2. 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
3. Polymers can be burned to reduce their volume but this often leads to the formation of toxic fumes, particularly CO, HCL, CL2 and dioxins

Question 22

How can recycling polymers help?

Answer 22

1. Reduces environmental impact by conserving finite fossil fuels and decreases amount going to landfill
2. 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

Question 23

What is PVC recycling and how can it help?

Answer 23

1. The disposal and recycling of PVC is hazardous due to the high chlorine content and the range of additives present in the polymer
2. Dumping it in a landfill is unsustainable and burning it relates HCL ( a corrosive gas) and other pollutants like toxic dioxins
3. 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

Question 24

How does using waste polymers as fuel help?

Answer 24

• 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

Question 25

How does feedstock recycling help?

Answer 25

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

Question 26

Why are bioplastics good?

Answer 26

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

Question 27

What are biodegradable polymers?

Answer 27

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

Question 28

What are photodegradable polymers?

Answer 28

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

Question 29

Why do alkenes undergo addition reactions?

Answer 29

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

Question 30

What is the mechanism for the reaction between but-2-ene and hydrogen bromide?

Answer 30

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

Question 31

What is the mechanism for the reaction between propene and bromine?

Answer 31

- 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

Question 32

What is Markownikoff's rule?

Answer 32

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

Question 33

What are the steps of electrophilic addition?

Answer 33

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

Question 34

What is the stability of carbocations?

Answer 34

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

Question 35

What affects the stability of carbocations?

Answer 35

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

Question 36

What is an unsaturated Hydrocarbon?

Answer 36

- An unsaturated compound contains only single covalent bonds - A hydrocarbon is a compound of carbon and hydrogen only

Question 37

What would be a bond angle in But-1-ene?

Answer 37

-109.5

Question 38

Explain the shape of an ethene molecule? What is the bond angle H-C-H?

Answer 38

- 3 areas of electron density surround each carbon atom and repel one another as far apart as possible - Bond angle: 120 degrees

Question 39

What is meant by structural isomer?

Answer 39

- Structural isomers are compounds with the same molecular formula - but different structural formula

Question 40

What is a stereoisomers?

Answer 40

- Stereoisomers have the same structural formula | - But different arrangements in space

Question 41

Explain why some alkenes have E/Z isomerism?

Answer 41

- 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

Question 42

What won't react with a nucleophile?

Answer 42

If a double bond as nucleophile is negative and negative repel negative

Question 43

What is true about recrystallisation?

Answer 43

1. The organic compound is more soluble in hot solvent | 2. The hot solution is cooled before the purified organic compound is collected

Question 44

What does unsaturated mean?

Answer 44

Contains at least one carbon-carbon double bond

Question 45

What is the problem of radical substitution?

Answer 45

More than one hydrogen is substituted / multisubsition by chlorine, or further substitution occurs

Question 46

Why is something suitable as a nucleophile?

Answer 46

It contains a lone pair that can be donated

Question 47

What are two ways that monomers are processed usefully?

Answer 47

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