Alkenes

Question 1

What is the structure of an alkene?

Answer 1

Unsaturated

Aliphatic alkenes with only one C=C bond have the general formula CnH2n.

Can be branched, cyclic or have more than one C=C bond.

If cyclic or have more than one C=C, the CnH2n formula does not apply.

Question 2

What is the nature of the double bond?

Answer 2

For each carbon atom of the double bond, three of the four valence electrons are used in sigma bonds. This leaves one electron not involved in a sigma bond.

This electron is in a p-orbital.

A pi bond is formed by the sideways overlap of two p -orbitals, one from each carbon atom of the double C=C bond.

Question 3

Where is the pi-electron density concentrated?

Answer 3

Above and below the line joining the nuclei of the bonding atoms.

Question 4

Why is the shape of an alkene fixed?

Answer 4

The pi bond locks the two carbon atoms in position and prevents them from rotating around the double bond.

Question 5

What is the shape around a C=C double bond?

Answer 5

Trigonal planar

There are three regions of electron density around each of the carbon atoms.

The three regions repel each other as much as possible; 120 degree bond angle.

All of the atoms are in the same plane.

Question 6

What is a stereoisomers?

Answer 6

Has the same structural formula but a different arrangements of atoms in space.

Question 7

What are the two types of stereoisomers?

Answer 7

E/Z isomerism (C=C)

Optical isomerism

Question 8

What is E/Z stereoisomerism?

Answer 8

Stereoisomerism around double bonds arises as the C=C is fixed but groups attached to each of the carbons of the double bonds can rotate.

This is because the pi bonds electron density above and below the plane of the sigma bond makes the C=C bond rigid.

Question 9

What is the E isomer?

Answer 9

Highest priority group (highest atomic number) are on opposite sides.

Question 10

What is the Z isomer?

Answer 10

Highest priority group (highest atomic number) is on the same side.

Question 11

What conditions does a molecule need to satisfy to be E/Z isomer?

Answer 11

A C=C double bond.

Different groups attached to each carbon atom of the double bond.

Question 12

What is cis-trans isomerism?

Answer 12

Still in the E/Z category, yet each carbon atom in the double bond must have a hydrogen atom attached to it.

Question 13

What E/Z isomer is the cis and trans isomer?

Answer 13

Cis - Z isomer

Trans - E isomer

Question 14

What is the CIP nomenclature step 1?

Answer 14

Assign priority; examine atoms that are directly attached to the C atoms of the double bonds and decide highest priority group.

Question 15

What is the CIP nomenclature step 2?

Answer 15

If the two atoms attached in a C=C are the same, find the first point of difference. Highest atomic number st the first point of difference has the highest priority.

Question 16

Why are alkenes more reactive than alkanes?

Answer 16

The C=C

As the C=C bond is made up of a sigma bond and a pi bond, where the pi electron density is concentrated above and below the plane of the sigma bond.

As the pi electron density on the outside is more exposed than the sigma bond electrons on the inside of the double bond, the pi electrons are more exposed than the sigma electrons, and are more readily available to react.

Question 17

What is bond enthalpy?

Answer 17

Energy required to break a bond.

Question 18

What is the bond enthalpy of a sigma bond? (C-C)

Answer 18

347kJ/mol

Question 19

What is the bond enthalpy of a pi + sigma bond? (C=C)

Answer 19

612kJ/mol

Question 20

What is the bond enthalpy of a pi bond?

Answer 20

265kJ/mol

Question 21

What can alkenes undergo addition reactions with?

Answer 21

Hydrogen (catalyst)

Halides

Hydrogen halides

Steam (catalyst)

Question 22

Describe the reaction conditions needed for the hydrogenation of alkanes?

Answer 22

Nickel catalyst
150 degrees Celsius 432k

E.g C3H6 H2 = C3H8

Addition reaction.

Question 23

Describe the reaction conditions needed for the halogenation of alkanes?

Answer 23

Alkene + halogen = haloalkane

Reactions with Cl are faster than that of iodine as Cl is a smaller molecule and has less shielding.

F is explosive.

Room temperature.

Addition reaction.

Question 24

How to test for the presence of alkenes?

Answer 24

Bromine water goes from orange to colourless.

Question 25

Describe the reaction of an alkene with a hydrogen halide?

Answer 25

Alkenes react with gaseous hydrogen halides at room temperature. If the alkene is also a gas, the reaction takes place when the gases mix together.. If the alkene is a liquid, the hydrogen halide is bubbled through it. If the alkene is asymmetrical, 2 possible products are formed. If it is symmetrical, only one product is formed. Addition.

Question 26

Describe the hydration of alkenes?

Answer 26

Alkene + steam = alcohol. Phosphoric acid catalyst H3PO4. Two possible products.

Question 27

What is an electrophile?

Answer 27

An electron pair acceptor. An electrophile is 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 (+) charge.

Question 28

Why do alkenes undergo electrophilic addition?

Answer 28

The double bond in an alkene represents a region of high electron density because of the presence of the -electrons. The high electron density of the m-electrons attracts electrophiles.

Question 29

How does the non polar electrophilic addition work for Br2?

Answer 29

Bromine is a non-polar molecule. 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 of the molecule becoming Bro+ and the other end of the molecule becoming Bro-. This is known as an induced dipole. 2 The electron pair in the Pi bond is attracted to the Bro+ end of the molecule, causing the double bond to break. 3 A bond has now been formed between one of the carbon atoms from the double bond and a bromine atom. 4 The Br-Br bond breaks by heterolytic fission, with the electron pair going to the Br° end of the molecule. A bromide ion (Br) and a carbocation are formed. In the final stage of the reaction mechanism the Br ion reacts with the carbocation to form the addition product of the reaction.

Question 30

How does electrophilic addition work for polar molecules?

Answer 30

Bromine is more electronegative than hydrogen, so hydrogen bromide is polar and contains the dipole H*-Br. 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. The H-Br bond breaks by heterolytic fission, with the electron pair going to the bromine atom. A bromide ion (Br) and a carbocation are formed. A carbocation contains a positively charged carbon atom. 6 In the final step the Br ion reacts with the carbocation to form the addition product.

Question 31

What is Markinkoff’s rule?

Answer 31

In the primary carbocation the positive charge is on a carbon atom at the end of a chain. In the secondary carbocation the positive charge is on a carbon atom with two carbon chains attached.

Question 32

Explain carbocation stability?

Answer 32

Carbocations are classified by the number of alkyl groups attached to the positively charged carbon atom (Figure 7). An alkyl group is normally represented by the symbol -R. Tertiary carbocations (with three R groups) are the most stable, and primary carbocations are the least stable. Carbocation stability is linked to the electron-donating ability of alkyl groups. Each alkyl group donates and pushes electrons towards the positive charge of the carbocation. The positive charge is spread over the alkyl groups. The more alkyl groups attached to the positively-charged carbon atom, the more the charge is spread out, making the ion more stable. Therefore tertiary carbocations are more stable than secondary carbocations, which are more stable than primary carbocations.

Question 33

What is the first synthetic polymer?

Answer 33

Bakelite (electrical insulation). 1907

Question 34

What is a polymer?

Answer 34

A large molecule formed from many repeating units of smaller molecules.

Question 35

What is a monomer?

Answer 35

A small molecule that reacts to form a polymer.

Question 36

How do alkenes form polymers?

Answer 36

Alkenes undergo addition reactions to produce saturated polymers. Pi bond breaks (lower bond enthalpy than sigma bond). The monomers join together to form a long chain.

Question 37

What do properties of polymers depend on?

Answer 37

The monomer(s) it is formed from.

Question 38

What are the conditions needed to make a polymer?

Answer 38

High temperatures High pressure Catalyst

Question 39

Where would you find poly(ethene)?

Answer 39

Supermarket bags, shampoo bottles, children's toys.

Question 40

Where would you find poly(chloroethene)?

Answer 40

AKA PVC poly(vinylchloride). Pipes Insulation sheathing Flooring Films & sheeting

Question 41

What is poly(propene) used for?

Answer 41

Children's toys, packing crates, guttering, UPVC windows, fibres for ropes.

Question 42

What is poly(styrene) used for?

Answer 42

Packaging materials, food trays, cups (high thermal insulator).

Question 43

What is poly(tetrafluoroethene) used for?

Answer 43

Coating for non-stick pans, permeable membrane for clothing and shoes, cable insulation.

Question 44

How does recycling polymers reduce environmental impact?

Answer 44

Recycling polmers reduces their environmental impact by conserving finite fossil fuels as well as decreasing the amount of waste going to landfill.

Question 45

Why is it difficult to dispose of waste polymers?

Answer 45

Many alkene-based polymers are non-biodegradable. The growing amount of polymer waste has serious environmental effects, for example, killing marine life.

Question 46

Why is the disposal of PVC difficult?

Answer 46

The disposal and recycling of PVC is hazardous due to the high chlorine content and the range of additives present in the polymer. Dumping PVC in landfill is not sustainable and, when burnt, PVC releases hydrogen chloride, a corrosive gas, and other pollutants like toxic dioxins.

Question 47

How are discarded polymers recycled?

Answer 47

Discarded polymers have to be sorted by type. The recycling process is undermined if polymers are mixed as this renders the product unusable. Once sorted, the polymers are chopped into flakes, washed, dried, and melted. The recycled polymer is cut into pellets and used by manufacturers to make new products.

Question 48

How is PVC recycled?

Answer 48

New technology uses solvents to dissolve the polymer. High-grade PVC is then recovered by precipitation from the solvent, and the solvent is used again.

Question 49

Why are some polymers difficult to recycle?

Answer 49

Some polymers are difficult to recycle. As they are derived from petroleum or natural gas, they have a high stored energy value.

Question 50

How are waste polymers used as fuel?

Answer 50

Waste polymers can be incinerated to produce heat, generating steam to drive a turbine producing electricity.

Question 51

What is feedstock recycling?

Answer 51

Feedstock recycling describes the chemical and thermal processes that can reclaim monomers, gases, or oil from waste polymers.

Question 52

How does feedstock recycling work?

Answer 52

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.

Question 53

What’s an advantage of feedstock recycling?

Answer 53

A major advantage of feedstock recycling is that it is able to handle unsorted and unwashed polymers.

Question 54

How are biodegradable polymers broken down?

Answer 54

Biodegradable polymers are broken down by microorganisms into water, carbon dioxide, and biological compounds. These polymers are usually made from starch or cellulose, or contain additives that alter the structure of traditional polymers so that microorganisms can break them down.

Question 55

What are bioplastics produced from?

Answer 55

Bioplastics produced from plant starch, cellulose, plant oils, and proteins offer a renewable and sustainable alternative to oil-based products.

Question 56

Describe compostable polymers?

Answer 56

Compostable polymers degrade and leave no visible or toxic residues. Compostable polymers based on poly (lactic acid) (Figure 11) are becoming more common as an alternative to alkene-based polymers.

Question 57

What are photodegradable polymers?

Answer 57

Where the use of plant-based polymers is not possible, photodegradable oil-based polymers are being developed. These polymers contain bonds that are weakened by absorbing light to start the degradation. Alternatively, light-absorbing additives are used.