Alkenes

Question 1

Define Alkene

Answer 1

Alkenes are unsaturated hydrocarbons containing at least one c=c double carbon bond.

Question 2

Describe alkene bonding (c=c)

Answer 2

Alkenes contain one double carbon bond which comprises of one π (pi) bond and one σ (sigma) bond. The sigma bond results from the overlap of two orbitals. The pi bond results from the sideways overlap of adjacet p orbitals.

Question 3

Why are alkene’s C=C bond rigid.

Answer 3

The pi bond prevents rotation about the double bond.

Question 4

Describe the shape around the double bond and why?

Answer 4

The shape around each carbon atom in the double bond is trigonal planar (120°). Since there are three regions of electron density around each carbon atom (in the C=C bond), the regions repel each other as far as possible .

Question 5

Define sterioisomers

Answer 5

sterioisomers are compounds with the same structural formula but with different arrangements of atoms in space.

Question 6

What two conditions must occur for E/Z sterioisomers?

Answer 6

• Compound must contain a C=C double carbon bond.
• Different groups must be attached to each carbon in the double bond.

Question 7

Draw the Z/cis isomer of But-2-ene

Answer 7

CH3 ‎ CH3
C = C
H ‎ ‎ ‎ ‎ H

Question 8

Define cis/trans isomers (3)

Answer 8

• Only occurs in compounds with C=C double carbon bonds.
• Different groups must be attaches to each carbon in the double bond
• One of the different groups attached to each double bonded carbon must be hydrogen.

Question 9

Draw the E/ trans isomer of But-2-ene

Answer 9

CH3 ‎ ‎ H
C = C
‎‎‎ ‎ ‎ H ‎‎ ‎ CH3

Question 10

Describe the reactivity of alkenes and why?

Answer 10

Alkenes have a high reactivity due to the π bond formed with the C=C double bond. Unlike the σ bond, the π bonds electrons are outside of the double bond and more exposed, allowing the π bond to easily break and react.

Question 11

What type of reaction do alkenes commonly undergo?

Answer 11

addition reactions

Question 12

Describe the process and enviroment for alkene hydrogenation

Answer 12

Alkene is mixed with hydrogen and passed over a nickel catalyst at 423K, forming an alkane.

Question 13

Describe the process and enviroment for alkene halogenation (Cl and Br)

Answer 13

Alkenes react with Chlorine and Bromine at room temperature, to form chloro-/ Bromo- alkanes.

Question 14

Describe how to test for an unsaturated alkene

Answer 14

Add bromine water drop wise to suspected alkene, bromine water should turn from orange to colourless.

Question 15

what two alkene addition reactions produce haloalkanes?

Answer 15

Halogenation and reactions with hydrogen halides.

Question 16

Describe the process and enviroment for alkene reaction with hydrogen halides

Answer 16

Alkenes react with gaseous hydrogen halides at room temperature to form haloalkanes. Or aqueous concentrated HCl and HBr.

Question 17

What is formed by the hydration of alkenes?

Answer 17

Alcohols

Question 18

Describe the process of hydrating alkenes

Answer 18

Alkenes react with steam in the presence of a phosphoric acid catalyst, to form an alcohol. At 300 degrees at 60 atmospheres.

Question 19

Define electrophile

Answer 19

an electron pair acceptor

Question 20

What is the mechanism for alkene addition reactions?

Answer 20

electrophilic addition

Question 21

Describe the process of HBr reacting with an alkene

Answer 21

1. Bromine is more electronegative than hydrogen making a dipole H+ br-
2. The electron pair in the π bond is attracted to the partially positive hydrogen atom, breaking the C=C double bond.
3. A bond forms between the Hydrogen of H-Br and a carbon from the double bond, causing the H-Br bond to break by heterolytic fission.
4. This produces a Bromine ion and carbocation.
5. The Bromine ion reacts with the carbocation to form the product.

Question 22

What does Markownikoff’s rule state?

Answer 22

When a halogen halide reacts with an unsymetrical alkene the hydrogen from the halogen halide attaches itself to the carbon with the most hydrogens (out of the C=C bond).

Question 23

What increases carbocation stability and why?

Answer 23

The more alkyl groups attached to the positive carbon atom the more stable the carbocation. The alkyl groups push their electrons towards the positive carbon, spreading the positive charge over the alkyl groups making the carbocation more stable.

Question 24

Derscirbe polymers

Answer 24

Large molecules formed from many small repeat units called monomers.

Question 25

How are alkenes made into polymers?

Answer 25

Alkenes undergo addition polymerisation to producd long saturated chains containing no double bonds. Under high temperatures and pressures.

Question 26

Why are polymers bad for the enviroment?

Answer 26

Polymers are very unreactive and do not easiy biodegrade, producing lots of waste, which can harm the enviroment.

Question 27

How does recycling work to help the enviroment?

Answer 27

Polymers are sorted and then chopped into flakes, washed and melted into pellets for new polymers. Reducing waste into landfill and less fossil fuel useage to produce new polymers.

Question 28

Downside of recylcing

Answer 28

Not all polymers can be recycled.

Question 29

How does using polymers as fuel help the enviroment?

Answer 29

Polymers have a high energy stored value, waste polymers are incinerated to produce heat, then steam, moving a turbine to produce electricity, reducing fossil fuel use and landfill waste.

Question 30

How does feedstock recycling work?

Answer 30

Reclaims monomers from polymers (e.g. oil), using chemical and thermal processes, the raw meterials can then be used to make new polymers.

Question 31

What is a mojor advantage of feedstock recycling?

Answer 31

Unsorted and unwashed polymers can be used.

Question 32

How does PVC recycling work and why is it god for the envirmonet?

Answer 32

Uses solvents to dissolve pvc to repurpose for new polymers. Prevents PVC from being burnt as it produces harmful Hydrogen chloride gas which is toxic and corrosive. saving the enviroment.

Question 33

What are biodegradable polymers?

Answer 33

Polymers that can be brocken down by mirco organisms

Question 34

What are photodegradable polymers?

Answer 34

Polymers that can be brocken down by absorbing light.

Question 35

what are the benefit of bio-/photo- degradable polymers?

Answer 35

Reduces landfill waste and the amount of harmful chemicals realesd into the enviroment. Also prevents dependancy on finite oil reserves.