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a-level chemistry yr 12 mine > 13 - Alkenes > Flashcards

13 - Alkenes Flashcards

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1
Q

What is a Polymer?

A

A long chain molecule formed from many thousands of repeat units of smaller molecules known as monomer.

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2
Q

What is a Monomer?

A

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

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3
Q

What is Addition Polymerisation?

A

When the monomers contain carbon-carbon double bonds and no other products are formed. Double bonds in Alkenes can open up and join together to make long chains called polymers.

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4
Q

What is polymerisation?

A

Polymerisation is the reaction where several molecules combine to form a long chain polymer.

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5
Q

What would the polymer be of the monomer Chloroethene?

A

Polychloroethene. (Add poly to the start)

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6
Q

What is the advantage and disadvantage with plastic unreactive?

A

Adv - food doesn’t react with it, does rot or corrode.
Disadv - Most polymers are not biodegradable and difficult to dispose of and can cause environmental damage disposing them.

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7
Q

What are the three ways waste plastics can be got rid of?

A
  • Buried in landfill
  • Plastic can be reused
  • Can be burned as fuel
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8
Q

What are the two ways plastics can be reused?

A
  • Recycled by melting and remoulding them.
  • Some plastics can be cracked into monomers, these can be used as an organic feedstock to make more plastics or other chemicals.
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9
Q

Why would you burn plastics?

A

In order for energy production through combustion, heat given off can be used to generate electricity.

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10
Q

Why does the burning of waste plastics have to be carefully controlled and how do they do this?

A

They need to remove toxic gases, such as HCl during the combustion (burning) of halogenated plastics like PVC.

The waste gases are passed through scrubbers which can neutralise gases such as HCl by reacting with a base.

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11
Q

What is the difference between Biodegradable polymers and normal waste plastics?

A

They decompose quickly in certain conditions because organisms can digest them.

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12
Q

What a biodegradable polymer made from?

A

Biodegradable polymer can be made from renewable raw materials such as starch or oil fractions.

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13
Q

What are some disadvantages of Biodegradable plastics?

A
  • They’re more expensive
  • Biodegradable polymers still need the right conditions to decompose, so they must be collected and separated from waste plastics to give them the right amount of oxygen and moisture.
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14
Q

What are photodegradable polymers?

A

These are polymers that decompose when exposed to sunlight.

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15
Q

What are the two characteristics of addition polymerisation?

A
  1. Monomer contains a C=C double bond.
  2. Backbone of polymer is a continuous chain of carbon atoms.
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16
Q

What is a stereoisomers?

A

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

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17
Q

Why do stereoisomers only happen for double bonds and not single bond?

A

There is restricted rotation around the double covalent bond so there is only two types of shapes. Single covalent bonds are rotational.

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18
Q

Why can’t atoms around C=C not rotate?

A

Because of the way the p orbitals overlap to form a π bond which locks the two carbons in place.

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19
Q

What are the two types of geometrical isomers?

A

Z (Cis)
E (Trans)

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20
Q

When can you use Cis and Trans?

A

Must have a C=C double bond and each carbon in the double bond must be attached to two different groups and one of the groups on each of the carbons must be hydrogen.

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21
Q

What is need of a molecule to be an E/Z isomer?

A

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

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22
Q

It this molecule E or Z isomer?

H3C CH3
\ /
C=C
/ \
H H

A

Z-but-2-ene

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23
Q

It this molecule E or Z isomer?

H3C H
\ /
C=C
/ \
H CH3

A

E-but-2-ene

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24
Q

What rule do we use when all the groups attached to the C=C are different?

A

Cahn-Ingod-Prelog (CIP) rule where atoms it’s a larger atomic number are given higher priority.

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25
Q

Is this molecule a E or Z isomer?

F Cl
\ /
C=C
/ \
H Br

A

E-isomer

26
Q

Is this molecule a E or Z isomer?

F Br
\ /
C=C
/ \
H Cl

A

Z-isomer

27
Q

What happens if two molecules have the same molecular formula? (E-Z isomers)

A

You look further along chain at the atom directly attached to this atom which has the largest atomic number, giving it the highest priority.

28
Q

Is this molecule a E or Z isomer?

H3C CH2CH2Cl
\ /
C=C
/ \
H CH2CH2OH

A

Z isomer

Left hand side C atom has large atomic mass than H so is priority.

Right hand side has the same Carbon atom so you continue down the group. Next higest priority (largest atomic mass on the atoms attached) is carbon for both so you go along the group again. Top group has Cl which has a larger atomic mass than O so is priority.

29
Q

Can you use Cis and Trans for this molecule?

H CH3
\ /
C=C
/ \
Br H

A

Yes you can use it as there are two H atoms on opposite sides of the double bond so is Trans.

30
Q

Are Alkenes saturated or unsaturated hydrocarbons?

A

Unsaturated hydrocarbons, because there is a at least one double carbon bond.

31
Q

What is a type of bonds make up a double bond?

A

A sigma bond and a Pi bond.

32
Q

How is sigma (σ) bond formed?

A

A sigma bond is formed when two s orbitals overlap.
They orbitals overlap directly between the bonding atoms in a straight line

33
Q

How is a Pi (π) formed?

A

A π bond is formed by the sideways overlap of two adjacent (next to each other) p-orbitals above and below the bonding atoms.
The P orbitals that overlap are dumbbell-shaped.

34
Q

Which bond is strong and which bond is weak out of a π and σ bond and why?

A

Pi bond is weaker than sigma bonds due to the fact that the electron density is spread put above and below the nuclei so the electrostatic attraction between the nuclei and shaheed pair of electrons is weaker. Whereas in a Sigma there is a highest possible electron density.

35
Q

What is the shape around a carbon in a carbon to carbon double bond?

Or double bond in general

A

Trigonal planar.

36
Q

Why is there a trigonal planar shape around the carbon atoms in a double bond?

A
  • Three regions of electrons density around each of the carbon atoms (sigma, sigma, sigma/Pi).
  • Three regions repel each other as far apart as possible, so the bond angle around each of the carbon is 120°.
  • All atoms are in the same plane.
37
Q

Explain the bonds in a C=C bond?

A

Carbon has four electrons in its outer shell. Three of these electrons form sigma bonds. One to the other carbon atom of the double bond and one for each of the r groups attached to that carbon.

The final electrons are in p-orbitals. Form a Pi bond between the sideways overlap of the two p-orbitals in the C=C.

38
Q

Which are more reactive Alkenes and Alkanes and Why?

A

Because Alkenes have a π bond.
A π-bonds electron density is concentrated above and below the plane of the sigma bond, so there on the outside the double bond so are more exposed. Meaning it has a lower bond enthalpy than sigma bonds so will break before sigma bonds. So will therefore break more readily.

39
Q

Why do alkanes not react easily?

A

As they only contain sigma bonds which have a high bond enthalpy which are the strongest type of covalent bonds.

40
Q

What is an electrophile?

A

An atom or group of atoms that is attracted to an electron-rich centre, where it accepts a pair electrons.
An electron pair acceptor.
They are normally positively charged ions, with partial positive charge δ+

41
Q

What is an electrophilic addition reaction?

A

An addition reaction in which the first step is attack by an electrophile on a region of high electron density.

42
Q

What is an addition reaction?

A

A reaction in which a reactant is added to an unsaturated molecule to make a one saturated molecule.

43
Q

What is heterolytic fission?

A

The breaking of a covalent bond forming a cation (positive ion) and an anion (negative ion). Movement of two electrons.

44
Q

What happens in an electrophilic addition reaction of an Alkene?

A

The carbon double bond opens up and atoms are added to the carbon.

45
Q

Why does electrophilic addition reaction take place for Alkenes?

A

The double bond has got a high electron density because of the presence of the Pi-electrons, this attracts electrophile which want to accept an electron pair.

46
Q

What are the 4 types of addition reaction for the Alkenes?

A
  1. Hydrogen in the presence of a nickel catalyst.
  2. Halogens
  3. Hydrogen halides
  4. Steam in the presence of an acid catalyst.
47
Q

Explain the hydrogenation of Alkenes with the presence of a nickel catalyst.

A

An Alkene is mixed with hydrogen and a nickel catalyst at 150°C. An addition reaction takes place, which hydrogen is added across a double bond.
All the C=C bonds will react with hydrogen in this way in the molecule.

48
Q

Write then equation for the hydrogenation of the Alkene, buta-1,3-diene.

A

. H H H H H H H H
| | | | | | | |
H—C=C—C=C—H + 2 H2 —> H—C—C—C—C—H
| | | |
H H H H

49
Q

Explain the hydration of Alkenes with steam in the presence of an acid catalyst.

A

Alkenes react with steam, H2O (g), in the presence of a phosphoric acid catalyst. And an alcohol is formed as the steam adds across the double bond.

50
Q

Write the products for the hydration of this Alkene with steam.
H H H
| | |
H—C—C=C—H + H2O (g) —>
|
H

A

. H H H H H H
| | | | | |
H—C—C—C—H H—C—C—C—H
| | | | | |
H H OH H OH H
(Propan-1-ol) (Propan-2-ol)

51
Q

How do Halogens react with Alkenes to form dihaloalkanes?

A

The halogens add across the double bond, and each of the carbon atoms end up bonded to one halogen atom. It’s an electrophilic addition reaction.

52
Q

Explain the mechanisms for an alkene (ethene) and bromine (Br2) ti make a diahaloalkane (dibromoethane).

A
  1. The double bond repels the electrons in Br2, polarising Br—Br. The Br closer to the double bond i δ+ , furthest away δ-.
  2. Heterolytic fission of Br2. The closed Br (to double bond) gives up the bonding electrons to the other Br and bonds to the C atom.
  3. You get a positively charged carbocation, and a :Br-.
  4. The lone pair on the :Br- then bonds to the carbocation (C+) forming a dibromoethane.
53
Q

How can you use bromine water to test for carbon double bonds?

A

When you shake an alkene with orange bromine water, the solution quickly decolourises.

54
Q

Why does a solution decolourise with an carbon double bond and orange bromine water?

A

The bromine is added across the double bond with electrophilic addition to a form colourless dibromoalkane.

55
Q

If a hydrogen halide adds to an unsymmetrical alkene how many product may form?

A

Two products, a major product and minor product.

56
Q

What determine which is the major product and minor product when adding hydrogen halides to unsymmetrical Alkenes?

A

How stable the carbocation formed in the middle of the reaction is. The more stable carbocation is the major product (much more likely to form).

57
Q

List this primary carbocation (1), secondary carbocation (2), tertiary carbocation (3) from least stable to most stable.
. H
|
1- R—C—H
+

. R
|
2. R—C—H
+

. R
|
3. R—C—R
+

A

Least Stable: Primary Carbocation
. Secondary Carbocation
Most Stable: Tertiary Carbocation

58
Q

Why are carbocations with more alkyl groups more stable.

A

Because the alkyl groups feed electrons towards the positive charge of the carbocation.

59
Q

What does Markownikoff’s rule say?

A

The major product from addition of a hydrogen halide (HX) to an unsymmetric alkene is the one where hydrogen adds to the carbon with the most hydrogens already attached.

60
Q

Explain which is the major product and minor product for when hydrogen bromide reacts with propene.

H CH3
\ / H δ+
C=C |
/ \ Br δ-
H H

A

Forms two products
. 1: 2:
. H CH3 H CH3
| | | |
H—C —C—H H—C—C—H
| + + |
H H

1: Is a Secondary Carbocation, Two alkyl groups attacking the carbocation. Therefore Major Product
2: Is a Primary Carbocation, one alkyl group attacking the carbocation. Therefore Minor Product

Finally,
1: This will form 2-bromopropane, so 2-bromoproane is the major product
2: This will form 1-bromopropane, so 1-bromopropane is the minor product.

a-level chemistry yr 12 mine (15 decks)

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