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chemistry: organic > topic 6C - alkenes > Flashcards

topic 6C - alkenes Flashcards

1
Q

What is the general formula for alkenes?

A

CnH2n.

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2
Q

Why are alkenes classified as unsaturated hydrocarbons?

A

They are unsaturated hydrocarbons as molecules contain only carbon and hydrogen atoms and at least one carbon-carbon bond is a double bond.

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3
Q

Why can alkenes form addition reactions?

A

As they are unsaturated, they can make more bonds with extra atoms in addition reactions.

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4
Q

What is the general formula for cycloalkenes?

A

CnH2n-2.

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5
Q

How are covalent bonds formed?

A

Covalent bonds form when atomic orbitals from different atoms overlap, causing the shared pair of electrons between the atoms which are electrostratically attracted to the nuclei involved.

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6
Q

What determines the type of covalent bond formed?

A

The way that the atomic orbitals overlap causes different types of bond to form.

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7
Q

How is a σ-bond formed?

A

A σ-bond is formed when two orbitals overlap in a straight line directly (head-on), giving the highest possible electron density between the two positive nuclei.

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8
Q

What kind of rotation can occur around a σ-bond?

A

Rotation can occur around a σ-bond.

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9
Q

Why do σ-bonds have high bond enthalpy?

A

A high electron density between the nuclei means there is a strong electrostatic attraction between the shared pair of electrons and the nuclei of the atoms involved. This means σ-bonds have a high bond enthalpy - they’re the strongest type of covalent bonds.

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10
Q

How is a π-bond formed?

A

A π-bond is formed when two lobes of two p-orbitals overlap sideways (end-to end).

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11
Q

Can rotation occur around a π-bond?

A

There is restricted rotation about a π-bond.

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12
Q

Why do π-bonds have low bond enthalpy?

A

A low electron density between the nuclei means there is a weak electrostatic attraction between the shared pair of electrons and the nuclei of the atoms involved. This means π-bonds have a low bond enthalpy.

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13
Q

What is a double bond composed of?

A

A double bond is made up of a sigma (σ-) bond and a pi (π-) bond. This means this bond is less than twice as strong as a single bond (just a σ- bond).

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14
Q

What types of bonds are found in alkenes?

A

In alkenes, the C-C and C-H bonds are all σ-bonds. The C=C bonds in alkenes contain both a σ- and a π-bond, where the π-bond leads to resultant high electron density above and below the line between the two nuclei.

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15
Q

What type of reactions do alkenes undergo?

A

Alkenes typically undergo addition reactions, in which small molecules (such as H2, Cl2 and HBr) add across the double bond to form a single product.

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16
Q

Why do alkenes undergo electrophilic addition reactions?

A

In an electrophilic addition, the alkene double bond opens up and atoms are added to the carbon atoms. These reactions happen because the π bond in the double bond breaks as they are exposed and have high electron density and is easily attacked by electrophiles.

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17
Q

What are electrophiles?

A

Electrophiles are electron-pair acceptors; any positive ion or molecule which is attracted to a region of high electron density.

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18
Q

What are examples of electrophiles?

A

They include positively charged ions and polar molecules (since the 𝛿+ atom is attracted to regions of high electron density).

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19
Q

What is the reagent and condition for the reaction of alkenes with hydrogen?

A

Reagent: Hydrogen gas; Conditions: Nickel catalyst, 150ºC.

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20
Q

What is the overall equation for the reaction of alkenes with hydrogen?

A

alkene → alkane.

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21
Q

What is the reaction type for the reaction of alkenes with hydrogen?

A

Hydrogenation.

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22
Q

What is catalytic hydrogenation used for?

A

This ‘catalytic hydrogenation’ process is used in the manufacture of margarine from unsaturated vegetable oils in palm seeds and sunflower seeds.

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23
Q

What effect does catalytic hydrogenation have on oils?

A

The conversion of C=C double bonds into C-C bonds turns the oily, unsaturated liquids into soft, saturated solids like margarine by raising the melting point.

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24
Q

How is the presence of C=C bonds tested?

A

This is the reaction used to test for C=C double bonds, because alkenes decolourise bromine water whereas alkanes do not – a dibromoalkane is formed.

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25
What is the reagent and condition for the reaction of alkenes with halogens?
Reagent: Halogen (Cl2, Br2, I2); Conditions: Room temperature and pressure.
26
What is the overall equation for the reaction of alkenes with halogens?
alkene → dihalogenoalkane.
27
What is the reaction type for the reaction of alkenes with halogens?
Addition.
28
What initiates the reaction of bromine with ethene?
The bromine molecule approaches the ethene molecule. The π-bond in the double bond of the alkene is a region of high electron density, which repels the electron pair of the covalent bond in the bromine molecule.
29
Why does the bromine molecule become electrophilic?
The bromine molecule now has an induced dipole. The π bond of the alkene are attracted to the positive bromine, so the positive bromine is acting as an electrophile.
30
How is the carbocation intermediate formed during the bromine reaction?
The electron pair forms a covalent bond to this bromine atom; at the same time the covalent molecule now breaks and the pair of electrons move on to the other bromine atom – heterolytic fission which forms a carbocation intermediate and a bromide ion.
31
How is 1,2-dibromoethane formed from the carbocation?
The electron pair on the bromide ion are attracted to the positive carbon atom in the carbocation intermediate, which forms a covalent bond and the final product 1,2 dibromoethane.
32
What is formed when alkenes react with hydrogen halides?
Alkenes also undergo addition reactions with hydrogen halides – a halogenalkane is formed.
33
What is the reagent and condition for the reaction of alkenes with hydrogen halides?
Reagent: Halogen halide (HCl, HBr, HI); Conditions: Room temperature and pressure.
34
What is the overall equation for the reaction of alkenes with hydrogen halides?
alkene → halogenalkane.
35
What is the reaction type for the reaction of alkenes with hydrogen halides?
Addition.
36
What initiates the reaction of a hydrogen halide with an alkene?
The hydrogen bromide molecule approaches the alkene. The positive hydrogen on the hydrogen halide is attracted to the high electron density of the double bond.
37
What role does the π-bond play in the reaction with hydrogen halide?
The π bond of the alkene are attracted to the positive hydrogen atom, so the positive hydrogen is acting as an electrophile.
38
How is the carbocation formed in the reaction with hydrogen halide?
The electron pair forms a covalent bond to this hydrogen atom; at the same time the covalent molecule now breaks and the pair of electrons move on to the halogen atom – heterolytic fission which forms a carbocation intermediate and bromide ion.
39
Why can two products form from hydrogen halide addition to unsymmetrical alkenes?
As the hydrogen bromide is added to an unsymmetrical alkene, there are two possible products. The amount of each produced depends on how stable the carbocation formed in the middle of the reaction is.
40
What makes a carbocation more stable?
Carbocations with more alkyl groups are more stable because the alkyl groups release electrons towards the positive charge. The more stable carbocation is much more likely to form as the electron released reduces the charge on the ion.
41
What does Markownikoff’s rule state?
The Markownikoff’s rule says that the major product from addition of a hydrogen halide (HX) to an asymmetrical alkene is one where hydrogen adds to the carbon with the most hydrogens already attached.
42
What product is formed from the catalytic hydration of ethene?
The catalytic hydration of ethene, for example, produces ethanol in a reversible reaction.
43
What is the reagent and condition for the reaction of alkenes with steam?
Reagent: H2O (g); Conditions: Concentrated H2SO4/H3PO4 and high temperature and pressure.
44
What is the overall equation for the reaction of alkenes with steam?
alkene → alcohol.
45
What is the reaction type for the reaction of alkenes with steam?
Addition (hydration).
46
What is observed when an alkene is shaken with acidified potassium manganate (VII)?
The purple solution is decolourised and the alkene is oxidised.
47
What is the reagent and condition for the oxidation of alkenes?
Reagent: KMnO4; Conditions: Acidified.
48
What is the overall equation for the oxidation of alkenes to diols?
alkene + H2O + [O] → diol.
49
What is the reaction type for the oxidation of alkenes?
Oxidation.
50
What is the reagent and condition for the reaction of alkenes with bromine water?
Reagent: BrOH (aq); Conditions: Room temperature.
51
What is the reaction type for the reaction of alkenes with bromine water?
Addition.
52
What is a polymer?
A polymer is a long-chain molecule made from many small molecules, called monomers.
53
Why are poly(alkanes) unreactive?
Poly(alkanes) like alkanes are very unreactive as the C-C and C-H bonds are relatively strong and non-polar.
54
What is an addition polymer?
An addition polymer is one in which there are no other products formed: all small alkene molecules (monomers) open up their double bonds and join together to form long-chain molecules.
55
How is the name of an addition polymer formed?
The name of the polymer is formed using the monomer in brackets and adding ‘poly’ in front of it.
56
What does the 'repeat unit' in a polymer represent?
The bit in brackets is the ‘repeat unit’ (or ‘repeating unit’) n represents the number of repeat units.
57
How do you find the monomer from a polymer repeat unit?
Take the repeat unit, add a double bond between the carbon atoms and remove the single bonds from each end.
58
How do you find the repeat unit from a monomer?
Change the C=C bond into a single bond, and add another single bond to each of the C=C carbons.
59
Why are polymers useful for storage?
One of the useful properties of polymers is that they are unreactive. This means they are suitable for storing food and other substances safely.
60
Why do polymers cause environmental problems?
They are not biodegradable as microorganisms cannot break them down. This means they cause a litter problem if disposed of carelessly.
61
When is landfill used to dispose of polymers?
Space in landfill sites is wasted if it is filled with non-biodegradable polymers, since it is used to dispose waste plastics when: difficult to separate from other waste, not in sufficient quantities to make separation financially worthwhile, or too difficult to technically recycle.
62
What happens during incineration of polymers?
Waste polymers are combusted at very high temperatures, producing carbon dioxide. Though, the released energy can be used for heating homes or generating electricity. The volume of rubbish is greatly reduced.
63
What are the advantages of recycling polymers?
Reduces reliance on landfill and incineration, conserving crude oil, a finite resource. This also reduces disposal problems.
64
What are the limitations of recycling polymers?
Polymers need collection and sorting, which is expensive in terms of energy and manpower. Polymers can only be recycled into the same type, so careful separation needs to be done. Thermoplastic polymers can be melted down and reshaped.
65
What is feedstock recycling of polymers?
Polymers are broken down into small molecules that can be used to make other chemicals and new polymers, saving raw materials.
66
How is toxic waste managed during polymer disposal?
Chemists have developed methods to remove toxic waste products like HCl before they are released into the atmosphere. Waste gases from incinerators are scrubbed/reacted with bases to neutralize acidic gases.
67
What are biodegradable polymers made from?
Chemists have developed biodegradable and compostable polymers made from natural, non-finite substances such as maize and starch, saving energy.
68
Why are biodegradable polymers considered carbon neutral?
Carbon dioxide is produced from biodegradation, CO2 absorbed by plants during growth (considered carbon neutral).
69
What are the challenges of using biodegradable polymers?
More expensive to produce than non-biodegradable equivalents, especially because they need collection and sorting, which is expensive in terms of energy and manpower. They also may require specific conditions to decompose effectively.
70
What principles guide sustainable polymer manufacturing?
Use reactant molecules that are as safe and environmentally friendly as possible. Use as few other materials, like solvents, as possible. Renewable raw materials should be used wherever possible. Energy use should be kept to a minimum. Catalysts are often utilised in polymer synthesis to lower energy use. Limit the waste products made, especially those which are hazardous to human health or the environment. Make sure the lifespan of the polymer is appropriate for its use to limit expenses.

chemistry: organic (6 decks)

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