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Chemistry > Chapter 13 > Flashcards

Chapter 13 Flashcards

1
Q

What are alkenes ?

A

Alkenes are unsaturated hydrocarbons as they contain at least one double carbon to carbon bond.

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2
Q

What is the general formula for an alkene?

A

CnH2n

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3
Q

What is the bonding in an alkene?

A

Each carbon atom in a double bond contains three sigma bonds, one to the other carbon atom in the double bond and the other two electrons to 2 other atoms. This leaves one electron in a carbon atom not involved with sigma bonds this electron is found in the
P- orbital bonded by a pi bond.

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4
Q

How is the pi bond formed?

A

It is formed by a sideways overlap of two P-orbitals one from each carbon atom in the double bond. Each carbon atom constitute one electron to the jumper in the pie bond. The pi electron density is concentrated above and below the line joining the nuclei of the bonding atoms.

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5
Q

Why is the geometry of an alkene different to an alkane?

A

The pi bonds locks the two carbon atoms in position and prevent them from rotating around the double bond. In alkanes rotation is possible around every atom.

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6
Q

What does a carbon to carbon double bond consist of?

A

One pi bond and one sigma bond.

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7
Q

What is the shape around a double bond?

A

The shape around each of the carbon atoms in the double bond is a trigonal planar as there are three regions of electron density around each carbon atom. Additionally the three regions repel each other as far as possible meaning it is 120° with all atoms being on the same plane.

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8
Q

What is stereoisomerism?

A

Stereoisomers have the same structural formula but arranged differently in 3-D space.

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9
Q

Why are alkenes fixed relative to each other?

A

As the position of the pi bonds electron density above and below the plane of the sigma bond restricted rotation of the double bond and the greats attached each other carbon atoms.

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10
Q

How does a alkene satisfy E/Z isomerism?

A

It must have a double carbon bond and have two different groups attached to each carbon atom of the double bond.

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11
Q

If the high priority groups are on the same side of the double bond what is the isomer?

A

Z

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12
Q

If the high-priority groups are diagonal cross the bond what is the isomer?

A

E

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13
Q

How do you assign priority?

A

The higher the atomic number, the higher the priority.

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14
Q

What happens if the first two atoms attached the carbon of the same when assigning priority?

A

You look for the first point of difference.

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15
Q

With cis trans isomerism when is trans-used?

A

When the two same groups attached to each carbon are opposite/across the double bond.

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16
Q

With cis trans-isomerism when is cis used?

A

When the same two groups are on the same side

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17
Q

How reactive are alkenes?

A

They are more reactive than alkanes due to the presence of the pi bond. As the pie electrons are outside the double bond it is more exposed than the electrons in the sigma bond. The pi bond readily breaks and the alkanes undergo addition reactions relatively easily. The pi bond is weaker than the sigma bond and it’s broken more readily.

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18
Q

What is an addition reaction?

A

It is when two reactant molecules joined together to form one product.

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19
Q

What alkene addition reactions do they undergo?

A

Hydrogen with the presence of a nickel catalyst, halogens, hydrogen halides and steam in the presence of an acid catalyst.

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20
Q

What is the hydrogenation of alkenes?

A

It is when an alkene reacts with hydrogen in the presence of a nickel catalyst (at 423 K) in an addition reaction forming an alkane. The hydrogen is passed over the double bond where it breaks the double bond.

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21
Q

What is halogenation of alkenes?

A

It is when alkenes undergo addition reactions with halogens (Br and Cl )at room temperature forming
dihaloalkanes.

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22
Q

How do you test for an unsaturated compound?

A

You add bromine water (an orange colour) to the sample where the bromine adds across the double bond. The orange colour disappears when a double bond is present. If this was carried out with a saturated compound no addition reaction will occur so no colour change.

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23
Q

What is the addition reaction of alkenes with hydrogen halides?

A

It is when alkenes react with gaseous hydrogen halides at room temperature where they form halo alkanes (they are mixed in gas form when they are gases and hydrogen halides are bubbled through liquids). Alkanes also react with concentrated hydrochloric a hydrobromic acid which a hydrogen halides and water.

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24
Q

What is the hydration reaction of alkenes?

A

It is when alkenes react with steam in the presence of a phosphoric acid catalyst forming an alcohol. This team adds across the double bond breaking it.

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25
Q

What happens if the alkene is unsymmetrical?

A

Two products are possible when forming compared to just the one.

26
Q

What is an electrophile?

A

It is a group of atoms or an atom that is attracted to an electron rich centre and accepts an electron pair. There are usually a positive ion or a molecule containing a partial positive charge. They are simply defined as an electron pair acceptor.

27
Q

What is the mechanism called when alkenes take part in a addition reaction forming saturated compounds?

A

Electrophilic addition.

28
Q

Why does electrophilic reactions occur in alkenes?

A

As the double bond represents a region of high electron density due to pi electrons being present. The pi electrons high density attracts the electrophiles.

29
Q

What do curly arrows indicate?

A

They indicate the movement of electron pairs.

30
Q

How is the major product formed in the addition of a hydrogen halide?

A

It is formed via the most stable carbocation intermediate.

31
Q

How are carbocations classified?

A

By the number of alkyl groups attached to the positive carbon atom.

32
Q

What carbocations are the most stable?

A

Tertiary carbocations are the most stable and primary carbocations are the least stable.

33
Q

What is a carbocation stability linked to?

A

The electron donating ability of the alkyl group. Each alkyl group donates and pushes electrons towards the carbocation. The positive charge is then spread over the alkyl groups. The more alkyl groups attached to the positive carbon atom the more the charge is spread out making the iron more stable.

34
Q

How are carbocations more stable?

A

They are more stable when more alkyl groups are attached to the positive carbon atom making the charge more spread out between the ion

35
Q

Where is the positive charge found on a primary carbocation?

A

At the end of the chain

36
Q

Where is the positive charge found on a secondary carbocation?

A

Where two carbon atoms are attached.

37
Q

What is the halogenation reaction mechanism?

A

It is when a halogen reacts with an alkene to form a saturated compound

38
Q

What reaction to unsaturated alkenes undergo forming long saturated chains?

A

Addition polymerisation

39
Q

What is a repeat in addition polymerisation?

A

It is the specific arrangement of atoms in a polymer molecule which repeats over and over again.

40
Q

What does the N indicate in addition polymerisation?

A

The large number of repeats

41
Q

What is the monomer of polythene?

A

Ethene

42
Q

What is the monomer of poly(chloroethene), PVC?

A

Chloroethene

43
Q

What is the monomer of poly(propene)/ polypropylene

A

Propene

44
Q

What is the monomer of poly(phenylethene)/ polystyrene?

A

Phenylethene

45
Q

What is the monomer of poly(tetrafluoroethene)/PTFE?

A

Tetrafloroethene

46
Q

What is polythene used in?

A

Shopping bags

47
Q

What is PVC used in?

A

Floor tiles, pipes, imitation leather.

48
Q

What is polypropylene used in?

A

Fibres for ropes, packaging crates

49
Q

What is polystyrene used in?

A

Packaging materials, food trays and cups

50
Q

What is PFTE used in?

A

Coating on non stick pans, permeable membrane for clothing and shoes.

51
Q

Are alkene based polymers biodegradable or non-biodegradable?

A

Nonbiodegradable

52
Q

How are polymers recycled?

A

Polymers are chopped into flakes and then washed and melted. The recycled polymer is cut into pellets and used to make new products. Discarded polymers have to be sorted into type with mixed polymers rendering the product unusable.

53
Q

Why is the disposal of PVC hazardous?

A

It contains a high chlorine content and the range of additives present in the polymer. Dumping PVC in a landfill is not sustainable and once burnt it releases toxic hydrogen chloride and other products like toxic dioxins.

54
Q

How is new technology disposing PVC?

A

It is using solvents to dissolve the polymer. High-grade PVC is then covered by precipitation from the solvent and then the solvent is then able to be used again.

55
Q

Why are some polymers difficult to recycle?

A

As they derive from petroleum or natural gas they have a high energy stored value. This means they can be incinerated to produce heat generating steam to drive a turbine.

56
Q

What is feedstock recycling?

A

It describes the chemical and thermal processes that can reclaim monomers, gases or oils from waste polymers.

57
Q

What is an advantage of using feedstock recycling?

A

The polymers can be handled unsorted and unwashed.

58
Q

What a bio plastics produced from?

A

Plant starch, Celulose, plant oils and proteins

59
Q

What are biodegradable polymers?

A

These are normally made from start cellulose or contain additives which alter the structure of traditional polymer an order for micro-organisms to break them down.

60
Q

How are biodegradable polymers broken down?

A

They are broken down by micro organisms forming water, carbon dioxide and biological compounds.

61
Q

What is polystyrene being replaced by?

A

Sugarcane fibres are placing polystyrene which has been made into compostable plates, cups and food trays.

62
Q

What are photodegradable polymers?

A

These polymers contain bonds which are weakened by absorbing light to start the degradation. Alternatively light absorbing additives are used.

Chemistry (23 decks)

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