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chemistry > C13 - Alkenes > Flashcards

C13 - Alkenes Flashcards

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1
Q

What are alkenes?

A

Hydrocarbon homologous series with a double carbon-to-carbon bond and a general formula of CnH2n

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2
Q

Are alkenes saturated or unsaturated hydrocarbons?

A

Unsaturated

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3
Q

What is the general formula for alkenes?

A

CnH2n

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4
Q

What are exceptions for alkene general formula?

A

Cyclic alkenes
Alkenes containing more than one double bond

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5
Q

What is a pi bond?

A

-Bond formed by the sideways overlap of two p-orbitals
-With electron density above and below the plane of the bonding atoms

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6
Q

What is a sigma bond?

A

-Bond formed by the overlap of one orbital from each bonding atom
-Consists of two electrons
-Electron density centred around a line directly between nuclei of the two atoms

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7
Q

How is the carbon double bond formed?

A

3 electrons used in 3 sigma bonds (2xC-H,1xC-C)
1 electron used in pi bond (1xC-C)

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8
Q

What does the pi bond do?

A

Locks the two C atoms in position
Prevents them rotating around the double bond

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9
Q

What is the shape around a double bond?

A

Trigonal planar because…
-3 regions of e- density
-no lone pairs
-3 regions repel equally

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10
Q

What is a stereoisomer?

A

Compound with the same structural formula but different arrangement of atoms in space

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11
Q

What is E/Z isomerism?

A

Type of stereoisomerism in which different groups attached to each C of a C=C bond may be arranged differently in space due to restricted rotation of C=C bond

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12
Q

What two conditions does a molecule need to have E/Z isomerism?

A
  • C=C double bond
  • Different group attached to each C atom
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13
Q

What is cis-trans isomerism?

A

Special type of E/Z isomerism where there are two non-hydrogen groups and two hydrogen atoms around the C=C bond
-Cis (Z) isomer: H atoms on same side
-Trans (E) isomer: H atoms on different sides

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14
Q

What is the cis isomer?

A

H atoms on the same side
Z isomer

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15
Q

What is the trans isomer?

A

H atoms on different sides
E isomer

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16
Q

How do you identify the E isomer?

A

Higher priority group are on different sides

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17
Q

How do you identify the Z isomer?

A

Higher priority group are on the same sides

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18
Q

When using E/Z system of naming how do you assign higher priority groups?

A

Higher atomic number takes priority

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19
Q

Are alkenes reactive?

A

Yes

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20
Q

Why are alkenes reactive?

A

C=C bond consists of a sigma and pi bond
Pi bond e- density is concentrated above and below plane of sigma bond
Pi bond is exposed and breaks more easily

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21
Q

Reaction conditions and products of hydrogenation of alkenes

A

Alkene + hydrogen –> alkane
Nickel catalyst
423K

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22
Q

Reaction conditions and products of halogenation of alkene

A

Alkene + halogen –> (di)haloalkane
Only Cl2/Br2

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23
Q

How do you test organic compounds for unsaturation?

A

1) Add bromine water dropwise to the sample
2) If unsaturated, bromine adds across double bond
3) Decolourises orange bromine water

24
Q

Reaction conditions and products of addition reaction of alkene with hydrogen halides

A

Alkene + hydrogen halide –> haloalkane

25
Q

Reaction conditions and products of hydration of alkenes

A

Alkene + H2O (g) –> alcohol
Phosphoric acid catalyst (H3PO4)

26
Q

What is electrophilic addition?

A

Addition reaction in which the first step is attack by an electrophile on a region of high electro density

27
Q

What is an electrophile

A

Electron pair acceptor

28
Q

Describe mechanism for electrophilic addition reaction

A

1) Br is more electronegative than H so HBr is polar (d- is Br)
2) Electron pair in pi bond attracted to partially positive H atom, causing double bond to break
3) Bond forms between H from HBr and C that was part of double bond
4) H-Br bond breaks by heterolytic fission, e- pair goes to Br
5) Br- ion and carbocation formed (+ve C atom)
6) Br- ion reacts with carbocation to produce final product

29
Q

What is a carbocation?

A

Ion that contains a positively charged carbon atom

30
Q

What is a primary carbocation?

A

Positive charge is on a carbon atom at the end of a chain

31
Q

What is a secondary carbocation?

A

Positive charge is on a carbon atom to which two carbon atoms chains are attached

32
Q

How do you identify the minor/major product?

A

Secondary carbocation - forms major product
Primary carbocation - forms minor product

33
Q

What symbol is an alkyl group represented by?

A

-R

34
Q

What does increasing number of alkyl groups do to carbocation stability?

A

Increases stability

35
Q

Why does increasing number of alkyl groups increase carbocation stability?

A

Each alkyl group donates and pushes electrons towards positive charge of carbocation
Positive charge spread over alkyl groups
More alkyl groups, more charge spread out, increasing stability

36
Q

What is a polymer

A

Large molecule formed from many thousands of repeat units of smaller molecules

37
Q

What is addition polymerisation

A

Formation of a very long molecular chain, by repeated addition reactions of many unsaturated alkene molecules

38
Q

What is a monomer?

A

Small molecule that combines with many other monomers to form a polymer

39
Q

What happens when unsaturated alkene molecules undergo addition polymerisation?

A

Produce long, saturated chains
Require high pressure and catalysts

40
Q

How are monomer units written in formula?

A

n on left side

41
Q

How are polymer (repeat) units written in formula?

A

Square brackets
n on right side of brackets

42
Q

What is a repeat unit?

A

Specific arrangement of atoms that occurs in the structure over and over again.

43
Q

Polymer of ethene?

A

Poly(ethene)

44
Q

What is poly(ethene) used in?

A

Supermarket bags
Shampoo bottles
Children’s toys

45
Q

What is poly(chloroethene) also known as?

A

poly(vinyl chloride) / PVC

46
Q

Why is disposing waste polymers harmful?

A

Most alkene based polymers are non-biodegradable and non-reactive
(kill marine life)

47
Q

How can PVC be recycled?

A

Solvents dissolve polymer
High-grade PVC is then recovered by precipitation from solvent

48
Q

Why is PVC hazardous to recycle?

A

When burnt, releases corrosive HCl gas

49
Q

How can waste polymers be reused?

A

Energy to produce electricity
Burnt to produce heat, generating steam to drive turbine

50
Q

What is feedstock recycling?

A

Chemical and thermal process that can reclaim monomers, gases, oils from waste polymers

51
Q

What is a benefit of feedstock recycling?

A

Can handle unsorted and unwashed polymers

52
Q

What are bioplastics made from?

A

Plant starch
Cellulose
Plant oils
Proteins

53
Q

What are biodegradable polymers broken down by and into?

A

Broken down by microorganisms in water into H20, CO2 and biological compounds

54
Q

What are photodegradable polymers broken down by?

A

Contain bonds that are weakened by absorbing light

55
Q

How do biodegradable and photodegradable polymers help the environment?

A

Conserves valuable oil reserves

chemistry (21 decks)

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