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A - Level Chemistry > C13 Alkenes > Flashcards

C13 Alkenes Flashcards

1
Q

Are alkenes unsaturated or saturated

A

Unsaturated

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2
Q

General formula of alkenes

A

CnH2n

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3
Q

What’s the bonding like for each of the C of C=C in alkenes

A

3 of 4 electrons used in 3 sigma bonds ( 1 to other C atom of C=C & other 2 electrons to other atoms)

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4
Q

What happens to 4th electron in each C of C=C in alkenes

A

1 electron (in p-orbital) on each C of C=C not involved in sigma bonds

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5
Q

Define pi bond

A

Sideways overlap of 2 p-orbitals, 1 from each C of C=C

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6
Q

Where is pi electron density concentrated in an alkene

A

Pi electron density concentrated above and below line joining nuclei of bonding atoms

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7
Q

Shape and bond angle in alkene

A

Trigonal planar

120*

3 regions of electron density around each of C atoms
- 3 regions repel each other as far apart as possible
- all of atoms are in same plane

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8
Q

Define stereoisomer

A

Compounds with same structural formula but with different arrangent of atoms in space

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9
Q

Conditions needed for E/Z isomerism

A

C=C

Diff groups attached to each C of C=C

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10
Q

Z isomer

A

Groups of higher priority on same side of C=C

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11
Q

E isomer

A

Groups of higher priority are diagonally opposite across C=C

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12
Q

What is the Cahn-Ingold-Prelog rule

A

Atoms attached to each C atom in C=C are given priority based on their atomic number

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13
Q

When can Cis-trans isomerism only be used

A

When each C atom in C=C is attached to a single H atom

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14
Q

Why’s there stereoisomerism around C=C

A

Rotation about C=C restricted, groups attached to each C fixed relative to each other

Reason fro rigidly - position of pi-bond electron density above + below plane of sigma bonds

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15
Q

2 types of isomerism

A

E/Z isomerism

Optical isomerism

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16
Q

Conditions needed for Cis-trans isomerism

A

C=C

1 of attached groups on each C of C=C must be the same

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17
Q

How to assign priority if 2 atoms attached to C=C are same

A

Find 1st point of difference

Group with higher atomic number at 1st point of difference is given higher priority

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18
Q

use enthalpy data explain why pi bonds break more readily than sigma bonds

A

In Ethene:

C-C (sigma bond) has BE = 347 kJ mol-1

C=C (pi and sigma) BE = 612 kJ mol-1

BE of pi bond : 612 - 347 = 265 kJ mol-1

Pi bonds weaker than sigma bond, so is broken more easily due to lower BE

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19
Q

Hydrogenation of alkene

A

Hydrogen

Ni at 423K

Form alkane

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20
Q

Are alkenes more/less reactive than alkanes

A

More reactive

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21
Q

Why are alkenes reactive

A

C=C made of sigma + pi bonds

Pi electron density conc above + below plane of sigma bond

Pi electrons outside of C=C —> so are more exposed than electrons in sigma bond

Pi bond breaks readily & undergo addition reactions easily

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22
Q

Hydration of alkenes

A

Steam/H2O(g)

H3PO4

Form alcohol

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23
Q

Reaction of alkene with hydrogen halide

A

Hydrogen halide (g)

Room temp

Form haloalkane

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24
Q

Test for unsaturation

A

Br2

Colour change = orange to colourless (bromine water decolourises)

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25
Halogenation of alkenes
Room temp Br2/Cl2 Form haloalkane
26
Type of bond fission in electrophilic addition
Heterolytic fission
27
What’s the most stable carbocation
Tertiary carbocation ( stability increases from primary to tertiary carbocation)
28
Explanation for electrophilic addition of but-2-ene with HBr
(Delta neg) Br more EN than H (delta pos) Electron pair in pi bond attracted to delta pos H C=C break Bond form between H of HBr and C of C=C H-Br break by heterolytic fission, electron pair go to Br Br- and carbocation form Br- react with carbocation to form addition product
29
Explanation of Br2 and propene
Pi electrons interact with electrons in Br2 Induces dipole on Br-Br Electron pairs in pi-bond attracted to delta pos Br, cause C=C to break Bond form between C and Br Br-Br break by heterolytic fission, electron pair for to delta neg Br Br- and carbocation form Br- react with carbocation to form product
30
define ectrophile
Atom/group of atoms that’s attracted to a electron rich centre & accepts an electron pair
31
Markownikoffs rule
When a H-halide reacts with an unsymmetrical alkene the H of H-halide attaches itself to C of alkene with greater number of H atoms and smaller number of C atoms
32
Why’s tertiary carbocation most stable
carbocation stability linked to e^- donating ability of alkyl groups each alkyl group donates + pushes e^-s towards pos charge of carbocation pos charge spread over alkyl groups more alkyl groups attached to pos-Charged C atom, more charge is spread out,making ion more stable
33
What’s the major product
more stable carbocation
34
Using wast polymer as fuel
some polymers made from natural gas and hard to recycle, they have high energy store incinerate waste polymer, produce heat, generate steam, drive turbine producing elelctrcity
35
Pros of polymers
Readily available Cheap Convenient for throwaway society Lack reactivity - suitable for storing food
36
Cons of polymers
Lack reactivity Enviro effect - killing marine life
37
Bioplastics
produced from plant starch, cellulose, plant oils, and proteins offer a renewable and sustainable alternative to oil-based products - protects enviro and conserves valuable oil refineries
38
PVC recycling
disposing PVC is hazardous due to high Cl content and additives it contains - when burnt releases Hydrogen chloride (corrosive gas) and pollutants
39
Compostable polymers
degrade and leave no visible/toxic residues - based on poly(lactic acid) becoming more common
40
photodegradable polymers
polymers that contain bonds that are weakened by absorbing light to start the degradation. alternatively, light absorbing additives are used
41
Feedstock recycling
chemical and thermal processes that can reclaim monomers, gases or oil from waste products - products made from this resemble the ones made from crude oil, so can be used as raw materials for production of new polymers - advantage = can handle unsorted and unwashed polymers
42
Recycling polymers
pros: - reduce enviro impact - conserve finite fossil fuels - decrease waste going to landfill .cons: - if polymer is mixed, it renders the product unusable so it can't be reused
43
Biodegradable polymers
- broken down by microorganisms into H20, CO2 and biological compounds - made from starch/cellulose or contain additives that alter structure of traditional polymers so microorganisms can break them down.
44
What are alkenes
Unsaturated HC that contain at least 1 C=C made up of a pi bond and sigma bond
45
What’s the general formula of alkenes
CnH2n
46
How is a pi bond formed
Electrons in the adjacent p orbitals overlap above and below the C atoms. They can only be made after a sigma bond is formed
47
What bond restricts rotation of C atoms
Pi bond
48
What’s the angle and shape of a double bond
Trigonal planar 120*
49
Are they more or less reactive than alkanes? Why?
More reactive due to high electron density of double bond and the fact the pi-bond is slightly easier to break
50
What IMFs do they have
Only LFs due to non-polar bonds
51
Are they soluble In water
No Non-polar bonds
52
What are the types of isomers that can be formed using alkenes
E/Z isomers - due to restricted rotation Cis-trans isomers - if 2 of the same substituents are attached to each C
53
What is an electrophile
Species that are electron pair acceptors
54
What is the most stable type of carbocation intermediate? Why?
Alkyl groups have a pos inductive effect, so the most stable carbocation is the one bonded to the most other C atoms, i.e. a tertiary carbocation
55
Major products will be formed from which kinds of carbocations
Tertiary (or the most stable available)
56
What conditions are needed for the electrophilic addition of H2O to an alkane? What is this type of reaction called?
Steam in the presence of an acid catalyst, usually phosphoric acid Reaction is called hydration
57
What are the products of the hydration reaction
An alcohol
58
What conditions are needed for the electrophilic addition of a hydrogen halide to an alkene
Hydrogen halide gases must be at room temp
59
What is the reaction called when a halogen is added to alkene
Halogenation
60
How does a molecule with non-polar bond react as if it is an electrophile
C=C with a high electron density induces a temporary dipole in the halogen molecule —> delta pos atom is attracted to double bond
61
How can an alkene be converted into alkane? What is the reaction called and what are the required conditions
Alkene + hydrogen = alkane Hydrogenation Conditions —> 150*C, nickel catalyst
62
What is an addition polymer
Many monomers bonded together via rearrangement of bonds without the loss of any atom or molecule
63
What are monomers? What form do they usually take?
Molecules which combine to form a polymer Usually have a C=C which breaks to leave a repeating pattern
64
What are the ways in which plastics can be disposed
Landfill Combustion Electricity generation Reuse Recycle Organic feedstock
65
What are the disadvantages of recycling
Plastic must be sorted into different types Expensive Labour intensive Requires high technology
66
How does photodegradable polymers break down
They are broken down chemically using energy with wavelengths similar to light. Once the break down begins it is not possible to stop the process
67
Explain what happens in organic feedstock
Plastics are separated and broken down into small organic molecules through a series of reaction. The molecules can then be used produce plastics and in other industries
68
Give a disadvantage of photodegradable polymers
May not be exposed to sufficient light

A - Level Chemistry (40 decks)

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