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a level chemistry > module 4: alkenes and alkanes > Flashcards

module 4: alkenes and alkanes Flashcards

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1
Q

what is homolytic fission

A

when a bond’s electrons are split equally, each atom receives one of the electrons, forming free radical fragments - electron movement is shown with a single- headed curly arrow

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2
Q

what are free radicals

A

a species with an unpaired electron, they are unstable, short lived and highly reactive

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3
Q

what is heterolytic bond breaking

A

when one atom takes both electrons from a covalent bond, leading to the formation of a positive and negative ion

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4
Q

how is it decided which molecule takes the electrons in heterolytic bond breaking

A

the more electronegative substance takes the electrons

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5
Q

what is an electrophile

A

an electron deficient ion/molecule that seeks an atom/molecule with an electron pair for bonding

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6
Q

what is a nucleophile

A

an atom or functional group with a pair of electrons that can be shared to another species (usually an electrophile) to form a chemical bond in a reaction

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7
Q

define stereoisomer

A

compounds with the same structural formulae but with a different arrangement in space

• E/Z isomers
• optical isomers

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8
Q

what is E/Z isomerism

A

it’s a form of stereoisomerism, in terms of restricted rotation about a double bond and the requirement for 2 different groups to be attached to each carbon atom of the C=C group

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9
Q

when are the CIP rules used

A

when naming stereoisomers

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10
Q

what are the CIP rules

A
  1. compare the atomic number of the atoms attached directly to the stereocentre (double bond); the group with a higher atomic number gains priority
  2. if there’s a tie, consider the atoms at the 2nd distance from the stereocentre
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11
Q

what does the complete combustion of an alkane result in

A

the production carbon dioxide and water it occurs when the alkane is reacted with a plentiful oxygen supply

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12
Q

what does the incomplete combustion of an alkane result in

A

the formation of carbon monoxide/soot and water, it occurs when there is not enough oxygen supplied

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13
Q

how and under what conditions do alkanes react with halogens

A

condition = when exposed to UV light
via a free radical substitution reaction, where a hydrogen is replaced by a halogen

eg. CH4 + Cl2 –> CH3Cl + HCl

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14
Q

what are the three stages of the mechanism of a free radical substitution reaction

A
  1. initiation
  2. propagation
  3. termination
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15
Q

explain the initiation, propagation and termination steps in a free radical reaction mechanism

A

initiation - the UV light and high temperature provides energy to break the halogen bond and form two free radicals by homolytic fission

propagation - the free radical from the initiation stage reacts with the neutral methane molecule, a chain reaction occurs

termination - the reaction terminates when two free radicals collide to form products

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16
Q

why do alkenes have a double bond and what are the characteristics of it

A

the double bond is due to the sideways overlap of a spare, unbonded, singly filled p-orbital on each C atom

the overlap produces a cloud of electrons above and below the molecule, known as a pi bond

Pi bonds are much weaker than sigma bonds
1. the electron density gets spread out above and below the nuclei
2. the electrostatic attraction between the nuclei and shared electron pair is weaker so they have relatively low bond enthalpy
the C=C bond has both a sigma and pi bond

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17
Q

what are the conditions for hydrogenation and what does it do

A

alkenes react with hydrogen at 150-180˚C, on the surface of a nickel catalyst

an alkene is turned into an alkane

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18
Q

when is hydrogenation used in life

A

it’s used in transforming vegetable oils into solid fats, eg. margarine

softer margin needs less hydrogen to react with the polyunsaturated oil, as that leaves some double bonds intact

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19
Q

what is the bond angle around double bonds

A

120˚, trigonal planar

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20
Q

what is the bond angle and name around a fully saturated carbon

A

109.5˚, tetrahedral

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21
Q

why do alkenes form stereoisomers

A

atoms can’t rotate around C=C double bonds because of how p-orbitals overlap sideways to form a pi bond

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22
Q

what happens when an alkene reacts with a halogen

A

halogenation, it’s an electrophilic addition reaction where the double bond is broken to fully saturate the carbon atoms

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23
Q

what does a tertiary carbocation look like

A
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24
Q

what does a primary carbocation look like

A
25
Q

what does a secondary carbocation look like

A
26
Q

order the carbocations from most to least stable

A

tertiary
secondary
primary

27
Q

why is the tertiary carbocation the most stable

A

it has the most alkyl groups which can stabilise the positive charge

28
Q

state Markovnikov’s rule

A

when a hydrogen halide reacts with an asymmetrical alkene, the hydrogen of the hydrogen halide attaches itself to the carbon atom of the alkene with the most hydrogen atoms

because that forms a more stable carbocation

29
Q

how can ethene be converted into ethanol

A

ethene is reacted with steam and phosphoric acid (H3PO4) catalyst

30
Q

why is steam used rather than water in the hydration of ethene

A
  1. reactions work better at higher temperature
  2. ethene is a gas
31
Q

how can synthetic polymers be disposed of

A
  1. buried
  2. reused/recycled
  3. burned
32
Q

why are waste plastics buried

A

landfill is generally used when the plastic is difficult to separate from other waste, quantities too small to make separation financially worthwhile

  • too technically difficult to recycle
  • we need to reduce use of landfill because we produce so much waste
33
Q

how are waste plastics reused

A

many plastics are made from non-renewable oil fractions so we should try to reuse them as much as possible

  1. they can be sorted, then melted and remoulded
  2. they can be cracked into monomers and then used as organic feedstock to make more plastics/other chemicals
34
Q

when and how are waste plastics burned

A

is recycling isn’t possible, they can be burned and the heat can generate electricity

the process has to be carefully controlled to reduce toxic gases

waste gases from combustion are passed through scrubbers which can neutralise gases allowing them to react with a base

35
Q

what are biodegradable polymers and how can they decompose

A

can be made from renewable materials like starch/oil fractions, but currently more ££ than non-biodegradable equivalents
used in plastic sheets for plants in frost, poly(ethene) is embedded with starch which microorganisms nom and the plastic turns to dust

can decompose quickly as organisms can digest them but they need moisture and oxygen to decompose, e.g. a compost bin

they need to be separated from non-biodegradable plastics

36
Q

what is the general formula of an alkane

A

CnH2n+2

37
Q

what determines an alkanes boiling point

A
  1. chain length
  • alkanes have covalent bonds inside the molecules; there are induced dipole-dipole interactions that hold the molecules together
  • the interactions get stronger as the length of the chain increases because there’s more surface contact and electrons to interact
  • longer molecules need more energy to overcome the interactions and the boiling point rises
  1. branched-chain vs straight chain
  • branched alkane has a lower boiling point than straight chain as there is less surface interaction between molecules, weakening induced dipole-dipole interactions
38
Q

what are the issues with free radical substitution reactions

A

the substitution can take part at any point along the carbon chain, leading to the formation of a mixture of isomers

you can also form a mixture of products rather than solely the one you want; not only the thing you want to get substituted will be substituted, you then have to separate the product you’re looking for from the mixture

39
Q

what is the general formula of an alkene

A

CnH2n

40
Q

how does a sigma bond form and what are it’s characteristics

A

two s orbitals overlap in a straight line to give the highest possible electron density between the nuclei and form a single covalent bond

the high electron density means there is strong electrostatic attraction between the nuclei and shared electron pair so sigma bonds have high bond enthalpy

41
Q

why are alkanes not very reactive

A

sigma bonds have high bond enthalpy

the bonds are also non-polar bc hydrogen and carbon have similar electronegativity so nucleophiles and electrophiles aren’t attracted

42
Q

why are alkenes more reactive than alkanes

A

the C=C double bond contains and a sigma and pi bond
it contains 4 electrons, giving it a high electron density and the pi bond sticks out above and below the molecule so it makes the bond likely to be attacked by electrophiles

the low bond enthalpy also increases reactivity

43
Q

what conditions are needed to turn an alkene into an alkane

A

hydrogen gas and a nickel catalyst

44
Q

how can bromine water be used to test for C=C

A

shaking an alkene with bromine water will cause to to decolourise as bromine is added across the double bond to form a colourless dibromoalkane

45
Q

what conditions are needed to form halogen free radicals

A
  1. uv light
  2. high temperature
46
Q

what part of an alkene is high electron density

A

the C=C bond

47
Q

what’s the difference between a major and minor product

A

major - forms from the more stable carbocation
minor - forms from the less stable carbocation

48
Q

how are photodegradable polymers broken down

A

they absorb light to weaken chemical bonds in the polymer

they are oil based

49
Q

what is addition polymerisation

A

the reaction where many monomers with at least one C=C double bond form long polymer chains
the pi bonds break and the monomers link together to form single C-C bonds

50
Q

what are optical isomers

A

molecules that are non-superimposable and non-identical mirror images of each other
shown by molecules with a chiral
carbon

51
Q

what is a chiral centre

A

a carbon atom bonded to 4 different functional groups
aka asymmetric carbons

they exhibit optical
isomerism (a type of stereoisomerism)

52
Q

what is E/Z vs cis/trans isomerism

A

cis/trans: used only when the alkene has two different groups on each C of the C=C and each C has one of the same group (looks at identical groups)

E/Z: based on the two highest priority groups

53
Q

what are the types of stereoisomers

A

E/Z and optical only!!!!!!

54
Q

define fission

A

the breaking of a covalent bond

55
Q

is cis/trans a stereoisomer

A

no loser!!!

56
Q

what is repulsion in an orbital

A

having 2 electrons in the orbital vs just one

57
Q

how do you work out the formula of a hydrocarbon based on the CO2 formed when given mass of both

A

find moles of CO2, this is equal to the moles of carbon

use to get mass
subtract from total hydrocarbon mass to get mass of H, find moles

use to get formula

58
Q

name the method of purifying an organic solid

A

recrystallisation

59
Q

how is an organic liquid purified

A
  1. distill
  2. shake and let settle in spectating funnel and tap off each layer
  3. add anhydrous salt until stop clumping
  4. redistill

a level chemistry (15 decks)

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