module 4: alkenes and alkanes

Question 1

what is homolytic fission

Answer 1

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

Question 2

what are free radicals

Answer 2

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

Question 3

what is heterolytic bond breaking

Answer 3

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

Question 4

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

Answer 4

the more electronegative substance takes the electrons

Question 5

what is an electrophile

Answer 5

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

Question 6

what is a nucleophile

Answer 6

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

Question 7

define stereoisomer

Answer 7

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

• E/Z isomers
• optical isomers

Question 8

what is E/Z isomerism

Answer 8

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

Question 9

when are the CIP rules used

Answer 9

when naming stereoisomers

Question 10

what are the CIP rules

Answer 10

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

Question 11

what does the complete combustion of an alkane result in

Answer 11

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

Question 12

what does the incomplete combustion of an alkane result in

Answer 12

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

Question 13

how and under what conditions do alkanes react with halogens

Answer 13

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

Question 14

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

Answer 14

1. initiation
2. propagation
3. termination

Question 15

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

Answer 15

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

Question 16

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

Answer 16

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

Question 17

what are the conditions for hydrogenation and what does it do

Answer 17

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

an alkene is turned into an alkane

Question 18

when is hydrogenation used in life

Answer 18

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

Question 19

what is the bond angle around double bonds

Answer 19

120˚, trigonal planar

Question 20

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

Answer 20

109.5˚, tetrahedral

Question 21

why do alkenes form stereoisomers

Answer 21

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

Question 22

what happens when an alkene reacts with a halogen

Answer 22

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

Question 23

what does a tertiary carbocation look like

Answer 23

Question 24

what does a primary carbocation look like

Answer 24

Question 25

what does a secondary carbocation look like

Answer 25

Question 26

order the carbocations from most to least stable

Answer 26

tertiary secondary primary

Question 27

why is the tertiary carbocation the most stable

Answer 27

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

Question 28

state Markovnikov's rule

Answer 28

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

Question 29

how can ethene be converted into ethanol

Answer 29

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

Question 30

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

Answer 30

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

Question 31

how can synthetic polymers be disposed of

Answer 31

1. buried 2. reused/recycled 3. burned

Question 32

why are waste plastics buried

Answer 32

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

Question 33

how are waste plastics reused

Answer 33

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

Question 34

when and how are waste plastics burned

Answer 34

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

Question 35

what are biodegradable polymers and how can they decompose

Answer 35

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

Question 36

what is the general formula of an alkane

Answer 36

CnH2n+2

Question 37

what determines an alkanes boiling point

Answer 37

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 2. 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

Question 38

what are the issues with free radical substitution reactions

Answer 38

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

Question 39

what is the general formula of an alkene

Answer 39

CnH2n

Question 40

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

Answer 40

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

Question 41

why are alkanes not very reactive

Answer 41

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

Question 42

why are alkenes more reactive than alkanes

Answer 42

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

Question 43

what conditions are needed to turn an alkene into an alkane

Answer 43

hydrogen gas and a nickel catalyst

Question 44

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

Answer 44

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

Question 45

what conditions are needed to form halogen free radicals

Answer 45

1. uv light 2. high temperature

Question 46

what part of an alkene is high electron density

Answer 46

the C=C bond

Question 47

what's the difference between a major and minor product

Answer 47

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

Question 48

how are photodegradable polymers broken down

Answer 48

they absorb light to weaken chemical bonds in the polymer they are oil based

Question 49

what is addition polymerisation

Answer 49

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

Question 50

what are optical isomers

Answer 50

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

Question 51

what is a chiral centre

Answer 51

a carbon atom bonded to 4 different functional groups aka asymmetric carbons they exhibit optical isomerism (a type of stereoisomerism)

Question 52

what is E/Z vs cis/trans isomerism

Answer 52

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

Question 53

what are the types of stereoisomers

Answer 53

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

Question 54

define fission

Answer 54

the breaking of a covalent bond

Question 55

is cis/trans a stereoisomer

Answer 55

no loser!!!

Question 56

what is repulsion in an orbital

Answer 56

having 2 electrons in the orbital vs just one

Question 57

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

Answer 57

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

Question 58

name the method of purifying an organic solid

Answer 58

recrystallisation

Question 59

how is an organic liquid purified

Answer 59

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