Module 4 Section 1: Basic Concepts and Hydrocarbons Flashcards
(122 cards)
1
Q
All the formula types and what they show
A
General formula - algebraic formula that describes any member or family of compound
Empirical - the simplest ratio of atoms of each element in a compound (cancel numbers down if possible)
Molecular - actual number of atoms of each element in a molecule
Structural - shows atoms carbon by carbon, with the attached hydrogens and functional groups (structure of each section)
Displayed - shows how all the atoms are arranged and all the bonds between them (like ball and stick model)
Skeletal - shows the bonds of carbon skeleton only, with any functional groups. The hydrogen and carbon atoms aren’t shown (zig zag, handy for drawing large complicated structures)
2
Q
all the formulas for butan - 1 - ol
A
General: CnH(2n+1)OH
Empirical: C4H10O ( ethane, C2H6, is CH3
Molecular: C4H10O
Structural: CH3,CH2,CH2,CH2,OH or CH3(CH2)3OH
Displayed: ball and stick type ( moly mod )
Skeletal: \ /\ /OH
3
Q
What is a homologous series
A
A group of compounds that have the same functional group and general formula
Consecutive (/successive) members of a homologous series differ by -CH2
4
Q
What is the simplest homologous series called
A
Simplest is the alkanes
These are straight chain molecules that contain carbon and hydrogen atoms only
5
Q
General formula for alkanes
A
CnH(2n+2)
6
Q
What are the first 10 alkanes
A
1 carbon atom: methane CH4
2 carbon atoms: ethane C2H6
3 carbon atoms: propane C3H8
4 carbon atoms: butane C4H10
5 carbon atoms: pentane C5H12
6 carbon atoms: hexane C6H14
7 carbon atoms: heptane C7H16
8 carbon atoms: octane C8H18
9 carbon atoms: nonane C9H20
10 carbon atoms: decane C10H22
7
Q
How to name any organic compound
A
1) Count the carbon atoms in the longest continuous chain which gives you the stem
2) The main functional group of the molecule usually gives you the end of the name
3) Number the longest carbon chain so that the main functional group has the lowest possible number, if there’s more than one longest chain, pick the one with the most side chains
4) Any side chains or less important functional groups are added as prefixes at the start of the name, put them in alphabetical order with the number of the carbon atom each it attached to
5) If there is more than one identical side chain of functional group, use di- (2), tri- (3) or tetra- (4) before that part of the name - but ignore this when working in alphabetical order
8
Q
Prefix, suffix and example for alkanes
A
Prefix or suffix: -ane
E.g. propane
9
Q
Prefix, suffix and example for branched alkanes
A
Prefix or suffix: alkyl-, -yl
E.g. methyl propane
10
Q
Prefix, suffix and example for alkenes
A
Prefix, suffix: -ene
E.g. propene
11
Q
Prefix, suffix and example for haloalkanes
A
Prefix, suffix: chloro-, bromo-, iodo-, ( no suffix )
Chloroethane
12
Q
Prefix, suffix and example for alcohols
A
Prefix, suffix: hydroxy-, -ol
E.g. ethanol
13
Q
Prefix, suffix and example for aldehydes
A
Prefix, suffix: oxo-, -al
E.g. ethanal
14
Q
Prefix, suffix and example for ketones
A
Prefix, suffix: oxo-, -one
E.g. propanone
15
Q
Prefix, suffix and example for cycloalkanes
A
Prefix, suffix: cyclo-, -ane
E.g. cyclohexane
16
Q
Prefix, suffix and example for Arenes
A
Prefix, suffix: benzene
E.g. ethylbenzene
17
Q
Prefix, suffix and example for esters
A
Prefix, suffix: oxycarbonyl-, -alkyl, -anoate
E.g. propyl ethanoate
18
Q
Prefix, suffix and example for Carboxylic acid
A
Prefix, suffix: Carboxy-, -oic acid
E.g. ethanoic acid
19
Q
Difference between aromatic and aliphatic compounds
A
Aromatic compounds contain a benzene ring
Aliphatic compounds contain carbon and hydrogen joined together in straight chains, branched chains or non - aromatic chains
If an aliphatic compound contains a ( non - aromatic ) ring, then it can be called alicyclic
20
Q
Difference between saturated and unsaturated compounds
A
Saturated compounds only contain carbon - carbon single bonds - like alkanes
Unsaturated compounds can have carbon - carbon double bonds, triple bonds or aromatic groups
21
Q
What is an alkyl group
A
An alkyl group is a fragment of a molecule with general formula CnH2n+1
( looks like a bit of an alkane attached to the side of the chain )
22
Q
What are isomers and what are the two we need to know
A
Two molecules are isomers of one another if they have the same molecular formula but the atoms are arranged differently
The two isomers we need to know are structural isomers and stereoisomers
23
Q
Structural isomers and the 3 types
A
In structural isomers, the atoms are connected in different ways
Although the molecular formula is the same, the structural formula is different
3 types include: chain isomers, positional isomers and functional group isomers
24
Q
Chain isomers and examples
A
The carbon skeleton can be arranged differently - e.g. as a straight chain, or branched in different ways
These isomers have similar chemical properties
But their physical properties, like boiling point , will be different because of the change in shape of the molecule
E.g. butane and methylpropane are chain isomers of eachother
25
Positional isomers and examples
The skeleton and the functional group could be the same, only with the functional group attached to a different carbon atom
These also have different physical properties, and the chemical properties might be different too
E.g butan - 1 -ol and butan - 2 - ol
26
Functional group isomers and examples
The same atoms can be arranged into different functional groups
These can have very different chemical properties
E.g. butanoic acid and methyl propanoate
27
Prefix, suffix and example for acid halide
Prefix/ suffix: halocarbonyl-, -oyl halide
e.g. ethanoyl chloride
28
Prefix, suffix and example for amides
Prefix/ suffix: carbamoyl-, -amide
e.g. methanamide
29
Prefix, suffix and example for nitrile
Prefix/ suffix: cyano-, -nitrile
e.g. propanenitrile
30
Prefix, suffix and example for thiol
Prefix/ suffix: sulfanyl-, -thiol
e.g. propanethiol
31
Prefix, suffix and example for amine
Prefix/ suffix: amino-, -amine
e.g. ethylamine
32
Prefix, suffix and example for sulfide
Prefix/ suffix: (no prefix), -sulfanyl (so this is for when secondary)
e.g. hexanethiol
33
Prefix, suffix and example for ether
Prefix/ suffix: (no prefix), -oxy-
e.g. methoxybutane
34
What is a functional group?
A group of atoms in a molecule responsible for the characteristic reactions of the compound
35
What is the functional group priority list
Carboxylic acid
Ester
Acid halide
Amide
Nitrile
Aldehyde
Ketone
Alcohol
Thiol
Amine
Imine
Ether
Sulfide
Alkene
Alkyne
Haloalkane
Nitro
36
What is heterolytic fission
The bond breaks unevenly with one of the bonded atoms receiving both electrons from the bonded pair
Two different substances are formed - a positively charged cation ( X+ ), and a negatively charged anion ( Y- )
37
What is homolytic fission
The bond breaks evenly and each bonding atom receives one electron from the bonded pair
Two electrically uncharged radicals are formed
38
What are radicals
Particles that have an unpaired electron
They are shown in mechanisms by a big dot next to the molecular formula
Radicals are very reactive due to the unpaired electron
39
How do halogens react with alkanes
Halogens react with alkanes in photochemical reactions
These are started by light - this requires ultraviolet light to start
A hydrogen atom is substituted by chlorine or bromine
This is a free radical substitution reaction
E.g. CH4 + Cl2 —UV—> CH3Cl + HCl
40
What are the three stages of reaction mechanism
Initiation reactions
Propagation reactions
Termination reactions
41
What happens in initiation reactions
Free radicals are produced by homolytic fission
UV provides energy to break Cl-Cl bond - this is photodissociation: Cl2 —UV-> 2Cl
The atom becomes a highly reactive free radical, Cl •, because of its unpaired electron
42
What happens in propagation reactions
Free radicals are used up and created in a chain reaction
Cl • attacks a methane molecule: Cl • + CH4 -> •CH3 + HCl
The new methyl free radical, •CH3, can attack another Cl2 molecule: •CH3 + Cl2 -> CH3Cl + Cl •
The new Cl • can attack another CH4 molecule, and so on, uncial all the Cl2 or CH4 molecules are gone
43
What happens in termination reactions
Free radicals are mopped up
If two free radicals join together, they make a stable molecule
There are lots of possible termination reactions
E.g.
Cl • + •CH3 -> CH3Cl
•CH3 + •CH3 -> C2H6
44
What’s the problem with free radical substitution
You do not always get the desired product, instead you get a mixture of products
E.g. if you’re trying to make chloromethane and there’s too much chlorine in the reaction mixture, some of the remaining hydrogen atoms on the chloromethane molecules will be swapped for chlorine atoms
The propagation reactions happens again to make dichloromethane, again to make trichloromethane and again to make tetrachloromethane
Also, it can take place at any point along the carbon chain
So a mixture of isomers can be formed e.g. reacting propane with chlorine will produce a mixture of 1-chloropropane and 2-chloropropane
45
How to reduce the problems with free radical substitution
Have an excess of methane so there’s a greater chance of a chlorine radical colliding only with a methane molecule and not a chloromethane molecule
46
What are alkenes and what is their structure
Unsaturated compounds with the general formula CnH2n
They are hydrocarbons as they are made of carbon and hydrogen atoms only
Alkenes all have at least one C=C double covalent bond
C=C double bonds make the molecule unsaturated as they can make more bonds with extra atoms in addition reactions
47
What is a σ bond and when is it formed
σ bond ( sigma bond ) is formed by two s orbitals direct overlap
The two s orbitals overlap in a straight line - gives the highest possible electron density between the two nuclei
This is is a single covalent bond
The high electron density between the nuclei means there is a strong electrostatic attraction between the nuclei and the shared pair of electrons
This means σ bonds have a high bond enthalpy - strongest type of covalent bonds
48
What are π bonds
π ( pi ) bonds are formed by sideways overlap of two adjacent p orbitals
It’s got two parts: one above and below the molecular axis
This is because the p orbitals which overlap are dumb bell shaped
π bonds are much weaker than σ bonds as the electron density is spread out above and below the nuclei
This means the electrostatic attraction between the nuclei and the shared pair of electrons is weaker, so π bonds have relatively low bond enthalpy
49
How are alkanes less reactive than alkenes
Alkanes only contain C - C and C - H σ bonds, which have a higher bond enthalpy and so are difficult to break
The bonds are also non-polar so they don’t attract nucleophiles or electrophiles
This means that they don’t react easily
50
How are alkenes more reactive than alkanes
Alkenes are more reactive than alkanes because the C = C bond contains both a σ bond and a π bond
The C = C double bond contains four electrons so has a high electrons density and the π bond also sticks out above and below the rest of the molecule
This means the π bond is likely to be attacked by electrophiles
The low bond enthalpy of the π bond also contributes to the reactivity of alkenes
Because the double bond’s so reactive, alkenes are good starting points for making other organic compounds or petrochemicals
51
How are carbon atoms in a C =C bond arranged
Carbon atoms in a C = C bond and atoms bonded to these carbons all lie in the same plane ( they’re planar )
This means that they are trigonal planar - atoms attached to each double-bonded carbon are at the corners of an imaginary equilateral triangle
However, smaller molecules like ethene are completely planar whereas in larger alkenes, only the C=C unit is planar
52
Why can’t the C=C rotate
The atoms can’t rotate around them like they can around single bonds
This is because of the way the p orbitals overlap to form a pi bond
This means they are rigid and don’t bend much either
However, the atoms can still rotate around the single bonds in the molecule
This restricted rotation around the C=C causes alkenes to form stereoisomers
53
What are stereoisomers
Have the same structural formula but a different arrangement in space
This happens because of the lack of rotation around the double bond
They occur when the two double bonded carbon atoms each have two different atoms or groups attached to them
One these isomers is called the E-isomer and the other is called the Z-isomer
54
Difference between E and Z isomers
The Z-isomer has the same groups either both above or both below the double bond
The E-isomer has the same groups positioned across the double bond
55
E/Z isomers of But-2-ene
Z isomer: same groups are both above double bond - Z-but-2-ene
H3C CH3
\ /
C = C
/ \
H H
E isomer: same groups across double bond - E-but-2-ene
H CH3
\ /
C = C
/ \
H3C H
56
What does the Z and E mean is stereoisomerism
Z stands for Zusammen: German for “together”
E stand for Entgegen: German for “opposite”
57
What are the CIP rules and how are these used
Cahn-Ingold-Prelog rule can work out which is the E isomer and which is the Z isomer for any alkene
58
How to use the CIP rules
1. Look at the atoms directly bonded to each of the C=C atoms
The atom with the higher ATOMIC NUMBER on each carbon is given higher priority
2. You can assign the isomers as E and Z by looking at how the groups of the same priority are arranged
3. You must be cautious if doing this for an alkene with only 3 different groups
The E/Z system gives the positions of the highest priority group on each carbon, which aren’t always the matching groups
59
Using the CIP rules for 1-bromo-1-chloro-2-fluoro-ethene
F Cl
\ /
C = C
/ \
H Br
1. Atoms attached to carbon one are bromine ( AN 35 ) and chlorine ( AN 17 ), bromine is higher priority
Atoms attached to carbon 2 are fluorine ( AN 9 ) and hydrogen ( AN 1 ), fluorine is higher priority
2. Higher priority groups are positioned across double bond so this is an E isomer
60
When may you have to look further along the chain to decide between E/Z isomers
If the atoms directly to the carbon are the same then you may have to look at the next atom in the group to work out priority
CH3
|
CH2 Cl
\ /
C = C
/ \
H3C Br
The first carbon is directly bonded to two carbon atoms, so you need to go further along the chain
The methyl carbon only has hydrogen atoms but the ethyl carbon is attached to another carbon atom so it has priority
61
What are cis-trans isomers
If the carbon atoms have at least one group in common ( like but-2-ene ), then you can call them cis or trans ( as well as E or Z )
62
What’s the difference between cis and trans isomers
Cis means the same groups are on the same side of the double bond
Trans means the same groups are on the opposite sides of the double bond
So E-but-2-ene can be called trans-but-2-ene and Z-but-2-ene can be called cis-but-2-ene
63
When can’t the cis-trans method work
If the carbon atoms both have totally different groups attached to them
64
What happens in an electrophilic addition reaction
The alkene double bond opens up and atoms are added to the carbon atoms
65
Why do electrophilic addition reactions occur
They happen because the double bond has got lots of electrons and is easily attacked by electrophiles
66
What are electrophiles
They are electron pair acceptors - they are usually short of electrons
This means they are attracted to areas where there are lots of electrons
67
What type of atoms or molecules are electrophiles
Positively charged ions ( e.g. H+ NO2+ )
Polar molecules ( partially positive atom is attracted to electrons
Polarisable bonds ( e.g. Br-Br, Cl-Cl )
68
What happens when hydrogen is added to alkenes
Adding hydrogen to C=C bonds produces alkanes
E.g. ethene will react with hydrogen gas in an addition reaction to produce ethane
This needs a nickel catalyst and a temperature of 150°C
69
What do halogens react with alkenes to make and what type of reaction is this
Halogens react with alkenes to form dihaloalknes
Halogens add across the double bond and each carbon atoms originally in the bond will bond to a halogen atoms
This is electrophilic addition reaction
70
General equation for hydrogen and alkenes
H2C=CH2 + H2 = CH3CH3
71
General equation for halogens and alkenes
H2C=CH2 + X2 = CH2XCH2X
72
Mechanism for the creation of a dihaloalkanes from alkenes ( electrophilic addition )
E.g. ethene and bromine molecules
Double bond repels electrons in Br2 which polarises Br - Br
Double bond donates electron pair to delta positive Br
Heterolytic fission of Br2 - closer Br gives up bonding electrons to the delta negative Br and bonds to carbon atom
Creates positively charged carbocation ( intermediate)
Br donates lone pair to carbocation, bonding to it, making 1,2-dibromoethane
73
What is a carbocation
Organic ion containing a positively charged carbon atom
74
How to test for carbon double bonds
When you shake an alkene with orange bromine water, the solution quickly decolourises
This is because the bromine is added across the double bond to form a colourless dibromoalkane
75
What type of molecules or atoms are nucleophiles
Anions ( Br-, OH- )
Pi bonds ( C=C )
Atoms with lone pairs ( H2O, NH3 )
76
How are alcohols made by steam hydration
Alkenes can be hydrated by steam at 300°C and a pressure of 60-70 atm
This needs a solid phosphoric acid catalyst
The reaction is reversible and the yield is low ( with ethene it’s 5% ) but the gas can be recycled which increases the yield much more
77
What are alkanes
Saturated hydrocarbons
General formula CnH2n+2
They are hydrocarbons as they only have carbon and hydrogen atoms
78
Why are alkanes saturated
Every carbon atom in an alkane has four single bonds with other atoms
All the carbon-carbon bonds are single bonds
79
What shape are alkane molecules around each carbon
Each atom has four pairs of bonding electrons around it
They all repel each other equally so the molecule forms a tetrahedral shape around each carbon
Each bond angle is 109.5°
80
What inter/intramolecular forces are present in alkanes
Alkanes have covalent bonds inside the molecules
Between the molecules, there are induced dipole-dipole interactions which hold them together
81
Trends with size of alkanes and boiling points
Smallest alkanes are gases at room temperature and pressure - very low boiling points
Larger alkanes are liquids at RTP - higher boiling points
The longer the carbon chain, the stronger induced dipole-dipole interactions
This is because there’s more surface contact and more electrons to interact
82
How does the boiling point increase with chain length
As the molecules get longer, it takes more energy to overcome the induced dipole-dipole interactions, and the boiling points rises
83
Why do branched alkanes have a lower boiling points than their straight chain isomers
Branched alkanes can’t pack closely together (so there is less surface contact) and they have smaller molecular surface areas - so the induced dipole-dipole interactions are reduced
84
What happens when you burn alkanes in oxygen
A combustion reaction occurs which produces carbon dioxide and water
85
What state do combustion reactions occur at
They happen between gases, so liquid alkanes have to be vaporised first
Smaller alkanes turn into gases more easily, so they’ll burn more easily too
86
Why do larger alkanes release more energy
Release more energy per mole because they have more bonds to react
87
Why are alkanes so good as fuels
They release lots of energy when they burn
88
How to use volumes to work out combustion equations
Example: 30cm3 of hydrocarbon X combusts completely with 240cm3 of oxygen. 150cm3 of carbon dioxide is produced, what is the molecular formula of hydrocarbon X?
Example: 30cm3 of hydrocarbon X combusts completely with 240cm3 of oxygen. 150cm3 of carbon dioxide is produced, what is the molecular formula of hydrocarbon X?
Using volumes provided, write a reaction equation 30X + 240O2 = 150CO2 + ?H2O
Can be simplified to X + 8O2 = 5CO2 + nH2O
8 moles of oxygen reactions to form 5 moles of CO2 and n moles of H2O. This means that n = (8x2) - (5x2) = 6
Combustion reaction is now X + 8O2 = 5CO2 + 6H2O - can be used to identify X
All carbon atoms from X form carbon dioxide molecules, and all hydrogen atoms from X are used to form water, so the number of carbon atoms in X is 5 and the number of hydrogen atoms is 12
Molecular formula of X is C5H12
89
What happens when the alkane is burned in limited oxygen
The alkanes will still burn, but it will produce carbon monoxide and water
90
How is carbon monoxide poisonous
The oxygen in your bloodstream is carried around by haemoglobin
Carbon monoxide is better at binding to haemoglobin than oxygen is, so it binds to the haemoglobin in your bloodstream before the oxygen can
This means less oxygen can be carried around your body, leading to oxygen deprivation
At very high concentration, carbon can be fatal
91
How do alkenes form addition polymers
The double bonds in alkenes can open up and join together to make long chains called polymers
92
What are the small alkenes called in addition polymers
Monomers
93
What is an example of addition polymerisation
Poly(ethene) is made by the addition polymerisation of ethene
94
How to draw displayed formula of polymers
Monomer:
H H
\ /
C = C n
/ \
H H
Polymer:
H H
\ /
-(- C - C -)-
/ \
H H n
Side links shows that both sides are attached to other units
The molecule in brackets is the repeating unit
n represents the number of repeat units
95
How does addition polymerisation occur
Alkenes must be heated at a high temperature and high pressure to open up their double bond and polymerise into long chains of repeating units
96
Name and uses of pol(chloroethene)
Also known as poly(vinyl chloride) or PVC
Can be used for: pipes, films and sheeting, bottles
97
Uses for poly(propene)
Children’s toys, guttering, rope fibres
98
Name and uses for poly(phenylethene)
Also called poly(styrene)
Can be used in packing material, insulating food trays and cups
99
Name and uses for poly(tetrafluoroethene)
Also called Teflon or PTFE
Can be used as coating for non stick pans, cable insulation
100
How to dispose of polymers
Burying
Reusing
Burning
101
How can waste plastics be buried and what should we do about this
Landfill is can be used to deal with waste plastics
Because the amount of waste generated is becoming more of a problem, there’s a need to reduce landfill
102
When is landfill used
Used when the plastic is difficult to separate from other waste
When the plastic is not in sufficient quantities to make separation financially worthwhile
When the plastic is difficult technically to recycle
103
Why is reusing plastics beneficial for the environment
Many plastics are made from non-renewable oil fractions so it’s beneficial to reuse plastics as much as possible
104
Different ways in which plastics can be reused
Can be recycled by melting and remoulding (e.g. polypropene)
Some can be cracked into monomers and used as organic feedstock to make more plastics or other chemicals
105
How can plastics be burned
Can be burned to produce heat used for electricity (if recycling isn’t possible)
Process must be controlled to reduce toxic gases (e.g. HCl can be produced when burning PVC)
106
How are waste gases from burning plastics controlled or removed
Passed through scrubbers which neutralise gases such as HCl by allowing them to react with a base
107
How do biodegradable polymers decompose
Organisms can digest them to form water, carbon dioxide and biological compounds
108
What can biodegradable polymers be made from
Made from renewable resources such as starch or oil fractions
These are more expensive than non-biodegradable equivalents
109
How to use biodegradable polymers responsibly
Do not decompose in landfill as there’s a lack of moisture and oxygen
Must be placed on a compost heap
Must be collected and separated from non-biodegradable plastics
110
Uses of biodegradable polymers
Can be used as sheeting to cover plants from frost
Made from polyethene with starch grains embedded in
Starch is broken down by microorganisms and remaining polyethene crumbles into dust
Scientists started to produce photodegradable polymers which decompose when exposed to sunlight
111
What are bioplastics made from
Produced from plant starch, cellulose, plant oils and proteins
112
Difference between primary, secondary and tertiary carbocations
Primary: positive carbon atom is connected to 1 other carbon atom
Secondary: positive carbon atom is connected to 2 other carbon atoms
Tertiary: positive carbon atom is connected to 3 other carbon atoms
113
How are the major and minor products formed from alkene addition reactions
In addition reactions of an unsymmetrical alkene, the major product is formed from the secondary carbocation
E.g. propene + HBr -> 2-bromopropane (major)
Whereas the primary carbocation intermediate goes on to form the minor product
E.g. propene + HBr -> 1-bromopropane (minor)
114
How do the yields between major and minor products compare
Yield of major product is much high than yield of minor product
115
How does markownikoff’s rule occur
When an unsymmetrical alkene (e.g. propene) reacts with an unsymmetrical molecules (e.g. hydrogen bromide)
116
How are carbocations classified
Classified by the number of R groups attached to positive carbon atom
The more R groups attached to the positive carbon atom, the more stable is it
Primary is the least stable, tertiary is the most
117
Why do more R groups make a carbocation more stable
Each alkyl group donates and pushes electrons towards positive charge of the carbocation
So the positive charge is spread over the alkyl groups
The more alkyl groups attached to the positively charged carbon atom, the more the charge is spread out
This makes the ion more stable
118
Why are the yields between major and minor product different
The major product is the most stable so more molecules form these molecules to achieve the most stable connection
119
What is markownikoff’s rule summarised as
The major product from an addition of a hydrogen halide (HX) to an unsymmetrical alkene is the one where the hydrogen adds to the carbon with the most hydrogen already attached
120
Equation for substitution involving haloalkane and ammonia
R - X + NH3 -> R - N+H3 + Br-
R - N+H3 + NH3 -> R - NH2 + NH4+
121
Mechanism of nucleophilic substitution from haloalkane to amine
Ammonia acts as Nucleophile and donates electrons to partially positive carbon
Heterolytic fission of C - X bond
Ammonia bonds onto carbon chain producing an amine salt with NH3+ at the end
Second ammonia nucleophile causes a hydrogen to split heterolytically and donate electron to N+ so charge is neutral
Hydrogen ion (proton) bonds to ammonia and forms NH4+ as other product
122
Mechanism of haloalkane to nitrile
CN- donates electron to carbon in C-X bond
Heterolytic fission of C-X bond
CN joins onto carbon chain and substitutes X
