Reaction Mechanisms Flashcards
1
Q
What does a reaction mechanism describe?
A
- how a reaction occurs and the steps involved
- the series of bond breaking and bond making steps that occur in a reaction
2
Q
what is the first step in any reaction mechanism?
how can bonds be broken?
A
- breaking bonds
- via homolytic or heterolytic fission
3
Q
what happens in homolytic fission?
what does homolytic fission form?
A
- the bond breaks equally and the electrons from the covalent bond are shared between the two atoms involved
- free radicals
4
Q
what are free radicals?
A
- are atoms with an unpaired electron
- they are very reactive particles
5
Q
what are intermediates?
A
- intermediates are a species that are formed in one step of a reaction mechanism and used in another step
6
Q
what is heterolytic fission
what does it form?
A
- the electrons from the bond are transferred to the more electronegative atom
- an electron-deficient atoms (positive) also known as electrophiles
- and an electron pair donor called a nucleophile
7
Q
alkanes react with…?
halogenalkanes react with…?
A
- electrophiles
- nucleophiles
8
Q
what else can be formed in heterolytic fission?
A
a positive carbon atom called a carbocation this only forms 3 bonds
9
Q
Alkanes react with chlorine via what reaction mechanism?
A
Free radical substitution mechanism
10
Q
What does the chlorination of alkanes produce?
A
A mixture of chlorinated alkanes and hydrogen chloride
11
Q
what conditions are required for chlorine to react with methane?
A
in the presence of ultraviolet light - will react explosively even at room temp or heat
12
Q
what is the equation for the chlorination of methane?
A
CH4 + CL2 ——–> CH3CL + HCL
methane + clorine —> chloromethane + hydrochloric acid
13
Q
what type of reaction is this?
A
substitution reaction - 1 of the chlorine atoms replaces one hydrogen
14
Q
this process occurs in three stages?
A
1.initiation
2. propagation
3. termination
15
Q
initiation step 1
A
the UV light provides the energy to break the covalent bonds in some of the chlorine molecules to produce free radicals by homolytic fission
CL2 —UV—–> CL*
16
Q
why is cl-cl bonds broken first then c-h?
A
cl-cl bonds are weaker
17
Q
propagation step 1
A
a highly reactive chlorine radical reacts with a methane molecule
cl* + CH4 ———> CH3* + HCL
18
Q
Propagation step 2
A
the methyl radical can then attack another chlorine molecule to produce another chlorine free radical
CH3* + CL2 ——-> CH3CL + CL*
19
Q
why is this a chain reaction?
A
in each propagation step, a radical is used up and a new radical is formed. hence chain reaction. the chlorine radicals are made and used until there is no more chlorine to react
20
Q
what is the overall reaction?
A
is the sum of the 2 propagation steps
CH4 + Cl2 ——> CH3Cl + HCl
21
Q
when does the reaction stop and ecome stable?
A
when 2 radicals combine, the product will be a stable molecule and the sequence of the reaction stops.
-once all the intermediates have been used up the reaction is finished
22
Q
termination step 1
A
the unpaired electrons in the radicals pair up to form a covalent bond e.g
CH3* + CL* ——> CH3CL
CH3* + CH3* ——-> CH3CH3
23
Q
What do the termination steps do?
A
‘mop’ up free radicals preventing further reactions
24
Q
why can chloromethane be further substituted?
A
- contains 3 hydrogen atoms that can be substituted via further pairs of propagation steps
25
further substitutions can lead to the formation of:
- dichloromethane (CH2Cl)
- trichloromethane (CHCl3)
- tetrachloromethane (CCL4)
26
How can further substitution be reduced?
By using an excess of methane
27
however industrial chemists would not use this method to prepare a sample of chloromethane why?
- the reaction is too messy.
- too many by-products are formed by further substitution and removal would be costly
better to prepare halogenalkanes from alkenes
28
trends in rate og halogenalkanes?
- methane reacts less readily with halogens as you go down group 7.
- as halogens are less reactive as you go down
-
29
why is chlorination of alkanes harmful to the ozone layer?
- some organic chloro-compounds escape into the atmosphere and eventually decompose to form chlorine free radicals
- these radicals react with ozone and contribute to the depletion of the ozone layer
30
What may protect the body from free radicals?
antioxidants are thought to protect the body from free radicals which may cause cell damage
31
uses of halogenalkanes?
solvents, refrigerants, aerosol propellants , insecticides and anaesthetics
32
why has their uses become controversial?
recent evidence has shown some of these halogen alkanes are toxic and have harmful effects on the atmosphere
33
what halogenoalkanes are used commonly but now banned?
1,1,1 - tricholorethane - used in tippex and as a dry-cleaning solvent
tetrachloromethane- used as a dry-cleaning solvent
34
why are CFC's very unreactive?
because c-f and c-cl bonds are very strong
35
properties of CFC's?
stable compounds, have not taste, no smell, usually gases at room temp - useful
36
what could they be used as?
refrigerants, aerosol propellants and in packaging materials such as expanded polystyrene
37
Since CFC's were considered very stable and unreactive they were....?
vastly released into the atmosphere and made
38
what is ozone? any distinct properties?
ozone is an allotrope of oxygen (03)
blue pale gas with a sharp smell
39
the ozone in the upper atmosphere is continually being formed and broken down by?
intense ultraviolet radiation
40
how is ozone formed and what does it break down into? (step 1)
ozone is formed when ultra-violet radiation from the sun breaks down oxygen molecules into 2 oxygen radicals
O2 ---uv----> 2 O*
41
2nd step into making ozone?
these oxygen radicals then react with more oxygen to form ozone
O* + O2 -------> O3
42
then with ultraviolet radiation ozone decomposes into...?
an oxygen molecule and an oxygen radical
O3 ----uv------> O2 + O*
43
then these oxygen radicals can react with more ...? to reform..?
ozone to reform O2 molecules
O* + O3 ------> 2O2
44
what is the overall result of these actions?
the presence of ozone in the upper atmosphere reduces the amount of harmful ultraviolet radiation from the sun that can reach the Earth's surface
so ozone formed naturally in the upper atmosphere is beneficial
45
what was a problem with the ozone layer?
it was becoming thinner - 1970s found that ozone layer in antartica was thinner than expected
46
what was the reason for the destruction of the ozone layer?
reaction of ozone with chlorine radicals
47
why are CFC's able to diffuse into the upper atmosphere?
they are extremely unreactive so do not get broken down by reacting with oxygen or water so they're are able to diffuse into the upper atmosphere
48
what happens in the upper atmosphere?
in the upper atmosphere there are large amounts of uv radiation, so the large amounts of uv radiation breaks down the C-CL bonds and chlorine radicals are produced
CF2CL2 ----uv------> CL* + *CF2CL
49
Why can the uv radiation not break down the C-F bonds?
uv radiaiton does not have sufficient energy to breaks down c-f bonds
50
then propagation steps occur?
CL* + O3 ----> ClO* + O2
CIO* + O3 ------> CL* + 2O2
51
Why are chlorine radicals not used up in these steps?
chlorine radicals are used up in the 1st step but are reformed in the 2nd step ∴ not used up
52
this means that...?
the chlorine radicals CATALYSE the decomposition of ozone and contribute to the formation of a hole in the ozone layer.
because the chlorine radicals are not used up it means that small amounts of chlorine radicals can continue to destroy ozone for many years
53
what was the Montreal protocol (1987)?
chemists supported the international agreement to ban the use of CFC's
54
what is an alternative to CFC's?
chlorine free CFC's called HFC'S
55
what are HFC's?
contain hydrogen flourine and carbon
56
why is this better?
because C- F bonds are stronger than c-cl bonds ∴ less likely to be broken down by ultraviolet radiaiton
57
what is the general formula for halogenoalkanes?
CnH2n+1 X
X = halogen
58
uses of halogenoalkanes?
refrigerants, solvents, pharmaceuticals
59
how are halogenoalkanes classified?
with primary, secondary, or tertiary depending on the number of carbon atoms directly attached to the carbon with the halogen atom
60
the halogen in the halogenoalkane is more electronegative so the bonds are...?
polar
61
explain how halogenoalkanes are polar?
the halogen is more electronegative, electrons are attracted towards the halogen atom.
the halogen becomes slightly negative , the carbon atom becomes slightly positive (electron deficient)
62
polar bonding means..?
halogenoalkanes have permanent dipole dipole forces between neighbouring molecules
63
what is the polar c-halogen bond responsible for?
chemical reactions of halogenoalkanes
hence makes them more reactive than alkanes
64
what are nucleophiles?
nucleophiles are a species that donates a lone pair to form a covalent bond
65
what happens in nucleophilic substitution?
- the lone pair on the nucleophile forms a covalent bond with the δ+ carbon atom
- the carbon halogen breaks via heterolytic fission
- the pair of electrons from the carbon-halogen bond pass to the carbon atom - forming a halide ion
-
66
What is nucleophilic substitution also referred to as?
hydrolysis
67
why do halogenoalkanes get more reactive as you go down group 7?
- bonds get weaker and are more easily broken , so more reactive the halogen
68
why does rate of reaction increase as you go down the group?
- the carbon-halogen bond energy decreases down the group
- therefore rate of reaction with nucleophiles increases down group 7
69
since C-F is very polar so why is it unreactive?
because the c-f bonds is so strong that it becomes very unreactive and do not undergo nucleophilic substitution
70
why are halogenoalkanes used in the synthesis?
- they can be prepared from different compounds
- they can be changed into many different homologous series
71
what type of ions does halogenoalkanes undergo nucleophilic substitution with?
- hydroxide ions (OH-)
- cyanide ions (CN-)
- ammonia (NH3)
72
explain what happens when halogenoalkanes react with warm aqueous sodium hydroxide/potassium hydroxide?
when halogenoalkanes react with warm aqueous sodium hydroxide or potassium hydroxide ALCOHOLS are formed
73
draw the reaction between 1-bromopropane and koh? [WHITEBOARD]
CH3CH2CH2Br + OH- -----------> CH3CH2CH2OH (PROPAN-1-OL)+ Br-
74
explain the product formed when halogenoalkanes are reacted with an aqueous alcoholic solution of potassium/ sodium cyanide
nitriles are formed
75
draw the reaction mechanism of iodopropane with KCN
CH3CH2CH2I + CN- --------------> CH3CH2CH2CN + I-
76
hydrolysis of nitriles?
nitriles are readily hydrolysed in acid solution to form carboxylic acids
77
propanetrile hydrolysis equation?
CHH3CH2CN + 2H20 + HCL ------------->
CH3CH2COOH + NH4CL
78
draw the Nitrile and carboxylic acid structure and ammonia structure
79
what products are formed when halogeneoalkanes are reacted with excess ammonia (warmed in a sealed container)?
primary amines are formed
80
draw the reaction mechanism with 1-chloropropane and 2NH3
[step 1]CH3CH2CH2CL + 2NH3 -------->
CH3CH2CH2NH2 (propylamine or 1-aminopropane)+ NH4CL
[step2] - a molecule of ammonia acts as a base so accepts h+ ions
81
structure of amines?
whiteboard drawing and answer in folder
82
what can amines do?
amines have a lone pair of electrons on the nitrogen atom so can act as nucleophiles themselves
83
why do we use an excess of ammonia
to prevent successive substitution. the excess ammonia reduces the chance of further reaction of the primary amine with the halogenoalkane to form secondary or tertiary amines or quaternary ammonium salts
84
nucleophillic substitution and elimination
85
when a halogenoalkane reacts with a hydroxide ions two types of reactions can occur?
- nucleophillic substitution
- elimination
86
the elimination reaction is favoured by...?
using ethanol as a solvent, using a higher temperature and by increased branching of the halogenoalkane ( so tertiary and also secondary)
87
the substitution reaction is favoured by...?
using warm aqueous alcoholic as the solvent
88
however, secondary alkanes undergo...?
primary undergo...?
- both substitution and elimination concurrently
- mainly substitution and very little elimination
89
outline the mechanism for the reaction between 2-bromobutane and potassium hydroxide (substitution and elimination)
Substitution forms - butan-2-ol
elimination forms - But-2-ene and But-1-ene
90
What happens when 2 - bromobutane is reacted with warm aqueous sodium hydroxide?
a mixture of organic products are formed - butan-2-ol, But-2-ene and But-1-ene
91
what is the general formula for alkenes
CnH2n
92
the carbon - carbon double bond is...?
planar and has an angle of 120 degrees
93
why are alkenes very reactive?
because the carbon-carbon double bond is electron rich. the double bond can therefore attack electron deficient species
94
in the carbon-carbon double bond there are 2 types of covalent bonds?
- (sigma) σ bond
- (pi) π bond
95
explain how they differ?
the differ in the way that the orbitals overlap so the electrons are shared differently in the two different bonds
96
what is the sigma bond?
it can be thought of as a conventional covalent bond
97
what is the π bond?
is a cloud of electron density above and below the molecule. the π bond is formed by the overlap of a spare unbonded, singly filled p-orbital present on each carbon atom in the bond
98
why is the carbon-carbon double bond planar?
due to the p-orbitals overlap leading to the π bond formation
99
E-Z stereoisomerism
(a.k.a geometrical isomerism or cis-trans isomerism)
100
what are stereoisomers?
stereoisomers are molecules which have the same structural formula but their bonds are arranged differently in space
101
what leads to the formation of e-z stereoisomers?
the double bonds can not rotate due to the π electron clouds above and below the plane of the bond. a large amount of energy is needed to rotate a double bond (not enough energy at room temperature) so we say the c=c has a restricted rotation leading to the formation of e-z stereoisomers
102
how do we differentiate between e-z stereoisomers?
Z form - both groups are on the SAME side of the double bond
E form - both groups are on the OPPOSITE side of the double bond
103
how do we differentiate e-z stereoisomers from more complex alkenes?
using Cahn-Ingold Priority rules
1. label the two carbon atoms of the c=c double bond
2. look at the two other atoms that are directly bonded to c1 and assign them a priority (higher or lower).
(the atom with the highest atomic number has the highest priority )
3. look at the other 2 atoms that are directly bonded to c2 and assigns them a priority
4. to work out their e-z isomer look at the positions of the higher priority groups
z form - higher priority groups are on the same side
e form - the higher priority groups are on the opposite side
104
explain the different properties of e-z isomers?
- z- isomers have higher boiling points because they are more polar than e-isomers
- e-isomers have a higher melting point because they pack together more closely than z-isomers
105
why can alkenes undergo combustion?
because they are hydrocarbons
106
what is the difference between complete and incomplete combustion?
- complete combustion occurs in a plentiful supply of oxygen whereas incomplete combustion occurs in a insufficient supply of oxygen.
- the products of complete combustion is c02 and water
- the products of incomplete combustion is carbon monoxide and water
107
Electrophilic Addition
108
why can alkenes undergo electrophilic addition?
- the alkene c=c double bond is electron rich
- the double bond can therefore attack electron deficient species (electrophiles)
- electrophiles have a partial positve charge and accepts a pair of electrons .
- alkenes undergo an addition reaction with an electrophile to make a saturated product
109
outline the reaction mechanism for the reaction of ethene with hydrogen bromide
- the hBr molecule ha a permanent dipole with a δ+ charge on the hydrogen atom
- a pair of electrons from the c=c double ond forms a bond with the δ+ hydrogen atom.
- the hydrogen-bromine bond breaks , leaving a bromide ion
- he addition of hydrogen to the alkene forms a carbocation intermediate
- carbocations are very reactive and are readily attacked by nucleophiles
- the carbocation is attacked by the bromide ion that acts as a nucleophile
110
What are the reactions conditions for reacting and alkene with H-Br?
- aqueous solutions / gas phase
111
alkenes react with cold concentrated sulphuric acid to form...?
alkyl hydrogensulphates
112
draw and outline the reaction mechanism of sulphuric acid to ethene
- electrophilic addition
- (forms ethyl hydrogensulphate)
113
hydrolysis of alkyl hydrogen sulphates with warmed water produces...?
an alcohol
114
what is the overall reaction of an alkene with concentrated sulphuric acid followed by hydrolysis represented by a single equation?
CH2=CH2 + H20 --------> CH3CH2OH
115
Why is sulphuric acid not included in the equation above?
because in this reaction sulphuric acid is considered a catalyst
116
how is ethanol produced
ethanol is not produced via this reaction but produced by fermentation of sugars and direct hydration of ethene
117
when alkenes react with bromine they form..?
dibromoalkanes
118
draw and ouline the addition reaction mechanism of bromine to ethene
- the bromine molecule is non-polar
- the high electron density of the c=c double bond induces a temporary dipole on the bromine molecule
- the electron deficient δ+ bromine atom is the electrophile
- a pair of electrons from the c=c double bond forms a bond with the δ+ bromine atom
- this forms a carbocation intermediate and a bromide ion
- the bromide ion then attacks the carbocation formed acting as a nucleophile
119
describe the test for alkenes
bromine water (orange-brown) to colourless
120
why do we only use a dilute solution of bromine water?
because bromine is a hazardous chemical
121
when alkenes react in water an alternative product is also formed...?
so when ethene reacts with bromine water
CH2BrCH2OH is formed
122
electrophilic addition to unsymmetrical alkenes
123
when we add an electrophile to an unsymmetrical alkene it forms two products..?
a major and a minor product
124
when is the major product formed?
via the most stable carbocation
125
how can a carbocation be labelled?
primary - have 1 carbon atom directly bonded to positve carbon atom
secondary - have two atoms directly bonded to the positve carbon atom
tertiary - have 3 carbon atoms directly bonded tot the positve carbon atom
126
what determines the stability of a carbocation?
by the presense of alkyl groups. the more alkyl groups directly attached to the positve carbon atom, the more stable the carbocation
127
state what is the order of stablity?
tertiary > secondary > primary
128
draw and outline the reaction mechanism of propene and hydrogen bromide, label the major and minor products formed
- hydrogen from the hBr molecule can bond to either of the carbon atoms in the c=c double bond
- this results in two different carbocations a primary and secondary
- the secondary carbocation is the most stable than the primary carbocation so the secondary carbocatin is mostly formed
- the bromide ion attacks the secondary carbocation and 2-bromopropane is formed as the major product
- and a small amount of 1-bromopropane is formed as the minor product
129
draw and outline the mechanism to show the reaction of propene with sulphuric acid, state the major and minor products formed
- hydrogen from the sulphuric acid adds adds on to the alkene forming a secondary and primary carbocation
- the secondary carbocation is more stable than the primary one
so the sulphuric acid ion attacks the secondary carbocation to form propan-2-ol the major product
- a small amount of propan-1-ol is also formed after hydrolysis as the minor product
130
FINISH
