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a level chemistry > Module 4 Section 2: Alcohols, Haloalkanes and Analysis > Flashcards

Module 4 Section 2: Alcohols, Haloalkanes and Analysis Flashcards

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1
Q

What is IR Spectroscopy

A

In Infrared Spectroscopy a beam of IR radiation is passed through a sample of a chemical
The IR radiation is absorbed by the covalent bonds in the molecules, increasing their vibrational energy (they vibrate more)
Bonds between different atoms absorb different frequencies of IR radiation.
Bonds in different places in a molecule absorb different frequencies too - the OH bond in an alcohol and the OH bond in a carboxylic acid absorb different frequencies

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2
Q

What does an infrared spectrometer show

A

It produces a spectrum that shows you what frequencies (wavenumber) of radiation the molecules are absorbing
This can be used to identify the functional groups in a molecule

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3
Q

How to tell if the functional group has changed using an infrared spectrometer

A

This also means that you can tell if a functional group has changed during a reaction
e.g., if an alcohol is oxidised to form an aldehyde then the OH bond will disappear from the spectrum, and a C=O absorption will appear
If it is oxidise it further to a carboxylic acid an OH peak at a slightly lower frequency than before will appear, alongside the C=O peak

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4
Q

Uses of IR spectroscopy

A

Can be used in breathalysers to workout if a driver is over the drink-drive limit
The amount of ethanol vapour in the driver’s breath is found by measuring the intensity of the peak corresponding to the C-H bond in the spectrum.
The C-H bond in particular is used because it is not affected by any water vapour in the breath

IR spectroscopy is also used to monitor the concentrations of polluting gases in the atmosphere
These include carbon monoxide (CO) and nitrogen monoxide (NO), which are both present in car emissions
The intensity of the peaks corresponding to the C=O (should be triple bond) or N=O bonds can be studied to monitor their levels

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5
Q

What is the finger print region?

A

Below 1500cm^-1
The finger print region contain unique peeks used to identify the particular molecule using a computer or comparing with a booklet of published spectra.
It is difficult to identify functional groups in this region.

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6
Q

What is on the X and Y axis of a IR spectrum

A

X - wavenumber cm^-1
Y - transmittance(%) (can sometimes be absorption)

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7
Q

What is a haloalkane

A

This is an alkane with at least one halogen atom in place of a hydrogen atom

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8
Q

Examples of haloalkanes

A

Trichloromethane
2-iodo-propane
2-bromo-2-chloro-1,1,1trifluoroethane

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9
Q

What bond is polar in haloalkanes

A

Halogens are generally much more electronegative than carbon
So the carbon-halogen bond is polar

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10
Q

How do nucleophiles interact with haloalkanes

A

The δ+ carbon is electron deficient
This means it can be attacked by a nucleophile

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11
Q

What is a nucleophile

A

This is an electron pair donor
It could be a negative ion or an atom with a lone pair of electrons
It donates an electron pair to somewhere without enough electrons

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12
Q

Examples of nucleophiles

A

OH-, CN- and NH3
Water is a nucleophile but reacts slowly

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13
Q

How can haloalkanes be hydrolysed and what do they make

A

This forms alcohols
This is a nucleophilic substitution reaction
A warm aqueous alkali e.g. sodium hydroxide (NaOH) or potassium hydroxide (KOH) must be used

General equation
R-X + OH- —(reflux)-> R-OH + X-

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14
Q

Reaction mechanism for hydrolysis

A

OH- is the nucleophile which provides a pair of electrons for the δ+ Carbon
The C-X bond breaks heterolytically - bond electrons from the bond are taken by the X-
X- falls off as OH- bonds to the carbon

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15
Q

How does water act as a nucleophile

A

Water molecule is a weak nucleophile so reaction will be slower
General equation:
R-X + H2O = R-OH + H+ + X-

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16
Q

What affects how quickly haloalkanes are hydrolysed

A

Bond enthalpy
Weaker carbon-halogen bonds break more easily so they react faster

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17
Q

What haloalkane hydrolysed the fastest and slowest

A

Iodoalkanes havw the weakest bonds
Fluoroalkanes have the strongest bonds so they hydrolyse the slowest

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18
Q

Mechanism of the experiment to compare the reactivity of haloalkanes

A

Mixing haloalkanes with water makes an alcohol
R-X + H2O = R-OH + H+ + X-
Adding silver nitrate makes a silver halide precipitate as the silver ions react with t(e halide ions as soon as they form
Ag+ + X- = AgX

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19
Q

Experiment to compare the reactivity of haloalkanes

A

Set up three test tubes each containing a different haloalkane, ethanol (as the solvent) and silver nitrate solution (this contains the water)
Haloalkanes must have all the same skeleton structure to make it a fair test
A pale yellow precipitate quickly forms with 2-iodopropane - so iodoalkanes must be the most reactive haloalkanes
Bromoalkanes react slower to from a cream ppt and chloroalkanes form a white ppt slowest

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20
Q

When are practical techniques used in chemistry

A

Used during synthesis of a product, or to purify it from unwanted by-products or unreacted reagents once it’s been made

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21
Q

Why reflux used

A

Organic reactions are slow and substances are usually flammable and volatile (low boiling points)
If they are put in a beaker and heated with a Bunsen burner they will evaporate or catch fire before they have time to react

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22
Q

What happens during reflux

A

The mixture is heated in a flask fitted with a vertical Liebig condenser - this continuous boils, evaporates and condenses the vapours and recycles them back into the flask, giving them time to react

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23
Q

Why are heating elements electrical in reflux

A

Hot plates, heating mantles or electrically controlled water baths are normally used
This avoids naked flames that might ignite the compounds

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24
Q

When is distillation used

A

Used to separate substances with different boiling points

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25
Q

How does distillation work

A

A mixture is gently heated in a distillation apparatus
The substances will evaporate out of the mixture in order of increasing boiling point
The thermometer shows the boiling points of the substances that is evaporating at any given time

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26
Q

What if you know the boiling point of the pure product in distillation

A

You can use the thermometer to tell you when it’s evaporating and therefore when it’s condensing

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27
Q

What happens if the product of a reaction has a lower boiling point than the starting materials in distillation

A

The reaction mixture can be heated so that the product evaporates from the reaction mixture as it forms

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28
Q

What happens if the starting material has a higher boiling point than the product

A

Aa long as the temperature is controlled, it won’t evaporate out from the reaction mixture

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29
Q

What happens when a product is left in the reaction mixture

A

It can go on to react further
E.g. when a primary alcohol is oxidised to an aldehyde and then oxidised to a Carboxylic acid
If the desired product is the aldehyde, the reaction can be done in the distillation equipment
The aldehyde product has a lower boiling point than the alcohol, so will distil out of the reaction mixture as soon as it forms
It is then collected in a separate container

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30
Q

How can volatile liquids be purified

A

Redistillation

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31
Q

How does redistillation work

A

If a product and the impurities have different boiling points, regular distillation equipment is used to heat an impure product, instead of a reaction mixture
When the desired liquid boils (when thermometer is at the boiling point of the liquid), place a flask at the open end of the condenser to collect your product
When the thermometer shows the temperature is changing, put another flask at the end of the condenser because a different liquid will be delivered

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32
Q

When to use separation

A

If a product is insoluble in water then separation is used to remove any impurities that do dissolve in water such as salts or water soluble organic compounds ( e.g. alcohols )

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33
Q

Process of separation

A

Once the reaction to form the product is completed, pour the mixture into a separating funnel, and add water
Shake the funnel and allow it to settle
The organic layer and the aqueous layer (which contains any water soluble impurities) are immiscible ( don’t mix ), so separate out into two distinct layers
You can then open the tap and run each layer off into a separate container

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34
Q

What usually happens when separation is used to purify a product

A

The organic layer will end up container trace amounts of water, so it has to be dried

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35
Q

How to dry out the organic layer after separation

A

Add anhydrous salt (e.g. magnesium sulfate or calcium chloride )
The salt is used as a drying agent - it binds to any water present to become hydrated
You can filter the mixture to remove the solid drying agent

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36
Q

What may happen when you first add the salt to the organic layer after separation

A

It may become lumpy
This means more must be added
You know that all the water has been removed when you can swirl the mixture and it looks like a snow globe

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37
Q

What are CFCs

A

Chlorofluorocarbons
Contain only chlorine fluorine and carbon - all hydrogens have been replaced

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38
Q

Properties of CFCs

A

Stable, volatile, non flammable and non toxic

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39
Q

Where are CFCs used

A

Fridges, aerosol cans, dry cleaning and air conditioning

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40
Q

Why were CFCs stopped

A

Scientists realised they were destroying the ozone layer

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41
Q

What is the chemical formula for ozone

A

O3

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42
Q

What is the purpose of ozone

A

Acts as a chemical blocker for UV radiation
Absorbs a lot of UV radiation which can cause sunburn or even skin cancer

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43
Q

How is ozone formed naturally

A

When an oxygen molecule is broken down into two free radicals by UV radiation
The free radicals attack other oxygen molecules forming ozone

O2 -UV light—> O + O ——> O2 + O —> O3

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44
Q

Why are holes in the ozone layer bad for earth

A

They allow more harmful UV radiation to reach the earth
This can cause an increase in skin cancer

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45
Q

When was the ozone layer found to be thinning

A

1970s and 80s was when scientists found the ozone layer above Antarctica and the Arctic was getting thinner

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46
Q

How are the holes in the ozone layer formed

A

CFCs in the upper atmosphere absorb UV radiation and split homolytically to form chlorine radicals
These free radicals catalyse the destruction of ozone - they destroy ozone molecules and are then regenerated to destroy more ozone

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47
Q

Initiation equations for how chlorine radicals catalyse the destruction of ozone

A

Cl• radicals are formed when C-Cl bonds in CFCs are broken down by UV radiation
CF2Cl2 -UV-> •CF2Cl + Cl•

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48
Q

Intermediate/ propagation equations for how chlorine radicals catalyse the destruction of ozone

A

Cl• + O3 -> O2 + ClO•
ClO• + O -> O2 + Cl•

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49
Q

How are chlorine radicals catalysts for the breakdown of ozone

A

Used in the first step to break down ozone and regenerated in the next reaction

50
Q

Overall reaction for breakdown of ozone

A

O3 + O -> 2O2
Cl• is the catalyst

51
Q

Where do nitrogen oxides come from and how are they formed

A

Nitrogen oxides are produced by car and aircraft engines and thunderstorms

This is formed when nitrogen from the air is heated up and it combines with oxygen in the combustion chamber to form nitrogen monoxide

52
Q

How do NO• free radicals from nitrogen oxides destroy the ozone

A

They act in the same way as chlorine radicals to breakdown ozone molecules and act as catalysts for these reactions

53
Q

General equation for how radicals break down ozone

A

R + O3 -> RO + O2
RO + O -> R + O2
R represents either Cl• or NO•

54
Q

What can be used instead of CFCs

A

HCFCs (hydrochlorofluorocarbons)
HFCs (hydrofluorocarbons)
Hydrocarbons

55
Q

Why are HCFCs slightly better than CFCs

A

They are broken down in the atmosphere in 10-20 years
Still damage the ozone layer, but the effect is less than CFCs

56
Q

Why are HFCs better than HCFCs

A

HFCs are still broken down in the atmosphere
They don’t contain chlorine, so they don’t affect the ozone layer

57
Q

Why are HFCs and HCFCs still bad

A

They are greenhouse gases that are 1000 times worse than CO2

58
Q

What is used instead of HFCs and HCFCs

A

Aerosols use pump spray systems of use nitrogen as the propellant
Many industrial fridges and freezers use ammonia as the coolant gas
Carbon dioxide is used to make foamed polymers

59
Q

What are the main greenhouse gases

A

Water vapour
Carbon dioxide
Methane

60
Q

What do the greenhouse gases do

A

The C=O, C-H and O-H absorb IR radiation (heat)
They re-emit it in all directions
Heat can be sent back to earth which keeps us warm

61
Q

How have humans enhanced the greenhouse effect

A

World population has increased and we’ve become more industrialised
This results in more CO2 being released and trees being cut down which absorb CO2
More food is being grown and cows and paddy fields release a lot of methane

62
Q

Consequences of enhanced greenhouse effect

A

Higher concentration of greenhouse gases mean more heat is being trapped and the earth gets warmer (global warming)
Global warming can cause climate change such as polar ice caps shrinking and less predictable weather

63
Q

How do scientists investigate climate change

A

Scientists collect air and sea water samples to investigate whether climate change is happening

64
Q

What evidence is there to show for climate change

A

Earths average temperature has increased dramatically in the last 50 years
CO2 levels have increased at the same time

65
Q

How do CO2 and global temperature correlate and show a causation

A

Correlation between CO2 and temperature is clear
This correlation alone does prove that one thing causes another
There must be a mechanism for how one change causes another (shown by enhanced greenhouse effect)

66
Q

How and why have governments agreed to limit enhanced greenhouse effect

A

Scientific evidence has persuaded governments to form a global agreement that climate change can be damaging to people, the environment and economies
They have tried to limit it
E.g. Kyoto protocol where industrialised countries agreed to reduced greenhouse gas emissions to agreed levels, this has since run out and been replaced by pledges to reduced emissions for the future
UK government has created policies to use more renewable energy supplies e.g. wind and solar farms

67
Q

General formula for alcohols

A

CnH2n+1OH

68
Q

Difference between primary, secondary and tertiary alcohols

A

1°: OH bonded to carbon which is bonded to one other carbon
2°: OH bonded to carbon which is bonded to two other carbons
3°: OH bonded to carbon which is bonded to three other carbons

69
Q

What properties does the OH functional group give alcohols

A

Alcohols are generally polar molecules
Due to electronegative hydroxyl group pulling electrons in the C-OH bond away from the carbon atom

70
Q

What can the polarity of alcohols result in

A

Electronegative oxygen in the polar hydroxyl group draws electron density away from the hydrogen making it δ+
Positive charge can attract lone pairs on an oxygen from a neighbouring molecule forming hydrogen bonds

71
Q

Why are alcohols soluble in water

A

When alcohols are mixed with water, hydrogen bonds form between the -OH and H2O
If the alcohol is small (methanol, ethanol or Propan-1-ol) then hydrogen bonding allows it to mix freely with water

72
Q

What is the trend with solubility and increasing chain length and why

A

As the chain length increases the solubility decreases
Larger alcohols have a larger non-polar carbon chain
Creates less attraction for polar H2O molecules

73
Q

Why do alcohols have low volatility

A

Alcohols can form hydrogen bonds between eachother
These intermolecular forces take a lot of energy to be overcome
So they don’t evaporate into a gas as easily as non polar compounds e.g. alkanes of similar sizes

74
Q

How do alcohols form haloalkanes

A

Will react with compounds containing halide ions in a substitution reaction
Hydroxyl group is replaced with halide ions to form a haloalkane

75
Q

What are the conditions needed for alcohols to form haloalkanes

A

Must be mixed with NaX and concentrated H2SO4
Room temperature

76
Q

What happens when alcohols are dehydrated

A

Alcohols are dehydrated to form alkenes and water
This is an elimination reaction
The water is made formed from the OH group and another hydrogen atom
Leaves two possible alkene products from a single elimination reaction depending on which side of the OH group the hydrogen atom is eliminated from

77
Q

What are the conditions needed for the dehydration of alcohols

A

Concentrated H2SO4 or concentrated phosphoric acid (H3PO4)
Must be heated

78
Q

What is the simplest way to oxidise alcohols

A

Combustion with oxygen to form carbon dioxide and water
Alcohol is completely oxidised and C-C and C-H bonds are broken

79
Q

How do alcohols burn

A

Pale blue flame

80
Q

What oxidising agent is used mildly to oxidise alcohols

A

The oxidising agent acidified dichromate(VI (Cr2O7 2-/H+)
E.g. K2Cr2O7 (potassium dichromate) and H2SO4

81
Q

What are the different alcohols oxidised to form

A

Primary alcohols: aldehydes and carboxylic acids
Secondary alcohols: ketones only
Tertiary alcohols: won’t be oxidised

82
Q

General formula for carbonyl compounds

A

E.g. aldehydes and ketones
CnH2nO

83
Q

How are primary alcohols oxidised

A

Oxidised twice to first form aldehydes and then oxidised further to form carboxylic acids

84
Q

General formula for oxidation of primary alcohols

A

R-CH2-OH +[O] —distil-> aldehyde + water + [O] —reflux-> carboxylic acid
O: oxidising agent potassium dichromate(VI)

85
Q

How to control how primary alcohols are oxidised
E.g. ethanol

A

Gently heating ethanol with potassium dichromate(VI) solute and H2SO4 produces ethanal (apple scented)
Hard to control amount of heat and aldehyde is usually oxidised to form ethanoic acid (vinegar scented)
To get just the aldehyde you heat excess alcohol with a controlled amount of oxidising agent in distillation apparatus
Aldehyde boils at a lower temperature than alcohol and distills off immediately
To produce a carboxylic acid, the alcohol must be vigorously oxidised
Alcohol is mixed with excess oxidising agent and heated under reflux

86
Q

How to produce ketones from secondary alcohols

A

Refluxing a secondary alcohol with acidified dichromate(VI) will produce a ketone

87
Q

Why does the temperature and conditions not need to be as controlled with oxidation of secondary alcohols

A

Ketones can be oxidised easily
Prolonged refluxing won’t produce anything more

88
Q

Why can’t tertiary alcohols be oxidised easily

A

Tertiary alcohols don’t react with potassium dichromate(VI) at all
Does not have any hydrogen atom next to OH on chain to react with
Solute stays orange
This is due to steric hinderance
Only way to oxidise tertiary alcohols is by burning them

89
Q

What colour does the dichromate(VI) solution go when the alcohol is oxidised

A

Orange dichromate(VI) ion is reduced to the green chromium(III) ion, Cr3+

90
Q

What is a synthetic route

A

This shows how to get from one compound to another
Shows all the reactions with the intermediate products, and the reagents needed for each reaction
This is useful for designing medicines

91
Q

Synthetic route from 2-bromopropane to propanone

A

2-bromopropane + KOH/H2O (reflux) > Propan-2-ol > + K2Cr2O7/H2SO4 (reflux) > propanone

92
Q

Typical properties and reactions of alkanes

A

Non-polar, unreactive
Radical substitution

93
Q

Typical properties and reactions of alkenes

A

Non polar, electron rich double bond
Electrophilic addition

94
Q

Typical properties and reactions of alcohols

A

Polar C-OH bond
Lone pair on O can act as a nucleophile

Nucleophilic substitution
Dehydration/ elimination

95
Q

Typical properties and reactions of haloalkanes

A

Polar C-X bond
Nucleophilic substitution

96
Q

Typical properties and reactions of aldehyde/ ketone

A

Polar C=O
Aldehydes will oxidise

97
Q

Typical properties and reactions of carboxylic acids

A

Electron deficient carbon centre
Esterification

98
Q

Synthetic route of alkane to haloalkane

A

X2 (halogen molecule)
UV light
Substitution

99
Q

Synthetic route of haloalkane to alcohol

A

Warm NaOH or KOH, H2O
Reflux
Substitution

100
Q

Synthetic route of alcohol to haloalkane

A

NaX, H2SO4
20°C
Substitution

101
Q

Synthetic route of alkene to dihaloalkane

A

X2 (halogen molecule)
20°C
Electrophilic Addition

102
Q

Synthetic route of alkene to alkane

A

H2, nickel catalyst
150°C
Addition

103
Q

Synthetic route of alkene to haloalkane

A

HX
20°C
Addition

104
Q

Synthetic route of alcohol to alkene

A

Conc H2SO4 or H3PO4
Heat
Elimination

105
Q

Synthetic route of alkene to alcohol

A

Steam, H3PO4, catalyst
300°C
Addition

106
Q

Synthetic route of alcohol to aldehyde

A

K2Cr2O7, H2SO4
Heat primary alcohol in distillation apparatus
Oxidation

107
Q

Synthetic route of aldehyde to carboxylic acid

A

K2Cr2O7, H2SO4
Reflux
Oxidation

108
Q

Synthetic route of alcohol to ketone

A

K2Cr2O7, H2SO4
Heat secondary alcohol in reflux apparatus
Oxidation

109
Q

How is the molecular ion formed in mass spectrometry

A

The molecules in the sample are bombarded with electrons
These remove an electron from the molecule to form a molecular ion, M+

110
Q

What does the molecular ion peak represent

A

This represents the relative molecular mass of a compound
The mass/charge value of the molecular ion peak is the molecular mass
This assumes the ion has a +1 charge, which it normally will have

111
Q

How do other fragments form in mass spec

A

The bombarding electrons make some of the molecular ions break up into fragments
The fragments that are ions show up on the mass spectrum, making a fragmentation pattern

112
Q

Molecular ion and fragments formed that show up on the mass spectrum

A

The propane molecular ion in CH3CH2CH3+
Breaks into fragments of CH3+ and CH3CH2+

This is because the CH3CH2CH3+ breaks into CH3CH2• + CH3+ and CH3CH2+ + CH3•
Only the ions show up and free radicals are lost

113
Q

Common fragments and their molecular mass

A

CH3+ : 15
C2H5+ : 29
CH3CH2CH2+ or CH3CHCH3+ : 43
OH+ : 17

114
Q

How to differentiate between two functional group isomers using mass spec
E.g. ketone and aldehydes

A

The two compounds will not produce the same set of fragments
E.g. propanal will have C2H5 fragment whereas propanone won’t

115
Q

How to accurately identify specific compounds from their mass specs

A

Different compounds produces a different mass spectrum
The spectrum is like a fingerprint for the compound
Large computer databases of mass spectra can be used to identify a compound from its spectrum

116
Q

How to work out the structure of a compound given its IR and mass spectra

A

Use the M+ ion to work out the molecular formula of the compound
Find the functional groups in the compound from the IR spectrum
Use the mass spectrum to work out the structure of the molecule

117
Q

General equation for oxidation of alcohols

A

Alcohol + 2O -> Carboxylic acid + H2O
Alcohol + O -> ketone/aldehyde + H2O

118
Q

Methods of CCS

A

Carbon Capture and Storage:
Stored deep in oceans
Storage in geological formations
Piped into disused or partially filled oil wells
Reaction with metal oxides to form solid carbonates

119
Q

Name this IR spectroscopy

A

Maybe IR spec for alkenes of strange ones you wouldn’t expect
Insert pic

120
Q

Diagram for distillation (drawing in exam)

A

Insert picture for Mat

121
Q

Diagram for reflux (drawing in exam)

A

Insert picture for Mat

a level chemistry (15 decks)

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