Module 4 Section 2: Alcohols, Haloalkanes and Analysis Flashcards
What is IR Spectroscopy
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
What does an infrared spectrometer show
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
How to tell if the functional group has changed using an infrared spectrometer
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
Uses of IR spectroscopy
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
What is the finger print region?
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.
What is on the X and Y axis of a IR spectrum
X - wavenumber cm^-1
Y - transmittance(%) (can sometimes be absorption)
What is a haloalkane
This is an alkane with at least one halogen atom in place of a hydrogen atom
Examples of haloalkanes
Trichloromethane
2-iodo-propane
2-bromo-2-chloro-1,1,1trifluoroethane
What bond is polar in haloalkanes
Halogens are generally much more electronegative than carbon
So the carbon-halogen bond is polar
How do nucleophiles interact with haloalkanes
The δ+ carbon is electron deficient
This means it can be attacked by a nucleophile
What is a nucleophile
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
Examples of nucleophiles
OH-, CN- and NH3
Water is a nucleophile but reacts slowly
How can haloalkanes be hydrolysed and what do they make
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-
Reaction mechanism for hydrolysis
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
How does water act as a nucleophile
Water molecule is a weak nucleophile so reaction will be slower
General equation:
R-X + H2O = R-OH + H+ + X-
What affects how quickly haloalkanes are hydrolysed
Bond enthalpy
Weaker carbon-halogen bonds break more easily so they react faster
What haloalkane hydrolysed the fastest and slowest
Iodoalkanes havw the weakest bonds
Fluoroalkanes have the strongest bonds so they hydrolyse the slowest
Mechanism of the experiment to compare the reactivity of haloalkanes
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
Experiment to compare the reactivity of haloalkanes
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
When are practical techniques used in chemistry
Used during synthesis of a product, or to purify it from unwanted by-products or unreacted reagents once it’s been made
Why reflux used
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
What happens during reflux
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
Why are heating elements electrical in reflux
Hot plates, heating mantles or electrically controlled water baths are normally used
This avoids naked flames that might ignite the compounds
When is distillation used
Used to separate substances with different boiling points
How does distillation work
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
What if you know the boiling point of the pure product in distillation
You can use the thermometer to tell you when it’s evaporating and therefore when it’s condensing
What happens if the product of a reaction has a lower boiling point than the starting materials in distillation
The reaction mixture can be heated so that the product evaporates from the reaction mixture as it forms
What happens if the starting material has a higher boiling point than the product
Aa long as the temperature is controlled, it won’t evaporate out from the reaction mixture
What happens when a product is left in the reaction mixture
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
How can volatile liquids be purified
Redistillation
How does redistillation work
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
When to use separation
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 )
Process of separation
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
What usually happens when separation is used to purify a product
The organic layer will end up container trace amounts of water, so it has to be dried
How to dry out the organic layer after separation
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
What may happen when you first add the salt to the organic layer after separation
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
What are CFCs
Chlorofluorocarbons
Contain only chlorine fluorine and carbon - all hydrogens have been replaced
Properties of CFCs
Stable, volatile, non flammable and non toxic
Where are CFCs used
Fridges, aerosol cans, dry cleaning and air conditioning
Why were CFCs stopped
Scientists realised they were destroying the ozone layer
What is the chemical formula for ozone
O3
What is the purpose of ozone
Acts as a chemical blocker for UV radiation
Absorbs a lot of UV radiation which can cause sunburn or even skin cancer
How is ozone formed naturally
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
Why are holes in the ozone layer bad for earth
They allow more harmful UV radiation to reach the earth
This can cause an increase in skin cancer
When was the ozone layer found to be thinning
1970s and 80s was when scientists found the ozone layer above Antarctica and the Arctic was getting thinner
How are the holes in the ozone layer formed
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
Initiation equations for how chlorine radicals catalyse the destruction of ozone
Cl• radicals are formed when C-Cl bonds in CFCs are broken down by UV radiation
CF2Cl2 -UV-> •CF2Cl + Cl•
Intermediate/ propagation equations for how chlorine radicals catalyse the destruction of ozone
Cl• + O3 -> O2 + ClO•
ClO• + O -> O2 + Cl•
How are chlorine radicals catalysts for the breakdown of ozone
Used in the first step to break down ozone and regenerated in the next reaction
Overall reaction for breakdown of ozone
O3 + O -> 2O2
Cl• is the catalyst
Where do nitrogen oxides come from and how are they formed
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
How do NO• free radicals from nitrogen oxides destroy the ozone
They act in the same way as chlorine radicals to breakdown ozone molecules and act as catalysts for these reactions
General equation for how radicals break down ozone
R + O3 -> RO + O2
RO + O -> R + O2
R represents either Cl• or NO•
What can be used instead of CFCs
HCFCs (hydrochlorofluorocarbons)
HFCs (hydrofluorocarbons)
Hydrocarbons
Why are HCFCs slightly better than CFCs
They are broken down in the atmosphere in 10-20 years
Still damage the ozone layer, but the effect is less than CFCs
Why are HFCs better than HCFCs
HFCs are still broken down in the atmosphere
They don’t contain chlorine, so they don’t affect the ozone layer
Why are HFCs and HCFCs still bad
They are greenhouse gases that are 1000 times worse than CO2
What is used instead of HFCs and HCFCs
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
What are the main greenhouse gases
Water vapour
Carbon dioxide
Methane
What do the greenhouse gases do
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
How have humans enhanced the greenhouse effect
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
Consequences of enhanced greenhouse effect
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
How do scientists investigate climate change
Scientists collect air and sea water samples to investigate whether climate change is happening
What evidence is there to show for climate change
Earths average temperature has increased dramatically in the last 50 years
CO2 levels have increased at the same time
How do CO2 and global temperature correlate and show a causation
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)
How and why have governments agreed to limit enhanced greenhouse effect
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
General formula for alcohols
CnH2n+1OH
Difference between primary, secondary and tertiary alcohols
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
What properties does the OH functional group give alcohols
Alcohols are generally polar molecules
Due to electronegative hydroxyl group pulling electrons in the C-OH bond away from the carbon atom
What can the polarity of alcohols result in
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
Why are alcohols soluble in water
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
What is the trend with solubility and increasing chain length and why
As the chain length increases the solubility decreases
Larger alcohols have a larger non-polar carbon chain
Creates less attraction for polar H2O molecules
Why do alcohols have low volatility
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
How do alcohols form haloalkanes
Will react with compounds containing halide ions in a substitution reaction
Hydroxyl group is replaced with halide ions to form a haloalkane
What are the conditions needed for alcohols to form haloalkanes
Must be mixed with NaX and concentrated H2SO4
Room temperature
What happens when alcohols are dehydrated
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
What are the conditions needed for the dehydration of alcohols
Concentrated H2SO4 or concentrated phosphoric acid (H3PO4)
Must be heated
What is the simplest way to oxidise alcohols
Combustion with oxygen to form carbon dioxide and water
Alcohol is completely oxidised and C-C and C-H bonds are broken
How do alcohols burn
Pale blue flame
What oxidising agent is used mildly to oxidise alcohols
The oxidising agent acidified dichromate(VI (Cr2O7 2-/H+)
E.g. K2Cr2O7 (potassium dichromate) and H2SO4
What are the different alcohols oxidised to form
Primary alcohols: aldehydes and carboxylic acids
Secondary alcohols: ketones only
Tertiary alcohols: won’t be oxidised
General formula for carbonyl compounds
E.g. aldehydes and ketones
CnH2nO
How are primary alcohols oxidised
Oxidised twice to first form aldehydes and then oxidised further to form carboxylic acids
General formula for oxidation of primary alcohols
R-CH2-OH +[O] —distil-> aldehyde + water + [O] —reflux-> carboxylic acid
O: oxidising agent potassium dichromate(VI)
How to control how primary alcohols are oxidised
E.g. ethanol
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
How to produce ketones from secondary alcohols
Refluxing a secondary alcohol with acidified dichromate(VI) will produce a ketone
Why does the temperature and conditions not need to be as controlled with oxidation of secondary alcohols
Ketones can be oxidised easily
Prolonged refluxing won’t produce anything more
Why can’t tertiary alcohols be oxidised easily
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
What colour does the dichromate(VI) solution go when the alcohol is oxidised
Orange dichromate(VI) ion is reduced to the green chromium(III) ion, Cr3+
What is a synthetic route
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
Synthetic route from 2-bromopropane to propanone
2-bromopropane + KOH/H2O (reflux) > Propan-2-ol > + K2Cr2O7/H2SO4 (reflux) > propanone
Typical properties and reactions of alkanes
Non-polar, unreactive
Radical substitution
Typical properties and reactions of alkenes
Non polar, electron rich double bond
Electrophilic addition
Typical properties and reactions of alcohols
Polar C-OH bond
Lone pair on O can act as a nucleophile
Nucleophilic substitution
Dehydration/ elimination
Typical properties and reactions of haloalkanes
Polar C-X bond
Nucleophilic substitution
Typical properties and reactions of aldehyde/ ketone
Polar C=O
Aldehydes will oxidise
Typical properties and reactions of carboxylic acids
Electron deficient carbon centre
Esterification
Synthetic route of alkane to haloalkane
X2 (halogen molecule)
UV light
Substitution
Synthetic route of haloalkane to alcohol
Warm NaOH or KOH, H2O
Reflux
Substitution
Synthetic route of alcohol to haloalkane
NaX, H2SO4
20°C
Substitution
Synthetic route of alkene to dihaloalkane
X2 (halogen molecule)
20°C
Electrophilic Addition
Synthetic route of alkene to alkane
H2, nickel catalyst
150°C
Addition
Synthetic route of alkene to haloalkane
HX
20°C
Addition
Synthetic route of alcohol to alkene
Conc H2SO4 or H3PO4
Heat
Elimination
Synthetic route of alkene to alcohol
Steam, H3PO4, catalyst
300°C
Addition
Synthetic route of alcohol to aldehyde
K2Cr2O7, H2SO4
Heat primary alcohol in distillation apparatus
Oxidation
Synthetic route of aldehyde to carboxylic acid
K2Cr2O7, H2SO4
Reflux
Oxidation
Synthetic route of alcohol to ketone
K2Cr2O7, H2SO4
Heat secondary alcohol in reflux apparatus
Oxidation
How is the molecular ion formed in mass spectrometry
The molecules in the sample are bombarded with electrons
These remove an electron from the molecule to form a molecular ion, M+
What does the molecular ion peak represent
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
How do other fragments form in mass spec
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
Molecular ion and fragments formed that show up on the mass spectrum
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
Common fragments and their molecular mass
CH3+ : 15
C2H5+ : 29
CH3CH2CH2+ or CH3CHCH3+ : 43
OH+ : 17
How to differentiate between two functional group isomers using mass spec
E.g. ketone and aldehydes
The two compounds will not produce the same set of fragments
E.g. propanal will have C2H5 fragment whereas propanone won’t
How to accurately identify specific compounds from their mass specs
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
How to work out the structure of a compound given its IR and mass spectra
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
General equation for oxidation of alcohols
Alcohol + 2O -> Carboxylic acid + H2O
Alcohol + O -> ketone/aldehyde + H2O
Methods of CCS
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
Name this IR spectroscopy
Maybe IR spec for alkenes of strange ones you wouldn’t expect
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Diagram for distillation (drawing in exam)
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Diagram for reflux (drawing in exam)
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