AS - Unit 2 - Alcohols, halogenoalkanes and analysis Flashcards
Name the two ways in which ethanol can be made
Hydration of ethene
Fermentation of sugars
How is ethanol produced industrially?
By the catalytic hydration of ethene
H3PO4 catalyst
300 degrees C
60atm pressure
Is the creation of ethanol from ethene reversible or not?
It is reversible
Give a brief summary of the fermentation process
Carbohydrates —> ethanol + carbon dioxide
What are the reagents and conditions of the fermentation of carbohydrates?
Occurs in the presence of yeast, catalysed by an enzyme in yeast called zymase
between 25 and 37 degrees C
How much ethanol is made when sugar is fermented?
14%
What type of respiration occurs when fermenting sugar and yeast?
Anaerobic
Give 4 ways alcohol can be used other than in alcoholic drinks
Perfumes
Aftershaves
Cleaning fluids
Methlyated spirits (alternative to petrol)
What is volatility?
The ease that a liquid turns into a gas. Volatility increases as boiling point decreases
What is the strongest type of intermolecular force?
Hydrogen bonding
Which alcohols are soluble in water?
The first three members of the alcohol homologous series
Why do larger chained alcohols not dissolve in water?
Because a larger part of the alcohol molecule is made up of a non-polar hydrocarbon chain
The hydrocarbon chain does not form hydrogen bonds with water molecules
What determines a primary alcohol?
No alkyl groups attached to the carbon the OH group is attached to.
Or one alkyl group
What determines a secondary alcohol?
Two alkyl groups attached to the carbon on which the OH group is attached to
What determines a tertiary alcohol?
The OH group is attached to a carbon bonded to three alkyl groups
What mixture is used to oxidise alcohols?
Potassium dichromate (K2Cr2O7) and sulfuric acid (H2SO4)
What happens which you oxidise a primary alcohol?
Aldehyde: Gently warm with potassium dichromate and distilled off immediately
Carboxylic acid: Stronger heating with an excess of potassium dichromate, under reflux
What is reflux?
The continual boiling and condensing of a reaction mixture to ensure that the reaction takes place without the contents of the flask boiling dry
What happens when you oxidise secondary alcohols?
A ketone is produced
What happens when you oxidise a tertiary alcohol?
Nothing, tertiary alcohols are resistant to oxidation so the oxidising agent remains orange in colour
What colour does an alcohol turn when it is oxidised?
Orange —-> green/black
What is esterification?
A reaction of an alcohol with a carboxylic acid to produce an ester and water
When naming an ester what component takes on which name?
The alcohol take on the alkyl part of the name and the carboxylic acid takes on the alkanoate part of the ester
How does the bond form when reacting an alcohol and a carboxylic acid to form an ester?
The H of the alcohol and the OH from the carboxylic acid a removed creating a H2O molecule and an ester bridge is formed
A bond between the two molecules joined by a single oxygen atom
What is dehydration?
An elimination reaction in which water is removed from a saturated molecules to make an unsaturated molecule
How can you prepare an ester in a test tube?
Add equal carboxylic acid and alcohol, add a few drops of H2SO4. place in hot water bath at 80 degrees C for five minutes
pour the product into a beaker of cold water
the oil floating on the surface is the ester
Outline the dehydration of an alcohol
Add conc H3PO4 or H2SO4 to the alcohol
heat under reflux for 40 minutes
An alkene and water is produced
What have halogenoalkanes been used for in the past?
Refrigerants
Aerosol propellants
Degreasing agents
Dry-cleaning solvents
Why are halogenoalkanes no longer used in aerosols?
They have a damaging effect on the ozone layer
Why do polar bonds occur in halogenoalkanes?
The halogen is more electronegative than the carbon atoms. The bonded electron pair is attracted more towards the halogen atom than towards the carbon atom
Explain the trend in polarity of the halogen-carbon bond in a halogenoalkane as you go down the group
Electronegativity of halogens decreases down the group resulting in a decrease in polarity of the carbon-halogen bond from fluorine to iodine
What is hydrolysis?
A reaction with water or aqueous hydroxide ions that breaks a chemical compound into two compounds
What happens when halogenoalkanes react with aqueous hot hydroxide ions?
A nucleophilic substitution reaction occurs and an alcohol product is produced
What is nucelophilic substitution?
A type of substitution reaction in which a nucleophile is attracted to an electron-deficient centre or atom, where it donates a pair of electrons to form a new covalent bond
During hydrolysis, how does the substitution take place?
The hydroxide ion has a lone pair of electrons
these are attracted and donated to the electron-deficient carbon atom in the halogenoalkane.
The donation of the electron pair leads to the formation of a new covalent bond between the oxygen atom of the hydroxide ion and the carbon atom.
The carbon-halogen bond breaks by heterolytic fission
both electrons from the bond move to the halogen, forming a halide ion
How can you test the rate of hydrolysis for different halogenoalkanes?
Halogenoalkane is heated with silver nitrate (aq) and ethanol is added. The water is the nucleophile, ethanol acts as a common solvent.
The silver nitrate (aq) reacts with any halide ions present, forming a precipitate of silver halide. The rate can be determined by calculating 1/time taken for the precipitate to occur
When testing for the rate of hydrolysis for halogenoalkanes how can you ensure a fair test?
Equal amounts of each halogenoalkane
Use halogenoalkanes with the same chain length
Water bath to ensure a constant temperature
Explain the procedure taken to test for the rate of hydrolysis of a halogenoalkane
Measure each halogenoalkane into separate test tubes, place in a water bath at 50 degrees
Put solution of water, ethanol and silver nitrate into a separate test tube, place in the same water bath
Once all tubes have reached the same temperature, add equal volumes of each mixture to each halogenoalkane
Time how long it takes for each precipitate to form
When testing for a chloroalkane, what colour precipitate is produced?
white precipitate
When testing for a bromoalkane, what colour precipitate is produced?
Cream precipitate
When testing for a iodoalkane, what colour precipitate is produced?
Yellow precipitate
What two factors can affect the rate of hydrolysis?
Polarity - the carbon-fluorine bond is the most polar amongst the halogenoalkanes so the C slightly + atom should attract the nucleophile most readily to give the fastest reaction
Bond enthalpy - The carbon-iodine bond is the weakest amongst the halogenoalkanes, so the C-I bond should be broken the most easily and give the fastest reaction
In the hydrolysis of halogenoalkanes which is more important: bond enthalpy or bond polarity?
Bond enthalpy
Which way does the rate of reaction go with the halogenoalkane groups?
The rate of reaction increases from the fluoroalkanes (slowest) to the iodoalkanes (fastest) as the carbon-halogen bond enthalpy weakens
What does the rate of reaction depend on?
The strength of the carbon-halogen bond!!! The weaker the bond the faster the rate
How is PTFE made?
The polymerisation of tetrafluoroethene
What is PTFE (Teflon) used for?
The non-stick on pans
What are polymer properties influenced by?
The strength of the carbon-halogen bonds in the long-chained polymer structure
Why is PTFE resistant to chemical attack?
Because the carbon-fluorine bonds are very strong, making it hard for them to be broken and any other chemical interact with it
Give 4 properties of PTFE
Inert
Heat resistant
Electrical insulating properties
Non-stick qualities
Give 5 things PVC is used for
Drainpipes Plastic window frames Sports equipment Children's toys Packaging
How is PVC made?
The polymerisation of chloroethene
Who developed the first CFC?
Thomas Midgley
How did Midgley demonstrate the first CFC was non toxic and non flammable?
He inhaled a lungful, the exhaled the gas onto a candle flame which was extinguished
How were CFCs used when they were first discovered?
As refrigerants
Propellants
blowing agents
solvents for the dry-cleaning industry
What problem was found with CFCs that made them unsafe?
They remained stable until they reached the stratosphere where they break down in the presence of UV radiation to form chlorine radicals
Chlorine radicals are thought the catalyse the breakdown of the ozone layer
How are chlorine radicals (a bi product of using CFCs) harmful to people on earth?
Chlorine radicals are thought the catalyse the breakdown of the ozone layer
Ozone absorbs much of the UV light from the sun, preventing it from reaching the earth’s surface
The depletion of the ozone layer has allowed harmful UV radiation to reach the lower atmosphere, increasing the prevalence of skin cancers
What alternative has been found to CFCs which are thought to be better for the ozone layer?
HFCs (hydrofluorocarbons) and HCFCs (hydrofluorohydrocarbons)
How are CFCs and HCFCs similar and different?
Similar:
Non toxic
Non fammable
Deplete the ozone layer
Different:
HCFCs depleting effect is only one-tenth or less than that of CFCs
HCFCs are a short term fix rather than better replacements
What has been used for propellants to replace CFCs?
Butane, they are not harmful to the ozone layer and are often seen labelled ‘ozone-friendly’.
Many hydrocarbons are being tested for use to replace CFCs however they are flammable so you have to take care
Give 5 reasons why 100% yield is rarely obtained
Reaction may be at equilibrium
Other side reaction may occur leading the by-products
Reactions may not be pure
Some of the reactants or products may be left behind i the apparatus used
Separation and purification may result in a loss of some of the product
Why is percentage yield calculated?
The measure the success of a laboratory preparation
How do you calculate percentage yield?
Actual amount (in mol) of product ----------------------------------------------------------- X100 Theoretical amount (in mol) of product
What is a limiting reagent?
The substance in a chemical reaction that runs out first
How do you calculate atom economy?
Molecular mass of the desired product
——————————————————————– X100
Sum of molecular masses of all products
What is done with the by-products from a reaction?
Often considered as waste and disposed of, this is costly and poses potential environmental issues
By-products may be sold on or used elsewhere in the chemical plant. This is likely to increase in the future as we are increasingly concerned about minimising waste and preserving the Earth’s resources
What is atom economy?
The efficiency of a reaction in terms of all the atoms involved. A reaction with a high atom economy uses atoms with minimal waste
How can atom economy benefit society?
By using processes with a higher atom economy, chemical companies can reduce the amount of waste produced. This is good as we are running out of landfill sites and about 5-10% of the total expenditure of a chemical company goes on waste treatment
What reaction has a 100% atom economy?
Addition reactions
What type of reactions have a 100% or less atom economy?
Substitution or elimination
What happens to the C-H bond as it absorbs infrared radiation?
It stretches and bends from the vibrations as a result of absorbing the IR radiation
What does the amount of bond vibration depend on?
The bond strength
The bond length
The mass of each atom involved in the bond
What frequency do most bonds vibrate at?
between 300 and 4000cm-1 in the infrared part of the electromagnetic spectrum
What does an infrared spectrum help you to do?
Determine details about a compound’s chemical structure
Indicated functional groups in the compound under investigation
How do modern infrared spectrometers work?
A beam of infrared radiation is passed through a sample of the material under investigation
the beam contains the full range of frequencies present in the infrared region
the molecule absorbs some of the frequencies and the emerging beam is analysed to plot a graph of transmittance against frequency
This is the infrared spectrum of the molecule
What does each peak in an IR spec represent?
The absorbance of energy from infrared radiation that causes the vibration of a particular bond in the molecule under investigation
Give three uses that rely on IR Spec
Monitoring the degree of unsaturation in polymers
Quality control in perfume manufacture
Drug analysis
One of the main methods for testing the breath of suspected drunken drivers
What does an O-H group mean in IR spec?
A hydroxyl group in alcohols
What does the C=O group mean in IR spec?
The carbonyl group in aldehydes and ketones
What does the COOH group mean in IR spec?
The carboxyl group in carboxylic acids
How did the first mass spectrometer work?
Ions were first generated in a discharge tube
Ions were then passed through a magnetic field and deflected from their original path
Ions were finally deflected on a photographic plate
What is mass spec used for?
To identify unknown compounds
To determine the abundance of each isotope in an element
To gain further information about the structure and chemical properties of molecules
Give for specific uses in industry for mass spec
Monitoring the breath of patients during surgery whilst under anaesthetic
Detecting banned substances such as steroids in athletics
Analysing molecules in space
Detecting traces of toxic chemicals in contaminated marine life
How does a modern mass spectrometer work?
Sample is introduced via a sample inlet
Sample is converted into ions
Ions are propelled into a mass analyser
Separated according to their mass-to-charge ratio
Ions are detected, mass spectrum is generated by computer software and displayed on a screen or printed
What does a mass spectrometer do?
It determines the mass of a molecule or isotope by measuring the mass-to-charge ratio of ions.
Can be used to find the molecular mass, determine the structural features of a molecule or to find the abundance of isotopes
Give three features of a mass spec graph for magnesium
Three peaks in the spectrum, so there are three isotopes of Mg
Heights of the peaks give the relative abundance of the isotopes in the sample (given as percentages)
Results can be used to find relative atomic mass
How do you calculate relative atomic mass from a mass spec graph?
You multiply the relative abundance of one of the isotopes by its mass, then do that for all of the peaks, add them all together and divide by 100
e.g. (24X79) + (25X10) + (26X11)
———————————————— = 24.32
100
What is a molecular ion?
The positive ion formed in mass spectrometry when a molecule loses and electron
What is fragmentation?
The process in mass spec that causes a positive ion to split into pieces, one of which is a positive fragment ion
Where is the molecular ion on a mass spec graph?
It is the peak with the highest m/z value in the mass spectrum
What is the m/z value for CH3+?
15
What is the m/z value for C2H5+?
29
What is the m/z value for C3H7+?
43
What is the m/z value for C4H9+?
57
What is the m/z value for an OH+ peak?
17
What is a radical?
A species with an unpaired electron
Which three mechanisms are you expected to be able to draw for AS?
Reaction of an alkane with a halogen
Reaction of an alkene with an electrophile
Reaction of a halogenoalkane with a nucleophile
What are the three steps of radical substitution?
Initiation
Propagation
Termination
What happens in the initiation stage of radical substitution?
Radicals are formed
UV radiation provides the energy to break the covalent bond in a halogen molecule e.g. bromine
Bond breaks by homolytic fission
What happens in the propagation stage of radical substitution?
The products of the reaction are made
There are a few things to notice in this stage:
Numbers of radicals are the same on each side of the equations
e.g. bromine radical is used in the first propagation step
bromine radical is regenerated in the second propagation step
What happens in the termination stage of radical substitution?
Any two radicals can combine
What type of reaction do alkenes take part in?
Electrophilic addition
What is an electrophile?
An electron pair acceptor
What type of reaction do halogenoalkanes take part in?
Nucleophilic substitution
What is a nucelophile?
An electron pair donor
