Alcohols Flashcards
What are some of The physical properties of the alcohols
Ethanol is a member of the homologous series of alcohols.
> present in mixtures produced by fermentation.
> commonly referred to as ‘alcohol’
Alcohols have functional group -OH
and the general formula CnH2n+1OH.
• Alcohols have a greater boiling point
- than alkanes with a similar relative molecular mass.
- due to presence of H bonding between alcohol molecules.
-
When a molecular covalent liquid is vaporised,
energy breaks intermolecular forces
»_space; Alkanes are held together by vdw forces.
»_space; Both vdw forces and H bonding (stronger) are present between alcohol molecules.
..
• -OH group has an effect on solubility of molecule, In short chain alcohols, like methanol and ethanol.
-
- Both are soluble in all water proportions while alkanes,
- wch have no -OH group are insoluble in water.
What is the Classification of alcohols
Alcohols are classified as primary (1°), secondary (2°) or tertiary (3°)
> depending on position of -OH group in carbon chain.
The classification can also be derived by
> considering the number of alkyl groups, R groups, attached to carbon
> bonded to the -OH group
DIAGRAM 1
alkyl groups include methyl,ethyl,etc
What happens and is produced by oxidation of primary alcohols and secondary
Primary alcohols are easily oxidised in the presence of oxidising agents (O)
> to produce aldehydes,
> wch can be further oxidised to carboxylic acids.
In the first oxidation,
- the primary alcohol loses two H atoms, from -OH group and from saturated carbon. (Forms aldehyde)
- In second oxidation an oxygen atom is added to remaining hydrogen. (Forms carboxylic acid)
»_space; attached to the carbon
When producing a carboxylic acid, the oxidising agent is in excess,
> encouraging complete oxidation of the alcohol (ensuring carboxylic)
> and reaction mixture is refluxed gently.
To ensure only aldehyde is produced,
> primary alcohol must be in excess
> product distilled off immediately
> to ensure oxidation is only partial and the carboxylic acid is not formed
WATER and ALDEHYDE = products
diagram 2
..
Secondary alcohols can be oxidised to ketones,
>wch do not undergo further oxidation.
(Same as diagram 2, but without carboxylic acid forming
And not aldehyde, but ketone)
Secondary alcohols must be refluxed gently
> with excess of oxidising
agent to produce ketone.
What is produced in oxidation of tertiary alcohols
Tertiary alcohols are not easily oxidised
- dont have two H atoms directly attached to carbon thats bonded to the -OH group.
-
- is oxidised using hot conc nitric acid.
- as oxidation of a tertiary alcohol requires breaking of strong C-C bonds.
» forms same as secondary ?
..
Aldehydes, ketones and carboxylic acids are further examples of organic homologous series.
- Aldehydes and ketones contain C=O; carbonyl group.
- In aldehydes, carbonyl group is at end of carbon chain,
- in ketones, is positioned along chain.
Aldehydes are named using the suffix-al.
Ketones are named with the suffix -one.
The suffix -oic acid denotes carboxylic acid.
diagram 3
What is refluxing
When a condenser is placed on the reaction vessel
- the volatile reactants are condensed back into reaction mixture.
It prevents the volatile reactants evaporating before they have a chance to react.
How to remove carboxylic acid from mixture ?
By distillation
What is a suitable oxidising agent
for oxidation of primary and secondary alcohols
Acidified potassium or sodium dichromate(VI) solution.
> The dichromate(VI) ion, Cr2O7-2, is orange in aq
> and is reduced to the green chromium(III) ion, Cr3+, as alcohol is oxidised.
The redox equation (revise 🚨)
Cr2O72- + 14H+ + 6e- (orange)
= 2Cr3+ + 7H2O (green)
Practical: Preparation of ethanal (bpt 21 °C)
Ethanal can be prepared in the laboratory using the following method.
• Place 7.5 g of sodium dichromate(VI) into a pear-shaped flask.
• Add 15 cm of water.
•Set up the apparatus for distillation and slowly add a mixture of 3 cm’ of conc sulfuric acid and 6 cm of ethanol from a tap funnel into the pear-shaped flask.
• Heat gently until approximately 6 cm of distillate is collected.
The distillate will contain a mixture of ethanal, water and ethanol.
Test for aldehydes and ketones (tollens reagent)
aldehydes can be further oxidised and ketones cannot easily be oxidised
> basis for a chemical test to distinguish between them.
• Tollens’ reagent - the silver mirror test
- Tollens’ reagent, a colourless solution of silver nitrate and dilute ammonia,
- also known as ammoniacal silver nitrate.
- contains the complex ion [Ag(NH3)2]+.
It is a mild oxidising agent.
..
- When Tollens’ reagent is warmed gently in the presence of an aldehyde,
- the aldehyde is oxidised to carboxylic acid and Ag+
- in the Tollens’ reagent are reduced to silver atoms, Ag(s)
RCHO + [O] → RCOOH
Ag+(aq) + e- → Ag(s)
The silver atoms are deposited on sides of the reaction vessel
..
Ketones will not react with Tollens reagent
> mixture remains colourless and silver mirror does not form.
Test for aldehydes and ketones (fehlings solution)
Fehling’s solution contains copper(I1) ions, Cu2+.
> also a mild oxidising agent.
When warmed gently with an aldehyde,
- the blue colour gradually disappears
- an orange-red precipitate of copper(I) oxide, Cu2O, forms.
Cu2+ + e → Cu+
The solution remains blue when warmed with a ketone
..
Fehling’s solution is freshly prepared in the laboratory.
- made initially as two separate solutions, Fehling’s 1 and Fehling’s 2,
»_space; wch are added together immediately before use.
Fehling’s 1 contains the copper(II) ions as aq copper sulfate;
Fehling’s 2 contains sodium hydroxide solution.
What are Elimination reactions of the alcohols (dehydration reaction)
An elimination reaction is where a small molecule is removed (eliminated)
- from a reactant molecule.
- A molecule of water can be eliminated from an alcohol to produce alkene.
also be described as a dehydration reaction.
- A dehydration reaction is chemical reaction where water mol is eliminated.
-
- Alcohols can be dehydrated using conc sulfuric acid as catalyst at 170°C
-
- or by passing alcohol vapour over a heated aluminium oxide catalyst, Al2O3, at 600°C (diagram 4)
..
The mechanism for the elimination of water from alcohols is shown
> uses sulfuric acid as a catalyst.
DIAGRAM 5
1) one lone pair of electrons on oxygen picks up H+ ion from H2SO4;is protonated.
2) HSO4 - ion is produced.
3) water molecule is lost from protonated alcohol forming carbocation.
4) Finally a HSO4 - ion removes an H+ ion from carbocation
5)a double bond forms between two carbon atoms.
> mechanism can be simplified for any acid catalyst using H+ instead of the full structure of sulfuric acid.
Elimination reactions in symmetrical and unsymmetrical alcohols - dehydration
When propan-2-ol (a symmetrical alcohol) is dehydrated in elimination reaction
- the oxygen and hydrogen atoms of -OH group are eliminated from mol
- with either H atom from the carbon on the right, or H from carbon on the left
It doesnt matter wch hydrogen atom is eliminated as product is always propene
- but when water mol is eliminated from asymmetrical alcohol
- a mixture of isometric products are formed
..
Eg. Dehydrated butan2ol
- three possible products.
• But-1-ene formed when -OH group and left H is eliminated (shown in red).
• But-2-ene formed when -OH group and right H is eliminated (shown in blue).
•
• But-2-ene is a pair of geometric isomers or E-Z isomers.
> product mixture will contain all three elimination products.
diagram 6
Industrial production of ethanol (by fermentation)
Ethanol is manufactured on an industrial scale
»_space; by fermentation of carbohydrates and by the reaction of steam with ethene.
Ethanol production by fermentation
• Fermentation occurs
- when yeast and bacteria convert sugars
- to alcohols, acids and various gases like carbon dioxide and methane.
-
- Eg. yeast converts glucose
- to ethanol and carbon dioxide.
C6H12O6 (aq) → 2C2H5OH (aq) + 2CO2 (g)
glucose → ethanol + carbon dioxide
..
Reaction is carried out in specific conditions..
1) In presence of yeast, yeast produces enzymes wch convert sugars into methanol.
2) in the absence of air (anaerobic conditions) to prevent oxidation of ethanol to ethanoic acid (vinegar).
3) at 35°C. Reaction is too slow below 25°C, and yeast enzymes denature above 40°C, hence a compromise temp is used.
4) in a neutral aqueous solution.
The yeast is killed by concs of ethanol above approximately 14%.
» ethanol is removed from reaction mixture by fractional distillation.
»_space;It can be used to make biofuel.
Industrial production of Ethanol by hydration of ethene
C2H4 + H2O(g) = C2H5OH
ethene + steam = ethanol
Conditions
1) catalyst of conc phosphoric acid absorbed on a solid silica surface
2) 60 atm pressure
3) 600 K temperature
4) excess ethane to give a high yield.
..
The mechanism for
the formation of ethanol by reaction of steam in presence of a phosphoric acid (H3PO4) catalyst is:
DIAGRAM 7
The mechanism is electrophilic addition.
- In mechanism the phosphoric acid is often represented as H+,
- carbocation forms then water adds on
to form a protonated alcohol.
- A proton (H+) is then removed, regenerating catalyst and forming alcohol.
-
- Remember for unsymmetrical alkene; stability of carbocation determines the major product.
(environmental/economic) advantages and disadvantages of both of the industrial processes used to produce ethanol
ECONOMIC
1) Process type
• fermentation
Batch process
- everythings put in a container
- then left until fermentation is complete.
- that batch is cleared out then
- a new reaction set up. This is inefficient.
• hydration of ethene
A continuous flow process.
- A stream of reactants is passed continuously over a catalyst.
- Continuous flow is a more efficient way of carrying out a chemical reaction
- on an industrial scale than batch process.
2) Rate of reaction
• fermentation
Very slow
• hydration of ethene
Very fast
3) Product quality
• fermentation
Produces very impure ethanol
> needs further processing
• hydration of ethene
Produces much purer ethanol
..
ENVIRONMENTAL
3) Reaction conditions
• fermentation
Uses gentle temps and atmopheric pressure
• hydration of ethene
High temps and pressures; lots of energy input
4) use of resources
• fermentation
Renewable plant based resources
• hydration of ethene
Non renewable resources based on crude oil
What us biofuel
Fermentation of carbohydrates produces dilute solutions of ethanol
- wch we know as wine or beer and also bioethanol, a biofuel.
-
- biofuel is any fuel made from living organisms (eg crops) or their waste
Bioethanol is ethanol made from the fermentation of sugar beet followed by fractional distillation.
- Theoretically bioethanol is a carbon-neutral fuel
»A carbon-neutral activity is one where theres no net annual emissions of carbon dioxide into atmosphere.
A carbon-neutral fuel uses the same amount of carbon dioxide from atmosphere in its production
> as is released into atmosphere upon its use.
CO2(emissions) - CO2(uptake) = 0
..
In theory,
- the mass of CO2 released into atmosphere by production and combustion of ethanol
- is equal to the mass of CO2 used during photosynthesis
Three equations.
- Production of glucose via photosynthesis
- Fermentation of glucose producing ethanol
- Combustion of ethanol to produce energy
Production of glucose via photosynthesis:
6CO2 + 6H2O → C6H12O6 + 6O2
(15.1)
Fermentation of glucose to produce ethanol:
С6H12O6 → 2С2H5ОH + 2CO2
(15.2)
Combustion of ethanol to produce energy:
CH5OH+ 3O2 → 2CO2 + 3H2O
(15.3)
To what extent is bioethanol considered a carbon neutral fuel
Adding Equation 15.1 to Equation 15.2 and (2 x Equation 15.3)
> to use all of the ethanol produced via the fermentation process
> shows that bioethanol can be considered a carbon neutral fuel.
..
However, Energy is required at each stage of production described by Equations 15.1-15.3.
- This energy at present is almost certainly produced by burning fossil fuels,
- wch releases carbon dioxide to atmosphere.
- The plants wch are grown to provide the glucose, have to be planted and cared for; - The production of fertilisers and pesticides used requires energy.
- The plant must be harvested and transported to fermentation facility.
- ethanol is extracted from fermentation mixture by fractional distillation
- after this energy intensive separation process, is transported to the fuel pumps.
..
Hence, bioethanol is not truly a carbon neutral fuel.
The fossil energy balance can be improved.
> eg. using the waste from the plants, like the stalks, to provide heat used in process.
How is fermentation much better and environmentally friendly compared to dehydration of ethene
Biofuels account for approximately 3% of global fuel use.
> Biobutanol, butan-1-ol, is more efficient fuel than ethanol, with longer hydrocarbon chain
> used in petrol engines without needing engine modification.
Apart from carbon neutrality
a major advantage of the use of glucose from crops
> as the crops for production of ethanol are renewable/sustainable
>
Also disadvantages to the use of crops for the production of ethanol.
> Our food supply may deplete, as increasingly land is used to grow crops for fuel; an ethical issue.
> Production of crops is subject to the weather and climate.
> long time to grow the crops.
> product is mixture of water and ethanol; requires separation/further processing.
..
Some research avenues involving ethanol and butan-1-ol are
- Alcohols will undergo elimination reactions to produce alkenes
- used to make polymers.
- Polymers have proven to have almost limitless uses
- Most polymers are derived from crude oil
Imagine still if we could use bacteria to ferment crops/waste organic material
> to produce alcohols with longer carbon chains and subject to dehydration
> to produce a greater range of alkenes
and extend range of polymers formed from non-crude oil sources.
