.5 Alcohols Flashcards
Alcohols: Functional group, mechanisms, soulbility, bonding strength(compare to alkanes), general formula, conditions. shape
Funct group: OH- hydroxyl
Mechanism: Elimination- Alcohol - H2O—->Alkene
Gen Formula: CnH2nOH
Bonding: Strong IMF- H bonding so stronger IMF than alkanes with VDW
Solubility: Due to the OH bond in the alcohols, hydrogen bonds can form between alcohol and water molecules increasing their ability to mix.
Shape: In alcohols, the oxygen atom has two bonding pairs of electrons and two lone pairs. The C-O-H angle is about 105° because the 109.5° angle of a perfect tetrahedron is ‘squeezed down’ by the presence of the lone pairs. These two lone pairs will repel each other more than the pairs of electrons in a covalent bond.
Primary, secondary and tertiary of alchols
Primary alcohols can be oxidised to aldehydes which can be further oxidised to carboxylic acids.
1 R, 2Hs
Secondary alcohols can be oxidised to ketones.
2Rs, 1H
Tertiary alcohols are not easily oxidised.
3R, No H
Compare fermentation and hydration, advantages of each
check in booklet
Fermentation:Easier: Lower temperature and pressure
Renewable materials
Hydration: Quick
Pure ethanol
The fermentation process, equation, conditions etc
During fermentation, carbohydrates from plants are broken down into sugars and then converted in to ethanol by the action of enzymes from yeast.
The carbohydrates come from crops such as sugar cane and sugar beet.
The key step is the breakdown of sugar in a process called anaerobic respiration:
enzymes from yeast C6H12O6(aq) 2CH5OH(aq) + 2CO2 (g)
- The rate of this chemical reaction is affected by temperature. It is slow at low temperatures but the enzymes are made ineffective if the temperature is too high. A compromise temperature of about 35 °C, a little below our body temperature, is used .
- Air is kept out of the fermentation vessels to prevent oxidation of ethanol to ethanoic acid (the acid in vinegar).
- Once the fermenting solution contains about 15% ethanol the enzymes are unable to function and fermentation stops. Ethanol may be distilled from this mixture by fractional distillation as its boiling temperature (78 °C) is less than that of water (100 °C).
Hydration of ethene process. draw mechanism
Alcohols react with water in the presence of a strong acid (phosphoric acid)- the acid acts as a catalyst
At present ethene is made from crude oil and then convened into ethanol. In the future it may become more economical to make ethene from ethanol made by fermentation
Ethanol as biofuel- biofuel and renewable definitions, downsides
- Ethanol prod by fermentation is renewable and a biofuel
- A RENEWABLE substance is one that can be replaced when it has been used
- A BIOFUEL is a renewable fuel made from plants
- Biofuel ethanol is carbon-neutral fuel- releases same CO2 when burned as when it was taken in from atmosphere when plants grew and photosynthesised ie no net CO2 emissions to the atmosphere
- However, in the process of going from crop to uding the fuel, there are other processes to consider including fuel for machinery and transport meaning it’s not 100% carbon neutral
- There are also growing concerns that too much agricultural land is being used for growing crops to make biofuels rather than to grow food
Oxidation of Primary Alcohols: What are they, draw structure, reagent used, do the three equations and important notes about the reactions (reagents colour changes mostly), tests to distinguish aldehydes and ketones
Primary- Have two or three Hs attached to carbon
ACIDIFIED Potassium dichromate (VI) and DILUTE Sulphuric Acid
Check equations in booklet
Important notes:
Potassium Dichromate - C2O7-2 ions (Cr +6) reduced to GREEN Cr3+ as they oxidise the alcohol/aldehyde
-To prevent Odation of aldehyde to carboxylic acid, it is removed by distillation as it is formed (aldehyde has lower bp than the alcohol as it is w/o H bonding)
Tollen’s Reagent- contains (Ag(NH3)2)+ ions (Ag +1) which are reduced to silver mirror (Ag 0) as it oxidises the aldehyde to a carboxylic acid
-Reaction is a simple test to distinguish aldehydes from ketones
Fehling’s solution (Benedict’s)- Blue solution containing Cu2+ ions which are reduced to red-orange precipitate of Cu2O as it oxidises aldehyde to carboxylic acid
-Reaction is a simple test to distinguish aldehydes from ketones
Oxidation of Secondary alcohols: structure, product, do equation, why ? reaction would be used instead of ?
1 H attached to C
Reflux instead of heating to stop products escaping as they are very volatile
No further oxidation
Look in booklet
Oxidation of Tertiary alcohols
No Hs attached directly to C
These are not easily oxidised (but they are if you set fire to them!) as you would have to break C-C bonds – all the other oxidations involve the breaking of C-H bonds. [This also explains why ketones are not easily oxidised].
What is an elimination reaction? What products formed in elimination reaction of alcohols? Do mechanism, catalyst, conditions and apparatus, benefits
One in which a small molecule is removed from a reactant molecule.
A molecule of water can be removed from an alcohol to form an alkene
This reaction can also be described as a dehydration reaction as the molecule being eliminated is water
Forms alkene
Mechanism= Elimination Type= Dehydration
Check in booklet
Heat (170 oC) with concentrated H2SO4 (as a catalyst) Or Pass the alcohol vapour over heated catalyst of Al2O3 at 600 oC
Alkenes produced by this method can be used to produce addition polymers without using monomers derived from crude oil.
Draw apparatuses for Preparation of ethanoic acid
Reflux: Lets product react even further. Products evaporate out of flask, cool, back into flask, continue to react.
Round bottom flask(contains sulphuric acid and sodium dichromate and AB granules) condenser, ethanol and water at the top
followed by…
Distillation: Contains excess potassium dichromate, removes product
Round bottom flask (cont reflux reaction products), condenser, into test tube
Check in booklet
Preparation of ethanal, draw apparatus, label, explain why cooled at the end
Check in booklet
Flask with conc H2SO4, water and AB granules, dropping funnel containing sodium dichromate, water and ethanol, condenser, conical flask in ice
In ice to stop ethanal evaporating - has BP of 25C
Check in booklet
Oxidation of Primary: How to get from Primary alcohol to aldehyde to carboxylic acid
How to get from Primary alcohol to carboxylic acid
Primary alcohol (K2Cr2O7 + H2SO4) (heat and distill off product)—–Aldehyde (K2Cr2O7 + H2SO4) (heat or reflux)—- Carboxylic acid
Primary alcohol (K2Cr2O7 + H2SO4)(reflux)—-carboxylic acid
Do Reactions of Alcohols 1 Task- very good practice esp 1, 5, 6, 11 but do all
In booklet
Write an equation for the oxidation reaction of propane-1,2-diol CH2(OH)CH(OH)CH3 under reflux, using (O) to represent the oxidising agent
Show the displayed formula of the organic product
In booklet
Draw a labelled diagram to show how you would set up apparatus for refluxing
In booklet-
Need flask attached to vertical condenser (no gaps)- condenser has jacket and open lid
Test for alcohols (distinguishes…from…)- why always heat alcohols in a water bath?
Test: Acidified dichromate
Place about 1cm3 of potassium dichromate (VI) in a solution test tube
Add 5cm3 of dilute sulphuric acid
Add a few drops of the organic compound
Heat in hot water bath for a few minutes
Positive result: Orange to green
Negative Result: Stays orange
Can distinguish out tertiary alcohols- they will give neg result as they don’t react
Also kind of secondary and primary because secondary makes acid quicker
Testing aldehydes and ketones- two tests and safety warnings
Tollen’s reagent: 1cm3 silver nitrate solution in clean test tube
Add drop of sodium hydroxide solution to form precipitate of silver oxide
Add dilute ammonia drop by drop until brown precipitate just redissloves (this is now Tollen’s reagent)
Add a few drops of the organic compound
Heat in hot water bath for a few minutes
Positve result: Aldehyde- silver mirror
Negative result: Ketone- No chnage- stays colourless
Fehling’s solution: 1cm3 Fehling’s sol A (or I) into boiling tube
Add Fehling’s sol B (or II) to it until blue precipitate just redissloves to give a deep blue solution
Add a few drops of the organic compound to the solution
Heat in hot water bath for few minutes
Positive: Aldehyde- red/orange precipitate
Negative: Ketone- no change (stays blue)
Safety: Silver nitrate and Fehling’s are corrosive
Ammonia is an irritant
Residues should be washed down fume cupboard sink with large amount of water
Harmful organic materials
Heat in water bath
Ethene is a vital industrial chemical; it is the starting material for poly(ethene) and many other important chemicals. Ethene can be produced by dehydrating ethanol made from sugar. giving a renewable
source or ethene.
At present ethene is made from crude oil and then converted into ethanol. In the future, what way may it become more economical to make ethene from ethanol?
fermentation
Combustion in alcohols, equation, does it burn, what is in picnic stoves
Alcohols bum completely to carbon dioxide and water if there is enough oxygen available. (Otherwise there is incomplete combustion and carbon monoxide or even carbon is produced.)
This is the equation for the complete combustion of ethanol: C2H5OH(l) + 3O2 (g) → 2CO (g) + 3H2O(l)
Ethanol is often used as a fuel, for example, in picnic stoves that burn methylated spirits.
Methylated spirits is ethanol with a small percentage of poisonous methanol added to make it unfit to drink. In this way it can be sold without the tax which is levied on alcoholic drinks. A purple dye is also added to show that it should not be drunk.
