topic 5: alcohols Flashcards
production of ethanol 2 ways
-fermentation: carbon neutral process
-hydration of ethene - quicker and continuous reaction
fermentation of glucose process to produce ethanol + equation
- plants photosynthesise to produce glucose
- carbohydrate crops (glucose) made from plants are converted into ethanol by the action of enzymes from yeast
- it is an exothermic process called anaerobic respiration
C6H12O6(aq) → 2C2H5OH(aq) + 2CO2
conditions of fermentation process
-temperature 35-40 degrees celsius used - to prevent yeast enzyme denaturing cant be too high or low
-occurs at 60 atm atmospher pressure
-anaerobic, without oxygen - to prevent oxidation of ethanol to ethanoic acid
advantages and disadvantages of fermentation
advantages
-fermentation is cheap
-uses low technology equipment
-renewable resources
disadvantages
-batch process which is slower reaction + high production costs
-produces impure ethanol which requires fractional distillation to purify it which takes extra time and money
-depletes land used for growing food crops
biofuel definition
a fuel that is made from biological material that has recently died
the overall equation and process for fermentation and burning
- Plants photosynthesise when they grow:
6CO2 + 6H2O →C6H12O6 + 6O2
2.Then the glucose is converted into ethanol during fermentation:
C6H12O6 → 2C2H5OH + 2CO2 - Fractional distillation is used to purify the ethanol as it isn’t pure as it contains water.
4.The ethanol is then combusted in cars as biofuel:
2C2H5OH(aq) + 6O2 →4CO2 + 6H2O
So the overall equation for fermentation and burning is:
C6H12O6 + 6O2 → 6CO2 + 6H2O
carbon neutral in fermentation
-6 moles of carbon dioxide are taking and released in this process therefore it is carbon neutral
-however fossil fuels burned to power machinery and grow crops (fertilisers, harvest, refine and transport) so not carbon neutral
hydration of alkenes in the presence of an acid catalyst process
- Ethene is produced when crude oil fractions are obtained and cracked thermally (zeolite catalyst).
- Ethene is then hydrated where water is added over the double bond.
- This reaction required the presence of an acid catalyst (phosphoric acid).
conditions for the hydration of ethene
-temperature high of 300 degrees celsius, so that the double bond can break
-the pressure needs to be high 60 atm pressure so water can break the double bond
advantages + disadvantages of hydration of ethene
advantages
-continuous process
-faster reaction
-makes ethanol that is very pure
disadvantages
-raw material is crude oil which is non renewable (will become more expensive when raw materials run out)
-high technology equipment needs so expensive costs
-high energy costs for pumping to produce high pressure
classification of alcohols
-primary: 1 alkyl group attached
-secondary: 2 alkyl groups attached
-tertiary: 3 alkyl groups attached
reactions of alcohols - oxidation
primary: alcohol —-> aldehyde —-> carboxylic acid
-primary alcohols can be oxidised to form aldehydes which can be further oxidised to carboxylic acids
secondary: alcohol —-> ketone
-secondary alcohols can be oxidised to ketones
tertiary: cannot be oxidised
reactions of alcohols - elimination
dehydration: alcohol —> alkene
oxidising agent
-acidified potassium dichromate K2Cr2O7 is an oxidisin agent, that causes alcohols to oxidise
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partial oxidation of primary alcohols - reagents, conditions + observations
PARTIAL OXIDATION
primary alcohol —-> aldehyde
reagents: acidified potassium dichromate and dilute sulfuric acid
conditions: use a limited amount of dichromate + warm gently and distill out the aldehyde as it forms which has a low bp so boils off quickly
observations: orange Cr6+ reduce to green Cr3+ ions as they oxidise the alcohol
full oxidation of primary alcohols
FULL OXIDATION
primary alcohol —-> carboxylc acid
conditions: use an excess of dichromate so there is excess oxygen to fully oxidise the alcohol and heat under reflux - distill of product after the reaction has finished
same reagents and observations
oxidation of secondary alcohols
secondary alcohols —-> ketone
reagents: acidified potassium dichromate solution and dilute sulfuric acid
conditions: heat under reflux
observations: Orange Cr+6 ions are reduced to green Cr3+ ions as they oxidise the alcohol
There is no further oxidation of the ketone under these conditions as there is only one hydrogen to be removed.
distinguish between aldehydes and ketones
-Aldehydes can be further oxidised to carboxylic acids whereas ketones cannot be further oxidised
-This is the chemical basis for two tests that are commonly used to distinguish between aldehydes and ketones.
-Can be done with Tollens’ reagent or Fehling’s solution.
tollens reagent
REAGENT: Tollens’ reagent formed by mixing aqueous ammonia and silver nitrate
CONDITIONS: heat gently
REACTION: aldehydes only are oxidised by Tollens reaegnt into a carbolxyic acid. The silver(I) ions are reduced to silver atoms (Ag+1 + e- →Ag)
OBSERVATION: with aldehydes, a silver mirror (Ag0) forms coating the inside of the test tube
-ketones result in no visible change
fehlings solution
REAGENT: Fehling’s solution containing blue Cu2+ ions.
CONDITIONS: Heat gently
REACTION: Aldehydes only are oxidised by Fehling’s solution into a carboxylic acid. The copper (II) ions are reduced to copper(I) oxide. (2Cu2+)
OBSERVATION: Aldehydes: blue Cu2+ ions in solution change to a red-orange precipitate of Cu2O (Cu+)
-Ketones do not react.
oxidation of tertiary alcohols
-cannot be oxidised at all by potassium dichromate - This is because there is no hydrogen atom bonded to the carbon with the -OH group
-There is no hydrogen to remove to replace with an oxygen
-this is why ketones cant be oxidised either
dehyration of alcohols elimination - reagents, conditions + reaction
alcohol —-> alkene
REAGENTS - Concentrated sulfuric or phosphoric acids (H2SO4 or H3PO4)
CONDITIONS - heat under reflux, need a H on the adjacent carbon where the alcohol group is.
REACTION - gives major and minor products.
