Alcohols Flashcards
(21 cards)
Primary, secondary, and tertiary alcohols
Intermolecular forces between alcohol molecules
In an alcohol, O is more electronegative than H - the O-H bond is polar with O being delta - and H being delta +.
This results in hydrogen bonding between molecules
Comparison of the boiling point of an alcohol with its alkene equivalent
Alcohols have Hydrogen bonding between molecules whereas alkenes have much weaker van der walls forces between molecules. More energy is required to overcome the strong Hydrogen bonds - alcohols have a higher boiling point
For a substance to be soluble in a given solvent, what needs to exist between solute and solvent molecules.
How does this apply to water and alcohols
Solute and solvent must have similar intermolecular forces.
Both alcohols and water consist of hydrogen bonds allowing alcohols to hydrogen bond with water
Methods of ethanol production
Fermentation of sugar
Hydration of Ethene
Methods of ethanol production - Fermentation
Reaction:
Glucose -> Ethanol + CO2
C6H12O6 -> 2C2H5OH +2CO2
Catalyst - Yeast
Plant sugar solution is used as glucose as plants can be regrown making it renewable
Anaerobic condition
37C is optimum temp. for enzymes
Low energy needed
Low cost
Rate of reaction: Slow - takes several days
Product quality: Impure - 15% ethanol max
Type of process - A batch process. Everything is put into a container and then left until fermentation is complete.
That batch is then cleared out and a new reaction set up. This is inefficient.
Methods of ethanol production - Hydration of ethene
Ethene + Steam -> Ethanol
C2H4 + H2O(g) -> C2H5OH
Catalyst - H3PO4
300C temp
70 atm
High energy needed
High cost
Rate of reaction: fast
Product quality: pure - 97% ethanol
Type of process - A continuous process - A stream of reactants is passed continuously over a catalyst. This is more efficient
How to make hydration of ethene more cost effective
The unreacted Ethene and water can be separated from the ethanol and can be reacted again
Why is biofuel thought to be more sustainable than fossil fuels.
More plants can be grown to replace the ones used to make biofuel making biofuel renewable - better than fossil fuels which are non renewable.
Why is biofuel thought to be carbon neutral
Plants carry out photosynthesis absorbing 6 moles of CO2 and producing 1 mole of glucose for the plants.
This glucose can be taken from plants and can be fermented with yeast producing 2 moles of ethanol and 2 moles of CO2.
These 2 moles of ethanol candies be combusted to release energy and 4 moles of CO2.
In total, 6 moles of CO2 are absorbed by the plant, and 6 are released back into the atmosphere.
Why is biofuel actually not carbon neutral
Energy is needed to produce biofuels:
Planting and harvesting crops - Fuel burnt in tractors and combine harvesters
Transportation of crops to production site - lorries burn diesel releasing CO2
Distillation to concentrate ethanol to make it suitable for use as a fuel - large amount of energy needed for this (this energy mat come from burning fossil fuels)
Clearing land to make it suitable for crop production - removes trees which store CO2 - burning them releases this into the atmosphere.
Oxidation of primary alcohol
Oxidation of Secondary alcohol
Oxidation of tertiary alcohols
Tertiary alcohols can’t be oxidised further as there is no H attached to the Carbon atom with the OH attached - H2O can’t form and reaction can’t proceed.
Practical process to oxidise a primary alcohol to form an aldehyde
Use Distillation
Heat the flash and evaporate the reaction mixture (this contains alcohol and oxidising agent - e.g. K2Cr2O7)
The aldehyde will distill first as it has the lowest boiling point
Aldehyde is collected in the conical flask, separating it from the oxidising agent preventing further oxidation.
Practical process to oxidise a primary alcohol to form a carboxylic acid
Heat under reflux
Aldehydes formed will evaporate but condense back into the condenser dripping back into the flask where further oxidation takes place.
Once this is done the carboxylic acid can be separated from the oxidising agent via distillation as it will have a different boiling point to the oxidising agent
Practical process to oxidise a Secondary alcohol to form a Ketone
Heat under reflux
Once this is done the ketone can be separated from the oxidising agent via distillation as it will have a different boiling point to the oxidising agent
??????? Why not just distillation in 1 go???
What is the chemical difference between Aldehydes and ketones
Aldehydes can be oxidised, ketones cannot
Test for aldehyde - fehling’s solution
Ina test tube, add the test substance, fehling’s solution (contains Cu2+ ions) and warm gently by putting test tube in a beaker of hot water. A brick red ppt should form if an aldehyde is present - solution goes from blue to brick red.
Test for aldehyde - Tollens reagent
Tollens’ contains [Ag(NH3)2]+ and is added warm to the test substance. In an aldehyde is present, tollens’ is reduced to silver which coats the inside of the flask. Meanwhile the aldehyde is oxidised to form a carboxylic acid.
Dehydration of alkenes mechanism and conditions
Elimination producing alkene
Catalyst - conc H2SO4 / H3PO4
Conditions - reflux at 180C
