2.10- Alcohols Flashcards
How do you name alcohols?
Have the ending -ol and if necessary the position number for the OH group is added between the name stem and the -ol
If the compound has an -OH group in addition to other functional groups that need a suffix ending then the OH can be named with the prefix hydroxy-
If there are two or more -OH groups then di, tri are used. Add the ‘e’ onto the stem though
What is the general formula for alcohols?
CnH2n+1OH
What are the bond angles in alcohols?
All the H-C-H bonds and C-C-O are 109.5° (tetrahedral shape), because there are 4 bond pairs of electrons repelling to a position of minimum repulsion
The H-O-C bond is 104.5° (bent line shape), because there are 2 bond pairs of electrons and 2 lone pairs repelling to a position of minimum repulsion
Lone pairs repel more than bond pairs so the bond angle is reduced
What are the 3 different types of alcohols?
Primary alcohols are alcohols where 1 carbon is attached to the carbon adjoining the oxygen
Secondary alcohols are alcohols where 2 carbon atoms are attached to the carbon adjoining the oxygen
Tertiary alcohols are alcohols where 3 carbon atoms are attached to the carbon adjoining the oxygen
What is the oxidising agent used when alcohols are oxidised?
Potassium dichromate K2Cr2O7
Partial oxidation of primary alcohols?
Reaction: primary alcohol –> aldehyde
Reagent: potassium dichromate (VI) solution and dilute sulphuric acid
Conditions: (use a limited amount of dichromate) warm gently and distil out as the aldehyde forms
Observation: the Orange dichromate ion reduces to the green Cr3+ ion
What is an aldehyde?
Always has the C-O double bond on the first carbon of the chain so does not need an extra number
Ends in -al
When writing the formulae of aldehydes in a condensed way write CHO and not COH
Eg CH3CH2CHO
How is distillation used in the partial oxidation of primary alcohols?
In general used as separation technique to separate an organic mixture from its reacting mixture. Need to collect the distillate of the approximate boiling range of the desired liquid.
Note the bulb of the thermometer should be at the T junction connecting to the condenser to measure the correct boiling point
Note the water goes in the bottom of the condenser to go against gravity. This allows more efficient cooling and prevents back flow of water
Full oxidation of primary alcohols?
Reaction: primary alcohol –> carboxylic acid
Reagent: potassium dichromate (IV) solution and dilute sulphuric acid
Conditions: use an excess of dichromate, and heat under reflux: (distill off product after the reaction has finished)
Observation: the Orange dichromate ion reduces to the green Cr3+ ion
How is reflux used in the full oxidation of primary alcohols?
Reflux is used when heating organic reaction mixtures for long periods. The condenser prevents organic vapours from escaping by condensing them back to liquids
Never seal the end of the condenser as the build up of gas pressure could cause the apparatus to explode. This is true of any apparatus where volatile liquids are heated including the distillation set up
Anti-bumping granules are added to the flask in both distillation and reflux to prevent vigorous, uneven boiling
Oxidation of secondary alcohols?
Reaction: secondary alcohol –> ketone
Reagent: potassium dichromate (IV) solution and dilute sulphuric acid
Conditions: Heat under reflux
Observation: the Orange dichromate ion reduces to the green Cr3+ ion
What are ketones?
End in -one
C-O double bond
When ketones have 5C’s of more in a chain then it needs a number to show the position of the double bond
Eg pentan-2-one
Why can’t tertiary alcohols be oxidised by potassium dichromate?
This is because there is no hydrogen atom bonded to the carbon with the OH group
How do you distinguish between aldehydes and ketones using tollen’s reagent?
Reagent: Tollen’s reagent formed by mixing aqueous ammonia and silver nitrate. The active substance is the complex ion of (Ag(NH3)2)+
Conditions: Heat gently
Reaction: aldehydes only are oxidised by tollen’s reagent into a carboxylic acid and the silver(I) ions are reduced to silver atoms
Observation: with aldehydes, a silver mirror forms coating the inside of the test tube
Ketones result in no visible change
How do you distinguish between aldehydes and ketones using Fehling’s 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 and the copper ions are reduced to copper(I) oxide
Observation: Aldehydes- blue Cu2+ ions in solution change to a red precipitate of Cu2O
Ketones don’t react
How can the presence of a carboxylic acid be tested?
By addition of sodium carbonate. It will fizz and produce carbon dioxide
Reaction of alcohols with dehydrating agents?
Reaction: alcohol –> alkene
Reagents: concentrated sulphuric or phosphoric acids
Conditions: warm (under reflux)
Role of reagent: dehydrating agent/catalyst
Type of reaction: acid catalysed elimination
What is a dehydration reaction?
Removal of a water molecule from a molecule
What advantage does producing alkenes from alcohols have?
Provides a possible route to polymers without using monomers derived from oil
What is fermentation?
When glucose is converted into ethanol and carbon dioxide
C6H12O6 –> 2CH3CH2OH + 2CO2
What are the conditions needed for fermentation?
Yeast
No air
Temperature 30-40°C
Why is the optimum temperature for fermentation around 38°C?
At lower temperatures the reaction is too slow
At higher temperatures the yeast dies and the enzymes denature
Why is fermentation done in an absence of air?
Because the presence of air can cause extra reactions to occur. It oxidised the ethanol to produce ethanoic acid (vinegar)
What are the advantages of fermentation?
Sugar is a renewable resource
Production uses low level technology/cheap equipment
What are the disadvantages of fermentation?
Batch process which is slow and gives high production costs
Ethanol made is not pure and needs purifying by fractional distillation
Depletes land used for growing food crops
What is hydration of ethene?
Ethene produced from cracking of fractions from distilled crude oil can be reacted with steam to produce ethanol
What are the essential conditions required for the formation of ethanol from the hydration of ethene?
High temperatures (300°C)
High pressure 70 atm
Strong acid catalyst of concentrated Phosphoric acid (H3PO4)
What are the advantages of the hydration of ethene?
Faster reaction
Purer product
Continuous process (which means cheaper manpower)
What are the disadvantages of the hydration of ethene?
High technology equipment needed (expensive initial costs)
Ethene is non-renewable resource (will become more expensive when raw materials run out)
High energy costs for pumping to produce high pressures
What is a biofuel?
A fuel produced from plants
Ethanol produced from fermentation is a biofuel
Is ethanol carbon neutral?
It can be argued that ethanol produced from fermentation is carbon neutral as any CO2 given off when the biofuel is burnt would’ve been extracted from the air by photosynthesis when the plant grew. There would be no net CO2 emission into the atmosphere
This does not take into account any energy needed to irrigate plants, fractionally distil the ethanol from the reaction mixture or process the fuel. If the energy for this process comes from fossil fuels then the ethanol produced is not carbon neutral
Proof of ethanol being carbon neutral?
Removal of CO2 by photosynthesis:
6CO2 + 6H2O –> C6H12O6 + 6O2
Production of CO2 by fermentation and combustion:
C6H12O6 –> 2CH3CH2OH + 2CO2
2CH3CH2OH + 6O2 –> 4CO2 + 6H2O
