Alcohols Flashcards
What are the 2 methods of industrially producing ethanol
Hydration of alkenes in the presence of an acid catalyst
Fermentation of glucose
What is the functional group of alcohols
-OH (hydroxyl group)
What is the general formula for alcohols
CnH2n+1OH
What is a primary alcohol
The carbon atom that is bonded to the -OH group is attached to one other carbon atom
Structural Isomers
Compounds that have the same molecular formula but different structural formula
Why do alcohols have higher boiling points than alkanes
There are hydrogen bonds between molecules of the alcohol which are stronger than Van der Waals forces between molecules of the alkane, so more energy is needed to separate molecules of the alcohol
Why are the lower members of alcohols soluble in water
They can form hydrogen bonds with water molecules
Why is methanol soluble in water
Methanol can form hydrogen bonds with water molecules because it has a polar -OH group
In an alcohol where is the hydrogen bond formed
Between the delta positive H atom of an alcohol molecule and the delta negative O atom of a water molecule.
Why are alcohols good solvents
They have a polar -OH group and a non-polar hydrocarbon chain so they can form both hydrogen bonds and VdW forces.
Why does the solubility of alcohols decrease as the Mr increases
As the molar mass increases, the non-polar hydrocarbon chain which is insoluble becomes larger, hence the solubility decreases.
What shaped molecules are non-polar and why
Trigonal planar
Tetrahedral
Octahedral
(and all the bonds are the same)
This is because the molecule is symmetrical and dipoles cancel
What shaped molecules are polar and why
Bent, trigonal pyrimidal or trigonal bipyrimidal
This is because the molecule in unsymmetrical and dipoles do not cancel
How is ethanol made by the catalytic addition of steam to ethene
By heating ethene with steam at 350 degrees and 60 atmospheres, in the presence of concentrated phosphoric acid as a catalyst
What type of reaction is the catalytic addition of steam to ethene
Continuous
Conditions needed for the catalytic addition of steam to ethene
High temp (350 degrees)
High pressure (60 atm)
H3PO4 catalyst
What type of reaction is making ethanol by fermentation
Batch process
How is ethanol made pure and separated from the reaction mixture
Fractional distillation
Reaction for fermentation
C6H12O6 (aq) ———–> 2C2H5OH(l) + 2CO2(g)
What are the conditions for fermentation
Yeast
25C <_ T <_ 42C
Anaerobic conditions
Advantages of making ethanol by fermentation
Uses renewable resources such as sugarcane
Carbon neutral
Disadvantages of making ethanol by fermentation
Slow reaction with low yield
Ethanol not pure so has be separated by fractional distillation
Large areas of land needed to grow sugarcane
Low atom economy process
Advantages of making ethanol by catalytic addition of steam to ethene
Continuous process therefore higher yield
Ethanol is pure as only one product formed
Atom economy of 100%
Disadvantages of making ethanol by the catalytic addition of steam to ethene
Ethene is made from crude oil which is a non-renewable resource
Expensive as it needs a high temp and pressure
Not carbon neutral
What is ethanol produced by fermentation used as after being separated by fractional distillation
A biofuel
Biofuel
Fuel produced from a renewable resource
Meaning of carbon-neutral in the context of biofuel
The fuel is carbon-neutral as the CO2 given out when the fuel burns almost equals the carbon dioxide that was taken in from the atmosphere by the plants. As a result there is no net carbon dioxide emission to the atmosphere when this fuel is burned.
What is formed when an alkene reacts with steam in the presence of an acid catalyst (concentrated phosphoric acid)
An alcohol
Describe the mechanism for the formation of an alcohol when an alkene reacts with steam in the presence of an acid catalyst.
- Draw the displayed formula of the alkene, with an arrow coming from the double bond to a single H+ ion and this will form a carbocation as the double bond breaks.
- Draw the carbocation reacting with a molecule of H2O draw out with the partial charges and the electron pair. There will be an arrow from the electron pair on the O going to the C+ on the carbocation.
- The H2O all add onto where the C+ is, on top of it and the O now has a + charge.
- There is now an arrow coming from the middle of the OH bond , to the O+ ion.
- The H with the arrow coming out of it leaves as a H+ ion and leaves an alcohol with a double bond on the oxygen atom.
What happens when alcohols burn in air/oxygen
They produce carbon dioxide and water and liberate heat energy
What 2 other products are formed in the incomplete combustion of alcohols
Carbon and Carbon monoxide
What observation can be made about the incomplete combustion of alcohols
Sooty flame / black smoke / black fumes
What is formed when a primary alcohol is oxidised
An aldehyde
What forms when an aldehyde is oxidised
A carboxylic acid
What is formed when a secondary alcohol is oxidised
A ketone
What is formed when a tertiary alcohol is oxidised
There is no reaction and no product as they are not easily oxidised
What is a suitable oxidising agent
Acidified potassium dichromate(VI) / potassium dichromate and dilute sulfuric acid
how do you show an oxidant in an equation where alcohols are being oxidised
[O]
What do aldehydes look like
There is a double bond coming off the carbon with an oxygen attached and a hydrogen atom coming off another single bond from the same carbon.
Ends in -al
What do ketones look like
They are always found in the middle and have just a single double bond coming off the carbon with an oxygen atom attached
ends in -one
What do carboxylic acids look like
Always on the end and have a double bond coming off the carbon atom with a oxygen atom attached and a single bond also coming off the same carbon atom with an OH attached
What is formed when a primary alcohol is partially oxidised
An aldehyde and water
Conditions for the oxidation of a primary alcohol to make an aldehyde
Heat and distil off the aldehyde as it forms
How do you ensure the maximum possible yield of the carboxylic acid is formed
Heat the mixture under reflux
Use excess oxidising agent
How do you prevent further oxidation of the aldehyde to the carboxylic acid
Distil off the aldehyde as it forms
Use excess alcohol
Explain how the method used to oxidise a primary alcohol determines whether an aldehyde or carboxylic acid is formed
Carboxylic acid :
Use excess acidified potassium dichromate instead of excess alcohol and heat the reacting mixture under reflux instead of distilling off the product.
What happens when a tertiary alcohol is oxidised
It doesn’t get oxidised and so there is no change and the solution remains orange
Describe the set-up for distillation
There is a round bottomed flask, whith a thermometer coming out of the top and heat being applied at the bottom ( electric heater)
The thermometer bulb should be level with the side arm leading to the condenser to measure the BP of the liquid that distils out
Attached to the flask is also a Liebig condenser, with water out on the top and water in on the bottom.
There is also a collection flask which is chilled with ice.
Why does water go in through the bottom of the condenser
To go against gravity to allow more efficient cooling and keep the condenser full of water
Why are electric heaters used to heat organic materials
Organic chemicals are flammable
Describe the setup for the laboratory preparation of an aldehyde
There is a round bottomed flash with an electric heater at the bottom.
There is a thermometer coming out of the round bottom flask.
There is a leigbig condensor attached to the round bottom flask (NOT VERTICALLY). The water in is through the bottom and the water out is through the top so that the condensor remains fully of water and there is efficient cooling.
There is an ice flask used to collect the aldehyde below the leibig condensor
Why is the collecting flask in distillation surrounded by an ice-water mixture
To prevent the aldehyde from evaporating as it has a low boiling point
Which method do you use to make an aldehyde from a primary alcohol
Distillation
Which method do you use to make a carboxylic acid from a primary alcohol
Heat under reflux
What does anhydrous sodium sulphate and anhydrous calcium chloride do / act as
It is added to the distillate as it is the drying agent and will remove water and dry the product.
When doing reflux, why is the end of the condenser never sealed
As the build up of gas pressure could cause the apparatus to explode
Primary alcohol + [O] =
aldehyde
Aldehyde + [O] =
Carboxylix acid
Secondary alcohol + [O] =
Ketone
Describe the test to distinguish between an aldehyde and ketone using Tollen’s reagent
Add Tollen’s reagent to the compound and warm.
Aldehyde - A silver mirror is formed
Ketone - There is no change. Remains colourless
What is Tollen’s reagent
A mild oxidising agent, which is a solution of silver nitrate in aqueous ammonia.
Why is the reaction of ethanal with Tollen’s reagent a redox reaction
Ethanal is oxidised to carboxylic acid and Ag+ ions are reduced to Ag
Why does propanone not react with Tollen’s reagent
It is a ketone and cannot be oxidised by Tollen’s reagent
Describe the test with Fehling’s solution to distinguish between an aldehyde and a ketone
Add Fehling’s solution to the compound and warm.
Aldehyde - A brick red precipitate is formed.
Ketone - There is no change. The solution remains blue
What is Fehling’s solution
A mild oxidising agent, which contains blue copper(II) ions which oxidise aldehydes and not ketones.
What is the reason for heating under reflux
Reflux allows heating for a long time without losing the volatile reactants and products. Refluxing helps speed up chemical reactions without losing the volatile reactants and products by evaporation.
What is the function of the condenser when heating under reflux
It condenses the hot vapour into liquid and the liquid drips back into the flask.
This prevents the escape of volatile reactants and products and allows the reaction to continue.
What happens and is formed when alcohols are dehydrated
When alcohols are dehydrated with excess hot concentrated sulfuric acid, an alkene is formed and water
Describe what happens generally in the dehydration of an alcohol (NOT THE MECHANISM)
- First remove the -OH
- Then remove the H from the C next to the C attached to the -OH
- An alkene is formed and water
What is formed when a secondary alcohol is dehydrated
A mixture of alkenes
Why is a mixture of alkenes formed when a secondary alcohol is dehydrated
Dehydration involves the removal of OH and H to make water, and H can be removed from either C1 or C3 so 2 alkenes and water are formed
Describe the mechanism for the dehydration of an alcohol
- Draw the partial charges on the O and H of the OH and the H+ ion
- Draw an arrow from the electron pair of the O on the OH to a H+ ion this forms just an alcohol with a + on the O and the H adds onto the O
- From this alcohol structure draw an arrow from the middle of the C-H on the adjacent C, to the C-C and then another arrow from the middle of the C-O bond to the O. (NORMAL ELIMINATION MECHANISM HERE)
- This forms an alkene and H+ and water
What is the name of the reaction between an alcohol and HBr/Cl/I
Nucleophillic substitution
What is the name of the reaction between an alcohol with K2Cr2O7 and dilute H2SO4
Oxidation/redox
What is the name of the reaction between an alcohol with concentrated H3PO4 / H2SO4
Elimination
