5. Alcohols Flashcards
General formula for alcohols?
CnH₂n₊₁OH
Are alcohols saturated or unsaturated?
Saturated
What is the functional group in alcohols?
-OH
What can the H in the O-H bond in alcohols do, in terms of bonding?
It can hydrogen bond with other alcohol molecules and with water
What can alcohols hydrogen bond to?
Other alcohol molecules and water
Why can alcohols form hydrogen bonds?
The H in the O-H bond
Why do alcohols have relatively high boiling points?
They can hydrogen bond
Why are many alcohols soluble in water?
They can hydrogen bond
What is it called when a molecule has a double bond and an O-H bond?
An alkenol
How can ethanol be manufactured industrially?
- hydration of alkenes
* fermentation to produce ethanol
What decides which method for ethanol to be manufactured?
Depends on desired purity of ethanol and the availability of the different raw materials in the country where is it made
What reaction takes place when alcohols are produced from alkenes?
Addition
Why does an addition reaction need to take place when alcohols are produced from alkenes?
To get rid of the double bond
What is the catalyst when alcohols are produced by hydration of alkenes?
Solid phosphoric acid catalyst (also possible with concentrated sulphuric acid)
What will be the purity of the ethanol produced by hydration of ethenes?
> 90%
What is the overall symbol equation for when ethanol is produced from ethene by hydration?
CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH
What is the overall word equation for when ethanol is produced from ethene by hydration?
Ethene + Water ⇌ Ethanol
What are the conditions for the production of alcohol through hydration of alkenes?
- solid phosphoric acid catalyst
- 300°C
- 60atm
What of type pressures and temperatures are required for hydration of alkenes?
High temperatures and pressure (300°C and 60atm)
How are alcoholic drinks produced?
Fermentation of glucose
What happens during the fermentation of glucose to form alcohol?
- carbohydrates from plants are broken down into sugars (e.g. glucose)
- this is converted to ethanol and carbon dioxide
What is the fermentation of glucose catalysed by?
The action of enzymes (zymase) in yeast
What is the optimum temperature for zymase enzymes in yeast?
30-40°C
What is the overall symbol equation for the fermentation of glucose to produce ethanol?
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Which alcohol is produced when glucose is fermented?
Ethanol
What temperature is the fermentation of glucose carried out at?
30-40°C
Why is the fermentation of glucose carried out at 30-40°C?
Any lower, reaction would be too slow and any higher, the enzymes would be denatured
Why must the fermentation of glucose be carried out in the absence of air?
To prevent oxidation of ethanol to ethanoic acid
What type of respiration is the fermentation of glucose?
Anaerobic
When can ethanol be classed as a biofuel?
When it has been produced by fermentation, as the raw material used is plant material
What is a biofuel?
A fuel produced from renewable biological resources
Is ethanol produced from ethene a biofuel?
No - ethene is from crude oil, obtained by fractional distillation
When fermenting glucose to form alcohol, what happens when the fermenting solution contains about 15% ethanol?
The enzymes are unable to function and fermentation stops
How is ethanol removed from the fermenting solution?
Fractional distillation
Why can ethanol be removed from the fermenting solution by fractional distillation?
Because ethanol and water have different boiling points
Is the combustion of ethanol exothermic or endothermic?
Exothermic
What is the balanced symbol equation for the combustion of ethanol?
C₂H₅OH + 3O₂ → 3H₂O + 2CO₂
What can the energy produced from the combustion of ethanol be used to power?
Vehicles, and is increasingly being used in cars
What is a carbon neutral activity?
When there are no net carbon emissions
Why is ethanol from fermentation of glucose said to be carbon-neutral?
- releases carbon dioxide when produced and burned
* but plant material from which it is made from absorbs carbon dioxide from the atmosphere during photosynthesis
How can ethanol produced by fermentation of glucose shown to be carbon-neutral using symbol equations?
- photosynthesis - 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (6 mols taken in)
- fermentation - C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (2 mols produced)
- combustion - 2C₂H₅OH + 6CO₂ → 4CO₂ + 6H₂O (4 mols produced)
What does proving that fermentation of glucose by symbol equations not take into account?
- planting & harvesting
- preparation of plants for fermentation
- fermentation process
- fractional distillation
- transportation
Advantages of producing alcohol by hydration of alkenes?
- cheap - ethene is cheap and not much wasted
- fast
- continuous process - low labour costs
- pure product
- no waste products
- doesn’t use crops that could’ve been used for food
- extracting is fast and reliable
Disadvantages of producing alcohol by hydration of alkenes?
- uses crude oil - non-renewable, although now fairly cheap to obtain, as resources run out it will be more expensive
- high amount of energy needed for the process
- expensive equipment needed for continuous process
Advantages of producing alcohol by fermentation of glucose?
- glucose is from plants which can be grown again - renewable
- could be seen as carbon neutral as when plant grows it takes in CO₂ for photosynthesis and balances the CO₂ released when fermented
- high energy not needed - low cost due to low temp
- places like Brazil - more employment, beneficial to economy
Disadvantages of producing alcohol by fermentation of glucose?
- 2 stage process
- very slow
- batch process
- when ethanol concentration reaches 15% yeast stops functioning and fermentation stops
- product obtained is not pure
- 15% mixture must be purified by distillation - extra cost
- CO₂ is produced
- ethical issue - whether we should use crops for ethanol where there are developing countries with no food
What are primary alcohols?
Alcohols which have one alkyl group attached to the carbon which has the -OH group attached
(ie functional group is at the end of the chain)
What are secondary alcohols?
Alcohols which have two alkyl groups attached to the carbon which has the -OH group attached
What are tertiary alcohols?
Alcohols which have three alkyl groups attached to the carbon which has the -OH group attached
What must you ensure when drawing the displayed formula for alcohols?
The bond between the O and H must be shown
What can oxidation in organic chemistry be regarded as?
The addition of oxygen; or the removal of hydrogen
How can the oxidising agent be represented in full equations?
[O]
Why is the oxidising agent represented as [O] in equations?
Full equations are quite complex
What can primary alcohols be initially oxidised to?
Aldehydes
What must be happening when alcohols are oxidised in order for aldehydes to form?
Must be distilled off
What happens in the oxidation of primary alcohols, if the aldehyde is not distilled off?
The reaction is carried out under reflux and a carboxylic acid is formed
What happens when a primary alcohol is oxidised during distillation?
An aldehyde is formed
What happens when a primary alcohol is oxidised under reflux?
A carboxylic acid is formed
When a primary alcohol is oxidised under reflux, is the oxidising agent limiting or in excess?
In excess
When reflux apparatus is set up to obtain carboxylic acid from a primary alcohol, what is the condenser for?
To stop the evaporation of the aldehyde
What is formed when a secondary alcohol undergoes oxidation?
A ketone
Why are carboxylic acids not formed from secondary alcohols?
Ketones cannot be oxidised
Why can’t ketones be oxidised?
There is no C-H bond which an oxygen atom can be inserted
Can tertiary alcohols be oxidised?
No
Why can’t tertiary alcohols be oxidised?
There no hydrogen on the carbon containing the alcohol group
What is the oxidising agent used when oxidising 1°, 2° and 3° alcohols?
Potassium dichromate
Symbol for potassium dichromate?
K₂Cr₂O₇
What colour does potassium dichromate go when it has oxidised with a 1° alcohol?
Green
What colour does potassium dichromate go when it has oxidised with a 2° alcohol?
Green
What colour does potassium dichromate go when it has oxidised with a 3° alcohol?
Remains orange
What happens when potassium dichromate is reduced?
Changes colour from orange to green
Why does potassium dichromate change colour from orange to green in the presence of 1° and 2° alcohols?
It is reduced
What happens to potassium chromate in the presence of 1° and 2° alcohols?
It reduces itself, and changes colour from orange to green
Why are Tollen’s and Fehling’s reagents used to identify aldehydes?
They are good oxidising agents
What do Tollen’s and Fehling’s reagents test for?
Aldehydes
What do Tollen’s and Fehling’s reagents do to aldehydes?
They oxidise them to carboxylic acids
How many tests can be used to test for aldehydes?
Two
What is Tollen’s reagent made of?
Ammoniacal silver nitrate
When will Tollen’s reagent oxidise aldehydes?
When boiled
What happens when Tollen’s reagent oxidises aldehydes?
The colourless Ag⁺ ions are reduced to metallic silver, which form on the surface of the test tube
What indicates an aldehyde is present when Tollen’s reagent is the oxidising agent?
A silver mirror
What is Fehling’s solution made of?
A complex solution contained Cu²⁺ ions
When will Fehling’s solution oxidise aldehydes?
When heated
What happens when Fehling’s solution oxidises aldehydes?
The blue Cu²⁺ ions are reduced to the red copper (I) oxide, Cu₂O
What indicates an aldehyde is present when Fehling’s solution is the oxidising agent?
A brick red precipitate (of Cu₂O)
What can undergo elimination?
- halogenoalkanes
* alcohols
As well as alcohols, what can also undergo elimination?
Halogenoalkanes
What is formed when alcohols undergo elimination?
Alkene
Under what conditions are alcohols eliminated to form alkenes?
Heating with a concentrated sulphuric acid catalyst
Why is elimination of alcohols also known as dehydration?
Alcohols lose water when they undergo elimination
What conditions is hydration favoured by?
Acidic conditions
Why are hydration reactions favoured by acidic conditions?
The -OH group becomes protonated by H⁺ ions
What happens in elimination in alcohols, after the -OH group becomes protonated by H⁺ ions?
The -OH group that has been protonated by H⁺ ions leaves as a water molecule - forming a carbocation
In elimination in alcohols, where does the H that is lost come from?
A carbon atom that is adjacent to the carbon atom attached to the OH group
Why can there be more than one product formed from the elimination of alcohols?
The H which is lost can come from different carbons, as long as the carbon is next to the carbon with the OH group
What can alkenes produced by hydration be used to make?
Polymers
How can polymers be created without using crude oil?
- elimination (hydration) of alcohols to produce alkenes
- these alkenes produce polymers
(provided the original ethanol was produced by fermentation)
General equation for the oxidation of a primary alcohol to an aldehyde?
RCH₂OH + [O] → RCHO + H₂O
What is the potassium dichromate (used as an oxidising agent for alcohols) acidified with?
Dilute sulphric acid
Conditions of oxidation of primary alcohols to aldehydes?
Warm, distillation
General equation for the oxidation of a primary alcohol to a carboxylic acid?
- RCH₂OH + [O] → RCHO + H₂O
* R-COH + [O] → R-COOH
Conditions of oxidation of primary alcohols to a carboxylic acid?
Heat, reflux
General equation for the oxidation of a secondary alcohol to a ketone?
R₁CH(OH)R₂ + [O] → R₁COR₂ + H₂O
Conditions of oxidation of secondary alcohols to ketones?
Heat, distillation
Reagent for elimination of alcohols to alkenes?
Concentrated sulphuric acid
