Alcohols Flashcards
(24 cards)
What is the general formula for alcohols?
CnH2n+1-OH
We can dehydrate alcohols to make…
Alkenes (only if the alcohol has been made via fermentation)
Explain how we can dehydrate alcohols to make alkenes.
We can eliminate water from an alcohol to produce an alkene ,the reaction involves the use of an acid catalyst (either phosphoric acid or sulfuric acid)
Which alkene is formed when a primary alcohol is dehydrated?
Ethene
Why are different alkenes formed when a non-primary alcohol is dehydrated?
The double bond can form either side of the hydroxyl group
What is in the impure product we get when we oxidise an alcohol?
An impure product that contains; alkene, alcohol, water and acid.
How can we separate the impure product formed from the dehydration of an alcohol and what is a limitation of this?
Distillation - separate the chemicals by boiling point.
Despite distillation, further separation and purification may be needed to separate the alkene from the impurities.
How do we make cyclohexene from cyclohexanol?
Distillation, purification and separation techniques
Making cyclohexene - distillation
- Add concentrated sulfuric/phosphoric acid and cyclohexanol to a round bottomed flask.
- Add several anti-bumping granules to allow a smooth boiling process
- Use a heating mantle to warm the reactants to 83°C (boiling point of cyclohexanol)
- Chemicals with boiling points less than 83°C will evaporate, enter the condenser, cool down and condense back into a liquid. The condenser has cold running water running through the wall.
- The product is collected in a vessel but will still have small amounts of impurities such as unreacted cyclohexanol and water.
Making cyclohexene - separation & purification
- Add the products from step 1 into a separating funnel and add water to dissolve soluble impurities and create an aqueous solution.
- After the solution settles , 2 layers will form.
Top layer - impure cyclohexene
Bottom layer - aqueous layer containing water soluble impurities, drain this layer. - Take the impure cyclohexene and add it to a round bottomed flask. Then add anhydrous calcium chloride, invert the flask and leave for 20-30 minutes.
What does fermentation use to create alcohol?
Fermentation convers glucose into ethanol + carbon dioxide using a yeast catalyst in anaerobic conditions. This reaction is EXOTHERMIC. The optimum temperature for yeast to work at is 30°C to 40°C.
Why is fermentation a cheap process?
It requires little equipment and uses renewable resources . The biggest cost is the fractional distillation at the end which requires energy and time.
What are biofuels?
Fuel made from dead biological matter. For example, ethanol is being used as a fuel in countries where there is a good supply of sugar cane.
What are the advantages of biofuels?
- They are renewable and so more sustainable than crude oil.
- Biofuels produce CO2 when they are burnt but are classed as carbon-neutral as the CO2 is absorbed by sugar cane when it is growing.
What are the disadvantages of biofuels?
- Expensive to convert existing petrol engines to take fuels with a higher concentration of ethanol.
- Land used to grow crops to make fuels could be used to grown crops to make food. Countries like India and Brazil harvest sugarcane for bioethanol but have food shortages.
Why are biofuels not completely carbon-neutral?
Fossil fuels are burnt when transporting the biofuel across the country, they are used to make fertilisers to put on the crops and on the machinery to harvest the crops.
What can alcohols be oxidised using?
Acidified potassium dichromate, it is a mild oxidising agent so it will reduce itself . It will turn from ORANGE to GREEN.
What can primary alcohols be oxidised into?
Aldehydes and then carboxylic acids.
How do we oxidise primary alcohols?
Aldehydes - it is difficult to extract just an aldehyde when heating in a boiling tube so we have to remove the aldehyde from the oxidising agent when it forms using distillation.
Carboxylic acids - Use reflux and excess oxidising agent to make carboxylic acid. Reflux allows strong heating without losing volatile products and reactants. Aldehydes will evaporate, condense and fall back into the flask.
What can secondary alcohols be oxidised into?
Ketones
How do we oxidise secondary alcohols?
Use reflux and an oxidising agent to make a ketone. A secondary alcohol will oxidise into a ketone when acidified potasssium dichromate is added.
What can tertiary alcohols be oxidised into?
They cannot be oxidised using potassium dichromate.
How can we use Fehling’s solution to distinguish between aldehydes and ketones?
It will oxidise an aldehyde but not a ketone. Add warm Fehling’s solution which goes from blue to a brick red precipitate if an aldehyde is present.
How can we use Tollens reagent to distinguish between aldehydes and ketones?
It will oxidise an aldehyde but not a ketone. Add warm Tollens reagent and it will be reduced to silver which coats the inside of the flask - silver mirror/precipitate. Only if an aldehyde is present.
