Alcohols Flashcards
what is the fuctional group of alcohols?
-OH
what is a primary alcohol?
1 alkyl group attached to the carbon with the OH on
what is a secondary alcohol?
2 alkyl groups attached to the carbon with the OH on
what is a tertiary alcohol?
3 alkyl groups attatched to the carbon with the OH on
how can alkenes be made from alcohols sustainably?
by using an alcohol that has been made via the fermentation of glucose from plants
what catalyst is involved in the dehyrdation of alcohols?
acid catalyst, e.g sulfuric acid (H2SO4) or phosphoric acid (H3PO4)
what are the products of the dehydration of alcohols?
alkene + water
what kind of reaction is the dehydration of alcohols? describe the mechanism for the dehydration of ethanol.
- elimination reaction
1. lone pair on oxygen will attach the H+ ion which has come from the acid catalyst
2. the intermediate formed has a positive charge on the oxygen, oxygen pulls electrons in the C-O bond strongly to break the bond, leaving an unstable carbon intermediate
3. the carbocation loses a H+, the electrons in C-H move to form a C=C
4. ethene is made and the H+ is reformed
what process needs to be carried out for the dehydration of alcohols? what are the products? what other processes may need to be carried out?
- distillation
- when alcohols undergo dehydration an impure product containing alkene, alcohol, water and acid is formed
- useful alkene needs to be separate from the impurities - can be done by distillation
- further separation and purification may be needed: separation funnel and drying agent may be used to help
outline the first step in the making of cyclohexene from cyclohexanol
- Distillation:
1. add concentrated sulfuric/phosphoric acid and cyclohexanol into a round bottomed flask
2. add several anti-bumping granules to allow a smooth boiling process
3. use a heating mantle (no bunsen burner as cyclohexanol flammable) to warm up reactants to 83c - bp of cyclohexanol
4. chemicals with bp less than 83c will enter the condenser, cool down + condense back into a liquid. condenser has cold water running through the wall
5. the product is collected in a vessel , e.g. conical flask but will still have small amounts of impurities e.g. unreacted cyclohexanol and water
outline the second step in the making of cyclohexene from cyclohexanol
- separation:
1. add products from distillation into separating funnel
2. add water to disolve soluble impurities + create aqueous solution
3. after allowing solution to settle, 2 layers will form :
- top layer: impure cyclohexene
- bottom layer: aqueous layer containing water soluble impurities
4. drain aqueous layer off (remember to remove stopper)
outline the third step in the making of cyclohexene from cyclohexanol
- purification:
1. add impure cyclohexene from step 2 into a round bottomed flask
2. add anhydrous CaCl2 and invert the flask
3. leave for 20-30 mins
how can alcohols be made from alkenes? what conditions are required?
- hydration
- steam and acid catalyst used
- temp of 300c and pressure of 60atm needed
outline the mechanism of the hydration of alkenes
- electrons in double bond used to form bond with H+ ion
- lone pair on water bonds to carbocation
- hydrogen lost from water
- alcohol and H+ ion produced
what enzyme is used on fermentation? what conditions are required? what type of reaction is fermentation?
- yeast
- anaerobic conditions
- exothermic reaction
give an equation for fermentation of glucose
C6H12O6 (aq) –> 2C2H5OH(aq) + 2CO2 (aq)
what are the advantages of using biofuels?
- renewable so more sustainable than crude oil
- produce CO2 when burnt but are classed as carbon neutral as the CO2 is absorbed by the sugar cane when it is growing
what are the disadvantages of using biofuels?
- expensive to convert existing petrol engines to take fuels with a higher concentration of ethanol
- land that could have been used to grow food is being used to make fuel, could lead to food shortages
why are biofuels considered nearly carbon neutral? why aren’t they considered completelyl carbon neutral?
- when ethanol burned, CO2 produced, but is takem in by growing plants for photosynthesis
- plants take in 6 moles of CO2 to make 1 mole of glucose during photosynthesis
- during fermentation, 2 moles of CO2 are produced
- combustion of ethanol produces 4 moles of CO2
- total number of moles of CO2 used up is same as number of moles produced so considered carbon neutral
- NOT completely carbon neutral as fossil fuels burnt to transport biofuels, make fertilisers that would be put on the crops and in the machinery that is used to harvest the crop
what can be used to oxidise alcohols? what colour change will occur and why?
- acidified potassium dichromate (K2Cr2O7)
- mild oxidising agent so is reduced itself
- will turn orange (Cr2O72- - dichromate ion) to green (Cr3+ - chromium ion)
what are primary alcohols oxidised to? what is this dependent on?
- products depend on method followed
- produces an aldehyde then carboxylic acid
- use reflux + excess oxidising agent to make carboxylic acid
- reflux allows strong heating without losing volatile reactants + products - aldehydes evaporate, condense and fall back into the flask
what are secondary alcohols oxidised to? why?
- ketones
- difficult to oxidise further
- use reflux and an oxidising agent to make a ketone, a secondary alcohol will oxidise to a ketone when acidified potassium dichromate is added
what reagents can be used to distinguish between aldehydes and ketones? how do they both work?
- Fehling’s solution (Benedict’s solution)
- Tollen’s reagent
- both are oxidising agents so oxidise aldehydes but NOT ketones
how can Fehling’s solution be used to distinguish between aldehydes and ketones?
- Fehling’s solution is blue, as it contains Cu2+ ions
- add warm Fehling’s solution to sample
- aldehydes: go from blue to brick red precipitate as Cu2O is present
- ketones: remain blue
how can Tollen’s reagent be used to distinguish between aldehydes and ketones?
- contains [Ag(NH3)2]+ and is added warm to the sample
- aldehydes: tollen’s reduced to silver, which coats the inside of the flask
- ketones: no silver precipitate formed
