3.5 Alcohols Flashcards
What is the functional group of an alcohol?
Hydroxyl group -OH
What is the general formula of an alcohol?
CnH2n+1OH
How do you name alcohols (one prefix, one suffix)?
Hydroxyl- or -OH
What kind of intermolecular forces do alcohols have? Why?
Hydrogen bonding, due to the electronegativity difference in the OH bond
How do alcohols’ m.p. And b.p. Compare to other hydrocarbons’ of similar C chain lengths? Why?
Higher, because they have hydrogen bonding → stronger than Van der Waals forces
Are alcohols soluble in water? Why does solubility depend on chain length?
Soluble when short chain - OH hydrogen bonds
to hydrogen bond in water
Insoluble when long chain - non-polarity of C-H
bond takes precedence
How can ethanol be made from crude oil?
Hydration of ethene via electrophilic addition (phosphoric acid catalyst H₃PO₄)
What are the advantages and disadvantages of hydration of ethene via electrophilic addition method?
Advantages - fast, continuous process, ethanol has a high purity
Disadvantages - not renewable as from crude oil
How can ethanol be made by fermentation?
Plant carbohydrates broken down and fermented by enzymes in yeast → ethanol
What conditions are needed for fermentation to take
place?
Enzymes in yeast as catalyst, 35°C, anaerobic conditions
What are the advantages and disadvantages of fermentation?
Advantages - renewable as from plants
Disadvantages - slow, batch process, enzymes stop working at 15% alcohol so solution is not pure, needed to be fractionally distilled
In the future, how might most ethene be made? Why is it not made like this at the moment?
Dehydrate ethanol made by fermentation → ethene
Not economical at the moment
Define carbon neutral
No net addition of CO2 to the atmosphere - carbon dioxide released when combusted = carbon dioxide absorbed as a plant
Explain how using ethanol in petrol engines could be considered to be carbon neutral.
Carbon dioxide released in fermentation and combustion = carbon dioxide absorbed when growing
Why would it probably not be entirely carbon neutral to use ethanol?
Other “carbon costs” associated with it e.g. Transport
What is a commercial fuel that uses ethanol? What else does it contain and why?
Methylated spirits - methanol (toxic, so it can’t be drunk)
What is an elimination reaction?
The removal of a smaller molecule from a larger one
Which group leaves the parent molecule in the case of alcohols?
OH and a H (to form water)
What physical conditions are needed for the elimination reaction from alcohols to alkenes? (2 alternatives)
Excess hot concentrated sulphuric acid or pass vapour over hot aluminium oxide
What forms if you partially oxidise a primary alcohol?
Aldehyde
What conditions are needed to partially oxidise a primary alcohol?
Dilute sulphuric acid, potassium dichromate (VI), distill product as it’s produced, gentle heating
What forms if you fully oxidise a primary alcohol?
Carboxylic acid
What conditions are needed to fully oxidise a primary alcohol?
Concentrated sulphuric acid, potassium dichromate (VI), reflux for about 20 mins, strong heating
What forms if you oxidise a secondary alcohol?
Ketone
Why can it not be oxidised further and why can’t a tertiary alcohol be oxidised?
A carbon-carbon bond would have to break
What conditions are needed for the oxidation of a secondary alcohol?
Concentrated sulphuric acid, potassium dichromate (VI), reflux for about 20 mins, strong heating
What does the Tollens’ test give a positive result for?
Aldehydes
What is in Tollens’ reagent? How does this react with the substance to be tested?
Silver nitrate in NH3 (aq) - oxidises aldehydes but
not ketones
Complex silver (I) ions reduced to Ag(s)
How do you carry out the Tollens’ test?
Add equal volumes of substance being tested and tollen’s reagent to a test tube, leave in water bath for 10mins and observe any changes
What is the result of the Tollen’s test for aldehydes and ketones?
Aldehydes - silver mirror forms (solid Ag) Ketones - no visible change
What does Fehling’s solution give a positive test result for?
Aldehydes
What is in Fehling’s? How does this react with the substance to be tested?
Blue copper (II) complex ions - gentle oxidising agent Reduced to Cu+ ions (brick red)
What conditions do you need to use the Fehling’s
solution?
Heat
What result do aldehydes and ketones give in the Fehling’s test?
Aldehydes- brick red ppt
Ketones- no visible change
