Alcohols Flashcards
What is the functional group of an alcohol?
Hydroxyl group -OH
What is the general formula of an alcohol?
Cn H2n+1 OH
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’ melting point and boiling point 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.
What makes an alcohol primary?
C bonded to OH is only bonded to one other C atom
What makes an alcohol secondary?
C bonded to OH is bonded to two other C atoms
What makes an alcohol tertiary?
C bonded to OH is bonded to three other C atoms
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 this 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 this reaction to take place?
Enzymes in yeast as catalyst, 35°C, anaerobic conditions.
Write an equation for the reaction which takes place.
C6H12O6 (aq) → 2C2H5OH + 2CO2
What are the advantages and disadvantages of this method?
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).
Write an equation for the combustion of ethanol.
C2H5OH (l) + 3O2 (g) → 2CO2 (g) + 3H2O (l)
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?
An 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.
Write an equation for the partial oxidation of ethanol
CH3CH2OH (l) + [O] → CH3CHO (g) + H2O (l)
What forms if you fully oxidise a primary alcohol?
A carboxylic acid
What conditions are needed for this reaction?
Concentrated sulphuric acid, potassium dichromate (VI), reflux for about 20 mins, strong heating.
Write an equation for the full oxidation of ethanol
CH3CH2OH (l) + 2[O] → CH3COOH (g) + H2O (l)
What forms if you oxidise a secondary alcohol?
A 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.
Write an equation for the oxidation of propan-2-ol.
CH3CH(OH)CH3 (l) + [O] → CH3COCH3 (g) + H2O (l)
What is an aldehyde? What is its functional group?
Molecule with C=O group at the end of a carbon chain, carbonyl functional group (C=O)
How do you name aldehydes? Give an example.
Suffix -al
e.g. ethanal
What is a ketone? What is its functional group?
Molecule with C=O group in the middle of a carbon chain, carbonyl functional group (C=O).
How do you name ketones? Give an example?
Suffix -one
e.g. propanone
What is a carboxylic acid? What is its functional group?
Molecule with a COOH group, which has to be at the end of a carbon chain.
Carboxyl functional group (made up of carbonyl C=O and hydroxyl -OH group).
How do you name carboxylic acids? Give an example
Suffix -oic acid
e.g. propanoic acid
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
