Chapter 14 - Alcohols Flashcards
What are the properties of alcohols compared to those of alkanes with the same carbon chain?
- less volatile
- higher melting points
- greater water solubility
Why do the boiling points of alkanes increase faster than the boiling points of alcohols?
As chain length increases, the contribution of the -OH group (hydrogen bonding) decreases and the alcohols resemble the alkanes more closely
Why is alcohol soluble in water?
As it can form hydrogen bonds with water and the polar -OH
Alkanes are non-polar and doesn’t form hydrogen bonds
What is the trend of solubility as chain length increases?
Influence of the -OH group becomes relatively smaller and the solubility of longer chain alcohols becomes more like that of hydrocarbons so solubility decreases
How are alcohols classified as primary, secondary…?
Depends on the number of hydrogen atoms and alkyl groups attached to the carbon atom that contains the alcohol functional group
Why are alcohols more likely to undergo complete combustion?
As the alcohol contains an extra oxygen
What are the uses of methanol?
Highly toxic:
- petrol additive (improves combustion of unleaded petrol)
- solvent
- raw materials (used as feedstock for important industrial processes)
- fuel
What are the uses of ethanol?
- alcoholic drinks s
- solvent (industrial alcohol/methylated spirits)
- biofuel (petrol substitute with limited oil reserves)
What are the two ways to make ethanol?
1) Fermentation of carbohydrates (over 90% of the world’s ethanol)
2) Reaction of ethene with steam
What is the equation and conditions for the fermentation of carbohydrates?
C6H12O6 -> 2C2H5OH + 2CO2
Temperature: 30 - 40°C (low)
Pressure: gentle
Catalyst: enzyme zymose in yeast
Other: absence of air, yeast
What are the raw materials and type of process used for the fermentation process?
Raw materials: renewable -> crops (sugar cane, sugar beet, corn rice and maize)
Process: batch process which in inefficient
What is the reaction rate and purity of the ethanol produced by the fermentation method?
Reaction rate: slower than the other reaction due to use of low temperature
Purity of ethanol: impure due to the mixture of yeast and glucose, purified using fractional distillation
What is the equation and conditions of ethene with steam?
CH2=CH2 + H2O -> CH3CH2OH
Temperature: 300°C
Pressure: 60 atm
Catalyst: phosphoric acid
What are the raw materials and type of process used for the hydration process?
Raw materials: non renewable -> crude oil (fractions, distillation and cracking of ethene)
Process: continuous process (higher labour costs)
What is the reaction rate and purity of the ethanol produced by the hydration method?
Reaction rate: higher speed due to higher temperature/pressure
Purity: purer
Which reaction has a higher atom economy?
Reaction of ethene with steam (no by-products)
What does carbon neutral mean? Example?
It releases the same amount of CO2 when it is burned as was taken in from the atmosphere when the plants grew and photosynthesised
Ex: using ethanol as fuel
What is the shape and angle of methanol around the C-O-H?
Non-linear
104.5 °C
What is dehydration?
Any reaction in which a water molecule is removed from the starting material
What type of reaction is the dehydration of alcohol?
Elimination reaction
What is the reaction of the dehydration of alcohols?
An alcohol is heated under reflux in the presence of an acid catalyst (concentrated sulfuric/phosphoric acid) and the product is an alkene and water
What are the advantages and disadvantages of fermentation?
ADV:
- low energy process
- renewable sources
- simple experiment and equipment
DISADV:
- slow
- impurer ethanol
- less efficient (batch process)
What are the advantages and disadvantages of hydration?
ADV:
- fast
- pure
- efficient (continuous process)
DISADV:
- high energy process
- expensive plant required
- non renewable fossil fuels used
What is a substitution reaction of alcohols?
Alcohols reacting with hydrogen halides to form haloalkanes
How does the substitution reaction of alcohols work? What is the equation?
The alcohol is heated under reflux with sulfuric acid and a sodium halide to form a hydrogen halide
The sodium halide reacts with the alcohol to produce the haloalkane
CH3CHOHCH3 + NaBr + H2SO4 -> CH3CHBrCH3 + NaHSO4 + H2O
What type of reaction is combustion of alcohols? What happens as the number of carbons increases?
- exothermic
As the number of carbon atones increases the quantity of heat released per mole also increases
What alcohols oxidise and what do they oxidise into?
Primary alcohols -> aldehyde -> carboxylic acids
Secondary alcohols -> ketones
Why don’t tertiary alcohols oxidise?
As there are no hydrogen atoms bonded to the carbon atom carrying the hydroxyl group so oxidation would involve breaking a C-C bond
How do you write an oxidising agent in general?
[O]
What is the oxidising agent usually made of?
Solution of potassium dichromate (VI) acified with dilute sulfuric acid and anti bumping granules
What happens if the alcohol is oxidised?
Orange solution containing dichromate ions is reduced to a green solution contains chromium ions
How are aldehydes made?
- gentle heating of primary alcohols with acidified potassium dichromate forms an aldehyde
- to ensure the aldehyde doesn’t oxidise further (into a carboxylic acid) the aldehyde is distilled
- dichromate ions change colour from orange to green
How are carboxylic acids made?
Primary alcohol is heated strongly under reflux with an excess of acidified potassium dichromate, a carboxylic acid is formed
What are secondary alcohols oxidised into and how are they?
Ketones
- secondary alcohol is heated under reflux with the oxidising mixture
- dichromate ions once again change colour from orange to green
What is oxidation used for?
To differentiate between primary, secondary and tertiary alcohols
What tests can be used to distinguish aldehydes and ketones?
Tollens reagent:
- aldehyde -> silver grey solid or ‘silver mirror’ is formed
- ketone -> no reaction
Fehling’s test:
- aldehyde -> brick red precipitate of copper (I) oxide
- ketone -> no reaction
