Organic - Alcohols Flashcards
What is ethanol?
Ethanol is possibly our oldest social drug as it is derived from the fermentation of sugars in fruits and so on. It is the alcohol in alcoholic drinks. It may, in moderation, promote a feeling of well-being and reduce normal inhibitions. It is in fact a nervous system depressant (i.e., it interferes with the transmission of nerve impulses). In larger amounts it leads to loss of balance, poor hand-eye coordination, impaired vision, and inability to judge speed. Large amounts can be fatal. Excessive long-term use can lead to addiction - alcoholism.
The ethanol in alcoholic drinks is absorbed through the walls of the stomach and small intestine into the bloodstream. Some is eliminated unchanged in urine and in the breath. The rest is broken down by the liver. The combined effect of these processes is that an average person can eliminate about 10cm3 of ethanol per hour. This is approximately the amount of ethanol in half a pint of beer, a small glass of wine (125ml) or a shot (25ml of spirits). So some simple arithmetic should enable you to work out how long it would take to sober up.
What is the general formula for alcohols?
Alcohols form a homologous series and have the functional group -OH attached to a hydrocarbon chain. They are saturated and only contain single carbon to carbon bonds. They are relatively reactive. The alcohol most commonly encountered in everyday life is ethanol.
The general formula of an alcohol is CnH2n+1OH. This is often shortened to ROH.
What is the shape of alcohols?
In alcohols, the oxygen atom has two bonding pairs of electrons and two lone pairs. The C-O-H angle is about 105 degrees because the 109.5 degrees angle of a perfect tetrahedron is ‘squeezed down’ by the presence of the lone pairs. These two lone pairs will repel each other more than the pairs of electrons in a covalent bond.
Alcohols have stronger intermolecular forces (hydrogen bonding) and are more soluble than alkanes as they are polar molecules.
How are alcohols classified?
Alcohols are classified as primary, secondary, or tertiary according to how many other groups are bonded to the carbon that has the -OH group.
What are primary alcohols?
In a primary alcohol, the carbon with the -OH group has one R group (and therefore two hydrogen atoms).
- Propan-1-ol is a primary alcohol.
- Methanol, where the carbon has no R groups, is counted as a primary alcohol.
A primary alcohol has the -OH group at the end of a chain.
What are secondary alcohols?
In a secondary alcohol, the -OH group is attached to a carbon with two R groups (and therefore one hydrogen atom).
- Propan-2-ol is a secondary alcohol.
A secondary alcohol has the -OH group in the body of the chain.
What are tertiary alcohols?
Tertiary alcohols have three R groups attached to the carbon that is bonded to the -OH (so this carbon has no hydrogen atoms).
- 2-methylpropan-2-ol is a tertiary alcohol.
A tertiary alcohol has the -OH group at a branch in the chain.
What are the physical properties of alcohols?
They have a gradation in their physical properties. As the chain length increases, the molecules get longer. As molecules get longer,
the van der Waal’s forces of attraction are stronger. As the forces are stronger, more energy is required to overcome these forces, and so the boiling point increases.
The -OH group in alcohols means that hydrogen bonding occurs between the molecules. This is the reason that alcohols have higher melting and boiling points than alkanes of similar relative molecular mass.
The -OH group of alcohols can hydrogen bond to water molecules, but the non-polar hydrocarbon chain cannot. This means that the alcohols with short hydrocarbon chains are soluble in water because the hydrogen bonding predominates. In longer-chain alcohols, the non-polar hydrocarbon chain dominates and the alcohols become insoluble in water.
What is the industrial chemistry of alcohols?
Alcohols are very important in industrial chemistry because they are used as intermediates. They are easily made and easily converted into other compounds. Methanol is made from methane (natural gas) and is increasingly being used as a starting material to make other organic chemicals.
Where is ethanol used?
Ethanol, C2H5OH, is by far the most important alcohol.
- It is used as an intermediate in the manufacture of other organic chemicals.
- In everyday life it is often the solvent in cosmetics, such as aftershave and perfumes, and it is used as a solvent to make many common substances such as detergents and pharmaceuticals.
- It is used in the manufacture of drugs, detergents, inks, and coatings.
- It can also be used as a fuel. In some countries, such as Brazil, it is used as fuel for cars. It is a biofuel because it is made from plants that are renewable.
How is ethanol made?
It is made industrially by reacting ethene (made from cracking crude oil) with steam, using a catalyst of phosphoric acid. It is also made from sugars and crops (such as sugar cane, sugar beet, corn, rice and maize) by fermentation, as in the production of alcoholic drinks. These are renewable raw materials because we can grow more to replace those which have been used.
Beers have about 5% ethanol and wines about 12%. Spirits, such as gin and whisky, contain about 40% ethanol - these have been concentrated by distillation.
How do you make ethanol from crude oil?
Alkenes react with water in the presence of strong acids to form alcohols. Overall, the acid acts as a catalyst.
Ethene is produced when crude oil fractions are cracked. Ethene is hydrated, which means that water is added across the double bond, using steam at high temperature and pressure using concentrated phosphoric acid as a catalyst. The ethanol produced this way is pure. There is also a high percentage yield as ethanol is the only product so it is favoured as an industrial process.
CH2=CH2 + H2O –(phosphoric acid catalyst)–> C2H5OH
In the mechanism, the alkene reacts with H+ to form a carbocation. The main product comes from the more stable carbocation intermediate.
How do you make ethanol by fermentation?
During fermentation, carbohydrates from plants are broken down into sugars and then converted into ethanol by the action of enzymes from yeast. The carbohydrates come from crops such as sugar cane and sugar beet and are used as food. Over 90% of ethanol is made this way.
The key step is the breakdown of sugar in a process called anaerobic respiration:
- The rate of this chemical reaction is affected by temperature. It is slow at low temperatures but the enzymes are made ineffective if the temperature is too high. A compromise temperature of about 35 degrees Celcius, a little below our body temperature, is used.
- Air is kept out of the fermentation vessels to prevent oxidation of ethanol to ethanoic acid (the acid is vinegar).
- Once the fermenting solution contains about 15% ethanol, the enzymes are unable to function and fermentation stops. The dead yeast is filtered off. Ethanol may then be distilled from this mixture by fractional distillation as its boiling temperature (75 degrees Celsius) is less than that of water (100 degrees Celsius).
What are the features of using the fermentation of carbohydrates?
- method: fermentation and distillation
- equation: C6H12O6(aq) -> 2C2H5OH(aq) + 2CO2(g)
- conditions:
• temperature = 308K
• pressure = 100kPa
• catalyst = enzymes in yeast
• other = absence of air - raw materials: carbohydrates/sugars
- raw material type: renewable
- type of process: batch
- reaction rate: slow
- purity of ethanol: impure as aqueous solution of ethanol is produced
- yield: 15%
- atom economy: 51.1%
- decomposition reaction
What are the features of using the hydration of ethene?
- method: cracking and hydration
- equation: C2H4(g) + H2O(g) -> C2H5OH(g)
- conditions:
• temperature = 570K
• pressure = 6500kPa
• catalyst = phosphoric acid (concentrated) - raw materials: ethene from crude oil
- raw material type: non-renewable
- type of process: continuous
- reaction rate: fast
- purity of ethanol: essentially pure
- yield: 95%
- atom economy: 100%
- addition reaction
Which method has lower equipment costs? Which has lower labour costs?
Fermentation because it can be carried out at a lower temperature. It has higher labour costs though.
However it has to be fermented in batches, meaning it is a much slower process with a lower percentage yield.
What is a biofuel?
A biofuel is fuel derived or produced from renewable biological sources.
How is ethanol a renewable source of ethene?
Ethene is a vital industrial chemical: it is the starting material for poly(ethene) and many other important chemicals. Ethene can be produced by dehydrating ethanol made from sugar, giving a renewable source of ethene.
At present ethene is made from crude oil and then converted into ethanol. In the future it may become more economical to make ethene from ethanol made by fermentation.
What is carbon neutrality?
One of great advantages of using ethanol as a fuel, besides it being renewable, is that it is regarded as being “carbon neutral”. By this we mean that it releases the same amount of CO2 when it is burned as was taken in from the atmosphere when the plants grew and photosynthesised.
However, in the process of going from crop to using the fuel, there are other processes to consider including fuel for machinery and transport meaning that it is not 100% carbon neutral. Plus, there are growing concerns that too much agricultural land is being used for growing crops to make biofuels rather than food.
How is ethanol produced by fermentation a biofuel?
- Ethanol produced by fermentation is renewable and a biofuel.
- A renewable substance is one that can be replaced when it has been used.
- A biofuel is a renewable fuel made from plants.
- Biofuel ethanol is regarded as a carbon-neutral fuel. This means that it releases the same amount of CO2 when it is burned as was taken in from the atmosphere when the plants grew and photosynthesised, i.e. there are no net CO2 emissions to the atmosphere.
- photosynthesis in crops: 6CO2 + 6H2O -> C6H12O6 + 6O2 (takes in 6CO2)
- fermentation to make ethanol: C6H12O6 -> 2C2H5OH + 2CO2 (releases 2CO2)
- burning ethanol: 2C2H5OH + 6O2 -> 4CO2 + 6H2O (releases 4CO2)
- However, in the process of going from crop to using the fuel, there are other processes to consider including fuel for machinery and transport meaning that it is not 100% carbon neutral.
- There are also growing concerns that too much agricultural land is being used for growing crops to make biofuels rather than to grow food.
What other factors are there to consider in carbon neutrality?
- manufacturing, and running, farm equipment and bioethanol factories
- manufacturing and transporting fertilizers
- distilling and dehydrating the ethanol
- distributing the fuel
