Download The reaction between bromine and alkenes is an example of a type

Alkanes -
The alkanes are a homologous series of hydrocarbons. This means that they have similar
chemical properties to each other and they have trends in physical properties. For example, as the chain
length increases, their boiling point increases.
The straight chain alkanes share the same general formula:
The general formula means that the number of hydrogen atoms in an alkane is double the number of carbon
atoms, plus two. For example, methane is CH4 and ethane is C2H6.
Alkane molecules can be represented by displayed formulae in which each atom is shown as its symbol (C
or H) and the covalent bonds between them by a straight line.
Here are the structures and names of the first five alkanes:
Notice that the molecular models on the right show that the bonds are not really at angles of 90°.
Methylpropane
Alkanes are saturated hydrocarbons. This means that their carbon atoms are joined to each other by single
bonds. This makes them relatively unreactive, apart from their reaction with oxygen in the air - which we
call burning or combustion.
Butane
Like other homologous series, the alkanes show isomerism. This means that their atoms can be arranged
differently to make slightly different compounds with different properties. For example, an isomer of butane
is methylpropane.
Substitution reactions
In a substitution reaction, one atom is swapped with another atom. These are very useful reactions in the
chemical industry because they allow chemists to change one compound into something more useful,
building up designer molecules like drugs.
Alkanes undergo a substitution reaction with halogens in the presence of light.
For instance, in ultraviolet light, methane reacts with halogen molecules such as chlorine and bromine.
For example:
methane + bromine → methylbromine + hydrogen bromide
CH4 + Br2 → CH3Br + HBr
This reaction is a substitution reaction because one of the hydrogen atoms from the methane is replaced by a
bromine atom.
Alkenes
Alkenes are a homologous series of hydrocarbons that contain a carbon-carbon double bond. The number of
hydrogen atoms in an alkene is double the number of carbon atoms, so they have the general formula
.
For example, the molecular formula of ethene is
, while for propene it is
.
Here are the names and structures of four alkenes:
Alkenes are unsaturated, meaning they contain a double bond. This bond is why the alkenes are more
reactive than the alkanes.
Testing for alkenes
The presence of the C=C double bond allows alkenes to react in ways that alkanes cannot. This allows us to
tell alkenes apart from alkanes using a simple chemical test.
Bromine water is an orange solution of bromine. It becomes colourless when it is shaken with an alkene.
Alkenes can decolourise bromine water, but alkanes cannot. The slideshow shows this process.
The reaction between bromine and alkenes is an example of a type of reaction called an addition reaction.
The bromine is decolourised because a colourless dibromo compound forms. For example:
ethene + bromine → dibromoethane
C2H4 + Br2 → C2H4Br2
Other addition reactions of alkenes:
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Hydrogen can be added to a C=C double bond. This has the effect of ‘saturating’ the molecule, and
will turn an alkene into an alkane. For example: C2H4 + H2 → C2H6
If steam (H2O) is added to an alkene, an alcohol is made. For example: C2H4 + H2O → C2H5OH
Alcohols
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The alcohols are a homologous series of organic compounds. They all contain the functional group –
OH, which is responsible for the properties of alcohols.
The names of alcohols end with ‘ol’, eg ethanol.
The first three alcohols in the homologous series are methanol, ethanol and propanol. They are
highly flammable, making them useful as fuels. They are also used as solvents in marker pens,
medicines, and cosmetics (such as deodorants and perfumes).
Ethanol is the alcohol found in alcoholic drinks such as wine and beer. Ethanol is mixed with petrol
for use as a fuel.
Here are the names and structures of the simplest alcohols:
Ethanol from ethene
Structure of ethanol
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Ethanol molecules contain carbon, hydrogen and oxygen atoms.
Ethanol from ethene and steam
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Ethanol can be manufactured by the hydration of ethene. In this reaction, ethene (which comes from
cracking crude oil fractions) is heated with steam in the presence of a catalyst of phosphoric acid (to
speed up the reaction):
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This reaction typically uses a temperature of around 300°C and a pressure of around 60–70
atmospheres.
Notice that ethanol is the only product. The process is continuous – as long as ethene and steam are fed
into one end of the reaction vessel, ethanol will be produced. These features make it an efficient
process. However, ethene is made from crude oil, which is a non-renewable resource.
Ethene from ethanol
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The reaction of ethene with steam to form ethanol can be reversed. This allows ethanol to be
converted into ethene. A catalyst of hot aluminium oxide is used to speed up the reaction.
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This is called a dehydration reaction.
Ethanol from sugars
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Ethanol can also be made by a process called fermentation.
Fermentation
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During fermentation, sugar (glucose) from plant material is converted into ethanol and carbon
dioxide. This typically takes place at temperatures of around 30°C. The enzymes found in singlecelled fungi (yeast) are the natural catalysts that can make this process happen:
Unlike ethene, sugar from plant material is a renewable resource.
Hydration of ethene v fermentation
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These are some of the advantages and disadvantages of making ethanol by hydration of ethene and
by fermentation.
The table compares making ethanol by hydration of ethene (ethene and steam) to making ethanol by
fermentation (sugar from plant material).
Fermentation
Type of raw
materials
Type of process
Labour
Rate of reaction
Renewable (glucose from plants)
Batch (stop-start)
A lot of workers needed
Slow
Warm (30°C), normal pressure (1
Conditions needed
atm)
Purity of product Impure (needs treatment)
Energy needed A little
Hydration of ethene
Non-renewable (ethene from crude oil)
Continuous (runs all the time)
Few workers needed
Fast
High temperature (300°C) and high pressure (6070 atm)
Pure (no by-products made)
A lot
Biofuels
With fossil fuels being non-renewable and contributing to global warming, biofuels are increasingly being
considered as a possible alternative for the future.
Biofuels are produced from natural products, often plant biomass containing carbohydrate. As biofuels are
produced from plants, they are renewable and theoretically carbon neutral.
Some biofuels are produced by using microorganisms to anaerobically ferment carbohydrate in the plant
material - as is the case with bioethanol and biogas production (each process uses different microorganisms).
Bioethanol
When ethanol is made by fermentation, sugar is converted into ethanol and carbon dioxide if conditions are
anaerobic. Single-celled fungi, called yeast, contain enzymes that are natural catalysts for making this
process happen.
In some countries, such as Brazil, the source of sugar is sugar cane - which yeast can directly ferment into
ethanol. In other countries, plants such as maize are used. Because maize contains starch rather than sugar,
the enzyme amylase must first break down the starch into sugar before the yeast can ferment it into ethanol.
The ethanol produced by yeast only reaches a concentration of around 15 per cent before the ethanol
becomes toxic to the yeast. In order to make it sufficiently concentrated to be burnt as a fuel, the ethanol
must be distilled.
Disadvantages of bioethanol
There are some disadvantages to growing biofuel crops (such as sugar cane and maize) to be used as
bioethanol:
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The demand for biofuel crops means greater demand on rainforest land.
Crops grow slowly in parts of the world that have lower light levels and temperatures, so growing
biofuel crops in these countries would not satisfy the demand for fuel.
For bioethanol to be burnt in a car engine, some engine modification is needed. Modern petrol
engines can use petrol containing up to 10 per cent ethanol without needing any modifications, and
most petrol sold in the UK contains ethanol.
Although biofuels are in theory carbon neutral, this does not take into account the carbon dioxide
emissions associated with growing, harvesting and transporting the crops, or producing the ethanol
from them. Therefore, overall, more carbon dioxide is emitted than is absorbed, which means that it
contributes to global warming.
Some people morally object to using food crops to produce fuels. For example, it could cause food
shortages or increases in food prices.
Biodiesel
Biodiesel is produced by reacting vegetable oils with methanol. The main product is a methyl ester of a long
chain fatty acid, and this is used as biodiesel. Glycerol is produced as a by-product.
Biodiesel can be used as a replacement fuel in most modern diesel engines - unlike raw vegetable oil, which
can only be used in converted or old-fashioned diesel engines.
Carboxylic acids
The carboxylic acids are a homologous series of organic compounds.
Carboxylic acids contain the carboxyl functional group (-COOH). Carboxylic acids end in '-oic acid'.
The carboxyl group will never have a position number in a carboxylic acid, as it is always on the end of the
carbon chain.
The basic rules of naming apply. Carboxylic acids take their names from their ‘parent’ alkanes. For
example, ethane is the ‘parent’ alkane of ethanoic acid. Ethanoic acid has the formula CH3COOH and this
structure:
Properties of carboxylic acids
Short carboxylic acids are liquids and are soluble in water. Longer carboxylic acids are solids and are less
soluble in water.
The boiling point of a carboxylic acid is higher than that of the alkane with the same number of carbon
atoms because the intermolecular forces are much stronger.
Carboxylic acids are weak acids, so they can donate a hydrogen ion (H+) in acid-base reactions:
This means that they will react with carbonates to produce a salt, water and carbon dioxide:
They will also react with reactive metals to produce a salt and hydrogen.
Making a carboxylic acid
Ethanoic acid can be made by oxidising ethanol (which is an alcohol). In this case, oxidation involves
adding an oxygen atom and removing two hydrogen atoms. This can happen:
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during fermentation if air is present
when ethanol is oxidised by an oxidising agent, such as acidified potassium manganate(VII)
Making an ester
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Esters occur naturally - often as fats and oils - but they can be made in the laboratory by reacting an
alcohol with an organic acid. A little sulfuric acid is needed as a catalyst.
The general word equation for the reaction is:
alcohol + organic acid → ester + water
For example:
methanol + butanoic acid → methyl butanoate + water
The diagram shows how this happens, and where the water comes from:
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So, to make ethyl ethanoate, you would need to react ethanol with ethanoic acid.
What esters smell like
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Different esters have different smells. These smells are often fruity. Take a look at the following
four examples:
Alcohol Organic acid
Ester made
Smell of ester
Pentanol Ethanoic acid Pentyl ethanoate Pears
Octanol
Ethanoic acid Octyl ethanoate
Bananas
Pentanol Butanoic acid Pentyl butanoate Strawberries
Methanol Butanoic acid Methyl butanoate Pineapples
Fats and oils
Fats and oils are naturally-occurring esters. Fats are solid at room temperature, whereas oils are liquids.
Vegetable oils
Vegetable oils are natural oils found in seeds, nuts and some fruit. The oil can be extracted. The plant
material is crushed and pressed and the oil, eg olive oil, is squeezed out.
Sometimes the oil is more difficult to extract and has to be dissolved in a solvent. Once the oil is dissolved,
the solvent is removed by distillation and impurities (such as water) are also removed. This leaves pure
vegetable oil, eg sunflower oil.
Structure of vegetable oils
Molecules of vegetable oils consist of glycerol and fatty acids.
The diagram shows how three long chains of carbon atoms are attached to a glycerol molecule to make one
molecule of vegetable oil.
The structure of a vegetable oil molecule
Plant oils and their uses
Vegetable oils in cooking - Vegetable oils have higher boiling points than water - so foods can be
cooked or fried in vegetable oils at higher temperatures than they can be if they are cooked or boiled in
water.
Food cooked in vegetable oils:
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cook faster than if they were boiled
have different flavours than if they were boiled
Vegetable oils are a source of energy in the diet. Food cooked in vegetable oils releases more energy when it
is eaten than food cooked in water. This can have an impact on our health and cause excess weight.
Some vegetable oils can be converted to biodiesel by reacting them with methanol. This allows certain crops
to be grown and used to make fuels for cars and lorries without needing to use fossil fuels. This can help to
make biodiesel carbon neutral.
Saturated and unsaturated fats and oils - The fatty acids in some vegetable oils are
saturated - they only have single bonds between their carbon atoms. Saturated oils tend to be solid at room
temperature, and are sometimes called vegetable fats instead of vegetable oils. Lard is an example of a
saturated oil.
The fatty acids in some vegetable oils are unsaturated - they have double bonds between some of their
carbon atoms. Unsaturated oils tend to be liquid at room temperature, and are useful for frying food. They
can be divided into two categories:
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monounsaturated fats have one double bond in each fatty acid
polyunsaturated fats have many double bonds
Unsaturated fats (rather than saturated fats) are thought to be a healthier option in the diet.
Emulsions - Vegetable oils do not dissolve in water. If oil and water are shaken together, tiny droplets
of one liquid spread through the other liquid, forming a mixture called an emulsion.
Emulsions are thicker than the oil or water they contain. This makes them useful in foods such as salad
dressings and ice cream. Emulsions are also used in cosmetics and paints.
There are two main types of emulsion:
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oil droplets in water (milk, ice cream, salad cream, mayonnaise)
water droplets in oil (margarine, butter, skin cream, moisturising lotion)
Emulsifiers - If an emulsion is left to stand, eventually a layer of oil will form on the surface of the
water. Emulsifiers are substances that stabilise emulsions, stopping them separating out. Egg yolk contains a
natural emulsifier. Mayonnaise is a stable emulsion of vegetable oil and vinegar with egg yolk.
Emulsifier molecules have two different ends:
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a hydrophilic (water-loving) ‘head’ that forms chemical bonds with water but not with oils
a hydrophobic (water-hating) ‘tail’ that forms chemical bonds with oils but not with water
Lecithin is an emulsifier commonly used in foods. It is obtained from oil seeds and is a mixture of different
substances. A molecular model of one of these substances is seen in the diagram:
Emulsifier molecules
The hydrophilic 'head' dissolves in the water and the hydrophobic 'tail' dissolves in the oil. In this way, the
water and oil droplets become unable to separate out.
Hydrogenation
Bromine water test - Unsaturated vegetable oils contain carbon-carbon double bonds. They can be
detected using bromine water, just as alkenes can be detected in this way. Bromine water becomes
colourless when shaken with an unsaturated vegetable oil, but it stays orange-brown when shaken with a
saturated vegetable fat.
Bromine water can also be used to determine the level of saturation of a vegetable oil.
Hydrogenation - Saturated vegetable fats are solid at room temperature, and have a higher melting
point than unsaturated oils. This makes them suitable for making margarine or for commercial use in the
making of cakes and pastry. Unsaturated vegetable oils can be ‘hardened’ by reacting them with hydrogen, a
reaction called hydrogenation.
The structure of part of a fatty acid
During hydrogenation, vegetable oils are reacted with hydrogen gas at about 60°C. A nickel catalyst is used
to speed up the reaction. The double bonds are converted to single bonds in the reaction. In this way,
unsaturated fats can be made into saturated fats – they are hardened.