Edexcel Chemistry A-Level

Topic 6: Organic Chemistry I Detailed Notes

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Topic 6A: Introduction to Organic Chemistry Hydrocarbons Organic chemistry mainly concerns the properties and reactions of hydrocarbons, compounds that contain only carbon and hydrogen. These form series of compounds with similar structures and formulas that can be represented in many different ways. Nomenclature Nomenclature is the set of rules that outline how different organic compounds should be named and how their formulas are represented. Formulas There are many different way of writing and representing organic compounds:

1. Empirical Formula - The simplest whole number ratio of atoms of each element in a compound.

2. Molecular Formula

- The true number of atoms of each element in a compound.

3. General Formula - All members of a homologous organic series follow the general formula. Example:

4. Structural Formula - Shows the structural arrangement of atoms within a molecule. Example:

5. Displayed Formula - Shows every atom and every bond in an organic compound. Example:

6. Skeletal Formula

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- Shows only the bonds in a compound and any non-carbon atoms. - Vertices are carbon atoms. - Hydrogen is assumed to be bonded to them unless stated otherwise. Example:

Homologous Series Organic compounds are often part of a homologous series, in which all members follow a general formula and react in a very similar way. Each consecutive member differs by CH2 and there is an increase in boiling points as chain length increases. Example: Each series has a functional group that allows that molecule to be recognised as being able to react chemically in a certain way as a result of that group.

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Example: Reaction Mechanisms These show the movement of electrons within a reaction, shown with curly arrows. Example: Mechanisms are used to show the reactions of organic compounds.

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Isomerism Isomers are molecules with the same molecular formula but a different arrangement of atoms within the molecule. Structural Isomers These have a different structural arrangement of atoms. They can be straight chains or branched chains but will have the same molecular formula. Example:

Position Isomers These have the functional group of the molecule in a different position of the carbon chain. Example:

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Functional Group Isomers These have a different arrangement of the same molecular formula so that the molecule has a different functional group. Example:

Stereoisomers These have a different spatial arrangement. A type of stereoisomerism is E-Z isomerism, where limited rotation around a double carbon bond means that groups can either be ‘together’ or ‘apart’. The E isomer (german for entgegen meaning apart) has these groups are apart. The Z isomer (german for zusammen meaning together) has these groups together on the same side. Example:

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Cahn-Ingold-Prelog (CIP) Priority Rules There is a priority of different groups in molecules that can display E-Z isomerism. The atom or group on each side of the double bond with the higher Ar or Mr is given the higher priority. These groups are used to determine if it is the E or Z isomer. Example:

Therefore this molecule is the Z isomer as the highest priority atoms are on the same side.

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Topic 6B: Alkanes Introduction to Alkanes Alkanes are saturated hydrocarbons where all carbon-carbon bonds are single bonds. They are part of a homologous series with the general formula CnH2n+2. Cycloalkanes are an exception to this general formula but are still saturated hydrocarbons. Fractional Distillation Crude oil is a mixture of different hydrocarbons. It can be separated into the separate molecules by fractional distillation as the different chain lengths of molecules result in them having different boiling points. Crude oil is separated in the following way:

1. The mixture if vapourised and fed into the fractionating column.

2. Vapours rise, cool and condense.

3. Products are siphoned off for different uses.

Products with short carbon chains have lower boiling points, meaning they rise higher up the column before reaching their boiling point. Therefore they are collected at the top of the column. Products with long carbon chains have higher boiling points, meaning they don’t rise very far up the column before reaching their boiling point. They condense and are collected at the bottom of the fractionating column. The compounds collected from the fractionating column are then broken down further via the method of cracking.

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Cracking Longer carbon chains are not very useful, therefore they are broken down to form smaller, more useful molecules. The carbon-carbon bonds are broken in order to do this, which requires quite harsh reaction conditions. There are two main types of cracking which result in slightly different organic compounds. Thermal Cracking This method produces a high proportion of alkanes and alkenes. High temperatures around 1200 K and pressures around 7000 kPa are used to crack the carbon chains. Catalytic Cracking This method produces aromatic compounds with carbon rings. Lower temperatures around 720 K are used along with normal pressure, but a zeolite catalyst is also used to compensate these less harsh conditions. Combustion of Alkanes Alkanes make good fuels as they release a lot of energy when burned. With sufficient oxygen present, they undergo complete combustion to produce carbon dioxide and water. Example:

If the oxygen present is insufficient, combustion is incomplete and carbon monoxide is produced alongside water. Example: Catalytic Converters Carbon monoxide is a toxic, gaseous product which is especially dangerous to humans as it has no odor or colour. Oxides of nitrogen and sulfur are also produced as a byproduct of alkane combustion along with carbon particulates from unburnt fuel. These gaseous products can be removed from systems using a catalytic converter. This uses a rhodium catalyst to convert harmful products into more stable products such as CO2 or H2O.

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Example: Carbon Particulates Incomplete combustion can also produce carbon particulates, small fragments of unburned hydrocarbon. Unless removed from the waste products in industry, these can cause serious respiratory problems as they pollute the air. Flue Gas Desulfurisation Sulfur impurities can lead to the acidification of water in the Earth’s atmosphere as they react to form a weak form of H2SO4. The impurities can be removed from waste products via flue gas desulfurisation. Calcium oxide and gypsum are used in this process. Example: Unless treated or removed, all of these pollutants can contribute to global warming, acid rain and health issues in humans.

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Alternative fuels are now being developed such as biofuels that release fewer, less harmful products when burned. Ethanol is a common biofuel produced in this way. It is said to be carbon neutral as the carbon given out when it is burned is equal to the carbon taken in by the crops during the growing process. Example: It is produced by fermentation, where enzymes break down starch from crops into sugars which can then be fermented to form alcohol. It has to be produced in batches, meaning it is a much slower process with a lower percentage yield but the environmental benefits make it viable. Chlorination of Alkanes Alkanes react with halogens in the presence of UV light to produce halogenoalkanes. The UV light breaks down the halogen bonds (homolytic fission) producing reactive intermediates called free radicals. Free radicals are species containing an unpaired electron which is shown using a single dot. These attack the alkanes resulting a series of reactions; initiation, propagation and termination. Example:

1. Initiation - the halogen is broken down.

Free radicals are shown using a dot.

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2. Propagation - a hydrogen is replaced and the Cl• radical reformed as a catalyst

3. Termination - two radicals join to end the chain reaction and form a stable product. The propagation step can continue many times to result in multiple substitutions, this is a chain reaction. The condition of the reaction can be altered to favour the termination step and limit the number of substitutions.

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Topic 6C: Alkenes Introduction to Alkenes Alkenes are unsaturated hydrocarbons with at least one carbon-carbon double bond. They are part of a homologous series with the general formula CnH2n. Cycloalkanes are saturated and follow this same general formula. Structure and Reactivity The carbon double bond is an area of high electron density making it susceptible to attack from electrophiles (species that are attracted to ∂-areas). It consists of a normal covalent bond and a π bond. Example: Bromine water is used to identify this double bond and other unsaturated compounds. It turns from orange-brown to colourless if a double bond is present in the substance.

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Electrophilic Addition Alkenes undergo electrophilic addition about the double bond. Electrophiles These are electron acceptors and are attracted to areas of high electron density. Some of the most common electrophiles are:

● HBr ● Br2

● H2SO4

They can be used in the presence of steam to form alcohols or with hydrogen to reproduce alkanes from alkenes. Electrophilic Addition This is the reaction mechanism that shows how electrophiles attack the double bond in alkenes. When the double bond is broken, a carbocation forms. This is a carbon atom with only three bonds, meaning it has a positive charge. Carbocations can have varying stability, with tertiary being the most stable and primary the least. The more stable the carbocation, the more likely it is to form. Therefore in an addition reaction, multiple products can form but the major product will always be the most stable possible.

Mechanism - Halogenoalkanes

The π bond causes the bromine molecule to gain a temporary

dipole so that electrons are transferred.

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Mechanism - Water

This product reacts with water to produce alcohols.

Addition Polymers Addition polymers are produced from alkenes where the double bond is broken to form a repeating unit. Example:

The repeating unit must always be shown with extended bonds through the brackets showing that it bonds to other repeating units on both sides.

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Uses of Polymers Polymers are unreactive hydrocarbon chains with multiple strong, non-polar covalent bonds. This makes them useful for manufacturing many everyday plastic products such a shopping bags (poly(ethene)). However, the unreactive nature of the bonds in addition polymers means they are not biodegradable and cannot be broken down by species in nature. Disposal of Polymers Addition polymers are non-biodegradable meaning disposal of them can be difficult. Waste polymers can be processed in different ways, some are recycled, some are used as feedstock for cracking and some are incinerated to produce energy for other industrial processes. This can release toxic gases which must be removed to reduce the impact on the environment. As well as this, scientists are developing biodegradable polymers to overcome these waste issues.

Topic 6D: Halogenoalkanes Introduction to Halogenoalkanes Halogenoalkanes contain polar bonds as the halogens are more electronegative than carbon atoms. This means electron density is drawn towards the halogen forming ∂+ and ∂- regions. Example: They can be primary, secondary or tertiary halogenoalkanes depending of the position of the halogen within the carbon chain. Example:

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The boiling point and reactivity of the halogenoalkanes reduces from primary to tertiary as the polarity of the molecule is reduced. As a result, van der waals forces between molecules are reduced in strength resulting in the reduced boiling points. Reactivity also varies depending on the halogen present in the molecule. Electronegativity of the halogens decreases down the group meaning that a carbon-fluorine bond is much more polar than a carbon-iodine bond. Therefore halogenoalkanes containing fluorine would be much more reactive than those containing iodine. Nucleophilic Substitution Nucleophiles These species are ‘positive liking’. They contain a lone electron pair that is attracted to ∂+ regions on molecules. Some of the most common nucleophiles are:

● CN:- ● -:NH3

● -:OH

They must be shown with the lone electron pair and a negative sign indicating they are nucleophiles. Nucleophilic Substitution This is the reaction mechanism that shows how nucleophiles attack halogenoalkanes. Aqueous potassium hydroxide is used produce alcohols, potassium cyanide to produce nitriles and ammonia to produce amines from halogenoalkanes.

Mechanism - Alcohol The nucleophile attacks the ∂+ carbon and the electrons are transferred to the chlorine.

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Mechanism - Amines

The intermediate formed has an N+ atom, so electrons are transferred to it causing a hydrogen to be lost too.

The greater the Mr of the halogen in the polar bond, the lower the bond enthalpy meaning it can be broken more easily. Therefore the rate of reaction for these halogenoalkanes is faster. Nucleophilic substitution reactions can only occur for 1o (primary) and 2o (secondary) alkanes. Elimination When a halogenoalkane is heated to high temperatures under alcoholic conditions, elimination occurs. In this reaction, the nucleophile acts as a base and accepts a proton, removing a hydrogen atom from the molecule. This results in the elimination of the halide too producing a carbon-carbon double bond, an alkene.

Mechanism Elimination reactions can only occur form 2o and 3o (tertiary) halogenoalkanes.

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Topic 6E: Alcohols Production of Alcohols Alcohols contain an -OH group and follow the general formula CnH2n+1OH. They can be produced via two main methods of fermentation or hydration. Hydration This method produces alcohols from alkenes in the presence of an acid catalyst. Phosphoric acid is commonly used as the catalyst under aqueous conditions at 300oC and high pressures. Example: This process has a very high percentage yield as ethanol is the only product. Therefore the hydration method is favoured as an industrial process. Fermentation In this process, enzymes break down starch from crops into sugars which can then be fermented to form alcohol. This method is cheaper than hydration as it can be carried out at a lower temperature. However it has to be fermented in batches, meaning it is a much slower process with a lower percentage yield. Ethanol is a common biofuel produced in this way. It is said to be carbon neutral as the carbon given out when it is burned is equal to the carbon taken in by the crops during the growing process. Example:

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Reactions of Alcohols Combustion When burned in air, alcohols react with oxygen to form carbon dioxide and water. Alcohols make good fuels by reacting in this way as lots of energy is also released. Example: Nucleophilic Substitution Alcohols can react with halogenating agents via nucleophilic substitution where the -OH group is replaced by a halogen, producing a halogenoalkane.

PCl5 is used to produce chloroalkanes. Concentrated sulfuric acid and potassium bromide produce bromoalkanes and red phosphorus with iodine produces iodoalkanes.

Oxidation of Alcohols Alcohols can be primary (1o), secondary (2o) or tertiary (3o). 1o and 2o alcohols can be oxidised to produce various products but 3o alcohols are not easily oxidised. Example: 1o alcohols can be heated in the presence of acidified potassium dichromate and distilled to produce aldehydes.

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Example:

When heated further under reflux conditions, 1o alcohols oxidise further to produce carboxylic acids. Example:

2o alcohols can be oxidised when heated in the presence of acidified potassium dichromate to produce ketones. Example:

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Potassium Dichromate (K2Cr2O7) This is used in the oxidation of alcohols as the oxidising agent. It is reduced as the alcohol is oxidised. This can be observed as a colour change from orange to green when the alcohol is oxidised. Example: Elimination Reactions Alkenes can be formed from the dehydration of alcohols, where a molecule of water is removed from the molecule. In order to do this, excess of hot sulfuric acid is added and aluminium oxide is used as a catalyst.

Mechanism

The H+ acidic ions are reformed in the reaction showing how they act as a catalyst.

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This reaction means that addition polymers can be produced from fermentation without the need for crude oil, a non-renewable resource. Fermentation produces the primary alcohol which is then dehydrated to produce an alkene used in the production of addition polymers.

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