Edexcel Chemistry - Topic 6: Organic I

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Organic Definitions

  • Hydrocarbon: A compound consisting of hydrogen and carbon only
  • Saturated: Carbon chains that contain the maximum number of hydrogens per carbon; only single C-C bonds
  • Unsaturated: Carbon chains that don't have the maximum number of hydrogens per carbon; contain C=C double bonds
  • Empirical formula: The simplest whole number ratio of atoms of each element in the compound
  • Molecular formula: The actual number of each type of atom in the compound
  • General formula: Algebraic formula for a homologous series
  • Structural formula: Shows the minimal detail of the arrangement of atoms in a molecule
  • Displayed formula: Shows all the covalent bonds present in a molecule
  • Skeletal formula: Shows the simplest organic formula of the carbon skeleton and associated functional groups
  • Homologous Series: Families of organic compounds with the same functional group and general formula
  • Functional group: the atom/group of atoms which provide the chemical properties of a substance
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Functional Groups

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Organic Nomenclature

  • Count the longest Carbon Chain and name appropriately (1 = meth-, 2 = eth-, 3 = prop-, 4 = but-, 5 = pent-)
  • Number the organic chain so that the numbers of functional groups are as low as possible
  • Find any side chains and name appropriately
  • Find any functional group and name/number appropriately
  • Ensure that the carbon with the highest priority functional group is numbered 1
  • Where there are multiple of the same functional group/side chain, use the prefixes di- tri or tetra-
  • Words are separated by numbers with dashes e.g. 2,2-dichloro-1-fluorobutane
  • Numbers are seperated by commas
  • If there are several functional groups, the functional groups are listed in alphabetical order
  • If there are any isomers that need to be labelled e.g. Stereoisomers or Chiral molecules the correct prefix is added e.g. E-but-2-ene
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Structural Isomers

  • Structural isomers: Same molecular formula, different structural formulas

Chain isomers: Compounds with the same molecular formular but different structures of the carbon skeleton

  • e.g. pentane, 2-methylbutane and 2,2-dimethylpropane

Position isomers: Compounds with the same molecular formula but different structures due to different positions of the same functional group on the carbon skeleton

  • e.g. 1-bromopropane and 2-bromopropane

Functional group isomers: Compounds with the same molecular formula but with atoms arranged to give different functional groups

  • e.g. Butanol (Alcohol) and Ethoxyethane (Ether)
  • Cyclohexane (Cycloalkane) and Hexene (Alkene)
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Stereoisomers

  • Stereoisomers have the same structural formulae but a different spatial arrangement of atoms
  • Alkenes can exhibit a type of stereoisomerism called E-Z stereoisomerism
  • E-Z isomers exist due to restricted rotation around the C=C bond and when two different group/atoms are attached to both ends of the double bond
  • But-1-ene cannot display stereoisomerism because there are two hydrogens on Carbon 1 but But-2-ene can as there are different groups on Carbon 2 and 3.
  • If the two groups are on the same side of the C=C bond, the isomer is refered to as a Z-isomer
  • If the two groups are on opposite sides of the C=C bond, the isomer is refered to as an E-isomer
  • If there are different groups on the Carbons, the highest atomic number group takes priority

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Alkanes: Properties

  • General Formula: CnH2n+2
  • Alkanes are saturated hydrocarbons
  • Alkanes are often used as fuels and are obtained from crude oil
  • Relatively unreactive
  • Nonpolar solvent; do not dissolve in water

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Refining Crude Oil

  • Petroleum is a mixture consisting of hydrocarbons
  • Petroleum fraction: a mixture of hydrocarbons with similar chain length and boiling point range
  • Fractional Distillation is the process of seperating out petroleum fractions based on their boiling points
  • Preheated oil is passed into a fractioning column
  • The temperature of the column decreases upwards so the lower boiling point molecules condense higher up the column
  • Smaller alkanes have lower boiling points as the Van der Waals Forces are weaker between the molecules so less energy is required to break the Intermolecular Forces
  • Petrol and Gasoline condense higher up the column than Diesel or Bitumen
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Cracking and Reforming

  • Cracking is the conversion of large hydrocarbons to smaller more useful molecules by breaking C-C bonds
  • C8H18 --> C3H8 + C3H6 + C2H4
  • Cracking is important as there are higher demands for shorter hydrocarbons as they are more useful and valuable (e.g. in the manufacturing of plastics)
  • High temperatures are required to break the covalent bonds so a catalyst is often used to help reduce activation energies of the process making it more efficient and reducing economic costs
  • Reforming involves turning straight chain alkanes into branched and cyclic alkanes and aromatic hydrocarbons
  • Branched and cyclic hydrocarbons burn more cleanly and are used to provide fuels with a higher octane number
  • The Octane Number of a fuel is the measure of the ignition quality of a fuel. A High Octane Number means the fuel is less susceptible to 'knocking' or premature burning in the combustion chamber
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Combustion of Alkanes

  • A fuel is a chemical store of energy that release heat when burnt
  • Alkanes readily burn in the presence of oxygen making them highly exothermic and useful as fuels
  • The complete combustion of a fuel in excess oxygen results in the production of Carbon Dioxide and Water only
    C8H18(g) + 12.5 O2(g) --> 8 CO2(g) + 9 H2O(l)
  • Incomplete combustion of a fuel occurs in limited oxygen and produces Carbon Monoxide and/or solid Carbon (soot)
    CH4(g) + 1.5 O2(g) --> CO(g) + 2 H2O(l)
    CH4(g) + O2(g) --> C(s) + 2H2O(l)
  • Incomplete combustion of a fuel produces less energy per mole than complete combustion
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Environmental Issues of Fuels

  • Carbon Monoxide is produced from the incomplete combustion of fuels and is toxic, binding with haemoglobin reducing oxygen flow to tissues
  • Soot is produced from incomplete combustion of fuels and causes respiratory problems and contributes to global dimming
  • Carbon Dioxide is produced from the complete combustion of fuels and contributes to global warming
  • Nitrogen Oxides form when Nitrogen in the air reacts at the high temperatures in the engine.
    NO is toxic and can form smog
    NO2 is toxic, acidic and forms acid rain
  • Sulphur Dioxide forms when impurities in fuels (e.g. coal) are burnt leading to acid rain
  • Unburnt hydrocarbons contribute to the formation of smog
  • Catalytic converters remove CO, NOx and unburned hydrocarbons from exhaust gases turning them into 'harmless' CO2, N2 and H2O
    2 CO + 2 NO --> 2 CO2 + N2
    C8H18 + 25 NO --> 8 CO2 + 12.5 N2 + 9 H2O
  • Converters have a ceramic honeycomb shape coated with a thin layer of Platinum, Palladium or Rhodium to give a large catalytic surface area.
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Biofuels

  • Fossil fuels are a non-renewable source of fuel that are diminishing
  • Alternative renewable fuels created from plants such as alcohols or biodiesel can be used
  • Alcohols such as ethanol can be produced from the fermentation of sugars from plants
  • Biodiesel is produced by reacting vegetable oils with a mixture of alkali and methanol
  • Advantages of using biofuels include:
    Reduce usage of fossil fuels which are finite resources
    Are renewable and carbon-neutral
    Allows fossil fuels to be potentially used for creating other organic compounds
    Reduces pollution from exploitation of fossil fuels
  • Disadvantages of using biofuels include:
    Less food crops may be grown, particularly in developing nations e.g. Brazil
    Rain forests have to be cut down to provide land
    Shortage of fertile soils for growing food crops
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Homolytic and Heterolytic Fission

  • Mechanisms show how electrons move in an organic reaction
  • Homolytic Fission splits a bond by providing one electron to each atom forming a free radical
  • Heterolytic Fission splits a bond by providing both electrons to one atom forming a positive and a negative ion.
  • A free radical is a reactive species which possess an unpaired electron
  • Curly arrows show electron movement in a reaction. If the arrow is single headed, homolytic fission occurs; if the arrow is double headed, heterolytic fission is occuring
  • Curly arrows will always start from a lone pair of electrons or the centre of a bond
  • Most organic reactions occur via heterolytic fission producing ions

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Free Radical Substitution Reactions

  • Alkane + Halogen --> Halogenoalkane + HCl
  • Conditions: UV light
  • Mechanism: Free Radical Substitution (Homolytic Fission)
  • A mixture of products are formed due to multiple termination stages (CH3• + CH3• --> C2H6)

Image result for free radical substitution of ethane and chlorine initiation stage

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Alkenes: Properties

  • General Formula: CnH2n
  • Alkenes are unsaturated hydrocarbons containing a C=C double bond
  • Alkenes are used in the production of poly(alkene) plastics e.g. polyethylene bags
  • Alkenes are more reactive than alkanes due to the high electron density in the double bond
  • Nonpolar solvent; doesn't dissolve in water
  • Can form E-Z Stereoisomers

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Formation of a Double Bond

  • The C=C double covalent bond consists of one sigma (σ) bond and one pi (π) bond
  • π bonds are exposed and have a high electron density and so are vulnerable to attack by electrophiles - a species that is attracted to an electron rich centre
  • One sp2 orbital from each carbon overlap to form a single C-C bond (a sigma σ bond)
  • One p orbital from each carbon can overlap to form a double C=C bond (a pi π bond)
  • Another p orbital can overlap to form another pi bond to form a triple carbon bond of an alkyne
  • There is restricted rotation around a pi bond which gives rise to stereoisomerism
  • The pi bond results in a high electron density between the two carbon atoms causing the alkenes to attract electrophiles and undergo electrophilic reactions.

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Electrophilic Addition Mechanism

  • Electrophiles are attracted to the high electron density of the double bond
  • A dipole is induced which causes the δ+ side of the electrophile to be attracted to the δ- double bond 
  • Heterolytic fission occurs, breaking the covalent bond and creating two ions.
  • The positive ion is attracted to the double bond to form an intermediate
  • The intermediate consists of a positively charge carbon atom - a carbocation
  • The carbocation attracts the negatively charged ion and the final product is formed
  • Markovnikovs Rule states that the more stable product (e.g. 2-bromopropane) is produced more often than the less stable product (e.g. 1-bromopropane) as the alkyl groups either side are electron releasing and reduce the charge on the ion it stabilises

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Reaction of Alkenes and Hydrogen

  • Reaction: Alkene --> Alkane
  • Reagent: Hydrogen
  • Conditions: Nickel Catalyst
  • Mechanism: Electrophilic Addition

Electrophiles are Electron Pair Acceptors
Addition Reactions occur where two molecules react to produce one molecule
Alkenes undergo addition reactions because the pi bond has a high electron density so are more accessible to electrophilic attack by electrophiles and so are more reactive

Image result for alkene to alkane reaction mechanism

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Reaction of Alkenes and Halogens

  • Reaction: Alkene --> Dihalogenoalkane
  • Reagent: Bromine (dissolved in organic solvent)
  • Conditions: Room temperature (not in UV light)
  • Mechanism: Electrophilic Addition
  • Observations: Colour Change: Brownish-red > Colourless

Bromine acts as an Electrophile
Markovnikovs Rule states that the Major product formed will be the more stable ion
Tertiary > Secondary > Primary

Image result for alkene and bromine reaction

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Reaction of Alkenes and Hydrogen Halides

  • Reaction: Alkene --> Halogenoalkane
  • Reagent: Hydrogen Chloride or Hydrogen Bromide
  • Conditions: Room temperature 
  • Mechanism: Electrophilic Addition

Hydrogen acts as the electrophile
HBr is a polar molecule as Br is more electronegative than H so the H is attracted to the electron rich pi bond

Image result for alkene to halogenoalkane reaction

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Reaction of Alkenes and Potassium Manganate(VII)

  • Reaction: Alkene --> Diol
  • Reagent: Potassium Manganate(VII) in an acidified solution
  • Conditions: Room temperature 
  • Mechanism: Oxidation
  • Observation: Colour Change: Purple > Colourless

Purple Colour is due to MnO4- ion
The reaction can be used as a test for alkenes as alkanes don't react

Image result for ethene to ethane-1 2-diol

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Reaction of Alkenes and Bromine Water

  • Reaction: Alkene --> Bromo-alcohol
  • Reagent: Bromine dissolved in water
  • Conditions: Room temperature 
  • Mechanism: Electrophilic Addition
  • Observation: Colour Change: Orange > Colourless

Orange colour is due to bromine water
The reaction can be used as a test for the alkene functional group

Image result for ethene and bromine water

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Reaction of Alkenes and Water

  • Reaction: Alkene --> Alcohol
  • Reagent: Water
  • Conditions: 300C, 60-70atm, Phosphoric(V) Acid Catalyst
  • Mechanism: Electrophilic Addition (Hydration)

High pressures mean this reaction can't be done in a laboratory
The process is preferred in industry as there are no waste products so an atom economy of 100% is achieved and the seperation of products is cheaper and easier to carry out
Hydration reactions are reactions where water is added to a molecule

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Poly(alkenes)

  • Poly(alkenes) are addition polymers formed from repeating monomers of an alkene
  • Poly(alkenes) are unreactive due to the strong C-C and C-H bonds that exist in them
  • A monomer is one repeating unit of a polymer e.g. Ethene in Poly(ethene)
  • Poly(ethene) is a flexible, easily moulded, waterproof plastic with a low density. It is used to make plastics bags, buckets and bottles
  • Poly(propene) is a stiffer polymer used in utensils, containers and fibres in ropes and carpets

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Disposal of Waste Polymers

Incineration

  • Rubbish is burnt and energy is produced to generate electricity
  • Some toxins can be released on incineration e.g. burning PVC releases HCl
  • Greenhouse gases are emitted through combustion contributing to global warming

Recycling

  • Saves crude oil supplies
  • Polymers need collecting and sorting which uses a lot of energy, time and manpower
  • Thermoplastic polymers can be melted down and reshaped

Feedstock for Cracking

  • Polymers can be cracked into smaller molecules which can be used to make other chemicals and new polymers - saving raw materials

Biodegradable Polymers

  • Made from renewable sources e.g. starch so can break down in UV light or rain
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Halogenoalkanes: Properties

  • General Formula: CnH2n+1X
  • Halogenoalkanes are based on an alkane but have a halogen attached to them
  • Halogenoalkanes are named as Fluoro- for F; Chloro- for Cl; Bromo for Br; Iodo for I
  • Halogenoalkanes are used in aerosoles and old refrigerants or in the manufacturing of strong polymers such as Polyvinyl Chloride (PVC) or Polytetrafluoroethene (PTFE/Teflon)
  • CFCs from aerosoles cause depletion of the ozone layer
  • Nonpolar solvent; doesn't dissolve in water
  • Halogenoalkanes can either be primary, secondary or tertiary depending on the number of carbons attached to the carbon that the halogen is bonded to.

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Nucelophilic Substitution Mechanism

  • Nucelophiles are Electron Pair Donators e.g. :OH-, :NH3
  • Substitution involves the swapping of one atom for another atom or group of atoms
  • Nucelophiles always have a lone pair of electrons.
  • The C-X bond breaks because the carbon has a slight positive charge due to the differences in electronegativity which causes the bond to be weaker
  • Iodoalkanes are the fastest to substitute and fluoroalkanes are the slowest as the C-F bond is very strong and so fluoroalkanes are very unreactive

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Hydrolysis of Halogenoalkanes

  • Hydrolysis is the splitting of a molecule by a reaction with water
  • Water is a poor nucleophile and reacts slowly with halogenoalkanes in a substitution reaction
  • Aqueous silver nitrate is added to a halogenoalkane so a silver halide precipitate forms.
  • This enables the rate of formation of the precipitate to be measured to determine the reactivity of the halogenoalkanes - the quicker the precipitate is formed, the faster the substitution reaction and the more reactive the halogenoalkane
  • The rate of these reactions depends on the strength of the C-X bond. The weaker the bond, the easier it is to break and so the faster the reaction
  • Iodoalkanes form a precipitate fastest as the C-I bond is weakest so hydrolyses the quickest

CH3CH2I + H2O --> CH3CH2OH + I- + H+

Ag+(aq) + I-(aq) --> AgI(s)

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Reaction of Haloalkanes with Aqueous Hydroxide ion

  • Reaction: Halogenoalkane --> Alcohol
  • Reagent: Potassium/Sodium Hydroxide
  • Conditions: In aqueous solution, heat under reflux
  • Mechanism: Nucleophilic Substitution

The hydroxide ions act as the nucleophile
The OH- is a stronger nucleophile than water as it has a full negative charge so is more strongly attracted to the δ+ Carbon
The aqueous conditions are needed for substitution; if alcoholic conditions are present, elimination occurs

Image result for bromoethane to ethanol

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Reaction of Haloalkanes with Ammonia

  • Reaction: Halogenoalkane --> Amine
  • Reagents: Ammonia dissolved in ethanol
  • Conditions: Heating under pressure in sealed tube
  • Mechanism: Nucleophilic Substitution

Ammonia acts as the Nucleophile
Further substitution reactions can occur between the halogenoalkane and the amines forming a lower yield of the amine. Using excess ammonia helps to reduce this.

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Reaction of Haloalkanes with Potassium Cyanide

  • Reaction: Halogenoalkane --> Nitrile
  • Reagent: Potassium Cyanide dissolved in alcohol
  • Conditions: Reflux
  • Mechanism: Nucleophilic Substitution

The Cyanide (CN-) ion acts as the Nucleophile

Image result for reaction of halogenoalkanes with potassium cyanide

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Elimination Mechanism

  • Elimination reactions involve the removal of a small molecule (usually water) from an organic molecule.
  • Elimination and Substitution reactions often produce a mixture of products

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Reaction of Haloalkanes with Alcoholic OH- Ions

  • Reaction: Halogenoalkane --> Alkene
  • Reagent: Potassium/Sodium Hydroxide
  • Conditions: In ethanol, heat
  • Mechanism: Elimination

The Hydroxide ions act as a Base
Alcoholic conditions are required for elimination to occur; aqueous conditions result in substitution

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Alcohols: Properties

  • General Formula: CnH2n+1OH
  • Alcohols have a hydroxyl (OH) group on the carbon chain
  • Alcohols can be either primary, secondary or teritary depending on how many carbons are attached to the carbon which the OH group is bonded to
  • The H-O-C bond is 104.5 degrees (bent shape) because there are 2 bonding pairs and 2 lone pairs so the lone pairs repel more until a position of minimal repulsion is reached
  • The OH group is polar and can form hydrogen bonds so alcohols dissolve in water and can dissolve polar/ionic solutes
  • Alcohols are used in drinks, fuels, perfumes and paint thinners

Image result for ethanol

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Reaction of Alcohols with Oxygen and Sodium

Combustion of Alcohols

  • Alcohols combust with a clean flame due to the presence of oxygen within the molecule itself

CH3CH2OH + 3 O2 --> 2 CO2 + 3 H2O

Reaction of Alcohols with Sodium

  • Sodium reacts with alcohols to form sodium-oxides (e.g. Sodium Ethoxide) and hydrogen

2 CH3CH2OH(l) + 2 Na(s) --> 2 CH3CH2O-Na+(l)  + H2(g)

Observations that are made include:

  • effervescene
  • sodium dissolving
  • a white solid being produced
  • a squeaky pop when a lit splint is applied to the gas
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Substitution reactions of Alcohols

  • Reaction: Alcohol --> Halogenoalkane
  • Reagents: Phosphorus(V) Chloride (PCl5)
                      Hydrogen Bromide (HBr)
                      Phosphorus(III) Iodide (PI3)**
  • Conditions: Room temperature
  • Mechanism: Substitution

CH3CH2OH + PCl5 --> CH3CH2Cl + POCl3 + HCl

CH3CH2OH + HBr --> CH3CH2Br + H2O

3 CH3CH2OH + PI3 --> 3CH3CH2I + H3PO3

Hydrogen Bromide is made from KBr and 50% concentrated H2SO4
** Phosphorus(III) Iodide is produced by reacting red phosphorus and Iodine

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Partial Oxidation of Primary Alcohols

  • Reaction: Primary Alcohol --> Aldehyde
  • Reagents: Potassium Dichromate(VI) and dilute Sulphuric Acid
  • Conditions: Limited amount of dichromate, gently warm and distil
  • Observations: Colour Change: Orange > Green

K2Cr2O7 is an oxidising agent an causes alcohols to oxidise
Distillation is used so as no further oxidation occurs

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Full Oxidation of Primary Alcohols

  • Reaction: Primary Alcohol --> Carboxyllic Acid
  • Reagents: Potassium Dichromate(VI) and dilute Sulphuric Acid
  • Conditions: Excess of dichromate, heat under reflux
  • Observations: Colour Change: Orange > Green

Reflux is carried out so complete oxidation is carried out
The product is distilled off to ensure the product is pure

Image result for ethanol to ethanoic acid

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Oxidation of Secondary Alcohols

  • Reaction: Secondary Alcohol --> Ketone
  • Reagents: Potassium Dichromate(VI) and dilute Sulphuric Acid
  • Conditions: Heat under reflux
  • Observations: Colour Change: Orange > Green

There is no further oxidation of ketones under these conditions
Tertiary alcohols cannot be oxidised at all by potassium dichromate as there is no hydrogen atom bonded to the carbon with the OH group

Image result for propan-2-ol to propanone

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Distinguishing between Aldehydes and Ketones

Fehling's Solution

  • Reagent: Fehling's Solution containing blue Cu2+ ions
  • Conditions: Heat gently
  • Reaction: Aldehydes only are oxidised by Fehling's Solution into a Carboxylic Acid and the copper ions are reduced to copper(I) oxide
  • Positive Observation (Aldehydes): Colour Change: Blue > Red
    Negative Observation (Ketones): Blue Colour Remains
    CH3CHO(aq) + 2 Cu2+(aq) + 2 H2O(l) --> CH3COOH(aq) + Cu2O(s) + 4H+

Tollens' Reagent

  • Reagent: Tollens' Reagent consisting of the complex ion 2Ag(NH3)2+
  • Conditions: Room temperature
  • Reaction: Aldehydes only are oxidised into a Carboxylic Acid and the complex ion reduced into Silver atoms
  • Positive Observation (Aldehydes): Silver Mirror forms
    Negative Observation (Ketones): No Mirror forms
    CH3CHO(aq) + 2 Ag+(aq) + 2 OH-(aq) --> CH3COOH(aq) + 2 Ag + H2O
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Reaction of Alcohols with Dehydrating Agents

  • Reaction: Alcohol --> Alkene
  • Reagents: Concentrated Phosphoric Acid
  • Conditions: Warm (under reflux)
  • Mechanism: Acid Catalysed Elimination

The Phosphoric Acid acts as a dehydrating agent and a catalyst
Dehydration Reactions are elimination reactions where a water molecule is removed
Forming alkenes from alcohols can give a possible route to forming polymers without using monomers derived from oil - renewable sources to make plastics

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Distillation and Reflux

  • Distillation is used as a seperation technique to seperate organic products from a mixture based on boiling points. A thermometer is used to allow the distillate of the approximate boiling point to be collected
  • Reflux is used when heating organic reaction mixtures for long periods. The condenser prevents organic vapours from escaping by condensing them back to liquids.
    Never seal the end of the condenser as the build up of gases can cause the apparatus to explode
  • Antibumping granules are added to prevent vigorous, uneven boiling
  • Electric heaters are often used to heat organic chemicals as organic substances are normally flammable

Image result for reflux and distillation apparatus

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Purifying an Organic Liquid

  • Put the distillate of impure product into a separating funnel
  • Wash product by adding either:
    Sodium Hydrogencarbonate solution, shaking and releasing the pressure from the CO2 formed
    Saturated Sodium Chloride solution
  • Allow the layers to separate in the funnel, and then run and discard the aqueous layer
  • Run the organic layer into a clean, dry conical flask and add three spatula loads of drying agent (anhydrous sodium sulphate) to dry the organic liquid
  • The drying agent should be insoluble and not react with the organic liquid
  • Carefully decant the liquid into the distillation flask and distill to collect pure product

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Measuring Boiling Point

  • The purity of a liquid can be determined by measuring a boiling point
  • This can be done in a distillation set up or by boiling a tube of the sample in a water/oil bath
  • Pressure should be noted as changing pressure can change the boiling point of a liquid
  • To get a correct measure of the boiling point, the thermometer should be above the level of the surface of the boiling liquid and be measuring the temperature of the saturated vapour
  • Measuring boiling point is not the most accurate method of identifying a substance as several substances may have the same boiling point

Image result for boiling point apparatus setup

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