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Edexcel Chemistry – Topic 18: Organic III

Benzene

  • Benzene is the simplest aliphatic (arene) compound
  • 6 carbon atoms are arranged in a hexagonal ring
  • The structure of benzene was first believe to be a ring of alternating C=C and C-C bonds (the Keklue Structure)
  • The Kekule Structure cannot be correct as the C=C bonds would be shorter than the C-C bond but this is not the case.

Benzene Delocalised Structure

  • The structure of benzene suggests that the carbon atoms are bonded to each other by sigma bonds leaving one unused electron on each carbon in the p orbital
  • The six pi electrons are perpendicular to the plane of the ring and can delocalisein a ring above and below the ring of carbon atoms
  • Benzene is a planar molecule with bond angles of 120 degrees
  • The bond length and bond energy of the C-C and C=C bond are the same

 

Enthalpies of Hydrogenation of Benzene

  • The enthalpy of hydrogenation for cyclohexene is -120kJ mol-1
  • Theoretically if the kekule structure existed, the enthalpy for hydrogenation would be 3 times greater (-360kJ mol-1) but instead is -208kJ mol-1
  • The 6 pi electrons are delocalised and are therefore more thermodynamically stable. The increased stability connected to delocalisation is called the delocalisation energy
  • Benzene doesn’t undergo addition reactions as this would break the delocalised structure – therefore benzene reactions involve substituting a hydrogen atom for a different group

Hydrogenation of Benzene

  • Reaction: Benzene –> Cyclohexane
  • Reagents: Hydrogen
  • Conditions: Nickel catalyst, temperature = 200-300C, pressure = 30atm
  • Mechanism: Addition and Reduction

Halogenation of Benzene

  • Reaction: Benzene –> Bromobenzene
  • Reagents: Bromine
  • Conditions: Iron(III) Bromide (FeBr3) Catalyst
  • Mechanism: Electrophilic Substitution

C6H6 + Br2 –> C6H5Br + HBr

This reaction can be done with Chlorine. The catalyst can be AlCl3 or FeCl3
AlCl3 + Cl2 –> AlCl4- + Cl+
AlCl4- + H+ –> AlCl3 + HCl

itration of Benzene

  • Reaction: Benzene –> Nitrobenzene
  • Reagents: conc. nitric acid in presence
  • Conditions: conc. sulphuric acid catalyst, temperature = 60C
  • Mechanism: Electrophilic Substitution
  • Electrophile: NO2+

The reaction is important in the synthesis of explosives (TNT) and dyestuffs

Combustion of Benzene

  • Benzene will combust with a very sooty flame. The lower the Carbon:Hydrogen ratio the sootier the flame
  • In limited oxygen, the combustion of benzene will produce Carbon Monoxide or Soot

Benzene + Oxygen –> Carbon Dioxide + Water
C6H6(l) + 7.5 O2(g) –> 6 CO2(g) + 3 H2O(l)

C6H6(l) + 4.5 O2(g) –> 6 CO(g) + 3 H2O(l)

C6H6(l) + 1.5 O2(g) –> 6 C(s) + 3 H2O(l)

Friedel Crafts Alkylation Reactions

  • Reaction: Benzene –> Alkylbenzene
  • Reagents: Chloroalkane
  • Conditions: Heat under reflux with an anhydrous AlCl3 or FeCl3 catalyst
  • Mechanism: Electrophilic Substitution

The H+ ion reacts with the AlCl4- to reform AlCl3 and HCl

Friedel Crafts Acylation Reactions

  • Reaction: Benzene –> Phenyl ketone
  • Reagents: Acyl Chloride
  • Conditions: Heat under reflux, AlCl3 or FeCl3 catalyst, temperature = 50C
  • Mechanism: Electrophilic Substitution

AlCl3 + CH3COCl –> CH3CO+ + AlCl4-
H+ + AlCl4- –> AlCl3 + HCl

Side Groups on Benzene

Electron releasing groups that are added to a benzene ring, release electrons into the delocalised system increasing electron density and making it more attractive to electrophiles

Chlorobenzene

  • The C-Cl bond is made stronger so typical halogenation reactions don’t occur
  • The electron rich benzene ring will repel nucleophiles

Phenylamine

  • Less basic than aliphatic amines as the lone pair is delocalised and less avaliable for accepting a proton

Phenol

  • Delocalisation makes the C-O bond stronger and the O-H bond weaker
  • Phenol doesn’t behave like an alcohol; it is more acidic and doesn’t oxidise

Phenol

  • Phenol consists of an OH group attached to a benzene ring
  • The lone pair of electrons on the oxygen is delocalised with the electron charge cloud of the arene ring
  • The delocalisation changes the reaction of the OH group and the areneringmaking it a weak acidic solution

Reaction of Phenol, Sodium and Sodium Hydroxide

  • Reaction: Phenol –> Sodium Phenoxide
  • Reagents: Sodium or Sodium Hydroxide
  • Conditions: Room temperature and pressure

2 C6H5OH + 2 Na –> 2 C6H5O-Na+  + H2

C6H5OH + NaOH –> C6H5O-Na+  + H2O

Sodium phenoxide is more soluble than phenol so solid phenol dissolves on the addition of sodium hydroxide

Reaction of Phenol and Bromine

  • Reaction: Phenol –> 2,4,6-tribromophenol
  • Reagents: Bromine water
  • Conditions: Room temperature and pressure
  • Observations: Formation of a white solid

Phenol doesn’t need an FeBr3 catalyst and undergoes multiple substitution reactions
In phenol, the lone pair of electrons on the oxygen (p-orbital) is partiallydelocalisedinto the ring, so the electron density increases and the Bromine is more polarised.

Amines: Properties

  • Prefix = amino- | Suffix = -amine
  • Amines are a nitrogenous organic substance that can act as Bronsted-Lowry bases
  • Amines can be either primary, secondary or tertiary depending on the number of alkyl chains attached to the nitrogen atom
  • Secondary and tertiary amines require the letter N infront of the side alkyl chainsnames
    e.g. N-methylpropylamine or N,N-dimethylpropylamine
  • Amines have a fishy smell
  • Amines can form hydrogen bonds and are soluble in water

Amides: Properties

  • Suffix = -amide
  • Amides are a nitrogenous organic substance derived from carboxylic acids
  • Amides can be primary, secondary or tertiary depending on how many alkyl chains are attached to the nitrogen atom
  • Secondary and tertiary amides require the prefix N- to show positioning of an alkyl chain
    e.g N-methylpropanamide or N,N-dimethylamide

Reaction of Amines with Acids

  • Reaction: Amine –> Ammonium Salt
  • Reagents: Hydrochloric Acid
  • Conditions: Room temperature and pressure

CH3NH2(aq) + HCl(aq) –> CH3NH3+Cl-(aq)

Addition of sodium hydroxide to the ammonium salt will convert it back into the amine.
These ionic salts will be solid crystals, if the water is evapourated, because of the strong ionic interactions
basic buffer can be made from combining a weak base (e.g. Methylamine) with a salt of the weak base (e.g. Methylammonium Chloride)

Formation of Complex Ions

  • The lone pair of electrons on the Nitrogen enables amines to act as ligands and form dative covalent bonds with transition metal ions to form colouredcomplex ions
  • Ethane-1,2-diamine is a common bidentate ligand e.g. [Ni(NH2CH2CH2NH2)3]2+

Reaction of Amines with Halogenoalkanes

  • Reaction: Primary Amine –> Secondary Amine –> Tertiary Amine
  • Reagent: Halogenoalkane
  • Conditions: Room temperature and pressure
  • Mechanism: Nucleophilic Substitution

On reacting a halogenoalkane with a primary amine, a secondary amine is formed. This secondary amine can further react with the halogenoalkane to form a teritary amine or a quaternary ammonium salt

Formation of Amines from Nitriles

  • Reaction: Nitrile –> Primary Amine
  • Reagents: Lithium tetrahydridoaluminate (LiAlH4) in ether
  • Conditions: Room temperature and pressure
  • Mechanism: Reduction

Formation of Primary Amides from Acyl Chlorides

  • Reaction: Acyl Chloride –> Primary Amide
  • Reagents: Ammonia
  • Conditions: Room temperature and pressure
  • Observations: White smoke of NH4+Cl- forming

Formation of Secondary Amides from Acyl Chlorides

  • Reaction: Acyl Chloride –> Secondary Amide
  • Reagents: Primary Amine
  • Conditions: Room temperature and pressure
  • Mechanism: Nucleophilic Addition-Elimination

Reducing Nitroarenes

  • Reaction: Nitrobenzene –> Phenylamine
  • Reagent: Tin or Iron and Hydrochloric Acid
  • Conditions: Heat
  • Mechanism: Reduction

As the reaction is carried out with HCl the salt C6H5NH3+Cl- forms. Reacting the salt with sodium hydroxide gives phenylamine.
The phenylamine produced is best seperated from the reaction mixture by steam distilation

Polyamides

  • Polyamides are a condensation polymer formed on the reaction of diamines and dicarboxylic acids
  • Polyamides are biodegradable and can be broken down by hydrolysis
  • Polyamides are reactive due to the presence of polar bonds which can attract attacking species i.e. Nucleophiles
  • Polyamides have permanent dipole interactions in addition to the Van der Waals Forces. Hydrogen bonding can also exist between the N-H and C=O bonds so polyamides have high melting points
  • Polyamide + Sodium Hydroxide –> Dicarboxylic Acid Salt + Diamine
  • Polyamide + Hydrochloric Acid –> Dicarboxylic Acid + Diamine
  • Common polyamides include Nylon 6,6 and Kevlar

Nylon 6,6

  • Nylon is a useful polymer because of its elasticity, strength, durability, avaliabilityand price
  • It is used in tights, toothbrushes and parachutes
  • The 6,6 stands for 6 carbons in each of the monomers

Kevlar

  • Kevlar is a strong polymer used in bullet proof jackets and helmets
  • Kevlar can be formed from reacting 1,4-diaminophenyl with either benzene-1,4-dioic acid or benzene-1,4-diacyl chloride
  • The reaction involving the carboxylic acid forms water and the reaction with the acyl chloride forms HCl

mino Acids: Properties

  • Amino acids consist of an amine, a carboxylic acid and an R group bonded to a central carbon atom
  • The R group is different for all 20 amino acids
  • Amino acids are zwitterions and are chiral molecules
  • Amino acids can form hydrogen bonds and are soluble in water
  • Amino acids are the building blocks of all life and combine to form proteins

Chirality of Amino Acids

  • All amino acids, except for glycine, are chiral as they have four different groupsaround a central carbon atom
  • Enantiomers of amino acids therefore have similar chemical and physical properties but rotate plane-polarised light in different directions

Zwitterions

  • Amino acids never form their neutral form but instead exist as zwitterions
  • Zwitterions have both a positive and a negative side (are dipolar)
  • Amino acids have high melting points due to the strong ionic interactions that exist between zwitterions

Acidity and Basicity of Amino Acids

  • The Amine group is basic | The Carboxylic Acid group is acidic
  • Amino acids can either become protonated or deprotonated from the isoelectric form by the addition of either an alkali or acid
  • At the isoelectric point, amino acids form a zwitterion
  • Amino acids can act as weak buffers and will only gradually chage pH if small amounts of acid or alkali are added.
  • If the R group contains either a carboxylic acid or an amine group, they will react and change in alkaline or acidic conditions

Image result for amino acids in acidic and basic conditions

Chromatography of Amino Acids

A mixture of amino acids can be seperated and identified by chromatography

Method

  • Draw a pencil line 1.5cm from the bottom of a piece of chromatography paper and mark spots for the amino acid to be placed
  • With a capillary tube, put a small drop of amino acid on the dots
  • Stand the paper in a large beaker of solvent ensuring the solvent doesn’t touch the pencil line
  • Allow the solvent to travel up the paper until it is about 1.5cm from the top of the paper
  • Spray the paper with ninhydrin and put in the oven for 10 minutes
  • Identify the distance travelled by the solvent and the amino acids and calculate the Rf value
  • Rf value = (Distance Moved by the Amino Acid) / (Distance moved by the solvent)

Ninhydrin is used to make the amino acids visible
A pencil line is used as an dyes in ink can be seperated out via chromatography
The solvent must be below the pencil line otherwise the amino acids will dissolve into the solvent

Proteins

  • Proteins are polymers of various combinations of amino acids bonded together with peptide links
  • Dipeptides are a combination of two amino acids joined together by a condensation reaction
  • The 3D structure of amino acids within the polypeptide chain is caused by intermolecular bonding.
  • Hydrogen bonds exist between the N-H and the C=O bonds in addition to the Van der Waals Forces which give arise to the specific 3D (Tertiary Shape)
  • Proteins can be hydrolysed and broken apart with dilute acid or alkali into their original amino acids

Grignard Reagent

  • Grignard reagent is used to increase the length of a carbon (alkyl) chain in a molecule
  • Grignad reagent is created by dissolving a halogenoalkane in dry ether and reacting it with magnesium
    CH3CH2I + Mg –> CH3CH2MgI (Ethyl magnesium Iodide)

Reactions with Carbonyls
Methanal produces a primary alcohol in presence of water
CH3CH2MgI + HCHO –> CH3CH2CH2OH + Mg(OH)I
Longer Aldehydes produce secondary alcohols in presence of water
CH3CH2MgI + CH3CHO –> CH3CH2CH(OH)CH3 + Mg(OH)I
Ketones produce tertiary alcohols in presence of water
CH3CH2MgI + CH3COCH3 –> CH3CH2C(CH3)(OH)CH3 + Mg(OH)I

Reaction with Carbon Dioxide
Carbon Dioxide forms a carboxylic acid in presence of water
CH3CH2MgI + CO2 –> CH3CH2COOH + Mg(OH)I

Steam Distillation

  • Steam distillation is used on organic substances that either have a high boiling point or decompose on heating
  • Steam distillation lowers the boiling point of an immisible product, allowing it to be distilled out of an impure mixture below its boiling point and before it decomposes
  • Water is heated until it evapourates. The steam is then passed into a flask containing the impure organic mixture which lowers the boiling point of the compounds in the mixture
  • If the organic product is less volatile than the components in the mixture you’re seperating it from, the organic product and the steam will evapourate out of the impure mixture together.
  • The organic product and water can then be condensed by a liebigcondensor and collected in a clean beaker. Water can then be seperated by further organic techniques

Solvent Extraction

  • Solvent extraction removes partially soluble compounds from water
  • Add the impure compound to a separating funnel and add some water and shake.
  • Add an organic solvent in which the product is more soluble in than water. Shake the separating funnel well so the product dissolves into the organic solventleaving impurities to dissolve into water
  • Add a salt (e.g. NaCl) to the mixture to cause the organic product to move into the organic layer, as it will be less soluble in the very polar salt and water layer
  • Open the tap and run off each layer into a seperate beaker
  • Water can also be removed by adding an anhydrous salt (e.g. MgSO4) to act as a drying agent

Image result for solvent extraction

Gravity Filtration

  • Used if you want a liquid (filtrate) and not a solid
  • Place a piece of fluted filter paper in a funnel that feeds into a conical flask
  • Gently pour the mixture to be seperated into the filter paper
  • The solution will pass through the filter paper into the conical flask and the solid will be trapped in the filter paper
  • Rinse the paper with a pure sample of the solvent present in the solution to ensure all soluble material has passed through the filter paper and has been collected in the conical flask

Filtration under Reduced Pressure

  • Used if you want to keep a solid and discard the liquid (filtrate)
  • Place a piece of filter paper, slightly smaller than the diameter of the funnel, on the bottom of the buchner funnel so it lies flat and covers all the holes
  • Wet the paper with a little solvent so that it sticks to the bottom of the funnel
  • Turn the vacuum on and pour in the mixture to the funnel
  • As the funnel is under reduced pressure, the liquid is pulled through the funnel into the flask, leaving the solid behind
  • Rinse the solid with a little of the solvent to wash of any of the original liquid from the mixture that stayed on the crystalls. This leaves you with a more pure solid
  • Turn off the vacuum and leave the solid to dry

Recrystallisation

  • Recrystallisation can be used to purify organic solids

Method

  • Add very hot solvent to the impure solid until it just dissolves – this should give a saturated solution of the impure product
  • Filter the hot solution by gravity filtration to remove any insoluble impurities
  • The filtrate solution is left to cool down slowlyCrystals of the product form as it cools while impurities remain in the solution
  • The crystals are removed by filtration under reduced pressure and washed with ice-cold solvent
  • The crystals are then dried, leaving a much purer organic solid

*The solvent used must be soluble when very hot but insoluble when very cold

 

Testing for Organic Functional Groups

Alkene – Bromine Water = Colour Change: Orange > Colourless

Carbonyl – 2,4-DNP = Orange/Red Crystals form

Aldehyde – Fehling’s Solution = Colour Change: Blue > Red
Tollens Reagent = Silver Mirror Forming

Aldehyde/Primary/Secondary Alcohol – Sodium Dichromate = Colour Change: Orange> Green

Alcohol/Carboxylic Acid – Phosphorous(V) Chloride = Misty fumes of HCl

Alcohol/Carboxylic Acid/Phenol – Sodium = Effevesence due to H2 gas

Carboxylic Acid – Sodium Carbonate = Effervesence due to CO2 gas

Acyl Chloride – Silver Nitrate = Misty fumes of HCl and white precipitate of AgCl

Halogeonoalkanes – Silver Nitrate = Chloro (White ppt); Bromo (Cream ppt), Iodo (Yellow ppt)

 

 

 

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