Benzene Structure and Azo Dyes

Draw the kekule’s structure of benzene and the accepted structure and list the simularities and differences

Both

• All bond angle are 120

• Planar structure

• 3 sets of electrons repelled

Kekule Structure

• Alternating single and double bonds (suggesting different bond lengths)

• Two dimethyl isomers expected

• Hydrogenation enthalpy 3x that on cyclohexene due 3 double bonds instead of 1

• Undergoes addition reactions

• Should decolourise bromine water

Actual Benzene

• C-C all the same length

• Only 1 isomer of dimethyl

• Hydrogenation enthalpy is less than 3x that of cyclohexene - less energy released so more stable due to delocalisation

• Undergoes substitution reactions

• Won’t decolourise bromine water

Explain, using a diagram, how benzene is delocalised

• Each carbon atom forms 3 sigma bonds

• This leaves the 4th electron in the p orbital of each carbon

• The 6 p orbitals overlap forming a pi cloud of electron density

• This is two rings of electrons one above and below the plane of the carbon atoms

• The electrons move freely within this pi system. They are delocalised

What is the difference between an aromatic compound and an arene

Aromatic compounds - contain a ring of delocalised electrons

Arenes - contain a benzene ring

Show the general mechanism for the electrophilic subsitution of benzene with elctrophile, E⁺

Give the reagents, conditions, electrophile, equations (generation of electrophile and regeneration at catalyst) and mechanism for the halogenation of benzene

Reagents - Cl₂ or Br₂

Conditions - Anhydrous AlFe₃/AlBr₃/FeCl₃/FeBr₃ Catalyst

Electrophile - Cl⁺ or Br⁺

Generation of electrophile - AlCl₃ + Cl₂ → AlCl₄^- + Cl⁺

Regeneration of catalyst - AlCl₄^- + H⁺ → AlCl₃ + HCl

Give the reagents, conditions, electrophile, equations (generation of electrophile and regeneration at catalyst) and mechanism for the nitration of benzene

Reagents - Concentrated HNO₃

Conditions - Concentrated H₂SO₄ (Catalyst and dehydrating agent) and less than 55C

Electrophile - NO2⁺

Give the reagents, conditions, electrophile and equation for the sulphonation of benzene

Reagents - Concentrated H₂SO₄

Conditions - Heat under reflux

Electrophile - SO₃H⁺

Give the reagents, conditions, electrophile and equation (including the generation of electrophile) for the Friedel-Crafts Reaction (adding carbon chains to a benzene ring)

Reagents - haloalkane with correct R group

Conditions - Anhydrous AlCl₃ catalyst, reflux

Electrophile - CH3⁺ (R⁺)

Generation of electrophile - AlCl₃ + CH₃Cl → CH₃⁺ + AlCl₄^-

Give the reagents, conditions, electrophile and equation (including the generation of electrophile) for the Friedel-Crafts Acetylation Reaction

Reagents - Acyl chloride

Conditions - Anhydrous AlCl₃ catalyst

Electrophile - e.g. CH₃CO⁺

Generation of electrophile - AlCl₃ + CH₃COCl → CH₃CO⁺ + AlCl₄^-

Show how benzene can be halogenated using halogen carrier

Reagents - Polarised halogen molecule, AlCl3 catalyst

Azo dyes contain an azo functional group. Draw the general structure of an azo functional group

R—N=N—R

Explain, using equations, how azo dyes form

Step 1: Formation of diazonium compound

Reagents - Aromatic amine, sodium (III) nitrate, dilute HCl

Conditions - <5C

Step 2: Coupling Reaction

Reagents - diazonium compound, phenol (or phenolamine), NaOH

Conditions - alkaline (NaOH), <5C

Observations - coloured ppt of azo dye formed

Explain how colour arises in organic molecules and what colour depend on

• Molecules must have an extended delocalised system (alternating single and double bonds)

• Radiation (usually visible light) is absorbed by electrons according to the energy gap (E=hv)

• Electrons in extended delocalised system move to higher energy levels

• Complementary colour is transmitted

• The energy gap and so frequencies depend upon

- Amount of delocalisation

-Size of chromophore

-Bonding and functional groups on chromophore

• Smaller delocalised system = larger energy gap and so higher frequency absorbed (e.g. UV light which will transmit all visible colours)

What is a chromophore and what are the impacts of its side groups on organic colour and solubility

Chromophore - the part of the molecule that gives it its colour i.e. all the parts with alternating double and single bonds

• Side groups can affect the colour

• Groups with lone pairs (e.g. OH) - delocalise in the pi system changing the energy gap and therefore the frequency absorbed

• Acyl groups and nitro groups contain double bonds and so extend the delocalised system

• Ionic groups like sulfonate groups allow ion-dipole bonds to form with water, increasing solubility as these are stronger than hydrogen bonds in water

How can dyes stick to fabric

• Ionic bonding - ionic bonds form between NH3+ o the fibres and the sulfonate group on dye molecules

• Covalent bonding

- Mordanting - Functional groups with lone pairs form a complex ion by dative bonding with a metal ion

- Fibre reactive dyes - Functional groups added to the dye molecules that will allow it to react with the fibre, e.g. condensation reaction

• Intermolecular bonding

- Hydrogen bonds form between OH on fibres and NH2 or OH groups of dye

- ID-IDs

How can aldehydes be prepared from primary alcohols

Reagents - primary alcohol and K₂Cr₂O₇/H⁺_(aq)

Conditions - Heat under distilation

Observations - Orange → Green

Equation - CH₃CH₂CH₂OH + [O] → CH₃CH₂CHO + H₂O

How can ketones be prepared from secondary alcohols

Reagents - secondary alcohol and K₂Cr₂O₇/H⁺_(aq)

Conditions - Heat under Reflux

Observations - Orange → Green

Equation - CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O

How can aldehydes and ketones be distinguished using K₂Cr₂O₇/H⁺

Warm with K₂Cr₂O₇/H⁺

Aldehyde - Orange → Green

Ketone - Stays orange as ketones can’t be oxidised further

How can aldehydes and ketones be distinguished using Fehlings Solution

Warm with Fehling’s solution

Aldehyde - Blue solution forms brick red precipitate (CuO)

Ketones - Solution stays blue

How can aldehydes and ketones be distinguished using Tollens Reagent

Warm with Tollens Reagent

Aldehyde - Silver mirror forms

Ketones - No change

Show the mechanism for the reaction between an aldehyde and HCN/KCN. Give the type of reaction, reagents and equation

Nucleophillic Addition

Reagents - acidified HCN or KCN

Equation - CH₃CH₂CHO + HCN → CH₃CH₂CH(OH)CN

Hydroxynitrile formed

Show how triesters (triglycerides) from glycerol and 3x fatty acids

What are saturated and unsaturated fatty acids

Saturated - single bonds only

Unsaturated - contains C=C

Unsaturated fatty acids can be cis (z) or trans (E), what does this mean

Cis (Z) - adjacent parts of chain are next to each other across the double bond

Trans (E) - adjacent parts of chain are on opposite sides across the double bond

Show how a triglyceride can be hydrolysed, give the reagents and conditions

Reagents - Concentrated NaOH

Conditions - heat

What is the mobile phase and stationary phase of Gas-Liquid Chromatography

Mobile - inert carrier gas, usually nitrogen

Stationary phase - high boiling point liquid on a porous support in a long capillary tube

How can GLC be used to find the concentration of components

• The area under each peak is proportional to the amount/conc of that component

• This can be compared with a calibration curve

What is retention time (GLC)

The time taken from the injection of the sample, for the component to leave to column - can be used to identify a component

What types of intermolecular bonds form between triglycerides and where do they form

• ID-IDs between fatty acid chains

• PD-PDs between C-O/C=O on glycerol