1

A2 Chemistry

Unit 1

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The Structure of Benzene

Kekulé’s Equilibrium Model of Benzene

Kekulé’s structure failed to explain benzene’s low chemical reactivity.

If C=C bonds were present, benzene should react similarly to alkenes.

Each C=C bond would be expected to decolourise bromine water.

Benzene does NOT take part in electrophilic addition reactions as expected from the C=C bonds.

C-C single bonds and C=C bonds have different bond lengths. Kathleen

Lonsdale found that all of the carbon bonds were the same length -

0.139nm. (between the lengths of C-C and C=C).

Hydrogenation of Benzene:

Cyclohexene enthalpy change of hydrogenation = -120kJmol⁻₁ .

Therefore benzene must have an enthalpy change of hydrogenation should be -360kJmol⁻₁ .(3x cyclohexene, 3x C=C)

The enthalpy change is actually -208kJmol⁻₁ .

The real structure of benzene is more stable than Kekulé’s structure.

This energy is known as the resonance energy of benzene.

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The Delocalised Model of BenzeneThe Delocalised Model has the following features:

o Cyclic hydrocarbon – 6 C molecules and 6 H molecules.

o Arranged in a planar hexagonal ring where each C is bonded to 2 other C atoms and 1 H atoms.

o The shape is a trigonal planar with a bond angle of 120°.

o Each C atom has 4 outer shell electrons. 3 of these e¯ bond to 2 other C atoms and 1 H atom. The bonds in this plane are called sigma bonds. The 4th outer shell e⁻ in a

2p orbital above and below the plane of the carbon atoms.

o The e⁻ in the p orbital overlap creating a ring of electron density above and below the plane of carbons.

o The pi-bonds spread over all 6 carbons and the ring is said to be delocalised.

Under normal conditions, benzene does not:

o Decolourise bromine watero React with strong acids such as HClo React with halogens such as bromine, chlorine or iodine.

Addition reactions will disrupt the delocalisation of the ring structure.

Instead, Benzene takes part in substitution reactions – a hydrogen (H) is replaced with another group. The organic product retains

the delocalised structure.

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Reactions of Benzene

Benzene’s high electron density attracts electrophiles.

To preserve the ring’s stability, benzene takes part in ELECTROPHILIC

SUBSTITUION reactions.

Conditions: conc. HNO₃, conc. H₂SO₄, 50°C

C₆H₆ + HNO₃ C₆H₅NO₂ + H₂O

Formation of NO₂⁺:HNO₃ + H₂SO₄ NO₂⁺ + HSO₄⁻ + H₂O

The H⁺ reacts with the HSO₄⁻ to reform H₂SO₄.

This is acting as a catalyst.

H ⁺ + HSO₄⁻ H₂SO₄

Nitration of Benzene

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Reactions of BenzeneHalogenation of Benzene

Benzene will react with halogens in the presence of a HALOGEN CARRIER.

Halogen Carriers Include:FeCl₃FeBr₃AlCl₃AlBr₃

Iron Metal

Chlorobenzene is used as a

solvent and in pesticides.

Bromobenzene is used in the preparation of

pharmaceuticals.

Formation of Br⁺ (or Cl):Br₂ + FeBr₃ Br⁺ + FeBr₄⁻

Regeneration of Br⁺ (or Cl):H⁺ + FeBr₄⁻ FeBr₃ + HBr

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Reactivity of Alkenes and Benzene

Benzene and Bromine:

Benzene has delocalised electrons spread over a ring structure. Alkenes

have localised electrons.

Benzene has LOWER ELECTRON DENSITY and CANNOT POLARISE Br₂.

Benzene is therefore resistant to reactions with non-polar halogens.

A halogen carrier is needed to generate a more powerful electrophile.

The greater charge on Br₂ can attract the pi-electrons from benzene so the

reaction can take place.

Cyclohexene and Bromine Water:

The pi-bond is localised – this gives cyclohexene HIGH ELECTRON DENSITY.

The pi-bond repels the electrons in the Br-Br bond inducing a dipole. The Br₂

molecule becomes polar.

The electrons in the double bond attract to the Br+ causing the double bond to

break. This forms a positive carbocation.

The Br-Br bond breaks via heterolytic fission forming Br⁻.

The Br⁻ is attracted to the intermediate carbocation forming a covalent bond.

Bromine +Benzene = orange

Bromine + Benzene + Iron fillings = decolourised and white fumes of

hydrogen bromide gas.

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Phenols

Only a phenol when an OH group is

directly linked to the ring.

Solid at room temperature and pressure.

Slightly soluble in water as OH group can make hydrogen bonds with water.

Benzene ring makes it less soluble than alcohols.

OH

O⁻ Na⁺

+ H₂O

+ H₂

O⁻ Na⁺NaOH

Na

Phen

ol

Sodium Phenoxide

Sodium Phenoxide

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Bromination and Uses of Phenols

Br

Br

Br

OH

Na

3Br₂ (aq)

+ 3HBr

Phen

ol

2,4,6-tribromophenol

OH Bromination of Phenol:

Lone pair of electrons on the O group of phenol.

This creates a HIGHER ELECTRON DENSITY.

This POLARISES the Br₂ which are more strongly attracted towards the ring

structure.

Surfactants and Detergents

Antiseptics and Disinfectants

Pharmaceuticals

Paints and Epoxy Resins

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Carbonyl Compounds

KetoneIn the middle of

the chain

AldehydeAt the end of the

chain

C-=

-H

O

C-

=

-

Oᶞ⁻

ᶞ⁺

Oxygen is more ELECTRONEGATIVE so electrons are more attracted to it than the carbon.

The functional

group is the part of

the molecule

responsible for its

chemical reactions.

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Reduction of Carbonyl Compounds

Reducing Agent [H] = NaBH₄ (Sodium Borohydride)

Water is the solvent

NaBH₄ readily generates hydride ions.

Ketone + [H] = Secondary AlcoholAldehyde+ [H] = Primary Alcohol

H

H

=

H CCO

H

:H⁻

H

H

H

:O

H CC H

H O H

H

H

H

OH

H CC H + OH⁻

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Oxidation of Carbonyl Compounds

Oxidising Agent [O] = Acidified Potassium Dichromate IonsK₂Cr₂O₇/H⁺

Primary Alcohol Aldehyde Carboxylic Acid

Heated under reflux before distillation.

Secondary Alcohol Ketone

NO COLOUR CHANGE

Reflux is the CONTINUAL BOILING AND CONDENSING of a mixture to ensure that it does not dry out.

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Chemical Tests on Carbonyl CompoundsTo detect the presence of a carbonyl compound:> 2,4-DNP

A solution of 2,4-DNP in a mixture of methanol & sulphuric acid is known as BRADY’S REAGENT.

Brady’s Reagent + Aldehyde/Ketone Yellow/Orange Precipitate.

CC₂H₅

O

H

=+

CC₂H₅

H

= + H₂O

Filter and recrystallise the 2,4-DNP derivative and record its melting point. This is compared against a

database.Difficult to distinguish between Heptan-2-one and Cyclohexanone as the b.ps are very similar.

The 2,4-DNP derivatives have different m.ps thus allowing easy identification.

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Chemical Tests on Carbonyl Compounds

Aldehyde or Ketone?

Tollens’ Reagent is a weak OXIDISING agent that distinguishes between aldehydes and ketones.

Aldehyde + [O] Carboxylic AcidSILVER MIRROR FORMED

Ketones are NOT OXIDISED by Tollens’ reagent.

Making Tollens’ Reagent:

NaOH + AgNO₃ until a brown precipitate of Silver Oxide is formed.

Dilute NH₃ added until precp. dissolves.

The colourless solution is aka Ammonical Silver Nitrate.

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Carboxylic Acids

Functional Group = COOHC-

=-

OH

O

Solubility:

As the carbon chain INCREASES, SOLUBILITY DECREASES.

Molecules become more NON-POLAR.

COOH’s are WEAK ACIDS and react with metals, bases and carbonates.

Salts formed from COOHs aka CARBOXYLATES.

C-=

-

O⁻

O Suffix ‘...oate’

Carboxylic Acid + Metal SALT + H₂(g)

Carboxylic Acid + Base SALT + H₂O

Carboxylic Acid + Carbonate SALT + CO₂ + H₂O

First part of carboxylate comes from metal, base or carbonate.

Carboxylic Acid Salt Formed

Methanoic Acid Methanoate

Ethanoic Acid Ethanoate

Propanoic Acid Propanoate

Butanoic Acid Butanoate

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EstersMaking esters is known as ESTERIFICATION.

COOHS + OHs produces an ESTER & H₂O

Acid Catalyst: H₂SO₄

e.g. Ethanol + Propanoic Acid = Ethyl Propanoate + Water

Carboxylic Acid + Alcohol

Acid AnhydrideAn acid anhydride is formed by the removal of H₂O from

2 molecules of carboxylic acids.

This produces an ESTER and a CARBOXYLIC ACID.

Esters from ACID ANHYDRIDES produce a GREATER YIELD.

This process requires gentle heating.

Ester Hydrolysis:

This is the reverse of esterification.

It ADDS WATER.

Acid Hydrolysis = reflux + aqueous acid, reversible

Alkaline Hydrolysis = reflux + aqueous alkali, makes sodium salt, non-reversible

Esters are used in perfumes and as flavourings.

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Fats and OilsBuilding Triglycerides

Fats are used for:InsulationAs an energy storeTo protect organs

Fats and Oils are esters of a long chained

carboxylic acid.

Fats m/p ABOVE room temp.Oils m/p BELOW room temp.

Triglycerides:Triglycerides are triesters of:Propane-1,2,3-triol (glycerol)

3 fatty acid molecules.

Naming Fatty Acids:Unsaturated fats with

MULTIPLE DOUBLE BONDSDouble bond commonly

between C9 and C10.

18:2 (9,10)No. C atoms

No. of = bonds Position of = bonds

Forming Triglycerides:Simple triglyceride derived from 2/3 of the SAME fatty acids.

Natural/Mixed triglycerides derived from 2/3 DIFFERENT fatty acids.

HDLs – carry cholesterol FROM ARTERIES back TO LIVERLDLs – carry cholesterol FROM LIVER TO TISSUES.

Fatty acids can also be used to make BIODIESEL.

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Amines

Amines are derivatives of ammonia.

Ammonia NH₃Primary Amine RNH₂Secondary Amine R ₂NHTertiary Amine R₃N

Adrenaline, Amphetamine, Phenylephrine

Naming Amines

Secondary Amines

N-methylpropylamine

Amines are WEAK BASES and ACCEPT protons.

Each lone pair on the N atom accepts a H⁺

:NH₃ + H⁺ NH₄⁺A DATIVE BOND forms between

the lone pair of the N atom and the H⁺.

Base + Acid SaltAlkylamine + Acid forms an

ALKYLAMMONIUM SALT.

e.g. Methylamine + Sulphuric Acid Methylammonium Sulphate

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Reactions of Amines

Preparing Amines:Conditions: Excess NH₃

Solvent = EthanalThis is a NUCELOPHILIC SUBSTITUTION

REACTION.

CH₃CH₂CH₂Cl + NH₃ CH₃CH ₂CH₂NH₂ + HCl

NH₃ + HCl NH₄⁺Cl⁻

Excess NH₃ is added so that it all reacts.Preparing Aromatic Amines:

Nitroarenes are reduced using a mixture of TIN AND CONC. HCl

Synthesis of Dyes from Phenylamines:1) Diazotisation

2) Coupling Reaction

+ HNO₂ + 2HCl (<10°C)

+ H₂O

Formation of HNO₂:NaNO₂ +HCl HNO₂ +NaCl

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Br Cl

NO₂ NH₂ N⁺ Cl⁻

N

NN

OH

BenzeneBromobenzene Chlorobenzene

Nitrobenzene Phenylamine Benzenediazonium chloride

Azo Dye

Br₂/FeBr₃ Cl₂/AlCl₃

Conc. HNO₃Conc. H₂SO₄

50°C

Sn/Conc. HCl

Reflux

NaNO₂/HCl (aq)<10°C

Phenol, NaOH

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OH

Br

Br

Br

O⁻ Na⁺

+ H₂O

+ H₂

O⁻ Na⁺

OH

NaOH

Na

3Br₂ (aq)

+ 3HBr

Phen

ol

2,4,6-tribromophenol

Sodium Phenoxide

Sodium Phenoxide

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Amino AcidsAmino acids make PEPTIDES and PROTEINS.

20 different amino acids in body.

They are α-amino acids and have a BASIC AMINE GROUP and an ACIDIC CARBOXYL GROUP.

Soluble in both acids and bases.

R-group is usually –OH, -SH, -COOH or –NH₂ EXCEPT GLYCINE which has H as the R group (simplest).

Carboxyl and Basic group can react to form a ZWITTERION (internal salt).

Carboxyl donates proton to basic group.

There is no overall charge as they cancel out.

The ISOELECTRIC POINT is the pH at which there is no net charge. The zwitterion exists in this pH.

Amino Acid Isoelectric Point

Glycine 5.97

Alanine 6.01

Leucine 5.98

Serine 5.68

Proline 6.48

H+ H₂O

H⁺pH 1

OH⁻pH 13

Amino acid acts as an ACID and

DONATES a proton to the hydroxide ion.

Amino acid acts as a BASE and ACCEPTS a

proton from the acid.

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Polypeptides and Proteins

Amino acids join together to from PEPTIDES

Amino acids join together in a CONDENSATION REACTION and so eliminate

H₂O.

Acid Hydrolysis of Polypeptides and Proteins:

Heated under reflux6 mol dm⁻₃

24 hoursAcid Solution

Peptide is separated and both become positive ions.

Alkaline Hydrolysis of Polypeptides and Proteins:

Alkaline SolutionAbove 100°C.

Peptide is separated into 2 original peptides and both become sodium

salts.Proteins are long polypeptides with more

than 50 amino acids.

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Optical Isomerism

STEREOISOMERS = same structural

formula, DIFFERENT ARRANGEMENT in

space.

CHIRAL CARBON= Carbon attached to 4 DIFFERENT

atoms

OPTICAL ISOMERS= stereoisomers that

are NON-SUPERIMPOSABLE

mirror images.

Optical isomers rotate the light CLOCKWISE & ANTICLOCKWISE.

A mixture of equal amounts of optical isomers is known as a RACEMIC mixture –

the rotations cancel each other out.

Optical isomers exist in all amino acids (expt. Glycine).

Only 1 of the isomers is synthesised naturally & only 1 will react with an enzyme.

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Chirality in Pharmaceutical Synthesis

Advantage of single isomer:

Risk from undesirable side effects reduced

Drug doses reduced

Separating optical isomers is difficult as they have the same properties.

How to separate optical isomers:

Use ENZYMES as biological catalysts

Chiral Pool Synthesis

Transition Element Complexes

Ibuprofen has 2 optical isomers.

It is sold as a mixture of both isomers.

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Condensation PolymerisationPolyesters

To from an ester:

A Carboxylic AcidAn Alcohol(either on 1 or 2 molecules)

-H lost from Alcohol, -OH lost from Carboxylic Acid

Elimination of a by-product – usually H₂O.

Ester Linkages

e.g. Terylene and Poly(lactic) Acid

C-=

-O

OEster Linkage

Uses of Polyesters:

oMachine-washableoMachine-dryableoResistant to stretching, shrinking and chemical attackoBurns easily

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Polyamides

To form a polyamide:

A Carboxylic AcidAn Amine(can have 2 different monomers or just 1 with both functional groups.)

Amide Linkage

-OH lost from Carboxylic Acid, -H lost from Amine to form H₂O

e.g. Nylon 6,6 and Kevlar

C

=_ NO

Amide Linkage

H

_ _ _

Nylon 6,6:Used widely in ClothingKevlar:-fire resistant-stronger than steel-fire fighter clothing and bullet-proof vests.

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PolyamideNylon 6,6

H0 – C – (CH₂)₄ - C – OH + H – N – (CH₂)₆ - N – H = =

O O

_

_

H

H

Hexane-1,4-dioic Acid 1,6-diaminohexane

C – (CH₂)₄ - C – N – (CH₂)₆ - N

= =

O O

_

_H

H

+ 2n-1 H₂O

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PolyamidesNylon 6

H-N-(CH₂)₅-C-OH

H O

=_

N-(CH₂)₅-C

H O=_

+ n-1 H₂O

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KevlarPolyamide

HO C

O

OHC

O

== + H N

H

HN

H

_

_

C

O

C

O

==

N

H

N

H

_

_ + (2n-1) H₂O

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TerylenePolyester

H

H

H

H

HO CC OH HO C

O

OHC

O

==+

H

H

H

H

O CC O C

O

C

O

==

+ (2n-1) H₂O

Na⁺COO⁻ COO⁻Na⁺

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Addition and Condensation Polymerisation

Addition polymers have 1 MONOMER and there is NO BY-PRODUCT.

Addition polymers contain a DOUBLE BOND.

Feature Addition Polymer Condensation Polymer

Polyester Polyamide

Functional Group C=C -COOH & -OH -COOH & NH₂

Monomer 1 1/2 1/2

Product Poly(alkene) Polyester + H₂O

Polyamide + H₂O

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Breaking down condensation polymers

Hydrolysis of Polyesters

NaOH/H₂O H⁺/H₂O

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Breaking down condensation polymers

Hydrolysis of Polyamides

NaOH/H₂O H⁺/H₂O

Na⁺OOC⁻ – C – (CH₂)₄ - C –COO⁻ Na⁺

= =

O O

⁺ ⁺

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Degradable Plastics

Degradable plastics break down into smaller fragments when exposed to

HEAT, LIGHT OR MOISTURE.

A biodegradable plastic breaks down COMPLETELY into CO₂ and H₂O

Biodegradable polymers have bonds than undergo hydrolysis.

Poly(lactic acid) – derived from corn starch

Poly(glycolic acid) – isolated from sugar cane and unripe grapes.

Photodegradable plastics are synthetic polymers designed to become weak and brittle when exposed to light for

prolonged periods.

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Separation by Chromatography

Chromatography is used to separate components in a mixture.

A mobile phase sweeps over a stationary phase.

Different components have different AFFINITIES for the phases.

The stationary phase slows the components down. They pass at different speeds thus

separating the compound.

THIN LAYER CHROMATOGRAPHYStationary Phase: SolidMobile Phase: Liquid

GAS CHROMATOGRAPHYStationary Phase: Solid/Liquid (silica)

Mobile Phase: Gas

A solid stationary phase separated by ADSORPTION.

ADSORPTION:The solid holds gas or liquid molecules ON THE SURFACE of

a solid.

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Thin Layer Chromatography

Rᶠ = distance moved by componentdistance moved by solvent front

Limitations:

Similar compounds have similar Rᶠ values.

Difficult to find solvent that separates all of the compounds in

a mixture.

A chromatogram is a visible record showing the results of separation of a mixture.

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Gas Chromatography

Stationary Phase: Thin layer solid/liquid coated on inside of

capillary tube.Mobile Phase: Inert carrier gas

e.g. helium

Each component leaves the column at different times and is

detected as it leaves the column.

Retention time in gas chromatography is the time for a

component to pass from the column inlet to the detector. The

area under the peak is proportional to the amount of

compound in a sample.

Limitations:

Similar retention times

Not all substances will be detected

No reference for unknown compounds.

Gas Chromatography Mass Spectrometry

Components are separated by GC

Detected by MS against a reference.

GC-MS used in...Forensics

Environmental AnalysisAirport Security

Space Probes

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NMRNuclear Magnetic Resonance

Chemical Shift ᵟ is a scale that compares the frequency of an NMR

adsorption with the frequency of the reference peak of TMS at 0ppm.

TMS is added so that the spectrometer can be calibrated

against the TMS reference peak.

TMS is chemically UNREACTIVE and is removed from the sample after

running the NMR.

Solvent: D₂OIsotope of Hydrogen

Produces no signal in spectrum as it has even number of nucleons.

Other solvents: CDCl₃ (C peak usually removed from spectrum via

evaporation)

C₂H₅O-H +D-O-D C₂H₅O-D + D-O-H

Won’t absorb radio waves

Low Resolution NMR Spectroscopy

High Resolution NMR Spectroscopy

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NMR in Medicine

An MRI scanner is like a large spectrometer in which the patient is the sample.

MRI scanners detect SOFT TISSUE DAMAGE and in the diagnosis of tumours because they contain a high % of water.

MRI scans are NON-IVASIVE so do NOT HARM the body’s cells.

Disadvantages: Expensive, High training required

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NMR Spectroscopy

Description Chemical Shift/ppm

No. Of Protons Identity Coupling Pattern

Example: EthanolTriplet 1.2 3 R-CH₃ next to 2 H

Quartet 3.6 2 H₂-CO next to 3 HSinglet 4.5 1 OH next to 0 H

m/z value Identity

15 CH₃⁺

17 OH⁺

28 C=0⁺

29 C₂H₅⁺ CHO⁺

31 CH₂OH⁺

35 ³⁵Cl⁺

37 ³⁷Cl⁺

43 C₃H₇⁺ or CH₃CO⁺

45 COOH⁺

57 C₄H₉⁺ or C₂H₅CO⁺

77 C₆H₅⁺

79 ⁷⁹Br ⁺

81 ⁸¹Br ⁺

91 C₆H₅OH⁺

Mass Spectrum = molecular ion (M) peak

furthest rightIdentifies fragments

No. of peaks = No. of Chemical Environments

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An addition reaction is one in which a reactant is

added to an unsaturated molecule to make a saturated molecule.

1 + 1 = 2

Electrophilic Substitution is a substitution reaction

where an electrophile is an atom/s attracted to an

electron rich centre where it accepts a pair of protons to form a new covalent bond.

Electronegativity is a measure of the attraction of a bonded atom for the pair of electrons

in a covalent bond.

Delocalised electrons are shared between MORE

THAN 2 atoms.

A reaction mechanism is a series of steps that,

together, make up an overall reaction.

A redox reaction is one in which both reduction and

oxidation take place.

An electrophile is an atom/s attracted to an

electron rich centre where it accepts a pair of protons

to form a new covalent bond.

A curly arrow shows the movement of an electron

pair in the breaking or formation of a covalent

bond.

The stem is the longest carbon chain present in an organic

molecule.

A suffix is the part of the name added after the stem.

In a substitution reaction, an atom/s is replaced with

a different atom/s.

The functional group is the part of an organic molecule responsible for its chemical

reactions.

Reflux is the continual boiling and condensing to make sure a reaction takes place without

the contents drying.

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An nucleophile is an atom/s attracted to an

electron deficient centre where it donates a pair

of protons to form a new covalent bond.

A zwitterion is a dipolar ionic form of an amino acid that is formed by the donation of a

H+ ion from the carboxyl group to the amino group. There is no overall charge.

A condensation reaction is one in which 2 small

molecules react together to form a larger molecule with

the elimination of water.

Esterification is the reaction of an alcohol

with a carboxylic acid to form an ester and water.

A peptide is a compound made of amino acids linked

by peptide bonds.

A chiral carbon is a carbon atom attached to 4 different

atoms.

Hydrolysis is a reaction with water or hydroxide

ions that breaks a chemical compound into

2 compounds. Can be catalysed with an acid or

alkali.

High Density Lipoproteins (HDL) remove cholesterol

from the arteries and carry it back to the LIVER for excretion or reuse.

Sterioisomers are species with the same structural formula

but with a different arrangement of atoms in

space.

Low Density Lipoproteins (LDL) carry cholesterol

and triglycerides from the LIVER TO THE TISSUES.

The isoelectric point is the pH value at which the amino acid

exists as a zwitterion.

Optical isomers (enantiomers) are sterioisomers that are

non-superimposable mirror images of each other.

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The repeat unit is the specific arrangement of atoms that

occurs in the structure repeatedly. It is in brackets with letter ‘n’ outside of it.

A biodegradable polymer is a polymer that breaks

down completely into CO₂ and H₂O.

A degradable polymer is a polymer that breaks down

into smaller fragments when exposed to light,

moisture or heat.Pharmalogical activity is the

beneficial or adverse effects of a drug on living matter.

A phase is a physically distinctive form of a

substance (e.g. Solid, liquid or gaseous states of

ordinary matter).

Adsorption is the process by which a solid holds

molecules of gas or liquid or solute as a thin film on the

surface of a solid/liquid.The mobile phase is the phase

that moves in chromatography.

The stationary phase is the phase that does not move

in chromatography.

A chromatogram is a visible record showing the results of separation of a mixture.

Rᶠ = distance moved by component

distance moved by solvent front

Retention time in gas chromatography is the time

for a component to pass from the column inlet to

the detector.

Chemical Shift ᵟ is a scale that compares the

frequency of an NMR adsorption with the

frequency of the reference peak of TMS at 0ppm.