a2 organic notes 2010[2].docx

7/29/2019 A2 organic notes 2010[2].docx

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Organic 1

A2 ORGANIC Chemistry

Isomerism & Naming organic compounds

Naming compounds has been covered in the AS course make sure youknow how to name compounds using the IUPAC rules.

Isomerism has been covered in the AS syllabus. Make sure you know thefollowing:

know and understand the meaning of the termstructural isomerism

know thatE-Z isomerismandoptical isomerismare forms ofstereoisomerism

know that anasymmetric carbonatom is chiral and givesrise to optical isomers which exist asnon super-imposablemirror imagesand differ only in their effect onplanepolarised light

understand the meaning of the termsenantiomerandracemate

understand whyracematesare formed

be able to draw thestructuralformulae anddisplayedformulae of isomers

Appreciatethat drug action may be determined by thestereochemistryof the molecule and that different opticalisomers may have very different effects e.g Thalidomide

Carbonyl Compounds

Compounds that contain the (C=O) functional group.

- Aldehydes & Ketones- Carboxylic Acids- Esters- Anhydrides- Acyl halides(chlorides)

Making Carbonyl compounds:

Aldehydes and KetonesThese are also known as carbonyl compounds and contain the carbonyl (C=O) functional group.

Functional Group:

C

O

H C

C

O

C ALDEHYDE KETONE


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Organic 2

Oxidation

Primary and secondary alcohols may be oxidised using an oxidising agent such as acidifiedpotassium dichromate(VI) or acidified potassium manganate(VII).

Primary Alcohols

Primary alcohols are oxidised first of all to an aldehyde (partial oxidation) and then the aldehyde isoxidised further to a carboxylic acid (complete oxidation).

Cr2O72-

is the oxidising agent and is therefore reduced during the reaction. Cr2O72-

is orange and itis reduced to the green Cr

3+(aq) ion. The half equation for the reduction is:

Cr2O72-

+ 14H+

+ 6e- 2Cr

3++ 7H2O

If the reaction mixture is heated under reflux ethanoic acidis obtained as the main product and the aldehyde is notusually isolated. However, it is possible to set up theexperiment so that the aldehyde is distilled off as soon as

it is formed and before it can be oxidised further.

If we look at the reactions in terms ofthe functional group change it is easierto generalise the reaction to othermolecules:

Secondary Alcohols

Secondary alcohols are also oxidised by acidified potassium dichromate(VI). They are oxidised toketones, which cannot be oxidised any further.

In terms of functional group change the reaction can be represented as:

C C

OH

H

C C C C

OCr2O72-/H+

HEAT

Carboxylic acids have the

C

O

O H functional group

Primaryalcohol

AldehydeCarboxylicacid

Cr2O72-/H+ Cr2O7

2-/H+

HEAT HEAT

Aldehydes contain the

C

O

H

functional group

Colour change: ORANGEGREEN

Aldehydes have a lower boiling pointthan alcohols as they do not have anH attached directly to an O and

therefore do not participate inhydrogen bonding.

C

O

O HC

O

HC

OH

H

HCr2O7

2-/H+ Cr2O72-/H+

HEAT HEAT

Cr2O72-/H+

HEAT

Secondaryalcohol

KetoneCr2O7

2-/H+

HEAT

Ketones contain the

C C C

O

functional group


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Organic 3

Tertiary Alcohols

Tertiary alcohols are resistant to oxidation.

H C

H

H

C

OH

CH3

C H

H

H

Cr2O72-/H+

HEAT

2-methylpropan-2-ol

No Reaction

Identification test for the carbonyl group

Addition-Elimination reactions

The reaction of aldehydes and ketones with 2,4-dinitrophenylhydrazine (2,4-DNPH) is anexample of an addition-elimination reaction and is used as a test for the presence of the carbonyl(C=O) group in aldehydes and ketones.

This reaction may be used to distinguish aldehydes and ketonesfrom other species. The product of the reaction with aldehydes andketones is yellow/orange crystals.

If the melting point of these crystals is determined and the valuescompared with those in tables exactly which aldehyde/ketone wasoriginally present may be determined.

Draw out the structures of the organic products formed when the followingaldehydes/ketones react with 2,4-dinitrophenylhydrazine.

1 butanone

2 2-methylpentanal

Water iseliminated.

Yellow-orange

crystals produced -test for carbonylgroup


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Organic 4

Distinguishing between Aldehydes and Ketones.

Use is made of the oxidation reaction to tell the difference between Aldehydes (can be oxidised)and ketones(cannot be oxidised)

1. Tollens Reagentsilver mirror(Ag) Uses ammonical silver nitrate solution (AgNO3/NH3)

2. Fehlings solution Brick red precipitate (Cu2O) Uses copper(II) chloride in sodiumhydroxide (CuCl2/NaOH)

Reduction of Aldehydes and Ketones (H)

Using (H-ion ) HYDRIDE ION , aldehydes and ketones can be reduced back into alcohols.

Reducing agent: sodium borohydride (NaBH4) or lithium aluminium hydride (LiALH4)

**(LiAlH4 reaction is carried out in a non aqueous environment at low temperature (exothermic))

*(NaBH4 can be carried out in water)

Reaction is an example of NUCLEOPHILLIC ADDITIONCarboxylic acids can also be reduced back into primary alcohols.

Example


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Organic 5

Aldehydes and ketones reaction with HYDROGEN CYANIDE (HCN)

Another example of nucleophillic addition involving the carbonyl group involves reactingaldehydes and ketones with hydrogen cyanide. The product is a HYDROXYNITRILEorCYANOHYDRIN.

** (HCN is a very weak acid In practice a KCN/H2SO4solution is used)

NOTE!! KCN is extremely toxic!!! (HCN is also VERY toxic and flammable) Death = 100ppm

Mechanism

Nitrile groups may be hydrolysed to form a carboxylic acid:

C NCH3 + H2O H3O+

+ CH3 C

O

O H

+ NH4+H2O/H

+reflux

hydrolysis

Or hydrolysis of a cyanohydrin:

Hydrolysis is also brought about by refluxing with aqueous alkali. In this case the product is thesalt of the acid and ammonia:

CH3 COH

CN

H + H2O + +H2O/OH

-reflux

hydrolysisCH3 C

OHH

COO-Na

+

NaOH NH3


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Organic 6

Carboxylic acids

Physical properties of carboxylicacids

Lower relative molecular mass carboxylic acids are generally soluble in water because

However, the solubility decreases as the length of the hydrocarbon chain increases because

Carboxylic acids readily liberate CO2 from carbonate compounds

Carboxylic acids react with alcohols in the presence of an acid catalyst to give an ESTER.

Condensation Reactions

These are reactions in which two molecules join together with the elimination of the elements ofwater. They are part of a more general class of reactions called addition-elimination reactions,where molecules other than H2O are eliminated.

Esterification

When an alcohol is heated with a carboxylic acid in the presence of a small amount ofconcentrated sulphuric acid as a catalyst an ester is formed:

For example:

The alcohol and the carboxylic acid have been joined together and water has been eliminated 1H atom from the alcohol andOH from the carboxylic acid molecule.

Functional Group: C

O

O H

Boiling Point of carboxylic acids, aldehydesandalcohols

carboxylic

acids

aldehydes

alcohols

-50

0

50

100

150

200

250

0 20 40 60 80 100 120 140

relative molecular mass

Boiling

point/oC

alcohol + carboxylic acidconc H2SO4

heatester + water


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Organic 7

Complete the table:

Alcohol Carboxylic Acid Ester

H C

H

H

O

H

C

O

OH

C

H

H

C

H

H

C

H

H

H

H C

H

H

C

H

H

C

H

H

O

H

C

O

O H

H

C C

H

H

H

H OH

C

H

H

H

C

O

O H

C

H

C

H

H

C

H

CH3

H

C

H

H

H

C C

CH3

H

H

H

CH2OH

C

H

H

H

C

O

O HC

H

HC

CH3

CH3

C

H

HH

C

H

O

CH3

C

H

C

O

H

CH3

CH3

C

CH3

C

H

H

OC

H

HC

O

C

H

H

C

H

HC

H

HH

H

CH3

C

CH3

O

C

H

CO

H

CH3

CH3

H


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Organic 8

Naming Esters

Esters are named according to the carboxylic acid from which they are derived.

C

H

H

O

C

H

H

CO

C

H

H

H

H

C

H

C

H

H

O

C

H

H

CO

C

H

H

C

H

C

H

H

HH

H

H

C

H

O

CH3

C

H

C

O

HCH3

H

C

H

O

C

H

CO

H

CH2

CH3

H

CH3

C

H

O

CH

2

C

CH3

C

O

H

H

H

CH3

C

CH3

C

H

H

O

C

H

H

CO

C

H

H

C

H

H

C

H

H

H

H

CH3

Uses of esters

Esters often have a sweet-fruity smell and they are used as artificial flavours and odours. Otheruses are as plasticizers (added to polymers to make them easier to process and reducebrittleness) and as solvents.

Animal Fats and Vegetable oils are esters of propane-1,2,3-triol (glycerol) and long chaincarboxylic acids (fatty acids). (Vegetable oils and animal fats can be hydrolysed to give soap,glycerol and long chain carboxylic(fatty) acids)

Esters can be HYDROLYSED back into carboxylic acid and alcohol. This is done byheating(refluxing) with either an acid or a base (H2SO4 or NaOH)

Biodiesel is a mixture of methyl esters of long chain carboxylic acids

Vegetable oils can be converted into biodiesel by reaction with methanol in the presence of anacid catalyst

Polyesters

Polyesters may be formed in a condensation polymerisationreaction when a dicarboxylic acid reacts with dihydric alcohol(alcohol with 2OH groups). It is the presence of twofunctional groups on each monomer that allows the productionof a polymer chain as an ester is formed on both sides of bothmonomers.

The type of polymerisation iscondensation polymerisation asa water molecule is eliminatedeach time two monomers arejoined together.


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Organic 9

It can be seen from this reaction that, when four monomer molecules join together three watermolecules are produced. The total number of water molecules is always one less than the totalnumber of monomer molecules that join together.

If we thenlook at aspecificexample

The polymer chain as a whole can be represented by the unit shown in brackets in the equation.This is called the repeat unit or repeating unit of the polymer. The whole polymer chain could bebuilt up by just joining these units together


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Organic 10

From which monomers could the following polymers be formed?

Acylation reactions Reactions involving Anhydrides and AcidChlorides (Acylating agents)

Acid Anhydrides

The basic structure of an acid anhydride is:

This can be regarded as being formed from two molecules of carboxylic acid with water removed:

Naming acid anhydrides:

CH3

C

O

O

CCH3

O

CH3

CH2

C

O

O

CCH3

CH2

O

Acid anhydrides are acylating agents, that is they react by adding the acylgroup to other species. The reactions are called addition-elimination

C

O

C

H

H

C

H

H

C

O

O C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

O C

O

C

H

H

C

H

H

C

O

O C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

O C

O

C

H

H

C

H

H

C

O

O C

O

C

H

H

C

H

H

C

CH3

H

C

H

H

C

H

H

C

O

O C

H

H

C

H

H

C

CH3

CH3

C

H

H

C

H

H

OC

O

C

H

H

C

H

H

C

CH3

H

C

H

H

C

H

H

C

O

O C

H

H

C

H

H

C

CH3

CH3

C

H

H

C

H

H

R C

O

O

CR

O

R C

O


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Organic 11

reactions

Acid anhydrides react with nucleophiles:

1 Reaction with waterAcid anhydrides react violently with water to from carboxylic acids

2 Reaction with alcohols

Acid anhydrides react with alcohols to form esters.


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Organic 12

Phenol does not react directly with acid anhydrides but must first be dissolved in sodium hydroxidesolution, to generate the phenoxide ion, which reacts readily with the anhydride to form an ester.

This reaction is used in the formation of aspirin from 2-hydroxybenzoic acid

** Ethanoic anhydride is used instead of ethanoyl chloride on an industrial scale because it ischeaper, easier to handle, reacts more slowly and does not produce HCl as a by product.

3 Reactions with ammoniaWhen concentrated ammonia solution is added to an acid anhydride a primary amide is formed.

4 Reactions with aminesWhen acid anhydrides are reacted with amines substituted amides are formed.

O

C

O

OH

CCH3

O

OH

C

O

OH

aspirin2-hydroxybenzoic acid


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Organic 13

This reaction is used in the formation of paracetamol.

Draw out equations, using structural formulae for the following reactions:

1 Butanoic anhydride + ammonia

2 Propanoic anhydride + water

3 Ethanoic anhydride + 2-methylbutylamine

OH

N

HC

O

CH3

OH

NH2

paracetamol


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Organic 14

Acyl chlorides

The basic structure of an acyl chloride is:

These are extremely reactive and react in basically the same way as acid anhydrides to add an

acyl groups to nucleophiles with the elimination of HCl.

1 Reaction with waterAcyl chlorides react violently with water to from carboxylic acids

2 Reaction with alcoholsAcyl chlorides react with alcohols to form esters.

Phenol does not react directly with acyl chlorides but must first be dissolved in sodium hydroxidesolution, to generate the phenoxide ion, which reacts readily with the acyl chloride to form anester.

*This reaction may also be used in the formation of aspirin from 2-hydroxybenzoic acid.(Frequent exam question)

R C

O

Cl


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Organic 15

3 Reactions with ammoniaWhen concentrated ammonia solution is added to an acyl chloride a primary amide isformed.

4 Reactions with aminesWhen acyl chlorides are reacted with amines substituted amides are formed.

The addition-elimination mechanismThe mechanism of these reactions involves two steps:

1 initial addition of a nucleophile2 elimination of a molecule/ion


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IB Further Organic Option 16

AROMATIC CHEMISTRY

Benzene and Aromatic compounds

Benzene has the formula C6H6 and is usually represented by a skeletal formula:Compounds which contain a benzene ring are called aromatic whereas

compounds without benzene rings are called aliphatic.

When determining molecular formulae or condensed structural formulae forcompounds containing benzene rings it must be remembered that there is a Cand, if there is nothing else attached, an H atom at each vertex.

Therefore methyl benzene

Has the molecular formula

The condensed structural formula may be written asC6H5CH3 or may be shown as a benzene ring with CH3attached.

As long as a particular functional group is not attached directly to the benzene ring thereactions of compounds containing a benzene ring will be basically the same as the reactionsencountered in other sections.

Benzene and Aromatic Compounds

The structure originally proposed for benzene (C6H6) was due to Kekul:

Kekul

Benzene

The structure was accepted for many years but eventually the weight of evidence against itbecame too great and a modified structure was proposed.

Evidence for a different structure of benzene:

X-ray crystallographic data

C-C bond lengths may be determined by x-ray crystallography.

All C-C bond lengths are equal inbenzene

Bond Compound Bond length/nm

C=C ethene 0.133

C-C ethane 0.154

C--C BENZENE 0.139


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Thermochemical evidence

+ H2

H = -120 kJ mol-1

+ 3H2

H = .

+ 3H2

H = -207 kJ mol-1

Actual benzene

Therefore actual benzene is 153 kJ mol -1 morestable than expected if it were made up of 3

C=C double bonds.

cyclohexane (C6H12)

1,3,5-cyclohexatriene


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Heat of Combustion Data

The enthalpy change for the combustion of Kekul benzene may be worked out from the followingbond enthalpies:

Bond Energy /kJ mol-1

C=C 612

C=O (CO2) 805

O-H 464

O=O 496

C-H 413

C-C 347

This value can be compared with the heat of combustion of actual benzene (in the gaseous state),-3242 kJ mol

-1.

The value for the extra stability of benzene here is not exactly the same as the value fromhydrogenation data. Why not? Which value is more reliable?

The extra stability of benzene comes from the

The spreading out of electrons (delocalisation) stabilises the molecule.

delocalised electron system


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Reactions of Benzene

A feature of the extra stability associated with the benzene aromaticring is that benzene does not react like alkenes, i.e. it does notundergo addition reactions under normal conditions (and will notdecolourise bromine water).

The stability of the aromatic ring means that it is regenerated in reactions, therefore benzene

undergoes SUBSTITUTION reactions.

e.g. benzene reacts with chlorine at room temperature in the presence of a catalyst such asaluminium chloride:

Naming aromatic compounds

CH3

NO2

OH

C

O

OH

Cl

C

O

CH3


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Organic 20

Relative rates of nucleophilic substitution in halogenated benzene compounds

Chlorobenzene does not undergo nucleophilic substitution (hydrolysis) reactions with sodiumhydroxide under normal conditions.

Chloroethane undergoes the reaction much more readily:

To all extents and purposes halogenated benzene compounds with the halogen atomattached directly to the ring are inert to nucleophilic substitution.

When the halogen atom is not attached directly to the benzene ring but is rather in the side-chain,hydrolysis occurs much more readily, e.g.

What is the organic product formed when the following compound is heated with excess sodiumhydroxide solution

C

H

CH

H

H

Cl

Cl C

H

Cl

H

Halogen atomattacheddirectly to thering

Cl

+ NaOH NO REACTION under normal conditio

overlap of a lone pair on the halogen

with the benzene system.

strengthens the C-Cl bond

nucleophiles are repelled from the Cattached to the Cl.

C

H

CH

H

ClH


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Organic 21

Electrophilic Substitution Reactions

General mechanism

The Nitration of Benzene

Formation of theelectrophile:

Mechanism:

NO2

HNO3 H2O+ +

Reflux 60 oC

conc. H2SO4

conc. HNO3

pale yellowliquid

Theelectrophile isthe NO2

+ ion

C6H6 + HNO3 C6H5NO2 + H2O

HNO3 + 2H2SO4 NO2+ + H3O

+ + 2HSO4-

Or HNO3 + H2SO4 NO2+

+ H2O + HSO4-


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Organic 22

Chlorination of benzene

Overall equation:

Benzene does not react with halogens in the dark because the halogen molecule is non-polar.Therefore a catalyst is required which polarises the molecule. The catalysts used are AlCl3 or

FeCl3, these are known as HALOGEN CARRIERS.

Aluminium chloride is electron deficient and accepts a pair of electrons from the Cl2:

Formation of the electrophile

Mechanism:

Alkylation (also known as Friedel Crafts alkylation)

Halogenoalkanes react with benzene in the presence of a halogen carrier catalyst:

Overall Reaction:

Formation of the electrophile:

The mechanism is the same as for halogenation.

Cl2 + AlCl3 AlCl4-

+ Cl+

For brominationFeBr3 would beused.


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Organic 23

Acylation (also known as Friedel Crafts acylation)

Acyl halides react with benzene in the presence of a halogen carrier:

Overall Reaction:

Formation of the electrophile:

The mechanism is the same as for alkylation.


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Organic 24

Naming substituted benzene rings

When more than one group is present in a benzene ring the groups are first of all numbered togive the lowest possible numbers and then in alphabetical order.

CH3

CH3

CH3

CH3

CH3

Cl

CH3

O2N

OH

Cl

OH

O2

N


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Organic 25

Reactions of methylbenzene

Methylbenzene reacts in the same way as benzene, via an electrophilic substitution mechanism.The conditions for the reactions of methylbenzene are slightly milder than those for the reactionsof benzene and the methyl group is an activating group. The methyl group donates electrondensity into the benzene ring (positive inductive effect). This increase the amount of electron

density in the ring so that it is more attractive to electrophiles and reacts more readily.

The methyl group is a 2,4-directing group and so the major products of substitution are:

Nitration of methylbenzene

The methylbenzene is heated under reflux with a mixture of concentrated sulphuric andconcentrated nitric acid:

Mono nitration occurs readily easily, further nitration is possible but because of thedeactivating effect of the nitro group the 2nd and 3rd nitrations are slow and moredifficult to achieve.

Multiple nitrated compounds:

2,4,6, trinitromethylbenzene(trinitrotoluene/TNT)

2,4,6 trinitrohydroxybenzene(trinitrophenol/picric acid)

CH3

X

CH3

X


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Organic 26

Chlorination of methylbenzene

Methylbenzene is reacted with chlorine in the presence of a halogen carrier catalyst (AlCl3) atroom temperature. This is chlorination in the ring.

If methylbenzene is reacted with chlorine in the presence of UV light then side chain substitutionoccurs. This involves a free radical substitution mechanism as for alkanes.

Alkylation of methylbenzene

Methylbenzene is reacted with a halogenoalkane in the presence of a halogen carrier catalyst(AlCl3) at room temperature.

Acylation of methylbenzene

Methylbenzene is reacted with an acyl chloride in the presence of a halogen carrier catalyst (AlCl3)at room temperature.


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Organic 27

***Reactions of Substituted Benzene Rings (NOTIN THE SYLLABUS)

Substituted benzene rings undergo basically the same reactions as a benzene ring, i.e.electrophilic substitution. The nature of the substituent determines the position of furthersubstitution and the rate of the reaction relative to unsubstituted benzene.

Substituents on a benzene ring may be divided into two group: those which cause substitution

predominantly at positions 2 and 4 (and 6) (orthoand para) and those which cause substitution atposition 3 (and 5) (meta).

Substituents that cause substitution faster than with benzene are called activating groups andthose that cause substitution to occur more slowly than with benzene are called deactivatinggroups.

Substituent Main Product of HalogenationRate of Substitutionrelative to benzene

-CH3

CH3

Cl

CH3

Cl

FASTER

-OH

OH

ClCl

Cl

OH

Cl

OH

Cl

FASTER

-NO2

NO2

Cl

SLOWER

Rate of substitution

The rate determining step in the reaction is the attack of the electrophile on the ring. Attack, andhence the reaction, occurs more quickly when there is more electron density in the ring so that anelectrophile is attracted more strongly. When the ring is deactivated by the withdrawal of electrondensity the electrophile is attracted less strongly and reaction occurs more slowly.

Thus methylbenzene reacts more readily than benzene due to the electron-releasing effect of the -CH3 group (positive inductive effect).

The methyl group activates the ring towards electrophilic substitution by donatingelectron density into the ring. This makes the ring more negative, i.e. moreattractive towards electrophiles and the reaction occurs more quickly than withbenzene.

2,4-directing groups normally cause substitution faster than in benzene and 3-directing groupsnormally react more slowly than benzene

CH3v


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Organic 28

- Back to syllabus

Phenol is a significantly stronger acid than aliphatic alcohols such as ethanol

Dissociation of phenol:

+ H2O

OH

Delocalisation results in stabilisation of the anion, making it more likely to be formed than thecorresponding ion with ethanol, where no delocalisation can take place.

In general, for compounds of the form shown, if X (in any position) is an electron-withdrawing group the compound will be a stronger acid than phenol. The electron-withdrawing group reduces the amount of electron density on the O in the conjugatebase the charge is more delocalised in the anion meaning that the H

+is less

strongly attracted. If X is an electron-donating group the compound will be a weakeracid than phenol. In this case the charge is less delocalised in the anion with thecharge more localised on the O the H

+is attracted back more strongly.

AZODYES

The nitration of aromatic compounds, followed by the reduction of the nitro group to an amine arethe first steps to producing organic dyes.

Step 1. Nitration

Step 2. Reduction

Consider the anion (phenoxide ion)

Delocalisation of negative charge from O-into ring

H+

not attracted back as strongly

Anion (conjugate base) stabilised Greater dissociation

Stronger acid

OH

X


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Organic 29

Step3. Producing the diazonium salt

Step4. Making the Azodyes


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Organic 30

Amines

Functional Group: -NH2 or -NHR or -NR2 orNR3+

Solubility: Lower members are soluble in water (H-bonding).

Higher members increasingly less soluble.

Boiling Points: Higher than alkanes of similar Mr (H-bonding) but lower than corresponding alcohols.

Acidity:

Preparation:Nucleophilic Substitution:

CH3 CH2Br + NH3conc. NH3

Heat in sealed tube

CH3 CH2NH2 + HB

Mechanism:

Further substitution is also possible to give secondary and tertiary amines and quaternary ammonium

salts(cationic surfactant).

Aliphatic amines can also be made by the reduction of nitriles:(Nitriles made by substitution of haloalkane with CN

-)

(i)LiAlH4/dry ether then H2O

CH3CN + 4[H] CH3CH2NH2

(ii) H2 and Ni catalyst

**Carbon chain extended by one carbon unit

Aromatic amines are made by the reduction of nitro compounds:

(i)Sn/HCl(conc)

C6H5NO2 C6H5NH2

(ii) H2/Ni

Amides

Functional Group:

C

NH2

O

HYDROGEN

BONDING

BASIC


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Organic 31

Solubility: Soluble but solubility decreases as length of hydrocarbon chainincreases.

Boiling Points: Solids except for methanamide. Higher boiling points thancorresponding carboxylic acids.

Acidity: Very weakly basic

Basicity (General )A base (Brnsted-Lowry definition) is a proton acceptor:

B: + H+ HB

+

The order of basicity for nitrogen-containing compounds is:

Compound ethylamine ammonia phenylamine ethanamideStructure

pKb 3.33 4.75 9.38 14.5

Kb /mol dm-3

5.4x10-4

1.8x10-5

5x10-10

1x10-15

Amides are very weak bases and will not affect red litmus paper.

The base strength depends on:

Ethylamine is a stronger base than NH3 because of the electron-donating effect of the ethyl

group:

Phenylamine and ethanamide are weak bases due to unavailability of the lone pair:

HYDROGENBONDING

The charge on the N atom. A greater negative charge meansthat the H

+is more strongly attracted and the compound is a stronger

base.

The availability of the lone pair. The less available the lonepair for donation, the weaker the base.

CH3 CH2 N

H

H

>-

H

NH H

-

ETHYLAMINE

alkyl group electron-donating

N more- H+ attracted more strongly


32/36

Organic 32

Formation of SALTS

Amines, like ammonia, form salts

with acids, e.g.

Syntheses of amines actually often result in the formation of a salt.

CH3NH3+Cl

-+ NaOH CH3NH2 + H2O + NaCl

The alkylammonium ion is acting as a Brnsted-Lowry acid.

Thus, if sodium hydroxide solution is added to a salt of an amine the free amine is released. This will be

noticed by a fishy smell or can be tested for by using moist litmus paper (amines are basic).

Peptides and ProteinsPeptides and proteins are naturally occurring polyamide polymers formed from amino acids ( bifunctionalcompounds.)

(-amino acids) contain an amine and an carboxylic acid functional group.

H

NH

C

H

C

O

O HH

H

NH

C

H

C

O

O HCH3

AMINOETHANOIC ACID

glycine

2-AMINOPROPANOIC ACID

Alanine

Delocalisation of the lone pair makes it less available for donation to H+

Amides: the N is also less - due to electron-withdrawing effect of the adjacent O atom

NH3 + HCl NH4+Cl-

CH3NH2 + HCl CH3NH3+Cl-

methylammonium chloride

(Surfactant)

The free amine may be releasedby adding a stronger base(e.g. NaOH)

2-amino acidsH

N

H

C

H

R

C

O

O H

All 2-amino acids except glycine (aminoethanoic

acid) are optically active (all naturallyoccurring optically active amino acids in proteins

are L-isomers)


33/36

Organic 33

Zwitterions

The carboxylic acid in amino acids donates its proton to the amine. This makes a doubly charged ion which

can act as both acid and base:

+ OH-

H2NCH2CO2H H3N+CH2CO2

-

Zwitterion

+ H+

At an intermediate pH it will exist as simultaneously as a cation and anionZwitterion. Each amino acid

has a unique pH value when this happensIsoelectric pH. (The existence of the zwitterion makes amino

acids crystalline solids at room temperature)

Separating amino acids: simple chromatography is used to separate mixtures of amino acids, as they are

colourless compounds a staining agent is used to develop the chromatogram (ninhydrin)amino acids stain

purple. This technique can be extended to a more sophisticated separative techniqueElectrophoresis.

In living organisms enzymes catalyse the polymerisation of amino acids to form peptides and proteins:

H

N

H

C C

O

O HH

C H2O H

H

N

H

C

H

C

O

O HC H3

H

N

H

C C

O

H

C H2O H

C H3 HO

O

C

H

C

H

N

+ H2O+

SERINE ALANINEa dipeptide

N

H

CC

O

OH H

CH2OH

CH3

O

C

H

C

H

N

H

H2O+H

N

H

CC

O

OH H

CH2OHH

N

H

C

H

C

O

OH CH3

+

The dipeptides are different depending on which way round the amino acids are joined together.

The functional group present in peptides and proteins is an amide group called a peptide link.

Proteins are macromolecules (large long chain molecules) formed when amino acids polymerise. Thesimplest level of protein structure is the sequence of the amino acids in the chain. This is known as primarystructure. There are 20 naturally occurring amino acids and, if just 100 are joined together to make apolymer, there are 20

100combinations.

part of a protein chain

There are three further levels of protein structure; secondary,

tertiary, quaternary.

N C

H

R2

C

O

CN C NC

H

O

C

H

R1 H

H

R3

O

H


34/36

Organic 34

Polymerisation

Addition Polymerisation

C C

H

H

H

HC C

H

H

H

H[ ]nO2 200 oC

2000 atm.n

MONOMER POLYMER

Ethene poly(ethene)

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

This is an example ofaddition polymerisation.

The mechanism is free radical addition involving opening of the double bond:

C C

H

H

H

H

C C

H

H

H

H

. .

HOMOLYTIC FISSION

PVC [Poly(vinylchloride)]

This is more properly known as poly(chloroethene)

C C

H

H

H

Cln

HEAT

peroxide initiator [ ]n

C C

H

H H

Cl

chloroethene poly(chloroethene)

Poly(propene)

Write an equation for the formation of poly(propene) and draw a length of the polymer chain showing three

repeat units.

** Addition polymers are non-biodegradeable


35/36

Organic 35

Condensation Polymerisation

PolyestersThe mechanism involved is addition-elimination, where a molecule of water is eliminated as each monomer islinked to the chain.

C C

O

O

O

OC C

H

H

H

H

O C C

H

H

H

H

C C

O

O

O

O C C

H

H

H

H

O

TERYLENE

The functional group present is the estergroup and the polymer is a polyester.In general, an diol and an dioic acid are required for formation of a polyester.

Polyamides

H

C

H

NH2 C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

NH2 C C

H

H

C

H

H

C

H

H

C

H

H

C

O

O

H

O

O H

C C

H

H

C

H

H

C

H

H

C

H

H

C

O OH

C

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

N

H

N

H+ (2n-1) H2O

[ ]n

amide link

CONDENSATION

POLYMERISATION

NYLON-6,6

C CO

O

O

OH H

C C

H

O

H

H

H

OH H

TERYLENE

C C

O

O

O

O C C

H

H

H

H[ ]n + (2n-1)H2O

HEATacid cat.

CONDENSATIONPOLYMERISATION

NEED 2 FUNCTIONALGROUPS ON THEMONOMERS

ethane-1,2-diolbenzene-1,4-dicarboxylic acid


36/36

Organic 36

C C

H

H

C

H

H

C

H

H

C

H

H

C

O O

N

H

H

C

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

N

H

H

C

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

N

H

C C

H

H

C

H

H

C

H

H

C

H

H

C

O O

N

H

The functional group present is the amide group, therefore nylon is a polyamide.

** Condensation polymers are biodegradeable, the ester and amide link can be broken by acid hydrolysis (and bioorganisms)

Note:

Know the arguments for and against the recycling of plastics

The advent of biodegradable plastics

Different methods used to dispose of plastics

a2 organic notes 2010[2].docx

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