Organic ChemistryOrganic Chemistry

IntroductionIntroduction

Important definitionsImportant definitions

HOMOLOGOUS SERIES – HOMOLOGOUS SERIES – a family of organic compounds which all fit the same general formula, neighbouring members differ from each other by CH2, have similar chemical properties and show trends in physical properties.

e.g. e.g. the alkanes all fit the general formula C CnnHH2n+22n+2, , members of the family include methane (CH (CH44), ),

ethane (Cethane (C22HH66) ) and propane (C propane (C33HH88).).

EMPIRICAL FORMULAEMPIRICAL FORMULA – the simplest ratio of moles of atoms of each element in a compound.

MOLECULAR FORMULAMOLECULAR FORMULA – the actual number of moles of atoms of each element in a compound.

STRUCTURAL FORMULASTRUCTURAL FORMULA (aka displayed formula or graphical formula) – shows all of the atoms and the bonds between them. e.g. pentane (Cpentane (C55HH1212)).

C

H

H

C

H

H

C

H

H

H HC

H

H

C

H

H

A condensed structural formula can also be used which omits the bonds. So for pentanepentane we can use:

CHCH33CHCH22CHCH22CHCH22CHCH33

or

CHCH33(CH(CH22))33CHCH33

The formulas in the data book are called SKELETAL formulas. These must NOT be used in the exam.

STRUCTURAL ISOMERSSTRUCTURAL ISOMERS – compounds with the same molecular formula but a different arrangement of atoms. e.g. CC44HH1010 represents

C

H

H

C

H

H

H HC

H

H

C

H

H

butane andbutane and

H

C C

H

H

H C

H

H

H

CH

H

H

2-methylpropane

FUNCTIONAL GROUP – FUNCTIONAL GROUP – atom or group of atom or group of atoms which gives an organic compound atoms which gives an organic compound its characteristic chemical properties.its characteristic chemical properties.

CC HH AlkanesAlkanes

suffix – anesuffix – ane

For exampleFor example

CHCH33CHCH33 ethaneethane

AlkenesAlkenes

suffix – enesuffix – ene

For exampleFor example

CHCH22=CH=CH22 etheneethene

CC CC

CC XX HalogenoalkanesHalogenoalkanes

prefix halo - prefix halo -

For exampleFor example

CHCH33CHCH22ClCl chloroethanechloroethane

X = Cl, Br or IX = Cl, Br or I

CC OHOH AlcoholsAlcohols

suffix – ol suffix – ol or prefix hydroxy- or prefix hydroxy-

For exampleFor example

CHCH33CHCH22OHOH ethanolethanol

CHCH33CH(OH)COOHCH(OH)COOH 2-hydroxypropanoic 2-hydroxypropanoic acidacid

AldehydesAldehydes

suffix – alsuffix – al

For exampleFor example

CHCH33CHOCHO ethanalethanal

CCHH

OO

CC OO KetonesKetones

suffix –one suffix –one or prefix or prefix oxo-oxo-

For exampleFor example

CHCH33COCHCOCH33propanonepropanone

CHCH33COCHCOCH22COOHCOOH 3 – oxobutanoic 3 – oxobutanoic acidacid

Carboxylic acidCarboxylic acid

suffix – oic acidsuffix – oic acid

For exampleFor example

CHCH33COOHCOOH ethanoic acidethanoic acid

CCOHOH

OO

CC NHNH22 AminesAmines

suffix – amine suffix – amine or prefix amino -or prefix amino -

For exampleFor example

CHCH33NHNH22 methylaminemethylamine

HH22NCHNCH22COOHCOOH aminoethanoic acidaminoethanoic acid

EstersEsters

suffix – oatesuffix – oate

For exampleFor example

CHCH33COOCCOOC22HH55 ethyl ethanoateethyl ethanoate

CCOROR

OO

Aromatic compoundsAromatic compounds

Contain the BENZENE ring

formula C6H6

CCC

CC C

H

H H

H H

H

Delocalised electrons

Some examplesSome examples

NO2

CH3CHCH2CH3

BrCl

CH3

Cl

COOH

nitro group

nitrobenzene

(1-methylpropyl) benzene

1

2

1-bromo-2-chlorobenzene

1

34

4-chloro-3-methylbenzenecarboxylic acid

Sometimes the benzene ring is not Sometimes the benzene ring is not regarded as the key part of the regarded as the key part of the molecule.molecule.

In these cases it is referred to as the In these cases it is referred to as the PHENYL group.PHENYL group.

For exampleFor example

NHNH22

phenylaminephenylamine

CHCH33CHCH22CHCH22CCOO

HH

aldehydealdehyde

butanbutanalal

CHCH33CHCH22CCHCCH22CHCH33

OOketoneketone

pentanpentan-3-one-3-one

carboxylic acidcarboxylic acid

3-methylbutan3-methylbutanoic acidoic acidCHCH33CHCHCHCH22CC

OO

OHOH

CHCH33

alcoholalcohol

alkenealkene

carboxylic acidcarboxylic acid

ketoneketone

CCH

C

OCH

OH

O

C

C

O

OHO

esterester

carboxylic acid

ether

nitrile

aldehyde

amine

CHCH2

CO OH

OCH

CH2C

N

NH2

C

O

OCH2

CO

H

ester

Physical Properties of Organic Physical Properties of Organic MoleculesMolecules

Alkanes worksheetAlkanes worksheet

What type of intermolecular force would What type of intermolecular force would you expect to find between alkanes, you expect to find between alkanes, halogenoalkanes, aldehydes, ketones, halogenoalkanes, aldehydes, ketones, alcohols and carboxylic acids?alcohols and carboxylic acids?

Use this information to deduce the relative Use this information to deduce the relative boiling points of these homologous series boiling points of these homologous series and their solubility in water.and their solubility in water.

The AlkanesThe Alkanes

The alkanes is an homologous series where all The alkanes is an homologous series where all members fit the general formula Cmembers fit the general formula CnnHH2n+22n+2. .

They have trends in physical properties e.g. They have trends in physical properties e.g. density and m.p. and b.p. all increase with Mdensity and m.p. and b.p. all increase with Mrr..

They all undergo similar chemical reactions.They all undergo similar chemical reactions.

Alkanes are SATURATED HYDROCARBONS. Alkanes are SATURATED HYDROCARBONS. i.e. they contain only single C to C bonds and are i.e. they contain only single C to C bonds and are made up of C and H atoms only.made up of C and H atoms only.

Alkanes are obtained from crude oil by Alkanes are obtained from crude oil by fractional distillation.fractional distillation.

They are mainly used as fuels.They are mainly used as fuels.

The large MThe large Mrr alkanes do not ignite easily alkanes do not ignite easily

so there is little demand for them as so there is little demand for them as fuels so they are CRACKED to make fuels so they are CRACKED to make smaller more useful alkanes and smaller more useful alkanes and alkenes.alkenes.

Apart from combustion alkanes undergo Apart from combustion alkanes undergo few chemical reactions. This is for two few chemical reactions. This is for two main reasons:main reasons:

1.1. The bonds in alkanes are relatively The bonds in alkanes are relatively strong.strong.

2.2. The bonds have a relatively low The bonds have a relatively low polarity as the electronegativity of C polarity as the electronegativity of C and H is similar.and H is similar.

As a consequence alkanes can be As a consequence alkanes can be used as lubricating oils, although they used as lubricating oils, although they do degrade over time. do degrade over time.

Reactions of Alkanes:Reactions of Alkanes:

Combustion:Combustion:

Alkanes burn exothermically to produce Alkanes burn exothermically to produce carbon dioxide and water if there is a carbon dioxide and water if there is a plentiful supply of oxygen. This is known plentiful supply of oxygen. This is known as complete combustion.as complete combustion.

e.g. CHe.g. CH44 + 2O + 2O22 CO CO22 + 2H + 2H22OO

Write equations for the complete Write equations for the complete combustion of butane and octane.combustion of butane and octane.

If there is a limited supply of oxygen If there is a limited supply of oxygen incomplete combustion occurs and incomplete combustion occurs and carbon monoxide or carbon are formed carbon monoxide or carbon are formed instead of carbon dioxide.instead of carbon dioxide.

e.g. e.g. CHCH44 + 1 + 1½½OO22 CO + 2H CO + 2H22OO

CHCH44 + O + O22 C + 2H C + 2H22OO

What problems do the gases released on What problems do the gases released on combustion of alkanes cause?combustion of alkanes cause?

ChlorinationChlorination

Methane does not react with chlorine in Methane does not react with chlorine in the dark but in the presence of ultraviolet the dark but in the presence of ultraviolet light reacts to form hydrogen chloride.light reacts to form hydrogen chloride.

CHCH44 + Cl + Cl22 CH CH33Cl + HClCl + HCl

The mechanism for this reaction is The mechanism for this reaction is known as free radical substitution.known as free radical substitution.

Substitution = replacement of an atom or Substitution = replacement of an atom or group of atoms by a different atom or group of atoms by a different atom or group of atoms.group of atoms.

Free radical = species with an unpaired electron.

Free radicals are formed by homolytic fission of bonds. In homolytic fission one electron from the shared pair goes to each atom.

So

ClCl

ClCl +

or Cl2 2Cl.unpaired electron

Heterolytic fission of Cl – Cl would result in the formation of Cl+ and Cl-.

There are three steps in the mechanism: initiation, propagation and termination

CHCH44 + Cl + Cl22 CHCH33Cl + HClCl + HCl

Overall reaction equationOverall reaction equation

ConditionsConditions

ultra violet light (breaks weakest bond)ultra violet light (breaks weakest bond)

excessexcess methane to reduce further methane to reduce further substitution substitution

i.e. i.e. homolytichomolytic breaking of covalent bonds breaking of covalent bonds

Free radical substitutionFree radical substitution

chlorination of methanechlorination of methane

CHCH44 + Cl + Cl CHCH33 + HCl + HCl

ClCl22 Cl + ClCl + Cl

CHCH33 + Cl + Cl22 CHCH33Cl + ClCl + Cl

CHCH33ClClCHCH33 + Cl + Cl

initiation stepinitiation step

twotwo propagation propagation stepssteps

termination steptermination step

UV LightUV Light

CHCH33CHCH33CHCH33 + CH + CH33minor minor termination steptermination step

Free radical substitution mechanismFree radical substitution mechanism

Also get reverse of initiation step occurring as a Also get reverse of initiation step occurring as a termination step.termination step.

CHCH33Cl + ClCl + Cl22 CHCH22ClCl22 + HCl + HCl

Overall reaction equationsOverall reaction equations

Conditions

CHCH22ClCl22 + Cl + Cl22 CHClCHCl33 + HCl + HCl

CHClCHCl33 + Cl + Cl22 CClCCl44 + HCl + HCl

ultra-violet lightultra-violet lightexcess excess chlorinechlorine

Further free radical substitutionsFurther free radical substitutions

Write down two propagation steps to explain Write down two propagation steps to explain the formation of dichloromethane.the formation of dichloromethane.

Methane reacts in exactly the same way with Methane reacts in exactly the same way with bromine to form hydrogen bromide together bromine to form hydrogen bromide together with bromomethane, dibromomethane, with bromomethane, dibromomethane, tribromomethane and tetrabromomethane.tribromomethane and tetrabromomethane.

Write down the mechanism for the reaction Write down the mechanism for the reaction between chlorine and ethane to form between chlorine and ethane to form chloroethane. Use the mechanism to chloroethane. Use the mechanism to explain why small amounts of butane are explain why small amounts of butane are formed. How could the formation of further formed. How could the formation of further substitution products be minimised?substitution products be minimised?

The AlkenesThe AlkenesAll fit the general formula CAll fit the general formula CnnHH2n2n. .

Are unsaturated hydrocarbons as they contain a C = Are unsaturated hydrocarbons as they contain a C = C.C.

Much more reactive than alkanes.Much more reactive than alkanes.

Industrial importance of alkenes:Industrial importance of alkenes:

1.1. Making polymers (plastics)Making polymers (plastics)

2.2. Hydrogenation of vegetable oils to make Hydrogenation of vegetable oils to make margarinemargarine

3.3. Hydration of ethene to make ethanol.Hydration of ethene to make ethanol.

When naming alkenes have to include When naming alkenes have to include position of double bond, for example:position of double bond, for example:

CHCH33CH=CHCHCH=CHCH33 is but - 2 - ene and is but - 2 - ene and

CHCH33CHCH22CH=CHCH=CH22 is but -1- ene is but -1- ene

Draw out and name Draw out and name allall of the alkenes with of the alkenes with the molecular formula Cthe molecular formula C66HH1212..

Alkenes undergo ADDITION reactions. Alkenes undergo ADDITION reactions. Two substances combine to form one Two substances combine to form one new substance. Unsaturated molecules new substance. Unsaturated molecules are converted to saturated molecules.are converted to saturated molecules.

Reactions of AlkenesReactions of Alkenes1.1. Addition of hydrogen (hydrogenation) Addition of hydrogen (hydrogenation)

Alkenes react with hydrogen in the Alkenes react with hydrogen in the presence of a nickel catalyst at 180 °C presence of a nickel catalyst at 180 °C to form an alkane. e.g.to form an alkane. e.g.

H – C – C – HH – C – C – H C = CC = C

HH

HH HH

HH+ + HH22

HH HH

HH HH

CC22HH44 + H + H22 C C22HH66

etheneethene ethaneethane

Most oils are esters of propane-1,2,3-triol Most oils are esters of propane-1,2,3-triol (aka glycerol) with 3 long chain carboxylic (aka glycerol) with 3 long chain carboxylic acids (aka fatty acids). The esters are acids (aka fatty acids). The esters are sometimes called triglycerides. sometimes called triglycerides. Hydrogenation of these oils produces Hydrogenation of these oils produces margarine.margarine.

The common fatty acids includeThe common fatty acids include• octadeca-9-enoic (oleic) acid – octadeca-9-enoic (oleic) acid – unsaturated acid found in most fats and unsaturated acid found in most fats and olive oilolive oil• octadeca-9,12-dienoic (linoleic) acid – octadeca-9,12-dienoic (linoleic) acid – unsaturated acid found in many vegetable unsaturated acid found in many vegetable oils such as soyabean and corn oiloils such as soyabean and corn oil

CHCH22OOC(CHOOC(CH22))77CH=CH(CHCH=CH(CH22))77CHCH33

CHOOC(CHCHOOC(CH22))77CH=CH(CHCH=CH(CH22))77CHCH33

CHCH22OOC(CHOOC(CH22))77CH=CH(CHCH=CH(CH22))77CHCH33

Above is the triglyceride formed between Above is the triglyceride formed between propan-1,2,3-triol and oleic acid. propan-1,2,3-triol and oleic acid. Hydrogenation using a nickel catalyst and Hydrogenation using a nickel catalyst and slight pressure removes some of the C=C. slight pressure removes some of the C=C. This enables the chains to pack together This enables the chains to pack together more closely which increases the van der more closely which increases the van der Waals forces and hence m.p. so the oils are Waals forces and hence m.p. so the oils are solidified forming margarine.solidified forming margarine.

2.2. Addition of halogens (halogenation)Addition of halogens (halogenation)Halogens react with alkenes at room Halogens react with alkenes at room temperature and pressure in a non-polar temperature and pressure in a non-polar solvent to form a dihalogenoalkane.solvent to form a dihalogenoalkane.e.g.e.g.

C = C

H

H H

H+ Br2 H – C – C – H

Br Br

H H

C2H4 + Br2 C2H4Br2

ethene 1,2-dibromoethane

Bromine water is used as a test for Bromine water is used as a test for unsaturation.unsaturation.

In the presence of an alkene, bromine water In the presence of an alkene, bromine water turns from red brown to colourless. Alkanes turns from red brown to colourless. Alkanes do not react with bromine water.do not react with bromine water.

C = CC = C – – C – C – C – C –

BrBr BrBr

+ Br2(aq)

colorless amber colorless

Bromine Water Test For AlkenesBromine Water Test For Alkenes

3. 3. Reaction with hydrogen halides Reaction with hydrogen halides (hydrohalogenation)(hydrohalogenation)

Alkenes react with hydrogen halides Alkenes react with hydrogen halides (HCl, HBr etc.) to form halogenoalkanes. (HCl, HBr etc.) to form halogenoalkanes. The reaction occurs at room The reaction occurs at room temperature and pressure. e.g.temperature and pressure. e.g.

C = CC = C

HH

HH HH

HH+ + HBrHBr H – C – C – HH – C – C – H

HH BrBr

HH HH

CC22HH44 + HBr + HBr CH CH33CHCH22BrBretheneethene bromoethanebromoethane

4.4. Hydration (reaction with water)Hydration (reaction with water)

This can be done in two ways:This can be done in two ways:

a) Addition of concentrated sulphuric acid a) Addition of concentrated sulphuric acid to form an alkyl hydrogensulphate. to form an alkyl hydrogensulphate. Water is then added to hydrolyse the Water is then added to hydrolyse the product and produce an alcohol. The product and produce an alcohol. The sulphuric acid is regenerated.sulphuric acid is regenerated.

C = CC = C

HH

HH HH

HH+ + HH22SOSO44

OSOOSO33HH

H – C – C – HH – C – C – H

HH

HH HH

OSOOSO33HH

H – C – C – HH – C – C – H

HH

HH HH

+ H+ H22OO

OHOH

H – C – C – HH – C – C – H

HH

HH HH

+ H+ H22SOSO44

CC22HH44 + H + H22SOSO44 CH CH33CHCH22OSOOSO33HH

CHCH33CHCH22OSOOSO33H + HH + H22O O CC22HH55OH + HOH + H22SOSO44

ethanolethanol

b) b) Alkenes can also undergo direct Alkenes can also undergo direct hydration to form an alcohol. Ethene can hydration to form an alcohol. Ethene can be converted to ethanol by reaction with be converted to ethanol by reaction with steam in the presence of a phosphoric(V) steam in the presence of a phosphoric(V) acid (Hacid (H33POPO44) catalyst at a pressure of 60 – ) catalyst at a pressure of 60 –

70 atm and a temperature of 300 °C.70 atm and a temperature of 300 °C.

C = CC = C

HH

HH HH

HH+ + HH22OO

OHOH

H – C – C – HH – C – C – H

HH

HH HH

What advantages and disadvantages does What advantages and disadvantages does this method have over production of ethanol this method have over production of ethanol by fermentation?by fermentation?

5. Addition Polymerisation5. Addition Polymerisation

The formation of polymers involves alkenes The formation of polymers involves alkenes reacting with themselves to form a long reacting with themselves to form a long chain molecule called a polymer. The chain molecule called a polymer. The individual molecules used to make the individual molecules used to make the polymer are called monomers. Ethene is polymer are called monomers. Ethene is polymerised to form poly(ethene)polymerised to form poly(ethene)

nCH2 = CH2 CH2 CH2 n

nn = about 100 to = about 100 to 10 00010 000 CH2 CH2

is the repeating unit

monomermonomer repeating repeating unitunit

polymerpolymer typical typical usesuses

CHCH22=CH=CH22 - CH- CH22 – CH – CH2 2 -- poly(ethene)poly(ethene)

polythenepolythene

film, bagsfilm, bags

poly(propene)poly(propene)

polypropylenepolypropylene

Moulded Moulded plastic, plastic, fibresfibres

poly(phenylethene)poly(phenylethene)

polystyrenepolystyrene

packaging,packaging,

insulationinsulation

CHCH22=CHCH=CHCH33 - CH- CH22 – CH - – CH -

CHCH33

CHCH22=CHC=CHC66HH55 - CH- CH22 – CH - – CH -

CC66HH55

monomermonomer repeating repeating unitunit

polymerpolymer typical typical usesuses

pipes, pipes, flooringflooring

CHCH22=CHCl=CHCl - CH- CH22 – CH - – CH -

ClCl

poly(chloroethene)poly(chloroethene)

polyvinylchloridepolyvinylchloride

PVCPVC

PolyPoly

(tetrafluoroethene)(tetrafluoroethene)

PTFEPTFE

CFCF22=CF=CF22 - CF- CF22 – CF – CF22 - - Non-stick Non-stick coating coating (Teflon)(Teflon)

Alcohols

Alcohols are the homologous series with the general formula CnH2n+1OH.

They all contain the functional group, OH, which is called the hydroxyl group.

Alcohols can be classified as primary, secondary or tertiary, depending on the carbon skeleton to which the hydroxyl group is attached.

Draw out the structure, name and classify all the alcohols with the formula C4H9OH.

RCHRCH22OHOH

1 alkyl 1 alkyl group on C group on C next to OH next to OH so primary so primary alcohol, 1°alcohol, 1°

RR22CHOHCHOH

2 alkyl 2 alkyl groups on C groups on C next to OH next to OH

so so secondary secondary alcohol, 2°alcohol, 2°

RR33COHCOH

3 alkyl 3 alkyl groups on C groups on C next to OH next to OH so tertiary so tertiary alcohol, 3°alcohol, 3°

OHH

H

C

H

H

C

H

H

C

H

H

C

H

Butan-1-ol Butan-1-ol primaryprimary

OH

H

H

C

H

H

C

H

H

C

H

H

C H Butan-2-ol Butan-2-ol secondarysecondary

OHH

H

C

H

H

C

H

H

C

CH3

OH

H

H

C

H

H

C

H

HC

CH3

2-methylpropan-1-ol 2-methylpropan-1-ol primaryprimary

2-methylpropan-2-ol 2-methylpropan-2-ol tertiarytertiary

Reactions of AlcoholsReactions of Alcohols

1.1. CombustionCombustion

In countries such as Brazil, ethanol is In countries such as Brazil, ethanol is mixed with petrol and used to power mixed with petrol and used to power cars. Ethanol is less efficient as a fuel cars. Ethanol is less efficient as a fuel than petrol as it is already partially than petrol as it is already partially oxidised but does make the country oxidised but does make the country less reliant on supply of petrol. As it less reliant on supply of petrol. As it can be produced by fermentation of can be produced by fermentation of sugar beet, many consider ethanol a sugar beet, many consider ethanol a carbon neutral fuel.carbon neutral fuel.

2.2. Oxidation of AlcoholsOxidation of Alcohols

Primary alcohols are oxidised first Primary alcohols are oxidised first to aldehydes, such as ethanal.to aldehydes, such as ethanal. A A suitable oxidising agent is acidified suitable oxidising agent is acidified potassium dichromate(VI)potassium dichromate(VI)

OHOHHH

HH

CC

HH

HH

CC

HH

++ HH22OOOO

HH

HH

CC

HH

CC

HH

ethanolethanol ethanalethanal

CrCr22OO77/H/H++

An aldehyde still has one hydrogen An aldehyde still has one hydrogen atom attached to the carbonyl carbon, atom attached to the carbonyl carbon, so it can be oxidised one step further so it can be oxidised one step further to a carboxylic acid.to a carboxylic acid.

OOHH

HH

CC

HH

CC

HH

OOHH

HH

CC

HH

CC

OHOH

ethanalethanal ethanoic ethanoic acidacid

CrCr22OO77/H/H++

In practice, a primary alcohol such as ethanol is In practice, a primary alcohol such as ethanol is dripped into a warm solution of acidified dripped into a warm solution of acidified potassium dichromate(VI). potassium dichromate(VI).

The aldehyde, ethanal, is formed and immediately The aldehyde, ethanal, is formed and immediately distils off, thereby preventing further oxidation to distils off, thereby preventing further oxidation to ethanoic acid, because the boiling point of ethanal ethanoic acid, because the boiling point of ethanal (23 °C) is much lower than that of either the (23 °C) is much lower than that of either the original alcohol ethanol (78 °C) or of ethanoic acid original alcohol ethanol (78 °C) or of ethanoic acid (118 °C). Both the alcohol and the acid have higher (118 °C). Both the alcohol and the acid have higher boiling points because of hydrogen bonding.boiling points because of hydrogen bonding.

If oxidation of ethanol to ethanoic acid is required, If oxidation of ethanol to ethanoic acid is required, the reagents must be heated together under reflux the reagents must be heated together under reflux to prevent escape of the aldehyde before it can be to prevent escape of the aldehyde before it can be oxidised further.oxidised further.

Secondary alcohols are oxidised to Secondary alcohols are oxidised to ketones. These have no hydrogen atoms ketones. These have no hydrogen atoms attached to the carbonyl carbon and so attached to the carbonyl carbon and so cannot easily be oxidised further.cannot easily be oxidised further.

HH

HH

CC

HH OHOH

HH

CC

HH

HH

CC HH

propan-2-olpropan-2-ol

HH

HH

CC

HH OO

HH

CC

HH

CC HH

propanonepropanone

CrCr22OO77/H/H++

When When orangeorange acidified potassium dichromate(VI) acidified potassium dichromate(VI) acts as an oxidising agent, it is reduced to acts as an oxidising agent, it is reduced to greengreen chromium(III) ions. chromium(III) ions.

Primary and secondary alcohols both turn acidified Primary and secondary alcohols both turn acidified dichromate(VI) solution from dichromate(VI) solution from orangeorange to to greengreen when when they are oxidised, and this colour change can be they are oxidised, and this colour change can be used to distinguish them from tertiary alcohols. used to distinguish them from tertiary alcohols.

Tertiary alcohols are not oxidised by acidified Tertiary alcohols are not oxidised by acidified dichromate(VI) ions, so they have no effect on its dichromate(VI) ions, so they have no effect on its colour, which remains colour, which remains orangeorange..

Distinguishing between 1°, 2° and 3° alcoholsDistinguishing between 1°, 2° and 3° alcohols

HalogenoalkanesHalogenoalkanes

Named by using the name of the alkane from Named by using the name of the alkane from which they are derived with the prefix which they are derived with the prefix chloro-, bromo- or iodo-.chloro-, bromo- or iodo-.

For example: For example:

CHCH33CHCH22Br is bromoethaneBr is bromoethane

(CH(CH33))22CHCHCHCH22Cl is 1-chloro-2-methylpropaneCl is 1-chloro-2-methylpropane

Remember the position of the halogen Remember the position of the halogen atom must be indicated using the atom must be indicated using the appropriate number soappropriate number so

CHCH33CHCH22CHCH22Cl is 1-chloropropane andCl is 1-chloropropane and

CHCH33CHClCHCHClCH33 is 2-chloropropane is 2-chloropropane

Halogenoalkanes can be classified in the Halogenoalkanes can be classified in the same way as alcohols.same way as alcohols.

Draw out, name and classify all the Draw out, name and classify all the isomers with the formula Cisomers with the formula C55HH1111Br.Br.

Key feature of halogenoalkanes isKey feature of halogenoalkanes is

C X C X where X = Cl, Br or Iwhere X = Cl, Br or I

What is notable about this bond What is notable about this bond compared with say, C – C and C – H?compared with say, C – C and C – H?

The halogen atom is The halogen atom is more electronegative more electronegative than C so the bond is than C so the bond is polarised:polarised:

C XC X

++ --

C ClC Cl

++ --

C IC I

++ --

ORDER OF BOND ORDER OF BOND POLARITIES:POLARITIES:

C BrC Br

++ --

>> >>

So is order of reactivity:So is order of reactivity:

chloroalkane > bromoalkanes > iodoalkanes?chloroalkane > bromoalkanes > iodoalkanes?

Is there another factor that Is there another factor that ought to be considered before ought to be considered before reaching a conclusion?reaching a conclusion?

BOND BOND ENERGIESENERGIES

Bond energies:Bond energies:

BondBond Bond energy in Bond energy in kJmolkJmol-1-1

C - ClC - Cl

C - BrC - Br

C - IC - I

346346

290290

234234

This suggests that the order of reactivity is:This suggests that the order of reactivity is:

iodoalkane > bromoalkanes > chloroalkanesiodoalkane > bromoalkanes > chloroalkanes

There’s only one way to find out which is best!There’s only one way to find out which is best!

Do an experiment, not fight!Do an experiment, not fight!

But what do halogenoalkanes react with?But what do halogenoalkanes react with?

The The + carbon atom is susceptible to attack by + carbon atom is susceptible to attack by NUCLEOPHILES.NUCLEOPHILES.

A nucleophile is a species with a lone pair of A nucleophile is a species with a lone pair of electrons.electrons.

e.g. OHe.g. OH--, NH, NH33, CN, CN--..

When attack by a nucleophile occurs, the carbon – When attack by a nucleophile occurs, the carbon – halogen bond breaks releasing a halide ion.halogen bond breaks releasing a halide ion.

A suitable nucleophile for experimentation is OHA suitable nucleophile for experimentation is OH-- from an aqueous solution of an alkali such as from an aqueous solution of an alkali such as sodium hydroxide.sodium hydroxide.

CH3CH2X + OH- CH3CH2OH + X-

OH has replaced the X so overall we have OH has replaced the X so overall we have NUCLEOPHILIC SUBSTITUTIONNUCLEOPHILIC SUBSTITUTION

X = Cl, Br or I.X = Cl, Br or I.

How can we follow the rate of this reaction so How can we follow the rate of this reaction so that we can determine the order of reactivity?that we can determine the order of reactivity?

Halide ions Halide ions coloured precipitates when silver coloured precipitates when silver nitrate is added.nitrate is added.

So we can measure how long it takes for a So we can measure how long it takes for a precipitate to form.precipitate to form.

See EXPERIMENT SHEET.See EXPERIMENT SHEET.

Mechanisms for nucleophilic substitutionMechanisms for nucleophilic substitution

SSNN1 = unimolecular nucleophilic substitution (only 1 = unimolecular nucleophilic substitution (only

one species in the slow step of the one species in the slow step of the mechanism, rate determining step)mechanism, rate determining step)

SSNN2 = bimolecular nucleophilic substitution (two 2 = bimolecular nucleophilic substitution (two

species in the slow step of the mechanism, species in the slow step of the mechanism, rate determining step)rate determining step)

Use of curly arrows:Use of curly arrows:

Curly arrows are used in reaction Curly arrows are used in reaction mechanisms to show the movement of mechanisms to show the movement of electron pairs.electron pairs.

to formto forma a lonelone pair pairof electronsof electrons

XX

eithereithera a bondbond pair pairof electronsof electrons

oror a a lonelone pair pairof electronsof electrons

eithereithernextnext to an atom to an atom

TAILSTAILS come from come from

HEADSHEADS point point

to form to form a a bondbond pair pair of electronsof electrons

ororatat an atom an atom

XX

-OH

CH3

H

OHC

H

Br-

CH3

H

BrC

H

CCH3

H

H

+ +

CCH3

H

H

+

slow

fast

SN1

Intermediate carbocation

Heterolytic fission of C – Br bond

--OHOH

HOHO

HH

CHCH33CC

HH

BrBr--

BrBrCC

CHCH33

HH

HH

++

CC

HH

CHCH33 HH

HOHO BrBr

--

SSNN22

TransitioTransition staten state

Which is best? SN1 or SN2?

For primary halogenoalkanes – SN2

For tertiary halogenoalkanes – SN1

3° halogenoalkanes cannot undergo the SN2 mechanism as 5 bulky groups would not fit around the C in the transition state - steric hindrance.

1° halogenoalknes are less likely to undergo SN1 as this would involve the formation of a primary carbocation as an intermediate. Alkyl groups push electron density to the C atom they are attached to (positive inductive effect) which stabilises the positive charge. More alkyl groups mean a more stable carbocation.

2° halogenoalkanes react via a mixture 2° halogenoalkanes react via a mixture of Sof SNN1 and S1 and SNN2. The mechanism 2. The mechanism

predominating depends upon the predominating depends upon the nature of the alkyl groups and the nature of the alkyl groups and the nature of the solvent.nature of the solvent.