[PDF] chemrevise Reactions of Halogenoalkanes. 1. Nucleophilic

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NamingHalogenoalkanesBased on originalalkane, with aprefixindicating halogen atom:Fluorofor F;ChloroforCl;Bromofor Br;Iodofor I.CCHHBrHCHHHHSubstituents are listedalphabeticallyCCCCClHCHHHHHHHHHHClassifyinghalogenoalkanesHaloalkanes can be classified as primary, secondary or tertiary dependingon the number of carbon atoms attached to the C-X functional group.CCHHBrHCHHHHCCHHHBrCHHHHCCCCClHCHHHHHHHHHH

Reactions ofHalogenoalkanes1. Nucleophilic substitution reactionsNucleophile: electron pair donator e.g. :OH-, :NH3, CN-Halogenoalkanesundergo eithersubstitutionoreliminationreactionsSubstitution: swapping a halogen atom for another atom or groups of atoms

The nucleophilesattack the positivecarbon atomThe carbon has a smallpositive charge becauseof the electronegativitydifference between thecarbon and the halogen

The rate of these substitution reactions depends on the strengthof the C-X bondThe weaker the bond, the easier it is to break and the faster the reaction.The iodoalkanes are the fastest to substitute and thefluoroalkanes are the slowest. The strength of the C-F bond issuch that fluoroalkanes are very unreactive

Organic reactions areclassified by theirmechanisms

The Mechanism: We draw (oroutline) mechanisms toshow in detail how a reaction proceeds:Nu represents any nucleophile-theyalways have alone pairand act aselectron pair donators

We use curly arrows in mechanisms (withtwo line heads) to show the movement oftwo electronsA curly arrow will alwaysstartfrom alone pairof electrons orthecentre of a bond

+ X-CCHH H H

HXNu:-δ+δ-CCHH

H H HNu

Primary halogenoalkaneOne carbon attached to thecarbon atom adjoining thehalogenSecondaryhalogenoalkaneTwocarbons attached to thecarbon atom adjoining thehalogenTertiaryhalogenoalkaneThreecarbons attached to thecarbon atom adjoining thehalogen

3.3Halogenoalkanes

Comparing the rate of hydrolysis reactionsWateris apoornucleophilebut it canreactslowlywith haloalkanes in asubstitution reactionHydrolysisis defined as the splitting of a molecule ( in thiscase a haloalkane) by a reaction with waterCH3CH2X + H2OCH3CH2OH + X-+ H+Aqueous silver nitrateis added to a haloalkane and thehalide leaving group combines with a silver ion to form aSILVER HALIDE PRECIPITATE.The precipitate only forms when the halide ion has left thehaloalkane and so the rate of formation of the precipitate canbe used to compare the reactivity of the different haloalkanes.CH3CH2I+ H2OCH3CH2OH +I-+ H+Ag+(aq)+I-(aq)AgI(s)-yellow precipitateThe iodoalkane forms a precipitate withthe silver nitrate first as the C-I bond isweakest and so it hydrolyses the quickestThequickertheprecipitateis formed, thefasterthesubstitutionreaction and themorereactivethe haloalkaneAgI(s)-yellow precipitateAgBr(s)-cream precipitateAgCl(s)-white precipitateforms fasterThe rate of these substitution reactions depends on the strengthof the C-X bond . The weaker the bond, the easier it is to breakand the faster the reaction.Nucleophilic substitution withaqueoushydroxide ionsChange in functional group:halogenoalkanealcoholReagent: potassium (or sodium) hydroxideConditions: Inaqueous solution; Heat under refluxMechanism:NucleophilicSubstitutionType of reagent: Nucleophile, OH-CHHCCHHHHHOHCHHCCHHHHHBrTheaqueousconditionsneeded is an important point.If the solvent is changed toethanolan eliminationreaction occursAlternative mechanism for tertiaryhalogenoalkanesTertiary haloalkanes undergo nucleophilic substitution in a different way

CCH3CH3CH3BrC+CH3CH3CH3:OH-CCH3CH3CH3OHThe Br first breaksaway from thehaloalkane to forma carbocationintermediateThe hydroxidenucleophile thenattacks the positivecarbon

Tertiaryhalogenoalkanesundergo this mechanism as thetertiary carbocation is stabilisedby the electron releasing methylgroups around it. (see alkenestopic for another example of this).Also the bulky methyl groupsprevent the hydroxide ion fromattacking thehalogenoalkaneinthe same way as the mechanismabove

You don"t need to learn this butthere have been application ofunderstanding questions on this CH3CH

HBrCH3CH

HOH-HO:+:Br-δ+δ-

Nucleophilic substitution with cyanide ions

Change in functional group:halogenoalkaneamineReagent: NH3dissolved in ethanolConditions: Heating under pressure (in a sealedtube)Mechanism: Nucleophilic SubstitutionType of reagent: Nucleophile, :NH3

Change in functional group:halogenoalkanenitrileReagent: KCNdissolved inethanol/water mixtureConditions: Heating under refluxMechanism: Nucleophilic SubstitutionType of reagent: Nucleophile, :CN-CHHCCHHHHHBrCCHHHHHCHCNH

Note: themechanism isidentical to theabove oneThis reaction increases the length ofthe carbon chain (which is reflected inthe name) In the above examplebutanenitrile includes the C in thenitrile group

Nucleophilic substitution with ammoniaCHHCCHHHHHBrCHHHCHHCHHNH2propylamineNaming amines:In the above examplepropylamine, the propyl showsthe 3 C"s of the carbon chain.Sometimes it is easier to use theIUPAC naming for amines e.g.Propan-1-amineFurther substitution reactions canoccur between thehalogenoalkaneand the amines formed leading to alower yield of the amine. Usingexcess ammonia helps minimise this

Note the naming: butanenitrile and notbutannitrile.Naming NitrilesNitrile groups have to be at the end of a chain. Startnumbering the chain from the C in the CNCH3CH2CN : propanenitrileCH3CHCH2CNCH33-methylbutanenitrile

CH3CH

HCNCH3CH

HBr-NC:+:Br-δ+δ-

CH2CH

HBrCH3

CH2CH

HNH2CH3

CH2CH

HN+CH3HHH3HN:δ+δ-:NH3

2. Elimination reaction ofhalogenoalkanesElimination: removal of small molecule(often water) from the organic moleculeElimination with alcoholic hydroxide ionsChange in functional group:halogenoalkanealkeneReagents:Potassium (or sodium) hydroxideConditions:Inethanol; HeatMechanism:EliminationType of reagent: Base, OH-CCHHBrHCHHHHCCHHHCHHHNote the importance ofthe solvent to the type ofreaction here.Aqueous: substitutionAlcoholic: elimination

With unsymmetrical secondaryand tertiaryhalogenoalkanestwo (or sometimes three)different structural isomers canbe formedCCCCClHCHHHHHHHHHH

CCCCCHHHHHHHHHHCCCCHCHHHHHHHHH2-methyl-2-chlorobutane can give2-methylbut-1-ene and2-methylbut-2-eneOften a mixture of products from both elimination and substitution occurs

The structure of thehalogenoalkanealso has an effect on thedegree to which substitution or elimination occurs in this reaction.Primary tends towards substitutionTertiary tends towards elimination

:OH-CCH

HCH3Br

HHCCHCH3HH

Ozone ChemistryThe naturally occurring ozone (O3) layer in the upperatmosphere is beneficial as it filters out much of the sun"sharmful UV radiationOzone in the lower atmosphereis a pollutant and contributestowards the formation of smogMan-made chlorofluorocarbons (CFC"s)caused a hole to form in the ozonelayer.Chlorine atoms are formed in the upper atmosphere when energy fromultra-violet radiation causes C-Clbonds in chlorofluorocarbons (CFCs) tobreakCl.+ O3ClO.+ O2ClO.+ O32O2+Cl.Overall equation2 O33 O2The chlorine free radical atomscatalysethedecomposition of ozone due to these reactionsbecause they are regenerated. (They provide analternative route with a lower activation energy)They contributed to the formation of a hole in theozone layer.

CF2Cl2CF2Cl+Cl

Legislation to ban the use of CFCs was supported bychemists and that they have now developedalternative chlorine-free compoundsHFCs (Hydrofluorocarbons) e.g..CH2FCF3are now used for refrigeratorsandair-conditioners. These are saferas they do not contain the C-Clbond

The regeneratedClradicalmeansthat oneClradical could destroymanythousands of ozone molecules The C-F bond is stronger than the C-Clbond and is not affected by UV

chloroalkanes and chlorofluoroalkanes can be used as solventsUses ofHalogenoalkanesHalogenoalkaneshave also beenused as refrigerants, pesticidesand aerosol propellantsCH3CCl3was used as the solvent in dry cleaningMany of these uses have now been stopped due to the toxicity ofhalogenoalkanesand also their detrimental effect on theatmosphere

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