Nucleophile: electron pair donator e g :OH-, :NH3, CN- Halogenoalkanes undergo either substitution or elimination reactions Substitution: swapping a halogen
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[PDF] 33 Halogenoalkanes - chemrevise
Nucleophile: electron pair donator e g :OH-, :NH3, CN- Halogenoalkanes undergo either substitution or elimination reactions Substitution: swapping a halogen
[PDF] Classifying Halogenoalkanes Reactions of - chemrevise
10 avr 2018 · The rate of these substitution reactions depends on the strength of the C-X case a halogenoalkane) by a reaction with water CH3CH2X +
[PDF] 27 Halogenoalkanes
Secondary halogenoalkanes can undergo SN2 reactions, but the reaction is generally slow because of steric hindrance Tertiary halogenoalkanes do not undergo
[PDF] Module 2 Alcohols, halogenoalkanes and analysis - Pearson
When halogenoalkanes react with an aqueous solution of hot hydroxide ions, a nucleophilic substitution reaction occurs The product of this reaction is an alcohol
[PDF] Questions on the reactions of halogenoalkanes with - Chemguide
HALOGENOALKANES: REACTIONS WITH HYDROXIDE IONS 50/50 mixture of ethanol and water, and a substitution reaction occurs (i) What do you
[PDF] answers HALOGENOALKANES: REACTIONS WITH - Chemguide
Chemguide – answers HALOGENOALKANES: REACTIONS WITH SILVER NITRATE SOLUTION 1 a) You get a substitution reaction in which the halogen
[PDF] 33 The Halogenoalkanes
This makes the halogenoalkanes more reactive Reactions of the halogenoalkanes: Reactivity This reaction converts a halogenoalkane to an alkyl nitrile
[PDF] Mechanisms 1) Free radical substitution – Alkane
3) Nucleophilic substitution of halogenoalkanes a) With aqueous hydroxide, OH- Hydrolysis – forming alcohols • This reaction converts a halogenoalkane to an
[PDF] Halogenoalkane Questions 1 Some reactions of halogenoalkanes
Some reactions of halogenoalkanes are show in the diagram below: a) Name The reaction to form compound A is called nucleophilic substitution Explain
[PDF] Halogenoalkanes and Alcohols - Physics & Maths Tutor
A nucleophile is a species (molecule or negative ion) which can donate an electron pair in a chemical reaction Halogenoalkanes undergo substitution reactions
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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 theirmechanismsThe 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 HHXNu:-δ+δ-CCHH
H H HNuPrimary 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 CH3CHHBrCH3CH
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
CH3CHHCNCH3CH
HBr-NC:+:Br-δ+δ-
CH2CHHBrCH3
CH2CHHNH2CH3
CH2CHHN+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-CCHHCH3Br
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 UVchloroalkanes 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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