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Classifying Halogenoalkanes Reactions Of 10 Halogenoalkanes H Naming Halogenoalkanes H H H H C H H H H Based on original alkane, with a prefix indicating halogen atom: H C C C H Fluoro for F; Chloro for Cl; Bromo for Br; Iodo for I. H C C C C H Br H H H Cl H H Substituents are listed alphabetically 1-bromopropane 2-chloro-2-methylbutane Classifying Halogenoalkanes Halogenoalkanes can be classified as primary, secondary or tertiary depending on the number of carbon atoms attached to the C-X functional group. H H H H H H H H C H H H H H C C C H H C C C H H C C C C H Br H H H Br H H Cl H H Primary halogenoalkane Secondary halogenoalkane Tertiary halogenoalkane One carbon attached to the Two carbons attached to the Three carbons attached to the carbon atom adjoining the carbon atom adjoining the carbon atom adjoining the halogen halogen halogen Reactions of Halogenoalkanes Halogenoalkanes undergo either substitution or elimination reactions Nucleophilic substitution reactions Substitution: swapping a halogen atom for another atom or groups of atoms - - Nucleophile: electron pair donator e.g. :OH , :NH3, CN :Nu represents any nucleophile – they The Mechanism: We draw (or outline) mechanisms to always have a lone pair and act as show in detail how a reaction proceeds electron pair donators Nu:- The nucleophiles The carbon has a small attack the positive H H H H positive charge because carbon atom of the electronegativity H C C X H C C + X- δ+ δ- Nu difference between the carbon and the halogen H H H H We use curly arrows in mechanisms (with A curly arrow will always two line heads) to show the movement of start from a of electrons or two electrons lone pair the centre of a bond The rate of these substitution reactions depends on the strength Bond enthalpy / of the C-X bond kJmol-1 The weaker the bond, the easier it is to break and the faster the reaction. C-I 238 C-Br 276 The iodoalkanes are the fastest to substitute and the fluoroalkanes are the slowest. The strength of the C-F bond is C-Cl 338 such that fluoroalkanes are very unreactive C-F 484 N Goalby chemrevise.org 1 Comparing the rate of hydrolysis reactions Hydrolysis is defined as the splitting of a molecule ( in this Water is a poor nucleophile but it can case a halogenoalkane) by a reaction with water react slowly with halogenoalkanes in a substitution reaction - + CH3CH2X + H2O CH3CH2OH + X + H - + Aqueous silver nitrate is added to a halogenoalkane and the CH3CH2I + H2O CH3CH2OH + I + H halide leaving group combines with a silver ion to form a Ag+ + I- AgI - yellow precipitate SILVER HALIDE PRECIPITATE. (aq) (aq) (s) The precipitate only forms when the halide ion has left the The iodoalkane forms a precipitate with halogenoalkane and so the rate of formation of the precipitate the silver nitrate first as the C-I bond is can be used to compare the reactivity of the different weakest and so it hydrolyses the quickest halogenoalkanes. The quicker the precipitate is formed, the faster the AgI (s) - yellow precipitate substitution reaction and the more reactive the haloalkane AgBr(s) – cream precipitate forms faster AgCl – white precipitate The rate of these substitution reactions depends on the strength (s) of the C-X bond . The weaker the bond, the easier it is to break and the faster the reaction. If the experiment is repeated using primary, secondary and tertiary halogenoalkanes, the order of reactivity will be: Tertiary > secondary > primary Reacts first Nucleophilic substitution with aqueous hydroxide ions H H H Change in functional group: halogenoalkane H H H alcohol H C C C Br Reagent: potassium (or sodium) hydroxide + KOH H C C C OH + KBr Conditions: In aqueous solution; Heat under reflux H H H H H H Mechanism:Nucleophilic Substitution 1-bromopropane propan-1-ol Role of reagent: Nucleophile, OH- The OH– is a stronger nucleophile than water as it has a full negative The aqueous conditions needed charge and so is more strongly attracted to the Cδ+ is an important point. If the solvent is changed to ethanol an elimination reaction occurs SN2 nucleophilic substitution mechanism for halogenoalkanes H CH3 H - H δ+ δ- H C C Br - 3 HO C Br H3C C OH + :Br -HO: H H H transition state. This mechanism occurs with primary halogenoalkanes N Goalby chemrevise.org 2 SN1 nucleophilic substitution mechanism for tertiary halogenoalkanes Tertiary haloalkanes undergo nucleophilic substitution in a different way CH CH Tertiary halogenoalkanes 3 3 CH3 undergo this mechanism as the + tertiary carbocation is made H C C Br H C C - H C C OH 3 3 :OH 3 stabilised by the electron CH CH3 CH3 3 releasing methyl groups around it. (see alkenes topic for another The Br first breaks example of this). The hydroxide away from the Also the bulky methyl groups nucleophile then haloalkane to form prevent the hydroxide ion from attacks the positive a carbocation attacking the halogenoalkane in carbon intermediate the same way as the mechanism above Primary halogenoalkanes don’t do the SN1 mechanism because they would only form an unstable primary carbocation. Nucleophilic substitution with ammonia Change in functional group: halogenoalkane H H H amine H H H Reagent: NH3 dissolved in ethanol + 2NH H C C C NH2 + NH Br Conditions: Heating under pressure in a sealed H C C C Br 3 4 H H H tube H H H Mechanism: Nucleophilic Substitution propylamine Type of reagent: Nucleophile, :NH3 Naming amines: In the above example H H H propylamine, the propyl shows + - the 3 C’s of the carbon chain. δ δ + - CH C Br :Br CH3 2 CH3 CH2 C N H Sometimes it is easier to use the 3HN: IUPAC naming for amines e.g. H H H Propan-1-amine :NH3 H CH C NH CH3 2 2 + NH4Br H Further substitution reactions can occur between the haloalkane and the amines formed leading to a lower yield of the amine. Using excess ammonia helps minimise this. H R Further reactions H H RX RX RX R N: H N: H N: R N: H R R R N Goalby chemrevise.org 3 Elimination reaction of halogenoalkanes Elimination: removal of small molecule (often water) from the organic molecule Elimination with alcoholic hydroxide ions H H H H H H Change in functional group: halogenoalkane H C C C alkene H C C C H + KOH + KBr + H2O Reagents: Potassium (or sodium) hydroxide H H Br H H Conditions: In ethanol ; Heat 1-bromopropane propene Mechanism: Elimination Role of reagent: Base, OH- Note the importance of the solvent to the type of reaction here. Aqueous: substitution Alcoholic: elimination Often a mixture of products from both elimination and substitution occurs 2-methyl -2- H chlorobutane can give H H C H With unsymmetrical secondary 2-methylbut-1-ene and H H H H C H 2-methylbut-2-ene and tertiary halogenoalkanes H H H C C C C H two (or sometimes three) different structural isomers can H C C C C H H H H H be formed H Cl H H H C H H H H H C C C C H H H The structure of the halogenoalkane also has an effect on the degree to which substitution or elimination occurs in this reaction. Primary tends towards substitution Tertiary tends towards elimination Uses of halogenoalkanes Haloalkanes have been used as refrigerants, fire retardants, pesticides and aerosol propellants chloroalkanes and chlorofluoroalkanes can be used as solvents Some halogenoalkanes have low flammability CH3CCl3 was used as the solvent in dry cleaning Many of these uses have now been stopped due to the toxicity of halogenoalkanes and also their detrimental effect on the ozone layer N Goalby chemrevise.org 4 6E Alcohols General formula alcohols CnH2n+1OH Naming Alcohols OH These have the ending -ol and if necessary the position Butan-2-ol number for the OH group is added between the name stem CH3 CH CH2 CH3 and the –ol 1 2 3 4 O 2-hydroxypropanoic acid If the compound has an –OH group in addition to other H3C CH C functional groups that need a suffix ending then the OH can be named with the prefix hydroxy-): OH OH Ethan -1,2-diol HO CH2 CH2 OH e If there are two or more -OH groups then di, tri are used. H2C OH Add the ‘e’ on to the stem name though propane-1,2,3-triol HC OH H2C OH Bond angles in Alcohols H The H-O- C bond is 104.5o (bent line All the H-C-H bonds and C- H shape), because there are 2 bond C-O are 109.5o (tetrahedral O H 109.5O pairs of electrons and 2 lone pairs shape), because there are 4 C 104.5O repelling to a position of minimum bond pairs of electrons H C repulsion. Lone pairs repel more than repelling to a position of H bond pairs so the bond angle is minimum repulsion. H reduced. Different types of alcohols H H H H O H H H H H H C C C H H C C C O H H C C C H H H H H O H H H H C H Propan-1-ol H H Primary Propan-2-ol Secondary methylpropan-2-ol Tertiary Primary alcohols are alcohols Secondary alcohols are alcohols Tertiary alcohols are alcohols where 1 carbon is attached to where 2 carbon are attached to where 3 carbon are attached to the carbon adjoining the the carbon adjoining the oxygen the carbon adjoining the oxygen oxygen N Goalby chemrevise.org 5 1.
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