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16 Halogenoalkanes

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16 Halogenoalkanes 16 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 Haloalkanes 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 H Br H Br H H H Cl H H Primary halogenoalkane Secondary halogenoalkane Two Tertiary halogenoalkane Three One carbon attached to the carbon carbons attached to the carbon carbons attached to the carbon atom adjoining the halogen atom adjoining the halogen atom adjoining the halogen Reactions of Halogenoalkanes Halogenoalkanes undergo either Organic reactions are substitution or elimination reactions classified by their mechanisms 1. 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 always The Mechanism: We draw (or outline) mechanisms to show have a lone pair and act as electron pair in detail how a reaction proceeds donators Nu:- The nucleophiles The carbon has a small attack the positive H H H H positive charge because of carbon atom the electronegativity H C C + X- H C Cδ+ X 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 from two line heads) to show the movement of start a of electrons or the two electrons lone pair centre of a bond The rate of these substitution reactions depends on the strength of the C-X Bond enthalpy / 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 such that fluoroalkanes are C-Cl 338 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 case a Water is a poor nucleophile but it can react haloalkane) by a reaction with water slowly with haloalkanes in a substitution reaction - + CH3CH2X + H2O CH3CH2OH + X + H - + Aqueous silver nitrate is added to a haloalkane and the halide CH3CH2I + H2O CH3CH2OH + I + H leaving group combines with a silver ion to form a SILVER HALIDE Ag+ + I- AgI - yellow precipitate PRECIPITATE. (aq) (aq) (s) The precipitate only forms when the halide ion has left the The iodoalkane forms a precipitate with the haloalkane and so the rate of formation of the precipitate can be silver nitrate first as the C-I bond is weakest used to compare the reactivity of the different haloalkanes. and so it hydrolyses the quickest The quicker the precipitate is formed, the faster the substitution AgI (s) - yellow precipitate reaction and the more reactive the haloalkane AgBr(s) – cream precipitate forms faster The rate of these substitution reactions depends on the strength of the AgCl(s) – white precipitate C-X bond . The weaker the bond, the easier it is to break and the faster the reaction. Nucleophilic substitution with aqueous hydroxide ions Change in functional group: halogenoalkane alcohol H H H H H H Reagent: potassium (or sodium) hydroxide H C C C Br + KOH + KBr Conditions: In aqueous solution; Heat under reflux H C C C OH Mechanism:Nucleophilic Substitution H H H H H H - Role of reagent: Nucleophile, OH 1-bromopropane propan-1-ol The OH– is a stronger nucleophile than water as it has a full negative The aqueous conditions needed is charge and so is more strongly attracted to the Cδ+ 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 Nucleophilic substitution with cyanide ions Change in functional group: halogenoalkane H H H H H H nitrile Reagent: KCN dissolved in ethanol/water mixture H C C C Br + :CN- H C C C CN + Br- Conditions: Heating under reflux H H H H H H Mechanism: Nucleophilic Substitution 1-bromopropane butanenitrile Type of reagent: Nucleophile, :CN- Note: the H H This reaction increases the length of mechanism is δ+ δ- the carbon chain (which is reflected in + Br - identical to the H3C C Br H3C C CN : the name) In the above example above one -NC: butanenitrile includes the C in the H H nitrile group Naming Nitriles Nitrile groups have to be at the end of a chain. Start numbering the chain from the C in the CN Note the naming: butanenitrile and not butannitrile. CH3CH2CN : propanenitrile H3C CH CH2 C N 3-methylbutanenitrile CH3 Elimination reaction of halogenoalkanes Elimination: removal of small molecule (often water) from the organic molecule Elimination with alcoholic hydroxide ions H H H Change in functional group: halogenoalkane H H H 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 2-bromopropane propene Mechanism: Elimination Type of reagent: Base, OH- N Goalby chemrevise.org 4 Uses of Halogenoalkanes chloroalkanes and chlorofluoroalkanes can be used as solvents Halogenoalkanes have also been used as refrigerants, pesticides CH CCl was used as the solvent in dry cleaning 3 3 and aerosol propellants Many of these uses depended on the unreactivity of Fluorine containing compounds. They often had low flammability Many of these uses have now been stopped due to the toxicity of halogenoalkanes to the atmosphere and also their detrimental effect on the atmosphere Ozone Chemistry The naturally occurring ozone (O3) layer in the upper Ozone in the lower atmosphere atmosphere is beneficial as it filters out much of the sun’s is a pollutant and contributes harmful UV radiation towards the formation of smog Man-made chlorofluorocarbons (CFC’s) caused a hole to form in the ozone layer. Chlorine atoms are formed in the upper atmosphere when energy from ultra-violet radiation causes C–Cl bonds in chlorofluorocarbons (CFCs) to CF2Cl2 CF2Cl + Cl break The chlorine free radical atoms catalyse the . Cl + O3 ClO + O2 decomposition of ozone due to these reactions . because they are regenerated. (They provide an ClO + O3 2O2 + Cl alternative route with a lower activation energy) Overall equation 2 O3 3 O2 They contributed to the formation of a hole in the ozone layer. The regenerated Cl radical means that one Cl radical could destroy many thousands of ozone molecules Legislation to ban the use of CFCs was supported by HFCs (Hydro fluoro carbons) e.g.. chemists and that they have now developed CH2FCF3 are now used for refrigerators alternative chlorine-free compounds and air-conditioners. These are safer as they do not contain the C-Cl bond The C-F bond is stronger than the C-Cl bond and is not affected by UV N Goalby chemrevise.org 5.
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