Nucleophilic substitution PAG7
Naming Haloalkanes Based on original alkane, with a prefix indicating halogen atom: Fluoro for F; Chloro for Cl; Bromo for Br; Iodo for I. C C H H Br H C H H H H 1-bromopropane Substituents are listed alphabetically C C C C Cl H C H H H H H H H H H H 2-chloro-2-methylbutane Classifying haloalkanes Haloalkanes can be classified as primary, secondary or tertiary depending on the number of carbon atoms attached to the C-X functional group. C C H H Br H C H H H H C C H H H Br C H H H H C C C C Cl H C H H H H H H H H H H PRIMARY Haloalkane One carbon attached to the carbon atom adjoining the halogen SECONDARY Haloalkane Two carbons attached to the carbon atom adjoining the halogen TERTIARY Haloalkane Three carbons attached to the carbon atom adjoining the halogenNucleophilic substitution reactions Nucleophile: electron pair donator e.g. :OH- , :NH3 , CNSubstitution: swapping a halogen atom for another atom or groups of atoms The nucleophiles attack the positive carbon atom The carbon has a small positive charge because of the electronegativity difference between the carbon and the halogen The rate of these substitution reactions depends on the strength of the C-X bond The weaker the bond, the easier it is to break and the faster the reaction. Bond enthalpy / kJmol-1 C-I 238 C-Br 276 C-Cl 338 C-F 484 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 very unreactive The Mechanism: We draw (or outline) mechanisms to show in detail how a reaction proceeds :Nu represents any nucleophile – they always have a lone pair and act as electron pair donators We use curly arrows in mechanisms (with two line heads) to show the movement of two electrons A curly arrow will always start from a lone pair of electrons or the centre of a bond H C C + XH H H H X Nu: – δ + δ – H C C H H H H Nu N Goalby chemrevise.org 2 Nucleophilic substitution with aqueous hydroxide ions Change in functional group: halogenoalkane alcohol Reagent: potassium (or sodium) hydroxide Conditions: In aqueous solution; Heat under reflux Mechanism:Nucleophilic Substitution Type of reagent: Nucleophile, OH- + KOH C H H C C H H H H H OH C H H C C H H H H H Br + KBr 1-bromopropane propan-1-ol The aqueous conditions needed is an important point. If the solvent is changed to ethanol an elimination reaction occurs H3C C H H Br H3C C H H OH -HO: + :Br δ – + δ – Comparing the rate of hydrolysis reactions Water is a poor nucleophile but it can react slowly with halogenoalkanes in a substitution reaction. Use reflux OR heat for more than 20 minutes Hydrolysis is defined as the splitting of a molecule ( in this case a haloalkane) by a reaction with water CH3CH2X + H2O CH3CH2OH + X- + H+ Aqueous silver nitrate is added to a haloalkane and the halide leaving group combines with a silver ion to form a SILVER HALIDE PRECIPITATE. The precipitate only forms when the halide ion has left the haloalkane and so the rate of formation of the precipitate can be used to compare the reactivity of the different haloalkanes. CH3CH2 I + H2O CH3CH2OH + I – + H+ Ag+ (aq) + I – (aq) AgI (s) – yellow precipitate The iodoalkane forms a precipitate with the silver nitrate first as the C-I bond is weakest and so it hydrolyses the quickest The quicker the precipitate is formed, the faster the substitution reaction and the more reactive the haloalkane AgI (s) – yellow precipitate AgBr(s) – cream precipitate AgCl(s) – white precipitate forms faster The rate of these substitution reactions depends on the strength of the C-X bond . The weaker the bond, the easier it is to break and the faster the reaction.
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4.2.2 Haloalkanes
Substitution reactions of haloalkanes (a) hydrolysis of haloalkanes in a substitution reaction: (i) by aqueous alkali (ii) by water in the presence of AgNO3 and ethanol to compare experimentally the rates of hydrolysis of different carbon– halogen bonds PAG7 (see also 6.3.1 c) (b) definition and use of the term nucleophile (an electron pair donor) (c) the mechanism of nucleophilic substitution in the hydrolysis of primary haloalkanes with aqueous alkali (see also 4.1.1 h–i) HSW1,2 Use of reaction mechanisms to explain organic reactions. (d) explanation of the trend in the rates of hydrolysis of primary haloalkanes in terms of the bond enthalpies of carbon–halogen bonds (C–F, C–Cl, C–Br and C–I)
CFCs and the ozone layer
Chlorofluorocarbons (CFC’s) CO2 is now used as a blowing agent for producing expanded polymers instead of CFC’s Legislation to ban the use of CFCs was supported by chemists and chemists have now developed alternative chlorine-free compounds N Goalby chemrevise.org 3 The Ozone Layer The naturally occurring ozone (O3 ) layer in the upper atmosphere is beneficial as it filters out much of the sun’s harmful UV radiation Ozone in the lower atmosphere is a pollutant and contributes towards the formation of smog Ozone is continuously being formed and broken down in the stratosphere by the action of ultraviolet radiation Ozone formation UV light causes an O2 molecule to split into free radicals O2 + UV-light → O + O When the free radical hits another O2 molecule ozone forms O + O2 → O3 Ozone depletion This is the reverse of the formation reaction.. The energy is supplied by ultraviolet light O3 + ultraviolet light O2 + O There is a continuous cycle of formation and depletion of ozone rate of ozone formation = rate of ozone removal So there is a constant amount of ozone in the atmosphere The frequency of ultra-violet light absorbed equals the frequency of biologically damaging ultra-violet radiation. These reactions therefore filter out harmful UV and allow life to survive on earth. UV light can increase risk of skin cancer and increase crop mutation. Radicals from CFCs, and NOx from thunderstorms or aircraft, may catalyse the breakdown of ozone O + O2 O3 Destruction of Ozone Layer Chlorine radicals are formed in the upper atmosphere when energy from ultra-violet radiation causes C–Cl bonds in chlorofluorocarbons (CFCs) to break CF2Cl2 → CF2Cl + Cl Cl. + O3 ClO. + O2 ClO. + O. O2 + Cl. Overall equation O3 + O. 2 O2 The chlorine free radical atoms catalyse the decomposition of ozone due to these reactions because they are regenerated. (They provide an alternative route with a lower activation energy) They contributed to the formation of a hole in the ozone layer. The regenerated Cl radical means that one Cl radical could destroy NO + O3 NO2 + O2 many thousands of ozone molecules NO2 + O. O2 + NO Overall equation O3 + O. 2 O2 Legislation to ban the use of CFCs was supported by chemists and that they have now developed alternative chlorine-free compounds HFCs (Hydro fluoro carbons) e.g.. CH2FCF3 are now used for refrigerators and air-conditioners. These are safer as they do not contain the C-Cl bond CFC’s still concern us because CFCs are still entering the atmosphere from disused items and are still used for some purposes and by some countries. CFCs have a long lifetime in the atmosphere and it takes a long time for CFCs to reach upper atmosphere. Many of these uses have now been stopped due to the toxicity of halogenoalkanes and also their detrimental effect on the ozone layer in the atmosphere C. CFC’s were developed for use as aerosols, refrigerants, and in air-conditioning due their low reactivity, volatility and non-toxicity. HFCs (Hydro fluoro carbons) e.g.. CH2FCF3 are now used for refrigerators and air-conditioners. These are safer as they do not contain the C-Cl bond
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4.2.2 Haloalkanes
Environmental concerns from use of organohalogen compounds (e) production of halogen radicals by the action of ultraviolet (UV) radiation on CFCs in the upper atmosphere and the resulting catalysed breakdown of the Earth’s protective ozone layer, including equations to represent: (i) the production of halogen radicals (ii) the catalysed breakdown of ozone by Cl• and other radicals e.g. •NO. Simple equations of the breakdown process are required, e.g. CF2Cl 2 → CF2Cl• + •Cl •Cl + O3 → •ClO + O2 •ClO + O → •Cl + O2 Learners could be expected to construct similar equations for other stated radicals. HSW9,10,11,12 Benefits of CFCs; acceptance of scientific evidence explaining ozone depletion leading to government legislation against CFC use.
Credits: Neil Goalby