sign in sign up
<<
Home , Haloalkane

4.2.2 Haloalkanes

H Naming Haloalkanes H H H H C H H H H Based on original , with a prefix indicating 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 haloalkanes

Haloalkanes can be classified as primary, secondary or tertiary depending on the number of atoms attached to the C-X . 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 haloalkane Secondary haloalkane Tertiary haloalkane One carbon attached to the Two attached to the Three carbons attached to the carbon atom adjoining the carbon atom adjoining the carbon atom adjoining the halogen halogen halogen

Nucleophilic substitution reactions Substitution: swapping a halogen atom for another atom or groups of atoms

- - : 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 The carbon has a small attack the positive H H H H positive charge because carbon atom of the 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 Nucleophilic substitution with aqueous ions

H H H Change in functional group: H H H haloalkane  H C C C B+r KOH  + KBr Reagent: potassium (or ) hydroxide H C C C OH H H H Conditions: In aqueous solution; Heat under reflux H H H Mechanism: Nucleophilic substitution 1-bromopropane propan-1-ol Type of reagent: Nucleophile, OH-

The conditions H H aqueous needed is an important point. δ+ δ- + Br - If the is changed to H3C C Br H3C C OH : an elimination -HO: H H reaction occurs

Comparing the rate of reactions Water is a poor nucleophile but it can Hydrolysis is defined as the splitting of a molecule ( in this react slowly with haloalkanes in a case a haloalkane) by a reaction with water . Use reflux OR heat for more than 20 minutes - + CH3CH2X + H2O  CH3CH2OH + X + H

- + Aqueous silver is added to a haloalkane 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 haloalkane and so the rate of formation of the precipitate can the silver nitrate first as the C-I bond is be used to compare the reactivity of the different haloalkanes. weakest and so it hydrolyses the quickest

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 The rate of these substitution reactions depends on the strength AgCl(s) – white precipitate of the C-X bond . The weaker the bond, the easier it is to break and the faster the reaction.

Chlorofluorocarbons (CFC’s) Many of these uses have now been stopped CFC’s were developed for use as aerosols, due to the toxicity of haloalkanes and also their , and in air-conditioning due their low detrimental effect on the layer in the reactivity, volatility and non-toxicity. atmosphere

Legislation to ban the use of CFCs was supported by chemists and chemists have now developed alternative -free compounds

HFCs (Hydro fluoro carbons) e.g.. CH2FCF3 are now used for refrigerators CO2 is now used as a blowing agent for producing and air-conditioners. These are safer expanded polymers instead of CFC’s as they do not contain the C-Cl bond

N Goalby chemrevise.org 2 The

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

Ozone is continuously being formed and broken down in the stratosphere by the action of ultraviolet radiation O + O2 O3

Ozone formation UV light causes an O2 molecule to split into free radicals This is the reverse of the formation reaction.. O2 + UV-light  O + O The energy is supplied by ultraviolet light When the free hits another O molecule ozone 2 O3 + ultraviolet light  O2 + O forms

O + O2  O3 There is a continuous cycle of formation and depletion of ozone The frequency of ultra-violet light absorbed equals the rate of ozone formation = rate of ozone removal frequency of biologically damaging ultra-violet radiation. These reactions therefore filter out harmful So there is a constant amount of ozone in UV and allow life to survive on earth. the atmosphere UV light can increase risk of skin cancer and increase crop mutation. Destruction of Ozone Layer Chlorine radicals are formed in the upper atmosphere when energy from ultra-violet Radicals from CFCs, and NOx from thunderstorms or radiation causes C–Cl bonds in aircraft, may catalyse the breakdown of ozone (CFCs) to break   CF2Cl2  CF2Cl + Cl

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 + O  O2 + Cl alternative route with a lower activation energy) Overall equation . They contributed to the formation of a hole in the O3 + O  2 O2 ozone layer. The regenerated Cl radical means that one Cl radical could destroy NO + O  NO + O many thousands of ozone molecules 3 . 2 2 NO2 + O  O2 + NO Overall equation . O3 + O  2 O2 HFCs (Hydro fluoro carbons) e.g.. Legislation to ban the use of CFCs was supported by CH2FCF3 are now used for refrigerators chemists and that they have now developed and air-conditioners. These are safer as alternative chlorine-free compounds 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.

N Goalby chemrevise.org 3