3.3 Halogenoalkanes

H Naming Halogenoalkanes 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 halogenoalkanes 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

H Br H Br H H H Cl H H

Primary halogenoalkane Secondary halogenoalkane Tertiary halogenoalkane 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

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

- - : 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 Comparing the rate of reactions

Hydrolysis is defined as the splitting of a molecule ( in this Water is a poor nucleophile but it can case a haloalkane) by a reaction with water react slowly with haloalkanes in a - + 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.

Nucleophilic substitution with aqueous ions

 H H H Change in functional group: halogenoalkane H H H H C C C Br  Reagent: potassium (or ) 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 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

Alternative mechanism for tertiary halogenoalkanes You don’t need to learn this but there have been application of Tertiary haloalkanes undergo nucleophilic substitution in a different way understanding questions on this

CH3 CH3 CH3 Tertiary halogenoalkanes + undergo this mechanism as the H3C C Br H3C C :OH- H3C C OH tertiary carbocation is stabilised by the electron releasing methyl CH CH CH3 3 3 groups around it. (see topic for another example of this). The Br first breaks The hydroxide Also the bulky methyl groups away from the nucleophile then prevent the hydroxide ion from haloalkane to form attacks the positive attacking the halogenoalkane in a carbocation carbon the same way as the mechanism intermediate above

N Goalby chemrevise.org 2 Nucleophilic substitution with ions

Change in functional group: halogenoalkane  H H H H H H 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

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

Nucleophilic substitution with

Change in functional group: halogenoalkane  H H H H H H Reagent: NH3 dissolved in ethanol + 2NH H C C C NH2 + NH4Br Conditions: Heating under pressure (in a sealed H C C C Br 3 H H H tube) H H H Mechanism: Nucleophilic Substitution propylamine

Type of reagent: Nucleophile, :NH3 Naming : In the above example H H H propylamine, the propyl shows + - the 3 C’s of the carbon chain. δ δ + - CH CH C Br CH C N H :Br 3 2 CH3 2 Sometimes it is easier to use the 3HN: IUPAC naming for amines e.g. H H H Propan-1-amine :NH3

Further substitution reactions can occur between the halogenoalkane H and the amines formed leading to a lower yield of the amine. Using CH CH C NH 3 2 2 + NH4Br excess ammonia helps minimise this H

N Goalby chemrevise.org 3 2. 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 H C C C H + KOH  + KBr + H2O : Potassium (or sodium) hydroxide Reagents H H Br H H Conditions: In ethanol ; Heat 1-bromopropane Mechanism: Elimination Type of reagent: Base, OH-

Note the importance of H H H H the solvent to the type of - reaction here. CH C C H + Br + H2O 3 CH3 C C H Aqueous: substitution Br H Alcoholic: elimination :OH-

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

N Goalby chemrevise.org 4 Uses of Halogenoalkanes chloroalkanes and chlorofluoroalkanes can be used as Halogenoalkanes have also been used as , pesticides CH CCl was used as the solvent in dry cleaning 3 3 and aerosol propellants Many of these uses have now been stopped due to the toxicity of halogenoalkanes and also their detrimental effect on the atmosphere

Ozone Chemistry

The naturally occurring (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 (CFC’s) caused a hole to form in the . 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 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

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