Halogenoalkanes

Reading list

The attached notes and the pages from your Chemistry 1 textbook are compulsory reading. The other references are very strongly recommended. Each author approaches the subject from a slightly different angle. Obviously, the more you read around a subject, more likely you are to truly understand the subject. Consequently, you are then more likely to achieve a top grade in your AS and A2 exams.

                 Chemistry 1                                           136- 143                       

                 Ramsden (3rd edition) (SB8)                    614- 634

                 Chemistry in Context                              503- 515           

Assessment outcomes

Candidates should be able to:

(a)        describe substitution reactions of  halogenoalkanes, typified by the following reactions of  bromoethane:

(i)      hydrolysis with hot aqueous alkali to form alcohols;

(ii)     reaction with excess ethanolic ammonia to form primary amines.

(b)        define the term nucleophile as an electron pair donor.

(c)         describe the mechanism of nucleophilic substitution in the hydrolysis of primary halogenoalkanes.

·            Candidates should show ‘curly arrows’ in this mechanism and include any relevant lone pairs and dipoles.

(d)        explain the rates of hydrolysis of primary halogenoalkanes in terms of the bond enthalpies of carbon–halogen bonds (C–F, C–Cl, C–Br and C–I). (See also 5.3.1(f).)

·            aqueous silver nitrate in ethanol can be used to compare these rates.

(e)        describe the elimination of hydrogen bromide from halogenoalkanes, typified by bromoethane, with hot ethanolic sodium hydroxide.

(f)          outline the uses of

(i)      fluoroalkanes and fluorohalogenoalkanes, for example: chlorofluorocarbons, CFCs (refrigerants, propellants, blowing polystyrene, dry cleaning, degreasing agents);

(ii)     chloroethene and tetrafluroethene to produce the plastics pvc and ptfe. (See also 5.2.4(g)–(m)) and 5.4.6(a).)

(iii)    halogenoalkanes as synthetic intermediates in chemistry.

(g)        outline the role of chemists in minimising damage to the environment by, for example, the development of alternatives to CFCs so that depletion of the ozone layer (see also 5.3.2 (i), (l)) can be reversed. The equations will not be tested in Unit 2812.)

 

Halogenoalkanes and nucleophilic substitution

The halogenoalkanes (often called haloalkanes) are a homologous series of compounds with the general formula CnH2n+1X where X is a halogen, i.e. F, Cl, Br, or I. e.g.

  • CH3CH2CI          chloroethane

·         CH3CHBrCH3     2-bromopropane


The halogenoalkanes can be classed as primary, secondary or tertiary depending on the carbon skeleton to which the halogen atom is attached, (R = any alkyl or aryl group; Hal = F, Cl, Br, or I):

Halogens atoms are very electronegative (see table below) so carbon—halogen bonds are polar. The electrons in the C-X bond are attracted towards the halogen atom which becomes δ-, leaving the carbon atom electron deficient or δ+.

Element

Electronegativity

C

2.5

F

4.0

Cl

3.0

Br

2.8

I

2.5

The δ+ carbon is then susceptible to attack by nucleophiles, i.e. ions or molecules with a lone pair of electrons which can donate the electron pair to another species, forming a dative covalent bond. When nucleophilic attack occurs, the carbon—halogen bond breaks and a halide ion is released. The nucleophile replaces the halogen atom in a nucleophilic substitution reaction, i.e.

Incidentally, the technical name for the mechanism between halogenoalkanes and hot, aqueous sodium hydroxide is a SN2 reaction, S because the reaction is a substitution, N because it is nucleophilic and 2 because the rate-determining step is bimolecular. More next year!

The rate of such reactions depends on the strength of the carbon—halogen bond (see table below). Fluoroalkanes are very unreactive, because the C—F bond is so strong; chloroalkanes are also fairly slow to react. Carbon—bromine bonds, however, are more easily broken so bromoalkanes react at a reasonable rate.

Bond

C—F

C—Cl

C—Br

C—I

Bond strength/ kJmol-1

484

338

276

238


Examples of nucleophilic substitution reactions

(i) hydrolysis with hydroxide ions

When primary halogenoalkanes are warmed with hot, aqueous sodium or potassium hydroxide, the primary halogenoalkane is hydrolysed and alcohols are formed, e.g.

CH3CH2Br  +  OH-          CH3CH2OH     +  Br-

                                                            ethanol

The hydroxide ion acts as a nucleophile, donating a pair of electrons to the carbon, forming a dative covalent bond. The reaction is called a hydrolysis because a similar reaction will occur without alkali present, when water acts as a nucleophile. Hydrolysis is the term for the decomposition of a substance by the action of water. The water is also decomposed:

CH3CH2Br  +  H2O   CH3CH2OH  +  HBr

As explained above, the rates of hydrolysis of primary halogenoalkanes with hot aqueous sodium hydroxide depends on the enthalpies of the carbon—halogen bonds.

The relative rates of reaction can be observed by adding aqueous ethanolic silver nitrate solution to the reaction mixture and timing the first appearance of the halide precipitate. This will form as soon as sufficient halide ions have been formed by the hydrolysis of the halogenoalkane. For example, chloroalkanes will react to form a white precipitate of silver chloride:

Ag+(eth/aq)  +  Cl-(eth,aq)   AgCl(s)

In the reaction between halogenobutanes and hot aqueous sodium hydroxide, 1-chlorobutane slowly produces a white precipitate of silver chloride; 1-bromobutane produces a cream precipitate of silver bromide rather more rapidly; and 1-iodobutane produces a yellow precipitate of silver iodide most rapidly.

 (ii) with cyanide ions

When halogenoalkanes are warmed with aqueous/alcoholic solutions of potassium cyanide, nitriles are formed, e.g.

CH3CH2Br  +  CN-           CH3CH2CN     +  Br-

                                                            propanenitrile

Nucleophilic substitution with cyanide ions adds an extra carbon atom to the chain.

(iii) with ammonia

When halogenoalkanes are warmed with an excess of ethanolic ammonia (i.e. a solution of ammonia dissolved in ethanol), primary amines are formed, e.g. ethylamine

CH3CH2Br  +  NH3          CH3CH2NH2     +  HBr

                                                            ethylamine

Since the acid HBr will immediately react with the base ammonia, this equation is better written as

CH3CH2Br  +  2NH3         CH3CH2NH2     +  NH4Br

 

The excess of ethanolic ammonia minimises the chance of further reaction of the primary amines to form secondary or tertiary amines or quaternary ammonium salts.

Reduction of nitriles, using hydrogen in the presence of a nickel catalyst, also forms primary amines.

Elimination reactions

Primary halogenoalkanes undergo nucleophilic substitution reactions with hot, aqueous solutions of sodium hydroxide to produce alcohols. However, primary halogenoalkanes react with hot, ethanolic solutions of sodium hydroxide via an elimination reaction to produce alkenes.

For example, bromopropane will eliminate hydrogen bromide when treated with hot, ethanolic sodium hydroxide to form propene:

CH3CH2CH2Br  +  NaOH(eth)    CH2=CH2CH3  +  NaBr  + H2O

The overall transformation involves the removal of the elements of hydrogen bromide by the hydroxide ion which is acting as a base (i.e. proton acceptor) with the simultaneous creation of a double bond:

The hydrogen bromide is neutralised by the alkali. Under these conditions, the rate of elimination is faster than the rate of nucleophilic substitution. At lower temperatures, the substitution reaction proceeds at a faster rate.

Competition between substitution and elimination

In the above formation of propene, the hydroxide ion is functioning as a base. This species is also nucleophilic so that substitution also takes place to give some propan-2-ol:

The relative importance of substitution and elimination depends on several factors:

·         the structure of the halogenoaIkane;

·         the base strength of the nucleophile;

·         the reaction conditions.


Primary halogenoaIkanes (RCH2X) give, predominantly substitution products whereas tertiary halogenoaIkanes (R3CX) generally favour elimination. Both substitution and elimination take place concurrently with secondary halogenoaIkanes (R2CHX). The likelihood of elimination increases as the base strength of the nucleophile increases. Higher reaction temperatures also lead to a greater proportion of elimination. In the case of sodium hydroxide, aqueous conditions usually favour substitution whereas solution in ethanol results in preferential elimination.

Elimination can be achieved, even with primary halogenoalkanes, via quaternary ammonium salts. Thus, for example, propene is formed when CH3CH2CH2N(CH3)3+Br- is heated with sodium hydroxide. In the decomposition, trimethylamine (N(CH3)3) is also liberated.

Depending on the structure of the halogenoalkane, more than one alkene may be formed. Thus, for example, elimination from 2-bromobutane produces both but-1-ene and but-2-ene. The latter compound can, of course, exist in two stereoisomeric forms (geometrical isomers). Similarly, elimination from 1-bromo-1-methylcyclohexane gives two alkenes. In this case, double-bond formation can occur both inside and outside the six-membered ring.

Where more than one alkene can be formed, the preferred product is the more stable structure. The stability of an alkene is related to the number of alkyl groups attached to the carbon atoms of the double bond; the more alkyl groups the greater the stability. For example, loss of HBr from 2-bromo-2-­methylbutane produces approximately 80% of 2-methyl­but-2-ene and 20% of 2-methylbut-1-ene.

Uses of halogeno- compounds

Halogenoalkanes are useful synthetic intermediates in the manufacture of pharmaceutical compounds (e.g. ibuprofen). Chloroethene and tetrafluoroethene are used to manufacture polymers (PVC and PTFE). Chlorofluorocarbons (CFCs) such as CF2Cl2 and CFCl3 have been used as refrigerants, aerosol propellants and blowing agents for in foams such as expanded polystyrene. Other CFCs such as CCl2FCClF2 are used as dry cleaning solvents and degreasing agents for printed circuit boards. A bromochlorofluoroalkane, CBrClF2 (or BCF) is used as a fire-fighting compound as it readily undergoes homolytic fission to produce radicals that interfere with combustion reactions.

 

Chlorofluorocarbons (CFCs) and the environment

CFCs were used for decades as refrigerants in air-conditioners and domestic fridges. However, they are responsible for the thinning of the protective ozone layer in the stratosphere. Ozone (O3) absorbs harmful ultraviolet radiation and protects us from skin cancer. The high stability of CFCs enabled high concentrations to build up in the atmosphere. When they reach the stratosphere, CFCs absorb ultraviolet light and undergo photodissociation (i.e. homolytic fission of the carbon-chlorine bonds), forming very reactive chlorine radicals, e.g.:

CF2Cl2(g)     CF2Cl•(g)  +  Cl•(g)

The chlorine radicals catalyse the decomposition of ozone to oxygen:

2O3(g)  3O2(g)

Once chemists were aware that the mechanism of the degradation of the ozone layer involved chlorine radicals from CFCs, many CFCs were banned from use. Other substances such CF3CH2F have since been developed as an ozone-friendly replacement.