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Advanced Level Organic Chemistry: Halogenoalkanes: physical properties, hazards, uses

Part 3. The chemistry of HALOGENOALKANES

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

Part 3.8 The physical properties, hazards and uses of halogenoalkanes

Sub-index for this page

(1) Boiling points of halogenoalkanes & intermolecular forces

(2) The solubility of halogenoalkanes & intermolecular forces

(3) Hazards associated with halogenoalkanes

(4) A selection of the uses of halogenoalkanes

Abbreviations used: mpt = melting point and bpt = boiling point (oC or K units will be quoted)

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(1) The boiling point of halogenoalkanes and intermolecular forces

(Intermolecular forces and physical properties of halogenoalkanes)

Chloromethane (CH3Cl) and chloroethane (CH3CH2Cl) are gases at room temperature (25oC).

Higher chloro-alkane molecules are liquids (graph of the boiling points of the homologous series of 1-chloroalkanes CnH2n+1Cl is shown below (carbon number n = 1, 2, 3, ...).

For the other halogenoalkane homologous series:

Bromomethane (CH3Br) is a gas at room temperature, but higher bromo–alkanes are liquids.

All iodo-alkanes (CnH2n+1I) are liquids at room temperature.

The boiling point trend of 1-chloroalkanes are now discussed in detail.

Graph 1 green line = 1-chloroalkanes

The red line graph shows the boiling point of alkanes from methane CH4 (boiling point -164oC/109 K)  to tetradecane C14H30 (boiling point 254oC/527 K). [Remember K = oC + 273]

Note: The red line represents linear alkanes in all the graphs 1-3 and is a useful baseline to compare the intermolecular bonding present in other homologous series of non-cyclic aliphatic compounds.

For the 'green line' of 1-chloroalkanes, the graph goes from chloromethane (bpt -24oC/249 K) to 1-chlorodecane (bpt 223oC/496 K)

A plot of number of electrons in any molecule of a homologous series versus its boiling point (K) shows a steady rise with a gradually decreasing gradient.

I consider this the best for comparison of the effects of intermolecular bonding between different functional groups.

I think Graph 1 is the best graph to look at the relative effects on intermolecular forces (intermolecular bonding) on boiling point because it is the distortion of the electron clouds (e.g. in non-polar alkanes), that gives rise to these, weak, but not insignificant forces, known as instantaneous dipole - induced dipole forces.

Halogenoalkanes have a weakly polar Cδ+–Xδ bond (X = halogen) due to the difference in electronegativities (Pauling values) of carbon and halogens, e.g. Cl(3.0) > C(2.5) giving Cδ+–Clδ.

This gives rise to a weak, but permanent dipole, hence the extra permanent dipole – permanent dipole intermolecular attractive forces raising the boiling point very slightly compared to alkanes with the same number of electrons.

BUT the effect is quite small, so, for chloroethane, despite the C–Cl polar bond, almost all the intermolecular attraction arises from instantaneous dipoles – induced dipoles.

attractions

Total intermolecular force = (instantaneous dipole – induced dipole) + (permanent dipole – permanent dipole) + (permanent dipole – induced dipole)

From Graph 1 you can see the effect of the permanently polar carbon - halogen bond (e.g. Cδ+-Clδ-) is quite a minor effect, despite the fact that permanent dipole - permanent dipole attractive forces will exist between halogenoalkane molecules.

You are comparing the red line (linear alkanes) with the green line (linear 1-chloroalkanes).

For a broader discussion see on boiling points and intermolecular forces see:

Introduction to Intermolecular Forces

Detailed comparative discussion of boiling points of 8 organic molecules

Boiling point plots for six organic homologous series

 

Graph 2 green line = 1-chloroalkanes

A plot of the molecular mass of the 1-chloroalkane molecules versus its boiling point (K) shows a steady rise with a gradually decreasing gradient.

 

Graph 3 green line = 1-chloroalkanes

A plot of the carbon number of the 1-chloroalkane molecules versus its boiling point (K) shows a steady rise with a gradually decreasing gradient.

For the same carbon number, the 1-chloroalkanes have significantly higher boiling points than alkanes, mainly due to the extra electrons from the chlorine atom - more electron clouds can be distorted, increasing the instantaneous dipole - induced dipole forces.

The increase in intermolecular attractive forces, means the molecules need a higher kinetic energy to escape from the liquid surface i.e. have a higher boiling point.

However, the effect of the C-X polar bond is minimal (see discussion for graph 1).

This argument applies to any series of halogenoalkanes.


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(2) The solubility of halogenoalkanes and intermolecular forces

(Intermolecular forces and physical properties of halogenoalkanes continued)

Halogenoalkanes usually insoluble in water - tiny traces if at all.

Although they are very weakly, albeit permanently, polar molecules, but they do not have a sufficiently partially positive hydrogen atom (Hδ+) to hydrogen bond with water molecules and disrupt the strong hydrogen bonding between water molecules.

However, they will dissolve in most organic solvents like hexane, ethanol, ethoxyethane ('ether') where the solute-solute, solute-solvent and solvent-solvent intermolecular forces are of a similar magnitude.


(3) Hazards associated with halogenoalkanes

Many haloalkanes and other halogen compounds are toxic if fumes breathed in or ingested.


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(4) A selection of the uses of halogenoalkanes

CFC/CFCs = chlorofluorocarbon; HCFC/HCFCs = hydrochlorofluorocarbon; HFC/HFCs = hydrofluorocarbon

 

Bromomethane, CH3Br (methyl bromide) is a controversial fumigant for killing pathogens.

 

Chloroalkanes and useful solvents in the laboratory or industry e.g. for removing grease from metal plates before electroplating.

e.g. 1,1,2-trichloroethane Cl2CH-CH2Cl, (sometimes referred to as 'trichloroethane' or just 'trichlor')

and CHCl3 trichloromethane.

However, they are still quite volatile and chlorohydrocarbon vapours can be harmful if breathed in.

 

Halogenoalkanes are used as refrigerant gases and aerosol propellants.

They have the advantage of being chemically inert, non-toxic and non-flammable.

Unfortunately, on escaping into the atmosphere they cause a major environmental problem by destroying ozone in the upper layer.

Originally these where CFCs like the molecule CCl2F2

CFCs are being replaced by less harmful HCFCs and HFCs

e.g. a HCFC is CHClF2 chlorodifluoromethane and a HFC is difluoromethane CH2F2

The chemistry of ozone depletion and how this environmental problem was partially solved

 

Halogenoalkanes are used as flame retardants

Bromoalkanes are quite effective flame retardants that can be added to combustible materials to make them less flammable when exposed to a source of ignition.

 

Haloalkanes are used in making aromatic hydrocarbons acting as intermediate compounds

e.g. benzene  +  chloromethane  == AlCl3 catalyst ==>  methylbenzene  +  hydrogen chloride

(c) doc b  +  CH3Cl  ====>  (c) doc b  +  HCl

Alkylation to give alkyl-aromatic hydrocarbons like methylbenzene [Friedel-Crafts reaction]

 


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Doc Brown's Advanced Level Chemistry Revision Notes

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HALOGENOALKANES chemistry notes INDEX

All Advanced A Level Organic Chemistry Notes

Index of basic Oil and Organic Chemistry Revision Notes

 

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