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

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Part 3. The chemistry of HALOGENOALKANES

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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)


(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 chloroalkanes, 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 for the same number of carbon atoms in the molecule.

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

This argument applies to any series of halogenoalkanes.

 

The boiling point for a given haloalkane molecular formula is lower the greater the carbon chain branching.

The molecule becomes more compact, reducing the intermolecular 'contact' forces, reducing the enthalpy of vapourisation hence reducing the boiling point.

Bpt. trend: CH3CH2CH2CH2-Cl  78oC >  (CH3)2CHCH2-Cl  68 >  CH3CH2CHClCH3  67oC >  (CH3)3C-Cl 51oC

 

The more halogen atoms in the molecule the higher the boiling point.

The main reason for this is the increase in electron clouds involved with creating the instantaneous dipole - induced dipole intermolecular forces.

The boiling points of the chloromethanes rise steadily with increase in substitution.

Bpt. trend: CH3Cl -24oC  <  CH2Cl2 40oC  <  CHCl3 61oC  <  CCl4 77oC


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

Therefore halogenoalkanes are immiscible with water.

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.

Halogenoalkanes will dissolve a wide range of organic compounds.


(3) Hazards associated with halogenoalkanes

There is growing evidence that many haloalkanes and other halogen compounds are harmful and potentially toxic if fumes breathed in or ingested.

molecular structure 1,1,1-trichloroethane 1,1,2-trichloroethane positional structural isomers of C2H3Cl3 diagram images

1,1,1-trichloroethane (above), trichloromethane, CHCl3, and tetrachloromethane, CCl4, were used as dry cleaning solvents to remove grease from clothing and as a thinner in correcting fluid in typing.

Both of these chloroalkanes are now banned from this type of use due to their toxicity.

The destruction of the ozone layer by halogenoalkanes is on a separate page.


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(4) A selection of the uses of halogenoalkanes

Halogenoalkanes (haloalkanes) have/had many applications e.g. aerosol propellants, anaesthetics, insecticides, refrigerant gases, solvents and intermediates in the synthesis of PVC.

However, as already mentioned in section (3), their use is becoming increasingly limited because of their potential and known harmful poisonous effects on the body.

Anaesthetics

Anaesthetics are used to induce temporary loss of consciousness during operations that would otherwise be painful. Many modern anaesthetics are halogenated hydrocarbons and three are quoted below.

anaesthetics halothane isoflurane enflurane molecular structure skeletal formula boiling point molecular mass advanced A level organic chemistry doc brown

F3C-CHBrCl, common name halothane/fluothane, 2-bromo-2-chloro-1,1,1-trifluoroethane (Mr 197, bpt. 50oC)

The C-H carbon atom is chiral, giving rise to R/S isomers.

It is unstable in the presence of light and stored in dark glass bottles.

F3C-CHCl-O-CHF2, (Mr 184, bpt 49oC), isoflurane (isomeric with enflurane)

The C-Cl carbon atom is chiral, giving rise to R/S isomers.

ClFCH-CHCl-O-CHF2, (Mr 184, bpt. 57oC), enflurane, now withdrawn from use due to dangerous cardiac side-effects.

The C-Cl carbon atom is chiral, giving rise to R/S isomers.

Isoflurane and enflurane can be considered haloalkanes with a difluoromethoxy substituent group too, so they can also be classified as halogenated ethers - based on methoxyethane.

Despite the relative high molecular masses, they are quite volatile liquids and readily evaporate and mix with the respiratory system of the patient during the operation.

The intermolecular forces are quite weak between the molecules - mostly due to the instantaneous dipole - induced dipole forces. Overall the molecule, are not sufficiently polar to add significant permanent dipole - permanent dipole forces to the overall intermolecular forces.

Unlike previous anaesthetics like ethoxyethane ('ether', CH3CH2OCH2CH3) ...

... they are not flammable!, because most the combustible hydrogen atoms have been replaced by 'non-combustible' halogen atoms,

... they have a very low solubility in water, although polar molecules, the intermolecular force interaction with water is not sufficient to overcome the hydrogen bonding between water molecules.

but, like ether, they are chemically very stable molecules and relatively inert in the context of their use in the human body and, ALL obviously, must not be harmful to the patient.

They are potential ozone destroying gases if released into the atmosphere because the C-Cl bond, is the weakest bond in two of the molecules, can be broken by uv photons in the upper atmosphere.

R3C-Cl  == uv  ==>  R3C•  +  Cl•

It the anaesthetic molecules reach the upper atmosphere, they can contribute to ozone destruction. See detailed atmospheric ozone chemistry notes.

However, this is partly compensated by the C-H bond which can be attacked by radicals in the lower atmosphere, so degrading the anaesthetic molecules.

Bond enthalpies kJ/mol: C-F 484,  C-H 412,  C-O 360,  C-C 348,  C-Cl 338,  C-Br 276

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

Chloroalkenes

Tetrachloroethene, Cl2C=CCl2, is used as a dry cleaning agent, a grease solvent less toxic than the solvents mentioned in section (3).

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.

The colourless gas chloromethane (CH3Cl, bpt. -24oC) is used as a solvent in the manufacture of rubber, but most of it is used to manufacture silicones, whose uses range from bathroom sealants to artificial body parts.

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

Fire extinguishing agents

BCF, bromochlorodifluoromethane (halon-1211), CBrClF2, is very effective in extinguishing fires.

BCF releases bromine radicals into the combustion zone of flames and inhibit the free radical combustion reactions.

CBrF3, bromotrifluoromethane (Halon-1301) is also an effective fire extinguishing agent.

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

For more details and examples see

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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