HALOALKANE Properties

 

Introduction

 

 

A

s has been shown earlier in the general introduction to haloalkanes, the halogen atom is what gives these compounds ‘reactivity’. The carbon on which the halogen is attached is electron deficient and is therefore very suitable for attack by nucleophiles – assuming of course that there is nothing to hinder the approaching species. The alpha hydrogens have also been ‘activated’ by inductive effects that is to say that the bonds moving away from the heteroatom have become deficient in electron density as well and this results in the so-called alpha hydrogens being particularly prone to attack by positive seeking (nucleophilic) species. The electron density has been channeled away from the alpha hydrogens towards the halogen atom through the connecting bonds.

The chemistry of haloalkanes is in fact generally based on these electron density shifts within these molecules, coupled with the fact that halogens are quite content to exist as ‘free’ halide ions (- after all the sea has had, for eons, a substantial amount of halide ions!). Halogen atoms, in chemical parlance, are said to be good ‘leaving groups’. All that remains therefore is a bit of push from the electron-rich nucleophiles either directly onto the carbon bearing the halogen (resulting in a substitution reaction) or via attack onto the alpha hydrogens. The abstracted alpha hydrogens readily transfer their bonding electrons to the carbon to which the heteroatom is attached (and this is precisely the direction in which the electron density has been shifted by the halogen atom anyway), releasing the halogen and forming an alkene in an elimination reaction. These are the reactions that will be described henceforth.

We must also remember that reactivity will depend on the ease with which the carbon to halogen bond breaks and this follows the order R-I is weaker than R-Br which is also weaker than R-Cl and the strongest being R-F. The reactivities are therefore

 

R-I    >    R-Br    >    R-Cl    >>    R-F (with the RF being extremely stable).

 

The nature of the haloalkanes also contributes to how fast the reaction will occur – e.g. when a tertiary haloalkanes reacts, the three electron-pushing alkyl groups stabilize the carbocation that may form as an intermediate species in the reaction. Thus

 

3O  R-X   >    2O  R-X     >    1O R-X

 

 

Similarly, in generally               Weak bases tend to give Substitutions while

                                       Strong bases favour eliminations

 

 

Chemical Properties

 

A.                                           Formation of Alkanes (RH)

i)                                          Reduction

 

Subjecting the haloalkanes to a reducing medium such as Zn + conc HCl,  Zn/Cu couple + ethanol, H2/Raney Ni etc will give a direct replacement of the halogen with an hydrogen atom.

 

 

 

ii)                                       Reaction with metallic sodium - Wurtz Reaction

 

Heating the haloalkanes with metallic sodium in an inert solvent gives alkanes which have an extended chain. (Wurtz reaction)

 

 

 

iii)                                    Reaction via Grignard

 

Haloalkanes are first made to give the Grignard reagent, which is then decomposed by reaction with water. Such a process changes a haloalkane into an alkane:

 

 

 

B.                                           Formation of Alkenes (R’CH=CHR)

 

When an elimination of hydrogen halide is desired, the haloalkane must be treated with a strong base such as the alkoxide (e.g. ethoxide, C2H5O-) and the mixture heated. The alkoxide is either obtained by dissolving KOH in alcohol, KOH(alc) or alcoholic KOH, or through dissolving metallic sodium (or potassium) in an alcohol.

 

Secondary and tertiary haloalkanes give elimination reactions in good yields. However, for the smaller primary haloalkanes (1-2 carbons) and with the smaller alkoxides (1-2 carbons), substitution reactions are preferred.

For the halomethane, elimination is not even possible and therefore substitution is the only route available. With haloethane, particularly when the alkoxide is the ethoxide (C2H5O-), substitution is also the main route followed but for the other haloalkanes, under these conditions, elimination reactions are preferred. These reactions are summarized in the following scheme:

 

 

 

C.                                          Formation of Alcohols (ROH)

 

Hydrolysis of haloalkanes gives the alcohols in good yields provided that the hydrolyzing agent is not a strong enough base to give, alongside, the competing elimination reactions. To minimize this side reaction, a weak base, in the form of dilute aqueous sodium hydroxide, is used. (For the more reactive tertiary haloalkanes, water is enough to bring about this hydrolysis!).

 

 

The general reaction being represented by the following equation

 

 

As before, the ease with which this reaction occurs depends on the C-X bond strength, thus

 

R-I  >  R-Br  > R-Cl  >>  R-F

 

Mechanism of reaction:

 

Two types of mechanisms have been recognized for these hydrolyses and the distinction comes mainly from an investigation of the kinetics of the reactions. This looks at how changes in the concentrations of the reagents affect the rate at which the reaction occurs. Essentially, what one does here is to start off with fixed but known concentrations of reagents and the rate of change of one of the reagents, after mixing, is monitored (by titration, gas pressure, colour changes, conductivity, pH etc). The concentrations of the reagents are then changed, one at a time, and the rate of reaction monitored as before.

When the haloalkane is primary, the rate of reaction is dependent both on the amount, or concentration, of the haloalkane as well as the amount of hydroxide present (see table). For tertiary haloalkanes, reaction appears to be totally independent of how much hydroxide is present.

 

1O RX (Mol.dm-3)

OH- (Mol. dm-3)

Relative Rate

3O RX (Mol.dm-3)

OH- (M Mol.dm-3)

Relative Rate

1

1

1

1

1

1

2

1

2

2

1

2

1

2

2

1

2

1

 

These results clearly indicate that the mechanism by which the substitution occurs is different for the two types of haloalkanes used. (The secondary haloalkanes react in ways that are either similar to the primary or the tertiary). Any mechanism proposed has to be consistent with kinetics data i.e. somehow, for the primary compounds, the two components are involved in steps leading to, and including, the slow rate determining step. In tertiary haloalkanes, the slow step involves only one species – the haloalkane!

The following mechanisms have been proposed and are now generally accepted :

 

 

The hydroxide ion, in a slow, rate-determining step, approaches the haloalkane from the opposite side to where the halogen atom is (because of repulsions!). As the OH- approaches the electron deficient carbon, the C-X bond lengthens.

 

A very unstable, intermediate, structure is reached where the leaving halogen X and the incoming hydroxide OH- are equidistant from the carbon centre – both bonds are longer than normal. As soon as this stage is reached, in a fast process, the halogen bond breaks.

 

A normal C-OH is now formed and the X- is released. The three-dimensional arrangement of the carbon is again tetrahedral but the structure flips inside out (- very much like an umbrella in strong wind!)

[Only one isomer forms]

 

 

 

 

 

In an unmediated event, the halogen ion, X-, leaves the tertiary haloalkane in a slow reaction step. (The reason for this happening is that the three electron-pushing alkyl groups stabilize the carbocation that results.)

Once formed, this highly reactive carbocation is readily attacked by any nucleophilic species in the surrounding environment – and this is just as likely to be water as OH-. (If water acts as the nucleophile it soon deprotonates and this is then effectively an OH-). Attack can be from either side.

 

Since the nucleophile can approach the carbocation from two opposite sides, a mixture of (potentially) optically active isomers is possible,

[An optically inactive, racemic mixture is obtained if a pure form is used initially

 

Both mechanisms involve Nucleophilic Substitutions and are denoted by SN. The molecularity of the reaction is then quoted and this is the number of species that are known to be involved in the rate-determining step. For primary haloalkanes, the reactions are said to proceed by a bimolecular nucleophilic substitution or SN2 mechanism whereas the tertiary haloalkanes react by a unimolecular nucleophilic substitution or SN1 mechanism.

 

Since reaction of 3O RX depends only on the presence of RX, it is not surprising that in relative reaction rates,

3O RX  >  2O RX > 1O RX

 

 

 

 

 

D.                                          Formation of Esters (RCOOR’)

 

If the reacting nucleophile is a carboxylate  (RCOO-) ion, the haloalkane is converted to an ester.

For reaction to occur, the carboxylate will need to be heated with the haloalkane in some inert solvent.

 

 

i.e. in general :

 

 

 

 

E.                                           Formation of Ethers (R-O-R)

 

The competing reactions occurring when haloalkanes are treated with alkoxides have already been mentioned when treating the formation of alkenes (see above). It was stated then that the reaction depends on the nature of the haloalkane as well as that of the alkoxide. This is still relevant here! One will have to consider carefully what to put together to get the desired reaction to take place. Thus if methoxypropane (CH3OCH2CH2CH3) is required, it would be by far better to react a propoxide with halomethane than a methoxide with a halopropane. Similarly, primary haloalkanes are better at giving substitution reactions than either the secondary or (even less so) the tertiary. Often other methods are better at formation of these compounds e.g. dehydration of alcohols with conc. H2SO4 (see later chapter on ethers).

 

 

 

The reaction leading to ether formation is nevertheless an important one as diethyl ether can readily be prepared in this fashion in a process known as the Williamson`s ether synthesis

 

 

 

F.                                           Formation of Nilriles (R-CN)

 

The cyanide ion is a good nucleophile – good enough in fact to attack the electron deficient carbon on which the halo-atom resides and substitutes the halogen. The reaction is generally carried out in a mixture of water, for the ionic cyanide, and either alcohol or propanone for the haloalkane. (The reaction will need to be heated to bring about the desired change.)

 

 

 

G.                                          Formation of Amines (R-NH2)

Another potent nucleophile is ammonia and it does not come as a surprise to find that ammonia reacts readily with haloalkanes to give, in the first instance, unsubstituted amines. The latter, having an electron pushing alkyl group are now better nucleophiles and hence, further substitution take place more readily  -  providing, of course, that the alkyl groups are not so bulky that they block the approach of the reacting species.

 

Remember that in non-aqueous solvents, the trisubstituted amines are stronger bases than the disubstituted amines and these are in turn better than the primary amines, themselves better than ammonia. In water however, the trisubstituted amines, lacking the all-important H attached to the electronegative nitrogen substantially lowers the H-bonding capabilities with water, and a consequence of this is that the trisubstituted amines take second place in nucleophilicity (basic character) ranking. Thus

 

Strength of bases :

 

In non-aqueous media  :      3O amines (R3N)  >  2O amines (R2NH)  >  1O amine (RNH2)  >  ammonia (NH3)

 

In aqueous media   :            2O amines (R2NH)  >  3O amines (R3N)  >  1O amine (RNH2)  >  ammonia (NH3)

 

 

 

H.                                          Formation of Grignard reagents (R-Mg-X)

 

An unusual, but extremely useful, reaction that haloalkanes undergo is what has often been termed the magnesium insertion, or Grignard, reaction.

 

 

Polar covalent bonding involving a metal such as magnesium results in compounds with electron-rich carbon atoms which are susceptible to electrophilic attack. (The parent haloalkane has a carbon that is a likely candidate for nucleophilic attack. In the grignard reagent, that very same carbon is now open to attack by electrophiles.)

 

 

 

I.                                            Formation of Nitroalkanes (R-NO2)

 

Under the right conditions, the nitrite ion from silver nitrite can be made to substitute a halogen atom from a primary haloalkane to yield the nitroalkane (and silver halide) although some alkyl nitrite is also produced.

 

 

J.                                            Reaction with Ag2O

 

i)                                          Moist Ag2O : Formation of Alcohols (R-OH)

 

Boiling a suspension of silver oxide in water with a haloalkane present converts the haloalkane to the corresponding alcohol (and silver halide). (As usual RI > RBr > RCl >>RF)

 

 

ii)                                       Dry Ag2O : Formation of Ethers (R-O-R)

 

Vigorous shaking of a haloalkane with dry silver oxide produces an ether and precipitates out silver halide.

 

 

 

 

DIHALOALKANES.

 

For the most part, dihaloalkanes behave as double ordinary or simple (mono)haloalkanes except that in some instances, secondary reactions can follow. To begin with, the vicinal dihalides, i.e. those dihalides where the halogen atoms are on adjacent carbons (such as the products obtained when alkenes react with alkenes), have the same chemistry as the monohalogenated alkanes (twice over) except for the Grignard reaction. Reaction with magnesium brings about the release of magnesium dihalide, MgX2, and an alkene. (Zinc powder behaves similarly with these compounds).

 

 

 

The geminal dihalides, those dihalides with two halogen atoms on the same carbon, similarly behave as though they were simple haloalkanes except that when these are hydrolysed, the geminal diols that form are unstable and rearrange to give the more stable carbonyl compounds. 

 

 

 

 

 

Tests and identification

 

Classically, chemical reactions aimed at showing up the presence of halogens in a compound, depended mainly on the sodium fusion, or Lassaigne`s, Test. This involves literally fusing a small sample of metallic sodium with the unknown compound in a crucible. (In this process, any nitrogen present is converted to inorganic cyanide, sulphur gives sulphides and the halogens simply give ionic halides). When the mixture cools to room temperature, water is added to it and a number of tests carried out. The presence of cyanide is shown by first adding NaOH and Fe2+(aq) and warming, a mixture that produces hexacyanoferrate(II) when CN- ions are present, and this gives a deep blue complex with FeCl3. The sulphides are tested with sodium nitroprusside (nitrosopentacyanoferrate(III)) that gives a purple colour with S2-. The halogens are then checked but before this can be done, any cyanides and sulphides will have to be removed by boiling the mixture with nitric acid for some time. Addition of silver nitrate to the resulting mixture shows up the presence of halogen ions by way of precipitates:

If Cl- is present a white precipitate of AgCl, soluble in dilute aqueous ammonia, is obtained.

If Br- is found in the mixture a pale yellow precipitate of AgBr, soluble in excess concentrated aqueous ammonia, is seen.

If I- is present in the mixture a yellow precipitate of AgI, insoluble in ammonia, will form. 

 

A simple method would be to take the unknown, warm it up with dilute aqueous NaOH for some time (to bring about hydrolysis of the haloalkanes, if present). The solution is then acidified with nitric acid and silver nitrate solution is added. Any precipitates obtained will have to be compared with the standard silver halides i.e. for colour and solubility in ammonia.

 

 

 

 

 

 

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