Halogenoalkanes
From your syllabus:
Learning outcomes Candidates should be able to:
(a) recall the chemistry of halogenoalkanes as exemplified by
(i) the following nucleophilic substitution reactions of bromoethane:
-
- hydrolysis
- formation of nitriles
- formation of primary amines by reaction with ammonia
(ii) the elimination of hydrogen bromide from 2-bromopropane
(b) *describe the mechanism of nucleophilic substitution (by both SN1 and SN2 mechanisms) in halogenoalkanes
(c) interpret the different reactivities of halogenoalkanes and chlorobenzene (with particular reference to hydrolysis and to the relative strengths of the C-Hal bonds)
(d) explain the uses of fluoroalkanes and fluorohalogenoalkanes in terms of their relative chemical inertness
(e) recognise the concern about the effect of chlorofluoroalkanes on the ozone layer
Halogenoalkanes
Halogenoalkanes are compounds in which one or more hydrogen atoms in an alkane have been replaced by halogen atoms (fluorine, chlorine, bromine or iodine). We will only study only compounds with one halogen atom
They are also called as haloalkanes or alkyl halides
Classification of haloalkanes
Haloalkanes can be classified depending on where the halogen atom is in the carbon chain.
We have Primary, secondary and tertiary haloalkanes
In the primary haloalkanes, the halogen is attached to the carbon at the end of the chain. This carbon may be attached to another alkyl group. (Chloromethane is a primary haloalkane)
Secondary haloalkanes: The halogen is attached to a carbon with two alkyl groups which can be equal or different.
Tertiary haloalkanes: The halogen is bonded to a carbon that has three alkyl groups attached.
Physical Properties of Haloalkanes
- Almost all haloalkanes and halogenoarenes (which have the halogen attached to a benzene ring), are volatile liquids that do not mix or dissolve in water.
- They are soluble in organic non polar liquids since their molecule is almost non polar. You can understand this since water molecules have hydrogen bonds and haloalkanes are slightly polar. The only intermolecular forces present in the carbon chain are Van der Waals (London Dispersion) forces, and the only polarity is present in the HALOGEN-CARBON bond. You can see that the chloromethane, Bromomethane and Chloroethane are gases. the rest are liquids at room temperature.
- The boiling point of the isomers decreases from the primary haloalkane to the secondary and the tertiary.
Chemical Properties of Haloalkanes
The chemical reactions that haloalkanes undergo are SUBSTITUTION REACTIONS where the halogen is replaced by something else.
The CARBON-HALOGEN bond breaks and the halogen keeps the electrons of the bond producing a carbocation.
The following table shows the C-X Bond length and enthalpy for each CARBON-HALOGEN
| BOND | BOND LENGTH
(pm) |
BOND ENTHALPY
(kJ mol-1) |
| C-F | 135 | 488 |
| C-Cl | 177 | 330 |
| C-Br | 194 | 288 |
| C-I | 214 | 216 |
As shown in the table above, the bond enthalpy decrease down the group, so it is easier to break the C-I bond than to break the C-F bond.
If you compare the electronegativities of the haloalkanes, you will think that the best forming carbocation would be produced by the fluorine, but the bond enthalpy prevails and the easiest carbocation will be formed when Iodine is attached to the carbon atom.
Reactions:
- Substitution reactions
- hydrolysis
- formation of nitriles
- formation of primary amines by reaction with ammonia
- Elimination reactions
- the elimination of hydrogen bromide from 2-bromopropane
HYDROLYSIS
This is the direct replacement of the bromine atom in the molecule with a hydroxide (-OH) group, giving a bromide ion.
This can be achieved in a variety of ways. Water is the main reactant needed, to provide the -OH group. Some sort of acidic catalyst is also needed to help remove the halogen atom –
(i) either an alkali (NaOH, KOH, etc.), which provides extra -OH ions (very common)
(ii) or silver nitrate solution, which reacts with the halide ions produced, removing them from solution, as a silver halide precipitate, and pushing the reaction equilibrium over to the products.
CH3CH2Br + -OH → CH3CH2OH + Br-
FORMATION OF NITRILES
Reaction with cyanide ions –
This is the direct replacement of the bromine atom with a nitrile group (-CN), giving a bromide ion.
The reactant is either hydrogen cyanide (HCN) or more likely an acidified solution of an alkali metal cyanide salt (e.g. NaCN, KCN ) in an alcoholic solvent.
CH3CH2Br + -CN → CH3CH2CN + Br-
FORMATION OF PRIMARY AMINES
Reaction with ammonia –
This is the direct replacement of the bromine atom with an amine group (-NH2) (see amines in chains and rings and spectroscopy), giving hydrogen bromide.
The reactant is ammonia at high pressure in sealed vessel.
CH3CH2Br + NH3 → CH3CH2NH2 + HBr
ELIMINATION OF HYDROGEN BROMIDE FROM 2-BROMOPROPANE
Reactions of 2-Bromopropane :
Elimination
This type of reaction involves the removal of a group of atoms from a compound, giving two neutral compounds.
In this case it is the removal of hydrogen bromide (HBr).
The reactants are virtually the same as with the hydrolysis of bromoethane, i.e. alcoholic alkali(aq) – with the added condition of reflux (i.e. heat to boiling).
CH3CHBrCH3 → CH3CH=CH2 + HBr
There is always a competition between the substitution and elimination reactions. The refluxing pushes the reaction over to elimination.
Haloalkanes – Reactions mechanisms
There are only two reaction mechanisms that concern us here. They are both variants on the theme of nucleophilic substitution –nucleo = nucleus, centre of +ve charge, region of low electron density
philic = attraction
substitution = the direct replacement of an atom, or group of atoms, with another atom or group of atoms
There is one mechanism for 1° haloalkanes and another slightly different one for 3° haloalkanes. They both depend on the polarisation of the carbon-halogen bond present in the molecule.
2. 1° haloalkanes : SN2.
This mechanism involves only one stage – the simultaneous attacking by the nucleophile and expulsion of the halogen atom from the molecule.
e.g. hydrolysis and reaction of cyanide ions and ammonia with bromoethane.
Two molecules are involved in the rate determining step (in fact the only step in the reaction) and therefore the mechanism is called Substitution Nucleophilic 2 or SN2.
2. 3° haloalkanes :
This variant mechanism involves two separate parts. Firstly, the breaking of the C-X bond and then theformation of a new C-Nu bond.
e.g. hydrolysis and reaction of cyanide ions and ammonia with 2-bromo-2-methylpropane.
One molecule is involved in the rate determining step and therefore the mechanism is labeled SN1. (SubstitutionNucleophilic1).
Haloalkanes – Reaction rates
(1) 1° vs. 2° vs. 3° haloalkanes :
The rate of reaction of 1°, 2° and 3° haloalkanes depends on the mechanism followed by the particular compound.The rate of any reaction can depend on a number of factors including heat and pressure. Another important factor is the ability for molecules to collide with one another in order to start a reaction.With 1° haloalkanes the mechanism followed is SN2, because the carbon-halogen bond can be attacked by the hydroxide ion,
Bromoethane 
CH3CH2Br + -OH → CH3CH2OH + Br-
With the 3° haloalkane the central carbon atom is hidden by the surrounding methyl groups,
2-bromo-2-methylpropane 
this prevents the hydroxide ion from attacking directly.
As previously mentioned SN2 depends on two molecules colliding with one another before any reaction can occur. This is inherently a slow process and depends on a chance occurrence.
3° haloalkanes on the other hand follow an SN1 pathway since the carbocation formed is stabilised by the alkyl groups attached to the central carbon atom. The 1° haloalkane does not form a stable carbocation so does not follow SN1.
SN1 relies on the spontaneous splitting apart of a single molecule. Whilst this itself is not particularly fast it is a lot faster than the chance collision of two molecules in SN2.
Therefore, 3° haloalkanes react a lot faster than 1° haloalkanes.
2° halo compounds follow a mixture of the two reaction mechanisms and therefore are faster than the 1° compounds but slower than the 3° compounds.
(2) -F vs. -Cl vs. -Br vs. -I :
The relative rate of reaction of the various halogen compounds depends on the strength and polarisation of the C-halogen bond.The average bond energies for the four types of C-halogen bond are –| C-F = | 467 kJmol-1 |
| C-Cl = | 346 kJmol-1 |
| C-Br = | 290 kJmol-1 |
| C-I = | 228 kJmol-1 |
(3) 2° chloroalkanes vs chlorobenzene :
Whilst technically chlorobenzene is a 2° halo compound, because there are two carbon atoms attached to the C-Cl group, it reacts in a totally different manner.The aromatic benzene ring prevents the normal substitution reactions that would occur with normal 2° haloalkanes. The C-Cl bond is a lot stronger than normal partly because of the increased overlap with the p-orbitals of the ring.Therefore chloroform will not undergo hydrolysis or react with ammonia or cyanide ions as any other 2° haloalkane will.