Chemical Properties of Haloalkanes
Haloalkanes are highly reactive compounds and undergo a number of reactions such as nucleophilic substitution reaction, elimination reaction, reactions with metals etc.
Nucleophilic Substitution Reactions
The halogen atom in haloalkanes is more electronegative than the carbon atom attached to it. As a result, haloalkanes are polar i.e., the carbon atom acquires a partial positive charge and the halogen atom acquires a partial negative charge.
Because of the presence of this partial positive charge on the carbon atom, the carbon atom is prone to getting attacked by nucleophilic reagents (electron rich species). When a strong nucleophile attacks this carbon atom, a new bond between the carbon atom and the incoming nucleophile is formed and the halide ion gets removed.
Nucleophilic substitution reaction can take place in two ways :
- SN1 (Substitution, nucleophilic, unimolecular)
- SN2 (Substitution, nucleophilic, bimolecular)
In SN1 reaction, the alkyl halide dissociates first to give carbocation and halide ion in a slow process.
The carbocation being unstable is highly reactive and immediately reacts with the nucleophile. This happens very fast.
Rate of SN1 reaction
The second step in SN1 reaction is too fast to affect the rate of reaction. Thus, the rate of reaction only depends on step 1 (slow step). Since only one reactant participates in slow step, the order is 1 (unimolar) and the rate is K[R−X].
The rate of SN1 reaction = K[R−X]
In SN2 reaction, the nucleophile attacks the carbon atom from back side of the leaving group (in this case : halide ion). The intermediate state is formed, which is highly unstable and decomposes into products.
Rate of SN2 reaction
Since the rate of reaction depends upon both alkyl halide and nucleophile, it is a second order reaction (bimolar).
The rate of SN2 reaction = K[R−X][Nu−]
Reactivity of haloalkanes
The bond dissociation energies of C-X bonds follow the order : C−F > C−Cl > C−Br > C−I. Since the higher the bond dissociation energy, the more difficult it is to break the bond. The reactivity follows the order : RI > RBr > RCl > RF.
Reactivity of haloalkanes towards SN1 and SN2 reactions
SN1 reaction : SN1 reaction depends on the stability of carbocation. The more stable the carbocation, the better its chance to form. The stability of carbocation is : 1° < 2° < 3° (why?) . Therefore, the order of reactivity follows the same order.
SN2 reaction : In SN2 reaction, the attack of the nucleophile occurs from the back at the α-carbon (the carbon atom attached to halide), the presence of any alkyl group at the α-carbon tends to hinder the approach of nucleophile. As a result, the SN2 reactivity decreases with increase in number of alkyl groups at α-carbon.
Reactivity towards SN1 and SN2
Try to answer the following questions. Pay attention as they are very important questions.
Arrange the given alkyl halides in order of increasing SN2 reactivity : CH3Br, CH3Cl, CH3CH2Cl, (CH3)2CHCl.
Arrange the given compounds in order of increasing reactivity towards SN1 and SN2 reactions : C6H5CH2Br, (C6H5)2CHBr, C6H5CH(CH3)Br, (C6H5)2C(CH3)Br.
In elimination reaction, some molecules leave the compound leading to the formation of a double or a triple bond.
Reactions of Halogens with Metals
Alkyl halides react with some metals to form compounds containing carbon-metal bond which are called organometallic compounds.
Formation of grignard reagent
We will learn more about nucleophilic substitution reaction, elimination reaction and reaction of halogens with metals in separate sections.
Question : Which alkyl halide from the following pairs would you expect to react more rapidly by SN2 mechanism?
- CH3CH2CH(Br)CH3 or (CH3)3Br
- (CH3)2CHCH2CH2Br or CH3CH2CH(CH3)CH2Br
- CH3CH2CH(Br)CH3 is a 2° alkyl halide while (CH3)3Br is a 3° alkyl halide. Since steric hindrance (crowding) is lesser in 2° alkyl halide than in 3° alkyl halide, CH3CH2CH(Br)CH3 will react faster than (CH3)3Br in SN2 reaction.
- Both (CH3)2CHCH2CH2Br and
CH3CH2CH(CH3)CH2Br are 2° alkyl halides and contain a methyl group.
However, the methyl group in CH3CH2CH(CH3)CH2Br is attached to the
2nd carbon w.r.t bromine whereas it is attached to 3rd carbon w.r.t bromine in
Question : In the following pairs of halogen compounds, which compound undergoes faster SN1 reaction?
- is a 3° alkyl halide whereas is a 2° alkyl halide. Since the SN1 reactivity depends on the stability of the carbocation, and the stability of carbocation follows the order: 3° > 2° > 1°; will react faster in SN1 reaction.
- is a secondary alkyl halide while is a primary alkyl halide. Therefore, reacts faster in SN1 reaction.
Question : Which one will react faster in SN2 reaction with -OH: CH3Br or CH3I?
Answer :CH3I reacts faster than CH3Br because I− is a better leaving group than Br− ion.
Question : Which one will undergo SN2 reaction
Answer : being a primary alkyl halide will undergo SN2 reaction faster.
Question : Vinyl chloride is unreactive in nucleophilic substitution reactions. Explain.
Answer : Due to resonance in vinyl chloride, the C-Cl bond has some double bond character which is difficult to break.