Physical & Chemical Properties of Haloalkanes - Practice Questions & MCQ

Most common bases that we have use are alc.KOH and aqueous NaOH. In alc.KOH, we have EtO^-(CH₃-CH₂-O^-) as the base while in aqueous NaOH, the base is OH^-. The strength of these two bases are given below:

CH₃-CH₂-O^- > OH^-
Because CH₃-CH₂- is the releasing group and hence it unstabilize the O^-, thus it reacts faster and hence it is stronger base. The reactions occurs as follows:

For alc.KOH:$\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{Br}+\mathrm{KOH}(\mathrm{alc}) \longrightarrow \mathrm{H}_2 \mathrm{C}=\mathrm{CH}_2+\mathrm{KBr}+\mathrm{H}_2 \mathrm{O}$

For aq.NaOH:

The general reaction occurs as follows:
$\mathrm{R}-\mathrm{CH}_2-\mathrm{Cl}+\mathrm{OH}^{-} \rightarrow \mathrm{R}-\mathrm{CH}_2-\mathrm{OH}$

Mechanism

• It is bimolecular nucleophilic substitution (SN₂) reaction. 
• Rate of reaction follows second order kinetics and depends upon the concentration of both the nucleophile as well as the substrate.

                            Rate $\propto[\mathrm{R}-\mathrm{X}]\left[\mathrm{Nu}^{-}\right]$

• Rate determinig step depends on how fast the transition state is formed and also the stability of the transition state
• Stronger nucleophile is required as it has to attack and make the leaving group leave
• Polar aprotic solvents favour SN₂ reaction as they do not facilitate formation of ions
• The reaction occurs in concerted mechanism and inversion of configuration (Walden Inversion) takes place if the leaving group and the nucleophile have the same priority
• Steric hinderance in the substrate decreases the reactivity of the subtrate towards SN₂ reaction

The general reaction occurs as follows:

$\mathrm{R}-\mathrm{CH}_2-\mathrm{Cl}+\mathrm{OH}^{-} \rightarrow \mathrm{R}-\mathrm{CH}_2-\mathrm{OH}$

• The mechanism occurs as follows:
  
• SN₁ reactions are nucleophilic substitution reactions, involving a nucleophile replacing a leaving group.
• SN₁ reactions are unimolecular. The rate of this reaction depends only on the concentration of one reactant and does not depend upon the strength of the nucleophile

                                    Rate $\propto[\mathrm{R}-\mathrm{X}]$

• The rate determining step depends on the stability of the intermediate carbocation which is obtained during the course of the reaction

                            Rate $\propto$ stability of carbocation

• Since the mechanism involves the attack of nucleophiles on an already-formed carbocation, the strength of nucleophiles is unimportant for the rate of the reaction
• The rate of formation of intermediate is independent of concentration of nucleophile and depends only on the concentration of reactants.
• Good ionising solvents (polar protic solvents) are required to carry out SN₁ reaction as there has to bea  formation of ions
• Configuration of the product may be same or inverted and in cases where the leaving group departs from a chiral centre, racemisation occurs.
  • If Nu^- attacks on the same side from where X^- leaves, then it is called 'Retention'.
  • If Nu^- attacks from the opposite side from where X^- leaves, then it is called 'Inversion'.
  • Racemic mixture is obtained when equal amount of retention and inversion products are formed in the reaction.
  • Generally, partial racemisation is seen in the reactions as it both SN₁ and SN₂ are both competing 

Elimination-Addition Mechanism(I)

Very stong base such as sodium or potassium amide react with aryl halide, even those without electron withdrawing substituents to give products corresponding to nucleophilic substitution of halide by the base.

Mechanism

1. Elimination stage: Amide ion is a very strong base and brings about the dehydrohalogenation of chlorobenzene by abstracting a proton from the carbon adjacent to the one that bears the leaving group. The product of this step is an unstable intermediate called benzyne.
   
2. Beginning of addition phase: Amide ion acts as a nucleophile and adds to one of the carbons of the triple bond. The product of this step is a carbanion.
   
3. Completion of addition phase: The aryl anion abstracts a proton from the ammonia used as the solvent in the reaction.
4. 

Aryl halides are extremely less reactive towards nucleophilic substitution reactions due to the following reasons:

• Resonance effect: In haloarenes, the electron pairs on halogen atom are in conjugation with π-electrons of the ring and the following resonating structures are possible.
  
  C—Cl bond acquires a partial double bond character due to resonance. As a result, the bond cleavage in haloarene is difficult than haloalkane and therefore, they are less reactive towards nucleophilic substitution reaction.
• Difference in hybridisation of carbon atom in C—X bond: In haloalkane, the carbon atom attached to halogen is sp³ hybridised while in case of haloarene, the carbon atom attached to halogen is sp²-hybridised. The sp² hybridised carbon with a greater s-character is more electronegative and can hold the electron pair of C—X bond more tightly than sp³ -hybridised carbon in haloalkane with less s-chararcter. Thus, C—Cl bond length in haloalkane is 177pm while in haloarene is 169 pm. Since it is difficult to break a shorter bond than a longer bond, therefore, haloarenes are less reactive than haloalkanes towards nucleophilic substitution reaction.
• Instability of phenyl cation: In case of haloarenes, the phenyl cation formed as a result of self-ionisation will not be stabilised by resonance and therefore, S_N1 mechanism is ruled out.
• Because of the possible repulsion, it is less likely for the electron rich nucleophile to approach electron rich arenes.

Chlorobenzene can be converted into phenol by heating in aqueous sodium hydroxide solution at a temperature of 623K and a pressure of 300 atmospheres.

The presence of an electron withdrawing group (-NO₂ ) at ortho- and para-positions increases the reactivity of haloarenes.