Haloalkanes And Haloarenes

(i) Sulphuric Acid is not used in the reaction of alcohol and KI:
Sulfuric acid is not used in the reaction of alcohol and KI because it is a strong acid and can react with the iodine, forming sulfur dioxide and water. This would reduce the amount of iodine available for the reaction with the alcohol. A mild acid like acetic acid is used instead to promote the reaction without interfering with iodine.
(ii) Haloalkanes form alkyl cyanide as chief product on reaction with KCN, while isocyanide as chief product on reaction with AgCN:
When haloalkanes react with KCN, the cyanide ion ( ) directly attacks the electrophilic carbon in the haloalkane, leading to the formation of alkyl cyanide. In contrast, when the same reaction occurs with AgCN, the silver ion (Ag ) coordinates with the cyanide ion, causing a rearrangement to form an isocyanide (also known as isonitrile). This rearrangement is facilitated by the interaction of the cyanide ion with the silver ion.
(iii) Chlorine is an electron withdrawing group, yet it is ortho- and para-directing in aromatic electrophilic substitution reactions:
Chlorine is an electron-withdrawing group via induction (due to its electronegativity), but it is also an electron-donating group through resonance (due to the lone pair of electrons on the chlorine atom). This electron donation via resonance makes the ortho- and para-positions of the aromatic ring more reactive to electrophilic substitution reactions, which is why chlorine is ortho- and para-directing in these reactions.

Haloalkanes undergo nucleophilic substitution reactions in which a nucleophile replaces the halogen atom. These reactions proceed by two different mechanisms:
SN1 (unimolecular) and
SN2 (bimolecular). 1. Unimolecular Nucleophilic Substitution (SN1 Mechanism): \begin{enumerate} \item The reaction takes place in two steps. \item
Step 1 (slow step): The C–X bond breaks heterolytically, forming a carbocation. \item
Step 2 (fast step): The nucleophile attacks the carbocation to form the product. \item Since the rate-determining step involves only the haloalkane, the reaction rate is: \item Follows first-order kinetics and usually occurs in
tertiary haloalkanes due to stable carbocation formation. \end{enumerate} 2. Bimolecular Nucleophilic Substitution (SN2 Mechanism): \begin{enumerate} \item The reaction takes place in a single step. \item The nucleophile directly attacks the carbon atom bonded to the halogen, from the backside, while the leaving group departs simultaneously. \item The transition state has a partial bond between carbon, halogen, and nucleophile. \item Since both haloalkane and nucleophile participate in the rate-determining step: \item Follows second-order kinetics and usually occurs in
primary haloalkanes. \end{enumerate} Conclusion: \begin{itemize} \item SN1 mechanism involves carbocation intermediate and is favored in polar protic solvents with tertiary haloalkanes. \item SN2 mechanism is a concerted reaction involving backside attack, favored in polar aprotic solvents with primary haloalkanes. \end{itemize}

(i) CH CH = CH-CH Br:
This compound is a bromine-substituted alkene. The IUPAC name is 1-bromo-2-butene.
(ii) (CH ) -C-CH (Br):
This is a bromoalkane. The IUPAC name is 2-bromo-3-methylbutane.
(iii) CH CH -C-CH(Cl)CH CH :
This compound is a chloroalkane with a substituent on the second carbon. The IUPAC name is 2-chloro-3-methylpentane.
(iv) (CH ) CBrCH CH :
This is a bromoalkane. The IUPAC name is 3-bromo-2-methylbutane.
(v) (CH ) Cl:
This is a simple chloroalkane. The IUPAC name is tert-butyl chloride.

(i) 2-chloro-3-methylpentane.
Pentane chain, Cl at C-2, CH at C-3: (ii) 1,4-dibromobut-2-ene.
4-carbon chain, double bond at C-2, Br at C-1 and C-4: (iii) 1-chloro-2-methylbenzene.
Benzene ring with Cl at position 1, CH at position 2 (ortho-chlorotoluene).
(iv) 1-chloro-4-ethylcyclohexane.
Cyclohexane ring with Cl at C-1 and –CH CH at C-4.
(v) 3-bromo-2-methylbut-2-ene.
4-carbon chain with double bond at C-2, CH substituent at C-2, Br at C-3:

(i) n-Butyl chloride + alc. KOH.
Dehydrohalogenation occurs, giving 1-butene: (ii) Methyl iodide + Mg (dry ether).
Grignard reagent formation: (iii) Methyl bromide + Na (dry ether).
Wurtz reaction gives ethane: (iv) Methyl iodide + KCN.
Forms methyl cyanide: (v) Chlorobenzene + aq. NaOH.
At high temp. and pressure forms phenol:

1. Reaction with Sodium (Wurtz Reaction):
When haloalkanes react with sodium in dry ether, higher alkanes are formed. Example: 2. Reaction with Magnesium:
Haloalkanes react with magnesium in dry ether to form Grignard reagents. Example: Conclusion:
- With sodium, haloalkanes undergo Wurtz reaction to form higher alkanes.
- With magnesium, they form important organometallic compounds known as Grignard reagents.

The Mechanism (Substitution Nucleophilic Bimolecular):
This reaction proceeds in a single concerted step (one-step mechanism) without the formation of any intermediate. Key Features:

• Attack of Nucleophile: The incoming nucleophile ( ) attacks the carbon atom bonded to the halogen (leaving group) from the side opposite to the leaving group (back-side attack).
• Transition State: A transition state is formed where the bond between the carbon and nucleophile starts forming, and the bond between carbon and halogen starts breaking simultaneously. In this state, the carbon atom is partially bonded to five atoms.
• Stereochemistry: The reaction results in the inversion of configuration (Walden inversion), similar to an umbrella turning inside out in a gale.

Mechanism Diagram: Example: Reaction of Chloromethane ( ) with Hydroxide ion ( ).

Step 1: Understanding the Concept:
In an reaction, the rate depends on the stability of the carbocation formed after the leaving group departs.
Greater the stability of the carbocation, greater will be the reactivity.
Carbocation stability order:
Step 2: Classification of Given Compounds:
A.
Forms a tertiary ( ) carbocation → Most stable.
B.
Forms a primary ( ) carbocation → Least stable.
C.
Also forms a primary ( ) carbocation → Slightly more stable than straight chain due to inductive effect, but still primary.
D.
Forms a secondary ( ) carbocation → More stable than primary.
Thus, increasing order of reactivity in reaction:
So,
Step 3: Final Answer:
The correct increasing order of reactivity in reaction is .