Haloalkanes and Haloarenes Lecture Solutions

Practice questions and answers covering nomenclature, reaction mechanisms (SN1, SN2), physical properties, and specific chemical tests for Haloalkanes and Haloarenes based on NCERT exercise solutions.

Why is sulphuric acid ($H_2SO_4$) not used during the reaction of alcohols with $KI$?

Sulphuric acid is an oxidizing agent that oxidizes the $HI$ produced in the reaction to $I_2$, preventing the reaction between alcohol and $HI$ to produce alkyl iodide.

Which acid is used as a substitute for sulphuric acid when reacting alcohols with $KI$?

A non-oxidizing acid such as $H_3PO_4$ is used.

Identify the isomer of $C_5H_{12}$ that yields a single monochloride upon photochemical chlorination.

Neopentane, because it contains only one type of H-atom.

Identify the isomer of $C_5H_{12}$ that yields three isomeric monochlorides upon photochemical chlorination.

n-pentane, which contains three different types of H-atoms.

Identify the isomer of $C_5H_{12}$ that yields four isomeric monochlorides upon photochemical chlorination.

2-methylbutane, which contains four different types of H-atoms.

How does the boiling point of alkyl halides change with the atomic mass of the halogen atom for the same alkyl group?

The boiling point increases with an increase in the atomic mass of the halogen atom ($Cl < Br < I$).

Arrange the following in increasing order of boiling point: Chloromethane, Bromomethane, Dibromomethane, Bromoform.

Chloromethane < Bromomethane < Dibromomethane < Bromoform.

How does branching in the carbon chain affect the boiling point of alkyl halides?

The boiling point decreases with an increase in branching in the chain.

Why does 1-bromobutane react more rapidly than 2-bromobutane in an $S_N2$ mechanism?

1-bromobutane is a $1^\text{o}$ alkyl halide and is less sterically hindered than the $2^\text{o}$ alkyl halide 2-bromobutane, making it easier for the nucleophile to approach.

Which compound undergoes a faster $S_N1$ reaction: 2-chloro-2-methylpropane or 3-chloropentane?

2-chloro-2-methylpropane, because it forms a more stable $3^\text{o}$ carbocation compared to the $2^\text{o}$ carbocation formed by 3-chloropentane.

What is the Wurtz reaction?

A reaction where an alkyl halide is treated with sodium ($Na$) in the presence of dry ether to obtain a hydrocarbon containing double the number of carbon atoms as the original halide.

Define ambident nucleophiles.

Nucleophiles that have two different nucleophilic sites through which they can attack, such as the nitrite ion ($NO_2^-$).

Arrange the following in increasing order of dipole moment: $CH_2Cl_2$, $CHCl_3$, $CCl_4$.

$CCl_4$ ($0D$) < $CHCl_3$ ($1.08D$) < $CH_2Cl_2$ ($1.60D$).

Why does carbon tetrachloride ($CCl_4$) have a resultant dipole moment of zero?

$CCl_4$ is a symmetrical molecule where the dipole moments of the four $C-Cl$ bonds cancel each other out.

State Saytzeff's rule as applied to dehydrohalogenation reactions.

In dehydrohalogenation reactions, the alkene having a greater number of alkyl groups attached to the doubly bonded carbon atoms is the preferred or major product.

Why are alkyl halides immiscible with water despite being polar?

The new force of attraction between alkyl halides and water molecules is weaker than the existing alkyl halide-alkyl halide and water-water (H-bond) forces of attraction.

Why must Grignard reagents be prepared under anhydrous conditions?

Grignard reagents are very reactive and will react with any present moisture to form alkanes.

What is the chemical name and primary usage of Freon-12?

Dichlorodifluoromethane ($CF_2Cl_2$); it is used as a refrigerant in refrigerators and air conditioners and as an aerosol spray propellant.

What is the primary use of DDT ($p, p' \text{-dichlorodiphenyltrichloroethane}$)?

It is used as an insecticide, particularly effective against mosquitoes and lice.

To what is the antiseptic property of iodoform ($CHI_3$) attributed?

The liberation of free iodine when the compound comes in contact with the skin.

How do the products differ when an alkyl halide is treated with aqueous $KOH$ versus alcoholic $KOH$?

Aqueous $KOH$ leads to substitution (forming alcohols), whereas alcoholic $KOH$ leads to elimination (forming alkenes) because the alkoxide ion ($RO^-$) acts as a strong base.