Comprehensive Study Guide to Haloalkanes and Alkyl Halides

Introduction to Haloalkanes

• Definition: Haloalkanes are halogen derivatives of aliphatic hydrocarbons.

• General Formula: The general formula for haloalkanes is $RX$.

  • $R = \text{Alkyl group}$

  • $X = F, Cl, Br, I \text{ (Halogens)}$

• Hybridization: The carbon atom attached to the halogen in a haloalkane is $sp^3$ hybridized.

• Formation Schema: An aliphatic hydrocarbon ($R-H$) reacts with a halogen ($+X$) and loses a hydrogen ($-H$) to form a haloalkane (alkyl halide), $R-X$.

Applications in Healthcare and Medicine

• Chloramphenicol: A chlorine-containing antibiotic that is an effective drug used for treating "Typhoid."

• Chloroquine: A halogen derivative used in the treatment of Malaria.

• Halothane ($CF_3CHBrCl$): Used during surgeries as an anesthetic.

Nomenclature and Classification

IUPAC vs. Common Naming Examples

• $CH_3Cl$

  • IUPAC: Chloromethane

  • Common: Methyl chloride

• $CH_3-CH_2-Br$

  • IUPAC: Bromoethane

  • Common: Ethyl bromide

• $CH_3-CH(Br)-CH_3$

  • IUPAC: 2-Bromopropane

  • Common: Isopropyl bromide

• $(CH_3)_3C-Br$

  • IUPAC: 2-Bromo-2-methylpropane

  • Common: tert-Butyl bromide

Vicinal and Geminal Halides

• Vicinal (vic) Halides: In the common system, when two halogen atoms ($X$) are attached to neighboring carbon atoms, it is known as a vicinal system.

  • Example: 2,3-dichloroheptane (vic-heptyl chloride).

• Geminal (gem) Halides: In the common system, if two halogen atoms ($X$) are attached to the same carbon atom, it is called a geminal or gem halide.

  • Example: 2,2-dichloroheptane (gem-heptyl chloride).

Structural Classifications

• Vinylic Halide: If the halogen ($X$) is attached to a $C$ atom involved in a double bond ($C=C$), it is called vinylic.

• Allylic Halide: If a double bond is present at the second carbon away from the halogen ($X$), it is called allylic.

Isomerism in Haloalkanes

• Chain Isomerism: Compounds sharing the same molecular formula but possessing different carbon chain lengths.

  • Example: A 4-membered chain with a chlorine (1-chlorobutane) vs. a 3-membered chain with a methyl branch (1-chloro-2-methylpropane).

• Position Isomerism: Compounds with the same molecular formula but different positions of the halogen ($X$) on the chain.

  • Example 1: Halogen on the 1st position vs. the 2nd position in a propane chain.

  • Example 2: Bromine on the 3rd position in a longer chain.

Representative Questions and Solutions

Question 1: Write IUPAC Names

• (i) $(CH_3)_3C-CH_2-Br$: 1-Bromo-2,3-dimethylpropane (Note: Based on naming assigned in transcript, though structurally $(CH_3)_3C-CH_2-Br$ is 1-bromo-2,2-dimethylpropane).

• (ii) Cyclopentane with $CH_2Cl$ and two $CH_3$ groups: 1-Chloromethyl-2,2-dimethylcyclopentane.

• (iii) Cyclobut-1-ene with $Br$ and $I$: 1-Bromo-2-iodocyclobut-1-ene.

• (iv) Cyclopentane with $Cl$ and $OCH_3$: 1-Chloro-3-methoxycyclopentane.

Question 2: NCERT Question (Naming and Structure)

• Structure 1: 4-Bromo-3-methylpent-2-ene.

• Structure 2: 1-Bromobut-2-ene.

Exercise: Life Application

• Draw all 8 structural isomers of $C_5H_{11}Br$:

  • 1-Bromopentane

  • 1-Bromo-2-methylbutane

  • 1-Bromo-2,2-dimethylpropane

  • (Note: Only 3 were explicitly drawn/named in this section of the transcript, but 8 exist in total).

Methods of Preparation of Alkyl Halides ($RX$)

From Alcohols (Grooves Process)

• General Reaction: $R-OH + HX \xrightarrow{\text{anhy. } ZnCl_2} R-X + H_2O$

• Role of $ZnCl_2$: $ZnCl_2$ is a Lewis acid (electron-deficient). It generates a positive charge on the Oxygen of the alcohol. As a result, the $C-O$ bond breaks with ease, providing easier dissociation and allowing the formation of the haloalkane.

• Mechanism (1-step view): $CH_3-O-H + ZnCl_2 \rightarrow [CH_3-O^{(+)}-H \dots ZnCl_2] \rightarrow CH_3^{(+)} + OH^- \text{ complexes} \xrightarrow{Cl^-} CH_3Cl$.

Reactivity Trends in Alcohol Reactions

• Types of Alcohols:

  1. Methyl Alcohol: $H_3C-OH$

  2. $1^\circ$ (Primary): $R-CH_2-OH$

  3. $2^\circ$ (Secondary): $R_2-CH-OH$

  4. $3^\circ$ (Tertiary): $R_3-C-OH$

• Tertiary ($3^\circ$) Alcohol Reactivity: These are extremely reactive and can react with $HCl$ even without anhydrous $ZnCl_2$ at room temperature.

• Order of Reactivity of Alcohols: 3^\circ > 2^\circ > 1^\circ > \text{methyl}.

  • Reason: The $+I$ (inductive) effect stabilizes the carbocation intermediate.

• Reactivity of Halogen Acids: HI > HBr > HCl

  • Reason: Correlates with Bond Length and Bond Dissociation Energy; $HI$ has the longest bond and lowest dissociation energy.

Case Study: Neopentyl Alcohol Rearrangement

• When neopentyl alcohol ($(CH_3)_3C-CH_2-OH$) reacts with $HCl/ZnCl_2$, it generates a less stable $1^\circ$ carbocation.

• Mechanism: The reaction undergoes a rearrangement via a [1,2-Methyl Shift] to form a more stable $3^\circ$ carbocation ($(CH_3)_2C^{(+)}-CH_2-CH_3$).

• Final Product: The main product is a rearranged $3^\circ$ chloride.

• Photochemical Exception: If the reaction is carried out in sunlight ($h\nu$), it follows a Free Radical Mechanism and does not undergo rearrangement, yielding neopentyl chloride directly.

From Alkanes (Photochemical Free Radical Reaction)

• General Reaction: $CH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + CH_2Cl_2 + CHCl_3 + CCl_4$.

• Disadvantage: This often yields a mixture of products which must be separated by fractional distillation due to different boiling points.

• Selectivity: Single products are formed when isomeric products are impossible, such as with Neopentane reacting with $Cl_2$ in sunlight to form Neopentyl chloride only.

• Radical Stability: The reaction proceeds through radicals. The stability order is: \text{Benzylic} > \text{Allylic} > 3^\circ \text{ (Alkyl)} > 2^\circ > 1^\circ > \text{Vinylic}.

  • Reason: Benzylic and Allylic radicals are highly stable due to Resonance stabilization.

From Phosphorus Halides

• Alcohol + $PCl_3$: $3 R-OH + PCl_3 \rightarrow 3 R-Cl + H_3PO_3 \text{ (Phosphorous acid)}$

• Alcohol + $PCl_5$: $R-OH + PCl_5 \rightarrow R-Cl + POCl_3 \text{ (Phosphorus oxychloride)} + HCl$

• Example with Ethanol: $3 CH_3CH_2OH + PBr_3 \rightarrow 3 CH_3CH_2Br + H_3PO_3$.

From Thionyl Chloride ($SOCl_2$)

• Reaction: $CH_3CH_2OH + SOCl_2 \rightarrow CH_3CH_2Cl + SO_2\uparrow + HCl\uparrow$

• Significance: This is considered the Best/Preferred method because the side products ($SO_2$ and $HCl$) are in the gas phase and escape easily, allowing for the obtainment of pure haloalkanes.

Preparation From Alkenes and Alkynes

From Alkenes (Electrophilic Addition)

• Symmetrical Alkenes: Addition of $HX$ can happen on either side as the molecule is uniform.

• Unsymmetrical Alkenes: Follows Markovnikov's Rule.

  • Rule: The negative part of the adductant ($X^-$) goes to the carbon with the lesser number of hydrogen atoms.

  • Mechanism: The reaction proceeds through the more stable carbocation (2^\circ > 1^\circ).

Anti-Markovnikov's Rule (Peroxide Effect / Kharasch Effect)

• Occurs in the presence of peroxide ($R-O-O-R$) with only $HBr$.

• Rule: The negative part of the adductant goes to the carbon with the greater number of hydrogen atoms.

• Mechanism: Proceeds through a radical mechanism ($2^\circ$ radical is more stable than $1^\circ$).

• Limitations: This effect is not possible with $HI, HCl, \text{ or } HF$ because their homolytic fission processes are endothermic.

From Alkynes ($C\equiv C$)

• Addition of $HX$:

  • Alkyne + $1 eq. HX \rightarrow \text{Vinyl Halide}$

  • Alkyne + $2 eq. HX \rightarrow \text{Geminal Dihalide}$

• Addition of Halogens ($X_2$):

  • Alkyne + $1 eq. X_2 \rightarrow \text{Vicinal Dihalide}$

  • Alkyne + $2 eq. X_2 \rightarrow \text{Tetrahalide}$

• Unsaturation Test: The addition of bromine ($Br_2$) to an alkene/alkyne results in the disappearance of the reddish-brown color (becomes colorless), serving as a test for unsaturation.

Allylic Substitution

• High-Temperature Halogenation: Heating an alkene with $Cl_2$ or $Br_2$ at $773\,K$ leads to allylic substitution instead of addition because the reaction proceeds via the resonance-stabilized allylic radical.

• NBS (N-Bromosuccinimide): Used for allylic bromination. It overrides the high-temperature requirement and works in sunlight ($h\nu$).

Halogen Exchange Methods

• Finkelstein Reaction: Used to prepare Iodoalkanes.

  • Reaction: $R-Br + NaI \xrightarrow{\text{acetone, reflux}} R-I + NaBr\downarrow$

  • The $NaBr$ precipitates and is removed, shifting equilibrium forward.

• Swarts Reaction: Used to prepare Fluoroalkanes using metallic fluorides like $Hg_2F_2$, $AgF$, or $SbF_3$.

  • Example: $2 CH_3CH_2Cl + Hg_2F_2 \rightarrow 2 CH_3CH_2F + Hg_2Cl_2$.

Reaction of Silver Salts

• Borodine Hunsdiecker Reaction: Synthesis of bromoalkanes by treating silver salts of carboxylic acids with $Br_2$ in $CCl_4$.

  • Reaction: $CH_3COOAg + Br_2 \rightarrow CH_3Br + CO_2 + AgBr$

• Birnbaum Simonini Reaction: Reaction of silver salts with $I_2$.

  • Reaction: $2 CH_3COOAg + I_2 \xrightarrow{CCl_4, \text{ reflux}} CH_3COOCH_3 + CO_2 + 2AgI$

  • Note: No haloalkane is formed; the product is an ester.

Questions & Discussion

• Q: Complete the reaction: Benzene ring with side chain + $NaI$ in acetone.

  • A: This is a Finkelstein halogen exchange; the chlorine is replaced by Iodine.

• Q: Complete: $CH_3CH_2Cl + SbF_3 \rightarrow \dots$

  • A: $CH_3CH_2F + SbCl_3$ (Swarts Reaction).

• Q: Complete: $CH_3CH_2CH_2OH + SOCl_2 \rightarrow \dots$

  • A: $CH_3CH_2CH_2Cl + SO_2\uparrow + HCl\uparrow$.