Haloalkanes and Haloarenes

Classification of Haloalkanes and Haloarenes

**Haloalkanes** (alkyl halides) are organic compounds where one or more hydrogen atoms of an aliphatic hydrocarbon are replaced by halogen atoms. **Haloarenes** (aryl halides) are compounds where halogen atoms are directly bonded to aromatic carbon atoms.

Classification by Number of Halogen Atoms

**Monohalocompounds**: Single halogen atom (e.g., CH₃Cl)

**Dihalogen compounds**: Two halogen atoms (e.g., CH₂Cl₂)

**Polyhalogen compounds**: Three or more halogen atoms (e.g., CCl₄)

Classification by Hybridization and Position of Carbon-Halogen Bond

**Compounds containing sp³ C—X bond:**

**Primary alkyl halides (1°)**: Halogen bonded to primary carbon with one other carbon substituent (e.g., CH₃CH₂Br). These are represented by the general formula CₙH₂ₙ₊₁X.

**Secondary alkyl halides (2°)**: Halogen bonded to carbon with two other carbon substituents (e.g., CH₃CHBrCH₃)

**Tertiary alkyl halides (3°)**: Halogen bonded to carbon with three other carbon substituents (e.g., (CH₃)₃CBr)

**Allylic halides**: Halogen bonded to sp³ carbon adjacent to C=C double bond (e.g., CH₂=CHCH₂Br, allyl bromide). The carbon bearing halogen is not part of the double bond.

**Benzylic halides**: Halogen bonded to sp³ carbon attached directly to aromatic ring (e.g., C₆H₅CH₂Br, benzyl bromide). The halogenated carbon is not aromatic.

**Compounds containing sp² C—X bond:**

**Vinylic halides**: Halogen bonded to sp² carbon of C=C double bond (e.g., CH₂=CHCl, vinyl chloride). The halogen is part of the double bond.

**Aryl halides**: Halogen directly bonded to sp² aromatic carbon (e.g., C₆H₅Cl, chlorobenzene)

The distinction is critical because **vinylic and aryl halides do NOT undergo typical nucleophilic substitution reactions** due to strong C—X bonds and resonance stabilization.

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IUPAC Nomenclature of Haloalkanes and Haloarenes

Rules for Haloalkanes

1. **Identify the parent chain** containing the halogen atom

2. **Number the chain** to give the halogen atom the lowest number

3. **Name as halogenated hydrocarbons**: prefix the halogen name (fluoro-, chloro-, bromo-, iodo-) before the alkane name

4. **For multiple halogens**: use numerical prefixes (di-, tri-, tetra-, etc.) and list alphabetically

5. **For dihalogen compounds**:

**Geminal halides (gem-dihalides)**: Both halogens on same carbon → dihaloalkane

**Vicinal halides (vic-dihalides)**: Halogens on adjacent carbons → dihalalkane

Examples

| Structure | IUPAC Name | Classification |

|-----------|-----------|-----------------|

| CH₃CH₂CH₂CH₂Br | 1-Bromobutane | 1° alkyl halide |

| CH₃CHBrCH₂CH₃ | 2-Bromobutane | 2° alkyl halide |

| (CH₃)₃CBr | 2-Bromo-2-methylpropane | 3° alkyl halide |

| CH₂=CHCl | Chloroethene | Vinylic halide |

| CH₂=CHCH₂Br | 3-Bromopropene | Allylic halide |

| C₆H₅Cl | Chlorobenzene | Aryl halide |

| CH₂Cl₂ | Dichloromethane | Geminal dihalide |

| CHCl₃ | Trichloromethane | Geminal trihalide |

| CH₃CHClCH₂Cl | 1,2-Dichloropropane | Vicinal dihalide |

Dihalogen Derivatives of Benzene

For benzene substitution, use **1,2- (ortho/o-), 1,3- (meta/m-), 1,4- (para/p-)** numbering in IUPAC (common names also acceptable):

**1,2-dichlorobenzene** = o-dichlorobenzene

**1,3-dichlorobenzene** = m-dichlorobenzene

**1,4-dichlorobenzene** = p-dichlorobenzene

---

Nature of C—X Bond

Bond Characteristics

**Carbon-halogen bonds are highly polarized** due to the high electronegativity of halogens. The carbon atom bears a **partial positive charge (δ+)** while the halogen bears a **partial negative charge (δ—)**.

Bond Properties Table

| Bond | Bond Length (pm) | Bond Enthalpy (kJ/mol) | Dipole Moment (Debye) |

|------|------------------|----------------------|----------------------|

| C—F | 139 | 452 | 1.847 |

| C—Cl | 178 | 351 | 1.860 |

| C—Br | 193 | 293 | 1.830 |

| C—I | 214 | 234 | 1.636 |

Trends

**Bond length increases** (F < Cl < Br < I) down the halogen group as atomic size increases

**Bond enthalpy decreases** (F > Cl > Br > I) down the group, making C—I bonds weakest and most easily cleaved

**Dipole moment** shows relative constancy across the series; bonds are all significantly polar

**Reactivity in substitution**: C—I most reactive, C—F least reactive (inverse of bond strength)

---

Preparation of Haloalkanes

6.4.1: From Alcohols

**Alcohols are the most common and practical starting material** for preparing alkyl halides. The C—OH bond is replaced by C—X.

**Method 1: Reaction with hydrogen halides**

**Mechanism**:

1. Protonation of OH group (forms good leaving group)

2. Carbocation formation (SN1 mechanism for 2° and 3° alcohols)

3. Nucleophilic attack by halide

**Reactions**:

**With HCl**: R—OH + HCl → R—Cl + H₂O (catalyst ZnCl₂ required for 1° and 2° alcohols; 3° alcohols react at room temperature)

**With HBr**: R—OH + HBr (48%) → R—Br + H₂O (requires heating; no catalyst needed for 3° alcohols)

**With HI**: R—OH + HI → R—I + H₂O (best results with dilute acid; HI is most reactive)

**Order of reactivity**: 3° > 2° > 1° (due to carbocation stability)

**Example**: (CH₃)₃C—OH + HCl (conc.) → (CH₃)₃C—Cl + H₂O

**Method 2: Reaction with phosphorus halides**

**PBr₃**: 3 R—OH + PBr₃ → 3 R—Br + H₃PO₃ (used for large-scale preparations; generates PBr₃ in situ using red phosphorus + Br₂)

**PI₃**: 3 R—OH + PI₃ → 3 R—I + H₃PO₃ (PI₃ prepared in situ using red phosphorus + I₂)

**Method 3: Reaction with thionyl chloride (preferred)**

**SOCl₂**: R—OH + SOCl₂ → R—Cl + SO₂↑ + HCl↑

**Why preferred**: Both gaseous byproducts (SO₂ and HCl) escape, driving reaction forward and leaving pure alkyl halide product. No catalyst needed; works for all primary, secondary, and tertiary alcohols.

**Important limitation**: These methods **cannot prepare aryl halides** because the C—O bond in phenols has **partial double bond character** (resonance stabilization) making it much stronger than a regular single bond and resistant to cleavage.

6.4.2: From Hydrocarbons

**Method 1: Free radical halogenation**

**Mechanism** (three steps):

1. **Initiation**: Cl₂ or Br₂ undergoes homolytic cleavage under UV light or heat → 2 X•

2. **Propagation**:

R—H + X• → R• + HX

R• + X₂ → R—X + X•

3. **Termination**: R• + R• → R—R (or other radical combinations)

**Reactions**:

**Chlorination**: CH₄ + Cl₂ → CH₃Cl + HCl (and further chlorination products CH₂Cl₂, CHCl₃, CCl₄)

**Bromination**: C₂H₆ + Br₂ → C₂H₅Br + HBr (UV light or heat required)

**Problems**:

**Mixture of products**: Both mono- and polychloro/polybromo isomers formed

**Low yield of single product**: Example, monochlorination of butane gives 4 isomeric monochloroproducts

**Poor selectivity**: Cannot easily isolate pure compounds

**Selectivity**: Tertiary C—H > Secondary C—H > Primary C—H (abstraction rates follow radical stability)

**Example**: Monochlorination of (CH₃)₂CHCH₂CH₃ yields four monochlorides:

(CH₃)₂CCl—CH₂CH₃ (from tertiary C—H)

(CH₃)₂CH—CHCl—CH₃ (from secondary C—H)

(CH₃)₂CH—CH₂—CH₂Cl (from primary C—H on CH₂)

CH₃—CHCl—CH₂—CH₃ (from primary C—H on CH₂)

6.4.3: From Alkenes

**Method 1: Addition of hydrogen halides**

**Mechanism**: Follows **Markovnikov's rule** (halogen adds to carbon of C=C with more hydrogen atoms)

**Reaction**: R—CH=CH₂ + HX → R—CHX—CH₃

**Example**: CH₃CH=CH₂ + HCl → CH₃CHClCH₃ (2-chloropropane, major; CH₃CH₂CH₂Cl not formed)

**Method 2: Addition of halogens**

**Reaction**: R—CH=CH—R + X₂ → R—CHX—CHX—R (forms **vic-dihalides**)

**Product**: Colorless liquid; used to detect C=C (Br₂ in CCl₄ decolorization test)

**Example**: CH₂=CH₂ + Br₂ → CH₂Br—CH₂Br (1,2-dibromoethane)

6.4.4: Halogen Exchange (Finkelstein Reaction)

**Converting one alkyl halide to another by halogen substitution**:

**Finkelstein reaction** (alkyl chloride/bromide → alkyl iodide):

R—Cl + NaI (dry acetone) → R—I + NaCl↓

R—Br + NaI (dry acetone) → R—I + NaBr↓

**Mechanism**: Nucleophilic substitution (SN2). NaCl and NaBr precipitate in dry acetone (low solubility), driving equilibrium forward via Le Chatelier's principle.

**Driving force**: Iodide is better nucleophile than chloride/bromide in polar aprotic solvents; precipitated NaCl/NaBr removes salts from solution.

**Swarts Reaction** (alkyl chloride/bromide → alkyl fluoride):

R—Cl + AgF (heating) → R—F + AgCl↓

R—Cl + SbF₃ (heating) → R—F + SbCl₃

**Alternative agents**: CoF₂, Hg₂F₂ also used

**Why needed**: Direct fluorination of alcohols/hydrocarbons not feasible; fluorine too reactive

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Preparation of Haloarenes

Method 1: Electrophilic Aromatic Substitution

**Chlorination and bromination** of benzene and substituted benzenes:

**Chlorination**: C₆H₆ + Cl₂ (FeCl₃ catalyst) → C₆H₅Cl + HCl

**Bromination**: C₆H₆ + Br₂ (FeBr₃ catalyst) → C₆H₅Br + HBr

**Mechanism**: Electrophilic aromatic substitution (SEAr):

1. Halogen activated by Lewis acid (FeCl₃ or FeBr₃) to form X⁺ species

2. Nucleophilic aromatic ring attacks electrophilic halogen

3. H⁺ eliminated to restore aromaticity

**Separation**: Ortho and meta isomers of dihalogenated benzenes have **very different melting points** (para isomers typically highest mp due to symmetry), allowing easy separation by fractional crystallization.

**Iodination**:

C₆H₆ + I₂ (HNO₃ or HIO₄ oxidant) → C₆H₅I + HI

**Requires oxidizing agent** because I₂ addition is normally reversible; oxidant converts generated HI to I₂, driving equilibrium forward

**Fluorination**: **NOT done by direct halogenation** because fluorine is too reactive and cannot be controlled; use Sandmeyer or other methods instead.

Method 2: Sandmeyer Reaction (From Primary Aromatic Amines)

**General sequence**:

1. **Diazotization**: Ar—NH₂ + NaNO₂ + HCl (cold, <5°C) → [Ar—N≡N]⁺Cl⁻ (diazonium salt) + H₂O

2. **Halogenation**: [Ar—N≡N]⁺Cl⁻ + CuCl (or CuBr) → Ar—Cl (or Ar—Br) + N₂↑ + Cu⁺

**Mechanism**: Arene diazonium salts are highly reactive and unstable; copper(I) halides catalyze C—N bond cleavage with C—X bond formation.

**Examples**:

**Chlorination**: C₆H₅—NH₂ + NaNO₂/HCl → [C₆H₅—N≡N]⁺Cl⁻ + CuCl → C₆H₅—Cl

**Bromination**: C₆H₅—NH₂ + NaNO₂/HCl → [C₆H₅—N≡N]⁺Cl⁻ + CuBr → C₆H₅—Br

**Iodination**: [C₆H₅—N≡N]⁺Cl⁻ + KI → C₆H₅—I (no copper needed; simply mix with KI)

**Advantage**: Allows introduction of halogen into positions already bearing activating/deactivating groups via nucleophilic replacement of amine

**Key conditions**:

Diazotization must be done **at cold temperature** (0–5°C) to prevent decomposition

Diazonium salt solution should be **freshly prepared** (unstable)

Reaction mixture should be **cold** during halogenation to prevent side reactions

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Physical Properties of Haloalkanes and Haloarenes

Melting and Boiling Points

**General trends**:

**Higher than parent hydrocarbons**: Due to polarity of C—X bond and stronger intermolecular forces (dipole-dipole interactions + van der Waals forces)

**Increase with molecular mass**: More electrons → stronger London forces

**State at room temperature**:

Gases: CH₃Cl, CH₃Br, etc.

Liquids: Most C₂–C₆ haloalkanes

Solids: Longer-chain and polyhalogenated compounds

**For same alkyl group, order of boiling points**: **R—I > R—Br > R—Cl > R—F**

Despite F forming strongest C—X bond, fluorine is smallest; I is largest with most van der Waals interactions

F compounds have lowest boiling point due to minimal van der Waals forces (small size)

**Effect of branching**: More branched isomers have **lower boiling points** due to decreased surface contact between molecules. Example:

n-Butyl chloride (CH₃CH₂CH₂CH₂Cl): bp 78°C

Isopropyl chloride ((CH₃)₂CHCl): bp 35°C

tert-Butyl chloride ((CH₃)₃CCl): bp 51°C

**Dihalogenated benzenes**:

Boiling points of ortho, meta, para isomers very similar

**Melting points vary significantly**: Para isomers highest (symmetrical, pack better in crystal lattice)

Example: o-dichlorobenzene (mp 17°C), m-dichlorobenzene (mp –25°C), p-dichlorobenzene (mp 53°C)

Solubility in Water

**Very low solubility** ("insoluble in water")

**Reason**:

Breaking hydrogen bonds between water molecules requires energy

Weak dipole-dipole attractions between haloalkane (C—X polar) and water cannot compensate

Unfavorable enthalpy and entropy change

**Solubility in organic solvents**: **High**

Intermolecular forces (van der Waals, dipole-dipole) between haloalkane and nonpolar/weakly polar solvents similar in strength

New attractions formed ≈ original attractions broken → favorable ΔH ≈ 0

Density

**Bromine and iodine derivatives**: Denser than water (sink in water)

**Chlorine derivatives**: Generally less dense than water

**Polychlorinated compounds**: Density increases with Cl number (CCl₄ = 1.595 g/mL)

**Trend**: Density increases with:

1. Number of halogen atoms

2. Atomic mass of halogen (F < Cl < Br < I in density contribution)

3. Molecular mass increase

| Compound | Density (g/mL) |

|----------|----------------|

| n-C₃H₇Cl | 0.89 |

| n-C₃H₇Br | 1.335 |

| n-C₃H₇I | 1.747 |

| CH₂Cl₂ | 1.336 |

| CHCl₃ | 1.489 |

| CCl₄ | 1.595 |

Color and Smell

**Color**: Colorless when pure; develop color (Br₂ orange, I₂ brown) on exposure to light due to photodecomposition

**Smell**: Many volatile haloalkanes have characteristic sweet smell (e.g., chloroform, tetrachloromethane); some toxic by inhalation

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Chemical Reactions of Haloalkanes: Nucleophilic Substitution

Overview of Nucleophilic Substitution

**Definition**: Reaction where a nucleophile (Nu⁻: electron-rich species) replaces the halogen (leaving group, X) in an alkyl halide substrate.

**General equation**: R—X + Nu⁻ → R—Nu + X⁻

**Substrate requirement**: **sp³ hybridized C—X bond** (haloalkanes, allylic, benzylic halides)

**Vinylic halides (C=C—X) and aryl halides (Ar—X) do NOT undergo nucleophilic substitution** (strong C—X bond; resonance stabilization prevents carbocation/carbanion character)

**Nucleophiles involved** (Table 6.4):

OH⁻ → alcohols

OR⁻ → ethers

I⁻ → alkyl iodides

NH₃ → primary amines (RNH₂)

RNH₂ → secondary amines (RNHR')

R₂NH → tertiary amines (RNR'R")

CN⁻ → nitriles (RCN)

NC⁻ → isonitriles (RNC)

NO₂⁻ → nitroalkanes (RNO₂)

ONO⁻ → alkyl nitrites (RONO)

RCOO⁻ → esters (RCOOR')

H⁻ → hydrocarbons (RH)

**Ambident nucleophiles**: Possess two reactive sites

**Cyanide (CN⁻)**: [C≡N⁻ ↔ C⁻=N] attacks through C (nitrile, favored) or N (isonitrile)

**Nitrite (NO₂⁻)**: [—O—N=O ↔ —O=N⁺—O⁻] attacks through O (alkyl nitrite, kinetic) or N (nitroalkane, thermodynamic)

SN2 Mechanism (Substitution Nucleophilic Bimolecular)

**Definition**: Bimolecular mechanism where rate depends on concentrations of both **nucleophile and substrate**.

**Rate law**: Rate = k[R—X][Nu⁻] (second order)

**Mechanism** (single-step):

Nucleophile attacks from **rear** (opposite side to leaving group), backside attack

C—X bond breaks as C—Nu bond forms simultaneously (concerted)

Transition state shows **partial bonds**: Nu···C···X

**Example with CH₃Cl + OH⁻**:

```

HO⁻ + CH₃Cl → HO···C···Cl → HO—CH₃ + Cl⁻

(transition state)

```

**Stereochemistry**: **Complete inversion of configuration** (Walden inversion)

If substrate is optically active (stereocenter), product has **opposite stereochemistry**

Example: (R)-2-Iodobutane + OH⁻ → (S)-2-butanol (inverted)

**Structural factors favoring SN2**:

1. **Primary alkyl halides (1°)**: MOST reactive

Least steric hindrance around C—X bond

Unhindered approach for nucleophile

Example: CH₃CH₂Br → fastest SN2

2. **Secondary alkyl halides (2°)**: Moderate reactivity

Intermediate steric hindrance

Example: (CH₃)₂CHBr → intermediate rate

3. **Tertiary alkyl halides (3°)**: **LEAST reactive** in SN2

Extreme steric hindrance (three bulky groups on C—X carbon)

Nucleophile cannot approach from rear

Example: (CH₃)₃CBr → extremely slow SN2

**Effect of leaving group**: **Better leaving groups favor SN2**

Order: I⁻ > Br⁻ > Cl⁻ > F⁻ > OH⁻

Larger, more polarizable halides are better leaving groups (stabilize negative charge in product)

**Effect of solvent**: **Aprotic polar solvents strongly favor SN2**

Aprotic polar (DMSO, acetone, DMF): Nucleophile not solvated by H-bonding; increased reactivity

Protic polar (H₂O, alcohols): Nucleophile solvated; decreased reactivity

Nonpolar (hexane, benzene): Very low reactivity (no solvation of ions)

**Effect of nucleophile strength**: Stronger nucleophiles (more negative, more polarizable) react faster in SN2

Order in aprotic solvent: I⁻ > Br⁻ > Cl⁻ > F⁻

Order in protic solvent: F⁻ > Cl⁻ > Br⁻ > I⁻ (reversed due to solvation effects)

**Key characteristics summary**:

Bimolecular kinetics

Single-step mechanism

Backside attack / inversion of stereochemistry

Favored by: 1° > 2° > 3° (primary most reactive)

Favored by: strong nucleophile, aprotic solvent, good leaving group

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Chemical Reactions of Haloalkanes: Elimination Reactions

Overview

**Definition**: Reaction where H and X are removed from adjacent carbons, forming a C=C double bond.

**General equation**: R—CHX—CH₂—R' → R—CH=CH—R' + HX

**Substrate requirement**: Requires **β-hydrogen** (hydrogen on carbon adjacent to C—X)

Primary halides with no β-H: undergo only substitution

Secondary and tertiary halides: both elimination and substitution possible

**Competition**: In same substrate, SN2, SN1, E1, and E2 all possible; products determined by **mechanism which dominates**

E2 Mechanism (Elimination Bimolecular)

**Definition**: Concerted bimolecular elimination where base attacks β-hydrogen as C—X bond breaks.

**Rate law**: Rate = k[R—X][Base] (second order)

**Mechanism** (single-step):

Base abstracts β-H while C—X bond breaks

**Anti periplanar geometry required**: H and X on opposite sides of C—C bond (dihedral angle ≈ 180°)

Concerted transition state with partial C—H and C—X bond breaking

**Example**:

```

(CH₃)₂CHBr + KOH → (CH₃)₂C=CH₂ + KBr + H₂O

(3° halide → E2 predominates)

```

**Stereochemistry**: **Anti elimination** (stereoelectronic requirement)

H and X must be **anti periplanar** (opposite sides on Newman projection)

In flexible molecules, can rotate to achieve anti geometry

Zaitsev product predominates: double bond forms with substitution on carbon having more alkyl groups

**Zaitsev's rule**: When multiple alkenes possible, major product is alkene with **more substituted double bond** (thermodynamically more stable)

Example: (CH₃)₂CHCHBrCH₃ + base → (CH₃)₂C=CHCH₃ + CH₃CH=C(CH₃)₂ (both possible; first is Zaitsev/major product)

Hofmann product (less substituted alkene) is minor

**Structural factors favoring E2**:

1. **Tertiary halides (3°)**: Most reactive (bulky, favors elimination over substitution)

2. **Secondary halides (2°)**: Moderate reactivity

3. **Primary halides (1°)**: **Cannot undergo E2** with bulky bases (no β-H abstraction; SN2 favored)

**Effect of base strength**: Stronger bases favor E2 over SN2

Strong bases (KOH, KOtBu, NaH): E2 predominates

Weak bases (Br⁻, I⁻): SN2 favored

Example: (CH₃)₃CBr + KOH → (CH₃)₂C=CH₂ (E2); (CH₃)₃CBr + Br⁻ → no reaction (3° halide resists SN2)

**Effect of solvent**: Aprotic solvents favor E2 (higher kinetic barrier to abstraction; base not solvated)

**Key characteristics**:

Bimolecular kinetics

Single-step, concerted mechanism

Anti periplanar H and X geometry

Zaitsev rule: more substituted alkene product

Favored by: strong base, high temperature, 3° halide

E1 Mechanism (Elimination Unimolecular)

**Definition**: Two-step elimination where carbocation intermediate forms first (rate-determining), followed by base abstraction of β-hydrogen.

**Rate law**: Rate = k[R—X] (first order, depends only on halide concentration)

**Mechanism**:

1. **Slow step**: C—X bond breaks → carbocation intermediate

(CH₃)₃CBr → (CH₃)₃C⁺ + Br⁻

2. **Fast step**: Base abstracts β-hydrogen → alkene

(CH₃)₃C⁺ + B⁻ → (CH₃)₂C=CH₂ + BH

**Driving force for step 1**: C—X cleavage driven by solvent polarity (polar solvent stabilizes carbocation)

**Conditions favoring E1**:

3° halides (most stable carbocation)

Polar protic solvents (H₂O, alcohols; solvate carbocation)

Mild heating (provides activation energy)

Weak or no base (no competing SN2)

Elevated temperature

**Rearrangement in carbocation**: If more stable carbocation can form via **hydride or methyl shift**, both E1 and rearranged product possible

Example: 1-bromo-1-methylethylcyclopropane → rearranged E1 product (carbocation ring opens)

**Stereochemistry**: **No specific requirement** (two separate steps allow free rotation)

**Regioselectivity**: **Zaitsev product predominates** (most substituted alkene from most stable carbocation)

**Competition E1 vs SN1**: Both proceed via same carbocation intermediate; product ratio depends on nucleophilicity of solvent vs base concentration

**Key characteristics**:

Unimolecular kinetics

Two-step mechanism with carbocation intermediate

Rearrangement possible

Favored by: 3° halide, polar protic solvent, heat, weak base

Zaitsev rule: more substituted alkene

Summary: SN1, SN2, E1, E2 Mechanistic Competition

| Mechanism | Order | Kinetics | Rate Law | Substrate | Base/Nucleophile | Solvent | Stereochemistry |

|-----------|-------|----------|----------|-----------|------------------|--------|-----------------|

| **SN2** | 1° > 2° > 3° | Bi | k[R-X][Nu] | Primary | Strong Nu | Aprotic | Inversion |

| **SN1** | 3° > 2° > 1° | Uni | k[R-X] | Tertiary | Weak Nu | Protic | Racemization |

| **E2** | 3° > 2°, 1° none | Bi | k[R-X][B] | Tertiary | Strong Base | Aprotic | Anti periplanar |

| **E1** | 3° > 2° > 1° | Uni | k[R-X] | Tertiary | Weak Base | Protic | No requirement |

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Grignard Reagents (Organomagnesium Compounds)

Preparation

**General synthesis**: Alkyl or aryl halides react with magnesium metal in dry ether solvent:

R—X + Mg (dry ether) → R—Mg—X (Grignard reagent)

**Examples**:

CH₃—Br + Mg → CH₃—Mg—Br (methylmagnesium bromide)

C₂H₅—I + Mg → C₂H₅—Mg—I (ethylmagnesium iodide)

C₆H₅—Br + Mg → C₆H