Alkyl Halides and Nucleophilic Substitution (Chapter 6 Study Notes)

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Alkyl Halides and Nucleophilic Substitution

Halogenated Organic Compounds

Halogenated organic compounds are classified based on the type of carbon to which the halogen is attached. These compounds are central to organic synthesis and reactivity.

Alkyl halides: Halogen is directly bonded to an sp3 carbon (e.g., CH3CH2Br, cyclohexyl chloride).
Vinyl halides: Halogen is bonded to an sp2 carbon of an alkene (e.g., CH2=CHCl).
Aryl halides: Halogen is bonded to an sp2 carbon on a benzene ring (e.g., bromobenzene).

Polarity and Reactivity

The carbon-halogen bond is polar due to the higher electronegativity of halogens compared to carbon. This polarity imparts reactivity to alkyl halides.

Partial charges: Carbon acquires a partial positive charge (δ+), while halogen becomes partially negative (δ-).
Nucleophilic attack: The electron-deficient carbon can be attacked by a nucleophile, leading to substitution reactions.

General substitution reaction:

IUPAC Nomenclature of Alkyl Halides

Alkyl halides are named as haloalkanes according to IUPAC rules.

Select the longest carbon chain, even if the halogen is not bonded to it.
Number the chain to give the lowest possible numbers to the halogen positions.
Use prefixes (fluoro-, chloro-, bromo-, iodo-) for halogens.

Examples:

2-chlorobutane: CH3CHClCH2CH3
4-(2-fluoroethyl)heptane: CH2CH2F attached to the fourth carbon of heptane
1,3-dichlorocyclohexane: Two chlorines on cyclohexane ring at positions 1 and 3

Common and Trivial Names

Some alkyl halides have common names, especially for small alkyl groups.

Isobutyl bromide: (1-bromo-2-methylpropane)
Sec-butyl bromide: (2-bromobutane)
Tert-butyl bromide: (2-bromo-2-methylpropane)
Isopropyl bromide: (2-bromopropane)
Ethyl bromide: (bromoethane)

Alkyl halides derived from methane have trivial names:

CH3Cl: methyl chloride (chloromethane)
CH2Cl2: methylene chloride
CHCl3: chloroform
CCl4: carbon tetrachloride

Alkyl Halides Classification

Alkyl halides are classified by the degree of substitution at the carbon bonded to the halogen.

Methyl halide: Halogen attached to a methyl group (CH3X).
Primary (1°) alkyl halide: Halogen attached to a carbon bonded to one other carbon.
Secondary (2°) alkyl halide: Halogen attached to a carbon bonded to two other carbons.
Tertiary (3°) alkyl halide: Halogen attached to a carbon bonded to three other carbons.

Types of Dihalides

Dihalides contain two halogen atoms and are classified as geminal or vicinal.

Geminal dihalide: Both halogens on the same carbon.
Vicinal dihalide: Halogens on adjacent carbons.

Uses of Alkyl Halides

Alkyl halides have diverse applications:

Industrial and household cleaners
Anesthetics: Chloroform (CHCl3) and Halothane (CF3CHClBr)
Freons: Used as refrigerants and foaming agents; environmental concerns have led to their replacement.
Pesticides: DDT is toxic to insects but persistent in the environment.

Physical Properties of Alkyl Halides

Dipole Moments

Dipole moments arise from differences in electronegativity and bond length.

Electronegativity order: F > Cl > Br > I
Bond length order: C–F < C–Cl < C–Br < C–I
Bond dipole order: C–Cl > C–F > C–Br > C–I

Molecular dipole: Depends on geometry; e.g., CCl4 is nonpolar due to symmetrical bond orientation.

Boiling Points of Alkyl Halides

Boiling points are influenced by intermolecular forces and molecular mass.

Greater molar mass leads to higher boiling point.
London dispersion forces increase with atom size.
Spherical shape decreases boiling point.

Alkyl Halide	Boiling Point (K)
CH3F	195
CH3Cl	249
CH3Br	278
CH3I	316

Densities

Alkyl fluorides and alkyl chlorides (with one Cl) are less dense than water (1.00 g/mL).
Alkyl chlorides with two or more Cl, alkyl bromides, and alkyl iodides are denser than water.
Examples: CH3CH2Cl = 0.921 g/mL; CH2Cl2 = 1.33 g/mL; ethyl bromide = 1.47 g/mL; ethyl iodide = 1.94 g/mL.

Preparation of Alkyl Halides

Free Radical Halogenation

Alkyl halides can be prepared by halogenation of alkanes via a free radical mechanism.

Chlorination is not selective; produces mixtures if hydrogens are nonequivalent.
Bromination is highly selective: 3° > 2° > 1° carbons.

Allylic Bromination

Bromination at the allylic position (carbon adjacent to a double bond) is achieved using Br2 or N-bromosuccinimide (NBS).

Mechanism involves free radicals and resonance stabilization of the allylic radical.
NBS keeps Br2 concentration low, favoring allylic bromination.

Reactions of Alkyl Halides

Types of Reactions

Nucleophilic substitution: Nucleophile replaces the halide.
Elimination: Halide and a hydrogen are removed, forming an alkene (covered in Chapter 7).

The SN2 Reaction

The SN2 (bimolecular nucleophilic substitution) reaction is a concerted, one-step process.

Rate law:
Backside attack by nucleophile leads to inversion of configuration (Walden inversion).
Transition state is highest in energy; product is lowest.

Classes of Compounds Synthesized via SN2

Nucleophile	Product	Class of Product
I-	R–I	Alkyl halide
OH-	R–OH	Alcohol
RO-	R–OR	Ether
SH-	R–SH	Thiol
NH2-	R–NH2	Amine salt
N3-	R–N3	Azide
CN-	R–CN	Nitrile

Nucleophilic Strength

Nucleophilic strength affects reaction rate.

Stronger nucleophiles react faster.
Strong bases are often strong nucleophiles, but not all strong nucleophiles are strong bases (e.g., I-).

Basicity Versus Nucleophilicity

Basicity: Ability to abstract a proton (H+).
Nucleophilicity: Ability to attack an electrophilic carbon atom.

Trends in Nucleophilicity

Negatively charged nucleophile is stronger than its neutral counterpart:
Nucleophilicity decreases left to right across a period.
Nucleophilicity increases down a group due to increased size and polarizability.

Polarizability Effect

Larger atoms (e.g., I-) are more nucleophilic than smaller ones (e.g., F-) due to their soft, easily polarizable electron shells.

Solvent Effects

Protic Solvents

Polar protic solvents (e.g., water, alcohols) have acidic hydrogens that can hydrogen bond and solvate nucleophiles, reducing nucleophilicity.
Nucleophilicity increases with atom size in protic solvents.

Aprotic Solvents

Polar aprotic solvents (e.g., acetonitrile, DMF, acetone, DMSO) do not hydrogen bond and do not solvate nucleophiles, allowing SN2 reactions to proceed faster.

Crown Ethers

Crown ethers solvate cations, increasing the nucleophilic strength of the anion (e.g., fluoride becomes a good nucleophile).

Leaving Group Ability

Best leaving groups are electron-withdrawing, stable (not strong bases), and polarizable.
Examples: halides, sulfonates, water.

Effect of Substrate Structure on SN2 Reactions

Relative rates: methyl > primary > secondary >> tertiary (tertiary halides do not react via SN2 due to steric hindrance).

Stereochemistry of SN2

Backside attack leads to inversion of configuration (Walden inversion).
Chiral centers invert configuration during SN2 reactions.

The SN1 Reaction

The SN1 (unimolecular nucleophilic substitution) reaction proceeds via a carbocation intermediate and is first order in alkyl halide.

Rate law:
Racemization occurs if the substrate is chiral.
Mechanism: (1) Formation of carbocation (slow, rate-determining), (2) Nucleophilic attack (fast), (3) Deprotonation if nucleophile is neutral.

Carbocation Stability

Order of stability: 3° > 2° > 1° > methyl
Stabilized by inductive effect and hyperconjugation.

Stereochemistry of SN1

Produces mixtures of enantiomers; more inversion than retention.

Carbocation Rearrangement

Carbocations can rearrange via hydride or methyl shifts to form more stable carbocations.

Comparison: SN1 vs. SN2 Mechanisms

Feature	SN2	SN1
Substrate	Methyl, 1°, 2°	3°, 2°
Nucleophile	Strong	Weak
Solvent	Aprotic	Protic
Rate Law
Stereochemistry	Inversion	Racemization

Practice Questions

Name the haloalkane shown, including R/S configuration if needed.
Provide the IUPAC name for the given compound with multiple halogens and stereochemistry.
Arrange alkyl halides in order of increasing boiling point.
Identify the alkyl halide with the lowest boiling point.
Draw resonance structures and the hybrid for an allylic radical intermediate.
Predict the major organic product for allylic bromination with NBS.
Provide the structure of the major product for free radical bromination.
Identify the type of intermediate in an SN2 reaction.
Draw the product of an SN2 reaction with cyanide and bromoethane.
Predict the major product for SN2 reactions with various nucleophiles.
Determine which alkyl halide reacts most rapidly via SN2 with NaCN.

Additional info: These notes cover the essential concepts and mechanisms for alkyl halides and nucleophilic substitution, including physical properties, nomenclature, preparation, and detailed mechanistic comparisons of SN1 and SN2 reactions.