Haloalkanes

Overview and Classification of Haloalkanes

Haloalkanes (also called alkyl halides) are organic compounds in which one or more hydrogen atoms of an alkane are replaced by halogen atoms (fluorine, chlorine, bromine, or iodine). Their general formula can be written as:

$$\text{C}_n\text{H}_{2n+1}\text{–X} \quad \text{or more generally} \quad \text{R–X}$$

where R is an alkyl group and X is a halogen.

Key features that make haloalkanes a distinct class:

• Strongly polarized $ \text{C–X} $ bond (especially for $ \text{C–F},\ \text{C–Cl},\ \text{C–Br} $).
• The carbon bonded to the halogen is often electrophilic (electron-poor), making haloalkanes typical substrates in substitution and elimination reactions.
• Halogens influence physical properties such as boiling point and solubility.

Haloalkanes are classified in several ways.

Classification by Degree of Substitution at the Halogen-Bearing Carbon

This classification concerns the carbon atom directly bonded to the halogen (the $\alpha$-carbon):

• Primary (1°) haloalkane: the $\alpha$-carbon is bonded to one other carbon (or none, in methyl halides).

Example: $ \text{CH}_3\text{CH}_2\text{–Br} $ (bromoethane).

• Secondary (2°) haloalkane: the $\alpha$-carbon is bonded to two other carbons.

Example: $ (\text{CH}_3)_2\text{CH–Cl} $ (2-chloropropane).

• Tertiary (3°) haloalkane: the $\alpha$-carbon is bonded to three other carbons.

Example: $ (\text{CH}_3)_3\text{C–Br} $ (2-bromo-2-methylpropane).

This classification is important because it strongly affects reaction mechanisms (for example, which substitution or elimination pathways are favored).

Classification by Number and Type of Halogen Atoms

Haloalkanes can also be categorized according to how many hydrogen atoms have been replaced by halogens on the same carbon:

• Monohaloalkanes: one halogen atom in the molecule.

Example: $ \text{CH}_3\text{CH}_2\text{–Cl} $ (chloroethane).

• Dihaloalkanes: two halogen atoms in the molecule.

Example: $ \text{ClCH}_2\text{CH}_2\text{Cl} $ (1,2-dichloroethane).

• Trihaloalkanes, tetrahaloalkanes, etc.

In di- and polyhaloalkanes, the halogens can be:

• On the same carbon (geminal dihalides, “geminal”).
• On adjacent carbons (vicinal dihalides, “vicinal”).
• On more distant carbons (further substituted).

The type of halogen matters as well:

• Fluoroalkanes: often very stable (strong $ \text{C–F} $ bond).
• Chloroalkanes and bromoalkanes: commonly used as synthetic intermediates.
• Iodoalkanes: usually more reactive in substitution/elimination (weaker $ \text{C–I} $ bond, better leaving group).

Nomenclature of Haloalkanes

Haloalkanes are named according to IUPAC rules (systematic names). The halogen is indicated as a substituent on the parent alkane.

IUPAC Naming Principles

1. Choose the parent chain: the longest continuous carbon chain containing the carbon bonded to the halogen.
2. Number the chain so that:
• The position of the halogen substituent(s) gets the lowest possible number.
• If other substituents (e.g. alkyl groups) are present, the lowest set of locants is assigned following standard IUPAC priority rules (halogens are treated like alkyl substituents for numbering purposes).
3. Name and locate the halogen substituents:
• Fluoro (F), chloro (Cl), bromo (Br), iodo (I).
• Use numbers to indicate positions and prefixes di-, tri-, tetra- when multiple identical halogens are present.
4. Assemble the name:
• List substituents (including halo groups) in alphabetical order.
• End with the name of the parent alkane.

Simple Examples

• $ \text{CH}_3\text{CH}_2\text{Cl} $:

Parent: ethane, substituent: chloro on carbon 1
Name: chloroethane (or 1-chloroethane; “1-” is superfluous here).

• $ \text{CH}_3\text{CHClCH}_3 $:

Parent: propane, halogen on carbon 2
Name: 2-chloropropane.

• $ \text{CH}_3\text{CH}_2\text{CHBrCH}_3 $:

Parent: butane, substituent on carbon 2 (or 3; choose the lowest number)
Name: 2-bromobutane.

• $ \text{CCl}_4 $:

Parent: methane, four chloro substituents on carbon 1
IUPAC name: tetrachloromethane.
Common name: carbon tetrachloride.

Multiple Halogens and Other Substituents

For molecules with multiple halogens or other substituents:

• Use multiple locants: e.g. $ \text{CH}_3\text{CHBrCHBrCH}_3 $ is 2,3-dibromobutane.
• If both halogens and alkyl substituents are present, all substituents are treated equally, but they are listed alphabetically in the name.

Example:
$ \text{CH}_3\text{–CH(CH}_3\text{)–CH}_2\text{–Cl} $
Parent: propane, substituents: methyl at C2, chloro at C3
Name: 3-chloro-2-methylpropane (chloro and methyl ordered alphabetically).

Structure and Bonding in Haloalkanes

The C–X Bond

The key structural feature is the polar $ \text{C–X} $ bond. Its character depends on the halogen:

• Electronegativity: F > Cl > Br > I.
  This means:
• $ \text{C–F} $ is strongly polarized and very strong.
• $ \text{C–Cl} $ and $ \text{C–Br} $ are moderately strong and polar.
• $ \text{C–I} $ is relatively weak (and still polar).
• Bond length and strength:

Going down the group from F to I:

• Bond length increases.
• Bond strength decreases.
• The halogen becomes a better leaving group in many reactions.

This polarity can be represented qualitatively by:

$$\delta^+ \text{C–X} \delta^-$$

The carbon carries a partial positive charge and is therefore susceptible to nucleophilic attack.

Geometry

Haloalkanes are typically based on tetrahedral $ \text{sp}^3 $ carbon centers:

• The carbon of the $ \text{C–X} $ bond is roughly $ \text{sp}^3 $ hybridized.
• Bond angles around this carbon are close to $109.5^\circ$.

This geometry becomes important for understanding stereochemical aspects of substitution and elimination reactions.

Physical Properties of Haloalkanes

Haloalkanes show characteristic physical properties compared to the parent alkanes.

Boiling and Melting Points

• Introducing a halogen generally raises the boiling point compared with the corresponding alkane, due to:
• Increased molecular mass.
• Increased polarizability, especially for Cl, Br, I.
• Stronger intermolecular interactions (dipole–dipole and London dispersion forces).
• For isomeric haloalkanes:
• More compact (branched) structures usually have lower boiling points than less branched isomers (similar trend as in alkanes, but modified by dipole effects).

Solubility

• In water:
• Most simple haloalkanes are only slightly soluble or essentially insoluble in water.
• They are less polar than, for example, alcohols, and they cannot form hydrogen bonds with water to a significant extent (except for some special cases and when strongly polarized).
• In organic solvents:
• Haloalkanes are usually well soluble in nonpolar and moderately polar organic solvents (e.g. hexane, ether).

Because of their density and limited water solubility, some chlorinated and brominated haloalkanes can form separate layers under water, often as heavier-than-water liquids.

Density

• Many haloalkanes, especially chlorinated and brominated ones, have densities greater than water (e.g. chloroform, carbon tetrachloride).
• Fluoroalkanes are often less dense than their heavier halogen analogues.

Preparation of Haloalkanes

Haloalkanes can be synthesized in a number of ways. Here, only the specific reactions leading to haloalkanes are outlined briefly; detailed mechanisms of the reaction types themselves are covered elsewhere.

From Alkanes: Free Radical Halogenation

Alkanes can react with halogens such as $ \text{Cl}_2 $ or $ \text{Br}_2 $ under the influence of light or heat to form haloalkanes:

Example:

$$
\text{CH}_4 + \text{Cl}_2 \xrightarrow{h\nu} \text{CH}_3\text{Cl} + \text{HCl}
$$

Features:

• Proceeds via a radical chain mechanism.
• Often gives mixtures of products due to multiple possible substitution sites (especially for larger alkanes).
• Bromination is more selective than chlorination; fluorination can be dangerously vigorous.

From Alkenes: Addition of Hydrogen Halides or Halogens

Haloalkanes can be formed by addition reactions of alkenes.

1. Addition of hydrogen halides (HX):

$$\text{CH}_2= \text{CH}_2 + \text{HBr} \longrightarrow \text{CH}_3\text{CH}_2\text{Br}$$

Regioselectivity often follows Markovnikov’s rule (the proton adds to the carbon with more hydrogens, the halide to the more substituted carbon).

2. Addition of halogens (X$_2$):

Yields vicinal dihaloalkanes:

$$
\text{CH}_2= \text{CH}_2 + \text{Br}_2 \longrightarrow \text{BrCH}_2\text{CH}_2\text{Br}
$$

From Alcohols: Substitution of the Hydroxyl Group

Alcohols can be converted into haloalkanes via substitution of the $ \text{OH} $ group:

1. Using hydrogen halides (HX):

$$\text{R–OH} + \text{HBr} \longrightarrow \text{R–Br} + \text{H}_2\text{O}$$

Reactivity depends on the structure of the alcohol; tertiary alcohols usually react more readily than primary ones.

2. Using phosphorus halides or thionyl chloride:
• $ \text{PCl}_3,\ \text{PCl}_5,\ \text{PBr}_3 $.
• $ \text{SOCl}_2 $ (thionyl chloride) commonly used to convert alcohols to chloroalkanes:

$$\text{R–OH} + \text{SOCl}_2 \longrightarrow \text{R–Cl} + \text{SO}_2 + \text{HCl}$$

These methods are important in laboratory synthesis because they provide more control and fewer side products than free radical halogenation of alkanes.

From Other Functional Groups

Haloalkanes can also be obtained by:

• Conversion of carboxylic acids to acyl halides followed by reduction, depending on the target structure.
• Substitution of suitable leaving groups (e.g. tosylates, mesylates) with halide ions.

Details of these transformations depend on the functional group being modified and belong to the general chemistry of functional group interconversions.

Typical Reactions of Haloalkanes

Because haloalkanes contain a polarized $ \text{C–X} $ bond with a potential leaving group (X$^-$), they are classic examples of compounds that undergo substitution and elimination reactions.

Here the focus is on:

• Recognizing haloalkanes as substrates for these reaction types.
• Seeing how structure (1°, 2°, 3°) and the halogen affect reactivity.

The detailed mechanisms of these reactions (e.g. SN1, SN2, E1, E2) belong to chapters on reaction types and will be treated there more comprehensively.

Nucleophilic Substitution Reactions

In nucleophilic substitution, a nucleophile (Nu$^-$ or Nu:) replaces the halogen:

$$\text{R–X} + \text{Nu}^- \longrightarrow \text{R–Nu} + \text{X}^-$$

Typical nucleophiles include:

• Hydroxide: $ \text{OH}^- $
• Alkoxide: $ \text{RO}^- $
• Cyanide: $ \text{CN}^- $
• Ammonia and amines

Haloalkanes are key starting materials for:

• Alcohols (via substitution with $ \text{OH}^- $).
• Ethers (with alkoxides).
• Nitriles (with $ \text{CN}^- $).
• Amines (with ammonia).

The type of haloalkane is important:

• Primary haloalkanes usually favor bimolecular substitution (SN2) with strong nucleophiles.
• Tertiary haloalkanes are more prone to unimolecular substitution (SN1) under suitable conditions (e.g. polar protic solvents).

These mechanistic details will be treated in the chapters on reaction types and kinetics; here it is enough to note that the structure of the haloalkane controls which pathway is possible or favored.

Elimination Reactions

Haloalkanes can undergo elimination to form alkenes when treated with a base:

General scheme (dehydrohalogenation):

$$\text{R–CH}_2\text{–CH}_2\text{X} + \text{Base}^- \longrightarrow \text{R–CH=CH}_2 + \text{HX}$$

Typical bases:

• Alkoxides (e.g. $ \text{NaOEt} $).
• Hydroxide in hot, concentrated solution.

Key points:

• Elimination competes with substitution.
• More substituted haloalkanes (especially tertiary) often favor elimination with strong bases, leading to more substituted alkenes (Zaitsev’s rule).

Again, the detailed distinction between E1 and E2 mechanisms belongs to the general reaction-type discussion; in this chapter, the important point is that haloalkanes are important precursors to alkenes via elimination.

Other Transformations

Because the halogen is a good leaving group, haloalkanes also participate in many other reactions:

• Formation of organometallic reagents (e.g. Grignard reagents $ \text{RMgX} $, organolithium compounds $ \text{RLi} $).
• Coupling reactions and other carbon–carbon bond-forming processes (using specialized catalysts and reagents).

These will be dealt with when discussing the respective reaction types and organometallic chemistry; haloalkanes serve as a common entry point for such transformations.

Stereochemical Aspects (Introduction)

Haloalkanes that possess a stereogenic (chiral) carbon center can display stereoisomerism. The presence of a halogen at a stereocenter can lead to:

• Formation of R/S enantiomers.
• Changes in configuration during certain reactions (e.g. inversion of configuration in SN2-type processes).

For haloalkanes:

• A carbon bearing four different substituents, one of which is a halogen, is chiral.
• Substitution reactions can either invert or racemize configurations depending on the mechanism.

The detailed stereochemical consequences of different reaction types are treated in the chapters on isomerism and reaction mechanisms; in this chapter it is essential only to recognize that haloalkanes can be stereochemically active substrates.

Environmental and Practical Aspects (Overview)

Haloalkanes are widespread in technology and everyday life, but some also pose environmental and health concerns.

Uses

• Solvents: e.g. dichloromethane, chloroform (though some uses have declined due to toxicity).
• Refrigerants and propellants: historically chlorofluorocarbons (CFCs).
• Intermediates in synthesis: large variety of pharmaceuticals, agrochemicals, plastics.

Environmental Concerns

• Some chlorinated and brominated haloalkanes are persistent and can undergo long-range transport in the atmosphere.
• Certain haloalkanes (especially older CFCs and halons) are known to play a role in ozone layer depletion.
• Some are toxic or carcinogenic, requiring careful handling and proper disposal.

These aspects connect haloalkane chemistry to environmental chemistry and applied chemistry; more in-depth treatment follows in the chapters on environmental chemistry and materials.

In summary, haloalkanes are a central class of organic compounds characterized by a polarized $ \text{C–X} $ bond. Their classification (1°, 2°, 3°; mono-, di-, polyhalo; type of halogen), their physical properties, and their roles as substrates in substitution and elimination reactions make them foundational for understanding the broader chemistry of organic functional groups and synthetic transformations.