10.1 Names and Structures of Alkyl Halides

Alkyl halides are organic compounds where a halogen atom (F, Cl, Br, or I) is bonded directly to an $sp^3$-hybridized carbon. Naming them correctly and understanding their structural features sets the foundation for predicting their physical properties and chemical reactivity, especially in the substitution and elimination reactions you'll encounter later in this unit.

Alkyl Halide Nomenclature and Structure

IUPAC nomenclature for alkyl halides

Halogens are treated as substituents (not part of the parent chain name), so the naming process follows the same general IUPAC rules you already know for alkanes, with a few specifics:

1. Find the longest continuous carbon chain that contains the carbon bearing the halogen. Name it using the standard prefixes (meth-, eth-, prop-, but-, pent-, hex-, etc.) with the suffix "-ane."

2. Number the chain from the end that gives the halogen the lowest possible locant.

3. Name the halogen as a prefix: fluoro, chloro, bromo, or iodo.

4. List all substituents alphabetically. Halogens are treated just like alkyl substituents for alphabetical ordering. For example, bromo comes before methyl, and chloro comes before ethyl.

   • Use multiplying prefixes (di-, tri-, tetra-) when two or more identical substituents are present. These prefixes don't affect alphabetical order.

5. Assemble the name: locants + substituent names + parent chain name, separated by hyphens and commas as needed.

For example, a four-carbon chain with a chlorine on C-2 and a methyl group on C-3 becomes 2-chloro-3-methylbutane.

Structural formulas from IUPAC names

Working backward from a name to a structure is a skill worth practicing deliberately:

1. Draw the parent chain. The suffix tells you the chain length ("pentane" = five carbons).
2. Number the carbons from left to right (you can always renumber later if needed).
3. Attach the halogen(s) at the specified positions ("2-bromo" = bromine on C-2).
4. Attach any other substituents at their positions ("3-methyl" = a $CH_3$ group on C-3).
5. Fill in hydrogens so that every carbon has four bonds total.

Try this with 2-bromo-3-methylpentane: draw a five-carbon chain, place Br on C-2, place a methyl group on C-3, then complete the hydrogens.

Classification of alkyl halides

Alkyl halides are classified by the type of carbon the halogen is bonded to:

• Primary (1°): The halogen-bearing carbon is attached to one other carbon (e.g., 1-bromobutane).
• Secondary (2°): The halogen-bearing carbon is attached to two other carbons (e.g., 2-bromobutane).
• Tertiary (3°): The halogen-bearing carbon is attached to three other carbons (e.g., 2-bromo-2-methylpropane).

This classification matters a great deal because it directly controls whether the compound favors $S_N1$, $S_N2$, $E1$, or $E2$ pathways.

Properties of halomethanes

The halomethanes ($CH_3F$, $CH_3Cl$, $CH_3Br$, $CH_3I$) are the simplest alkyl halides and illustrate how the identity of the halogen affects physical properties.

• Bond lengths increase as the halogen gets larger: C–F < C–Cl < C–Br < C–I. A larger halogen has a larger atomic radius, so the bonding orbital overlap occurs at a greater distance.
• Bond strengths follow the opposite trend: C–F > C–Cl > C–Br > C–I. Shorter bonds mean better orbital overlap and stronger bonds. The C–F bond (~485 kJ/mol) is one of the strongest single bonds in organic chemistry, while the C–I bond (~234 kJ/mol) is relatively weak and breaks easily.
• Dipole moments depend on the electronegativity difference between carbon and the halogen, as well as the bond length. The dipole moment is defined as $\mu = Q \times r$, where $Q$ is the magnitude of the partial charge and $r$ is the bond length. Fluorine is the most electronegative halogen, so $CH_3F$ has the largest dipole moment despite having the shortest bond length. The trend is: $CH_3F > CH_3Cl > CH_3Br > CH_3I$.
• Boiling points increase with molecular mass: $CH_3F < CH_3Cl < CH_3Br < CH_3I$. Heavier molecules have stronger London dispersion forces, which require more energy to overcome.
• Water solubility decreases as the halogen gets larger. Smaller halomethanes can interact with water through dipole-dipole forces, but as the nonpolar hydrocarbon portion grows relative to the polar C–X bond, solubility drops.

Reactivity of alkyl halides

The carbon-halogen bond is polar because the halogen is more electronegative than carbon. This makes the carbon electrophilic (electron-poor) and the halogen a potential leaving group.

• In substitution reactions, a nucleophile (an electron-rich species) attacks the electrophilic carbon and replaces the halogen.
• In elimination reactions, a base removes a hydrogen from a carbon adjacent to the C–X bond, forming a double bond (alkene) as the halogen leaves.

The strength of the C–X bond influences how readily the halogen departs. Because C–I bonds are the weakest, iodide is generally the best leaving group among the common halogens, and fluoride is the worst. You'll explore how substrate classification (1°, 2°, 3°), the nucleophile/base, and the solvent all interact to determine which reaction pathway dominates in the sections ahead.