Organic Chemistry: Alkanes and Halogenoalkanes

Alkanes and halogenoalkanes form the bedrock of organic reaction mechanisms. Understanding them is essential because alkanes, sourced from crude oil, are our primary fuels and feedstocks, while halogenoalkanes are versatile intermediates that bridge simple hydrocarbons to a vast array of functional groups. Mastering their nomenclature and characteristic reactions—free radical substitution and nucleophilic substitution—provides the logical framework for predicting and explaining the behavior of more complex molecules.

IUPAC Nomenclature: The Systematic Language of Chemistry

Before diving into reactions, you must be able to name compounds unambiguously. The International Union of Pure and Applied Chemistry (IUPAC) nomenclature provides a standardized set of rules. For alkanes—saturated hydrocarbons containing only single C–C and C–H bonds—the name is based on the longest continuous carbon chain. Prefixes indicate the number of carbons (meth-, eth-, prop-, but-, etc.), and the suffix "-ane" denotes an alkane. Branches (alkyl groups like methyl or ethyl) are named and their positions indicated by numbering the main chain to give the lowest possible numbers.

For halogenoalkanes (or alkyl halides), the halogen is treated as a substituent. The root name is derived from the alkane chain, and halogens are indicated using prefixes like fluoro-, chloro-, bromo-, and iodo-. For example, a three-carbon chain with a chlorine on the middle carbon is 2-chloropropane. The priority level of substituents does not affect numbering in halogenoalkanes; you simply number the chain to give the halogen the lowest number.

Free Radical Substitution: Halogenating Alkanes

Alkanes are relatively unreactive due to the strength of their C–C and C–H bonds. However, they undergo free radical substitution with halogens like chlorine or bromine in the presence of ultraviolet (UV) light. This mechanism involves three distinct stages: initiation, propagation, and termination. A free radical is a highly reactive species with an unpaired electron.

The initiation step occurs under UV light, which provides the energy to break the halogen-halogen bond homolytically. For chlorine: $${\text{Cl}}_2\stackrel{\text{UV light}}{\to }2{\text{Cl}}^{\bullet }$$ This produces two chlorine radicals.

The propagation steps are chain reactions that consume reactants while regenerating radicals. First, a chlorine radical abstracts a hydrogen from methane, forming hydrogen chloride and a methyl radical: $${\text{Cl}}^{\bullet }+{\text{CH}}_4\to \text{HCl}+{\text{CH}}_3^{\bullet }$$ The methyl radical then collides with a chlorine molecule, forming chloromethane and a new chlorine radical: $${\text{CH}}_3^{\bullet }+{\text{Cl}}_2\to {\text{CH}}_3\text{Cl}+{\text{Cl}}^{\bullet }$$ This new chlorine radical can then begin the cycle again, leading to a chain reaction.

Termination steps occur when two radicals combine, removing the reactive intermediates from the mixture. Examples include: $${\text{Cl}}^{\bullet }+{\text{Cl}}^{\bullet }\to {\text{Cl}}_2$$ $${\text{CH}}_3^{\bullet }+{\text{Cl}}^{\bullet }\to {\text{CH}}_3\text{Cl}$$ $${\text{CH}}_3^{\bullet }+{\text{CH}}_3^{\bullet }\to {\text{C}}_2{\text{H}}_6$$ A key limitation is the lack of selectivity; with longer alkanes, a mixture of isomeric mono-substituted and poly-substituted products is formed due to the similar reactivity of different C–H bonds.

Nucleophilic Substitution: The Reactivity of Halogenoalkanes

The polar C–X bond (where X is a halogen) in halogenoalkanes makes them susceptible to attack by nucleophiles. A nucleophile is a species with a lone pair of electrons that is attracted to a region of positive charge. Two primary mechanisms exist: $S_{N}2$ and $S_{N}1$.

The $S_{N}2$ mechanism (Substitution, Nucleophilic, Bimolecular) is a one-step, concerted process. The nucleophile attacks the carbon atom bonded to the halogen from the side opposite the leaving group (backside attack). This causes an inversion of configuration at the carbon centre, akin to an umbrella turning inside out. The rate equation is second order: Rate = $k[\text{RX}][{\text{Nu}}^{-}]$. This mechanism is favoured for primary halogenoalkanes and with strong, small nucleophiles (e.g., ${\text{OH}}^{-}$, ${\text{CN}}^{-}$) in polar aprotic solvents like acetone.

In contrast, the $S_{N}1$ mechanism (Substitution, Nucleophilic, Unimolecular) occurs in two distinct steps. First, the slow, rate-determining step is the spontaneous ionization of the halogenoalkane to form a planar, positively charged carbocation intermediate and the halide ion. Second, the nucleophile rapidly attacks the carbocation from either side. The rate equation is first order: Rate = $k[\text{RX}]$. $S_{N}1$ is favoured for tertiary halogenoalkanes, as the tertiary carbocation intermediate is most stable due to hyperconjugation and inductive effects. It is also promoted by polar protic solvents (like water or ethanol) that can stabilize the ionic intermediates.

The choice between $S_{N}1$ and $S_{N}2$ depends on the structure of the halogenoalkane (primary favouring $S_{N}2$, tertiary favouring $S_{N}1$, secondary competing), the strength and nature of the nucleophile, and the solvent.

Elimination versus Substitution Competition

Halogenoalkanes reacting with nucleophiles that are also strong bases (like hydroxide, ${\text{OH}}^{-}$) face a competition. The base can act as a nucleophile, leading to substitution (forming an alcohol), or it can remove a $\beta$-hydrogen (a hydrogen on a carbon adjacent to the C–X bond), leading to elimination and forming an alkene.

The outcome is controlled by reaction conditions and substrate structure. Primary halogenoalkanes predominantly undergo $S_{N}2$ substitution with ${\text{OH}}^{-}$ unless conditions are harsh (high temperature, concentrated ethanolic ${\text{OH}}^{-}$), which promotes elimination. Tertiary halogenoalkanes primarily undergo elimination with strong bases like ${\text{OH}}^{-}$, as the bulky substrate hinders $S_{N}2$ attack and the $S_{N}1$ pathway can be outcompeted by the base abstracting a proton. Secondary substrates give a mixture of both products. The general rule is that high temperature, strong bases, and steric hindrance at the $\alpha$-carbon favour elimination over substitution.

Common Pitfalls

1. Incorrectly Applying Nomenclature Rules: A common error is failing to find the longest chain or incorrectly numbering it. Remember, the goal is to give the lowest set of numbers to the substituents, not necessarily to have a substituent on carbon 1. For halogenoalkanes, halogens have equal priority with alkyl branches; numbering is based on the first point of difference.
2. Confusing Radical and Polar Mechanisms: Students often mistakenly use curly arrows to show the movement of single electrons in free radical mechanisms. Curly arrows show the movement of electron pairs and should only be used for polar mechanisms like $S_{N}1$ and $S_{N}2$. For radical propagation, use "fishhook" arrows to denote single electron movement.
3. Oversimplifying $S_{N}1$ vs. $S_{N}2$ Predictions: While the primary/tertiary guideline is helpful, it's a pitfall to ignore the role of the nucleophile/base and solvent. For example, a tertiary halogenoalkane in a weakly basic, ionizing solvent may still proceed via $S_{N}1$, not elimination. Always consider all factors: substrate, nucleophile/leaving group, and solvent.
4. Misunderstanding the Elimination/Substitution Product Mixture: Expecting a single pure product from reactions of secondary halogenoalkanes with ${\text{OH}}^{-}$ is incorrect. You must be prepared to identify and explain the formation of both alkene (elimination) and alcohol (substitution) products, and rationalize how conditions shift the balance.

Summary

• IUPAC nomenclature provides systematic names based on the longest carbon chain, with substituents named and numbered to give the lowest possible locants.
• Free radical substitution of alkanes with halogens proceeds via a three-step chain mechanism (initiation, propagation, termination) requiring UV light and producing mixtures of products.
• Halogenoalkanes undergo nucleophilic substitution via two main mechanisms: the one-step, stereospecific $S_{N}2$ pathway (favoured for primary substrates) and the two-step $S_{N}1$ pathway that goes through a carbocation intermediate (favoured for tertiary substrates).
• The choice between $S_{N}1$ and $S_{N}2$ is influenced by the structure of the halogenoalkane, the nature of the nucleophile, and the solvent used.
• With strong bases, halogenoalkanes face competition between substitution and elimination; high temperature, strong bases, and hindered centres favour the formation of alkenes via elimination.