Haloalkanes and Nitriles: Structure and Naming

Haloalkanes (alkyl halides) are alkanes in which one or more hydrogen atoms have been replaced by a halogen, and nitriles are compounds containing the cyano group (–C≡N); both are important building blocks because the polar C–X and C≡N groups are easily converted into other functional groups. In brief: 1-bromopropane reacts with potassium cyanide to give butanenitrile, which on hydrolysis gives butanoic acid, a way to lengthen a carbon chain by one atom.

Haloalkanes have been used as solvents, refrigerants, anesthetics and the raw materials for plastics, and their release has caused environmental problems such as the damage to the ozone layer. Nitriles are important in synthesis and in polymers such as acrylic fibers. This article explains how to name and classify these compounds, compares the carbon–halogen bonds in a table, describes their properties and main reactions and shows examples of synthetic sequences.

How are haloalkanes named and classified?

The halogen is named as a prefix (fluoro-, chloro-, bromo-, iodo-) on the alkane chain, with its position given by a number, in alphabetical order with other substituents: 1-chlorobutane, 2-bromopropane, 2-bromo-2-methylpropane, 1,2-dichloroethane. Common names such as methyl chloride and tert-butyl bromide are still used. Haloalkanes are classified by the carbon bearing the halogen, like alcohols:

• Primary: the carbon is bonded to one other carbon (1-bromopropane).
• Secondary: bonded to two carbons (2-bromopropane).
• Tertiary: bonded to three carbons (2-bromo-2-methylpropane).

How do the carbon–halogen bonds compare?

The C–X bond is polar, with a partial positive charge on carbon, which makes it an electrophilic center. Yet the reactivity in substitution follows the bond strength, not the polarity: iodoalkanes react fastest and fluoroalkanes slowest, since the C–I bond is the weakest and I⁻ is the best leaving group. This can be shown by warming a haloalkane with aqueous silver nitrate: silver iodide (yellow) forms fastest, then silver bromide (cream) and then silver chloride (white).

What are the physical properties?

Haloalkanes are polar but cannot form hydrogen bonds, so they are not very soluble in water, but are miscible with organic solvents. Their boiling points are higher than those of the alkanes with the same number of carbon atoms, and increase with the size of the halogen and the number of halogens: CH₃Cl −24.2 °C, CH₃Br 3.6 °C, CH₃I 42.4 °C, CH₂Cl₂ 39.6 °C, CHCl₃ 61.2 °C and CCl₄ 76.7 °C. Many are denser than water.

What are the reactions of haloalkanes?

• Nucleophilic substitution (the halogen is replaced): with aqueous NaOH or KOH to give alcohols (C₂H₅Br + OH⁻ → C₂H₅OH + Br⁻), with KCN in ethanol to give nitriles, with ammonia to give amines and with alkoxides to give ethers.
• Elimination: with hot ethanolic KOH to give alkenes: C₂H₅Br + KOH → C₂H₄ + KBr + H₂O.
• Formation of Grignard reagents: RBr + Mg in dry ether → RMgBr, a reagent for forming new carbon–carbon bonds.

What are nitriles?

Nitriles contain the group –C≡N, in which the carbon and nitrogen are sp hybridized and the group is linear. They are named by adding -nitrile to the alkane name, counting the nitrile carbon in the chain: CH₃CN is ethanenitrile (acetonitrile), CH₃CH₂CN is propanenitrile and C₆H₅CN is benzonitrile. A nitrile is also named as a carbonitrile or a cyano- prefix on rings or in compounds where another group has priority. The group is very polar (the dipole moment of ethanenitrile is 3.9 D), so nitriles have relatively high boiling points: ethanenitrile 81.6 °C, propanenitrile 97 °C and benzonitrile 191 °C. Ethanenitrile is miscible with water, and is a common solvent in chemical analysis (HPLC).

Preparation and reactions

• Preparation: RBr + CN⁻ → RCN + Br⁻ (heated under reflux in ethanol). This adds one carbon to the chain.
• Hydrolysis: RCN + 2H₂O + H⁺ → RCOOH + NH₄⁺ (acid) or RCOO⁻ + NH₃ (base).
• Reduction: RCN + 4[H] → RCH₂NH₂, with LiAlH₄ or H₂/Ni, giving a primary amine.

Worked examples

Example 1: naming

CH₃CHBrCH₂CH₃ is 2-bromobutane (secondary). (CH₃)₃CCl is 2-chloro-2-methylpropane (tertiary). CH₃CH₂CH₂CN is butanenitrile, with four carbons including that of the nitrile group.

Example 2: a three-step synthesis

Prepare propanoic acid from bromoethane: (1) C₂H₅Br + KCN → C₂H₅CN (propanenitrile); (2) hydrolysis with dilute HCl under reflux gives CH₃CH₂COOH. The product has one more carbon atom than the starting haloalkane.

Example 3: yield of nitrile

From 10.0 g of 1-bromopropane (M = 123.0 g/mol, 0.0813 mol) and an excess of KCN, the theoretical yield of butanenitrile (M = 69.11 g/mol) is 0.0813 × 69.11 = 5.62 g. If 4.10 g is obtained, the percentage yield is 4.10 / 5.62 × 100 = 73 %.

Example 4: ranking reactivity

Rank 1-chlorobutane, 1-bromobutane and 1-iodobutane by their rate of hydrolysis. The order of the bond enthalpies is C–Cl > C–Br > C–I, so the rate of substitution is 1-iodobutane > 1-bromobutane > 1-chlorobutane.

Where are they used and what are the hazards?

• Solvents: dichloromethane (paint stripper, extraction), trichloromethane (chloroform) and tetrachloromethane were widely used, but are now restricted because of their toxicity, and carbon tetrachloride because of its effect on the ozone layer and liver.
• Polymers: chloroethene gives PVC, and tetrafluoroethene gives PTFE (Teflon).
• Anesthetics: halothane (CF₃CHClBr) was widely used from the 1950s.
• Refrigerants and propellants: CFCs were replaced by HCFCs and HFCs.
• Nitriles: acrylonitrile (CH₂=CHCN) is polymerized to acrylic fibers and used in rubber and ABS plastic; ethanenitrile is a laboratory solvent. Many nitriles, and the cyanide ion used to make them, are toxic.

What is the history of CFCs?

Chlorofluorocarbons such as CCl₂F₂ (Freon-12) were developed by Thomas Midgley and his colleagues in 1928 as safe, non-flammable refrigerants to replace ammonia and sulfur dioxide. In 1974 Mario Molina and F. Sherwood Rowland showed that CFCs, which are inert in the lower atmosphere, release chlorine atoms in the stratosphere under ultraviolet light, and these catalyze the destruction of ozone. The Montreal Protocol of 1987 phased out their production, and Molina, Rowland and Paul Crutzen received the Nobel Prize in Chemistry in 1995. The ozone hole above Antarctica is now slowly recovering.

Which mistakes are common?

• Counting the nitrile carbon wrongly: it is part of the chain, so CH₃CN has two carbons and is ethanenitrile.
• Assuming that the strongest bond is the most reactive: reactivity follows the weakness of C–X, so iodides are the most reactive.
• Using aqueous KCN: water gives alcohols as a side product; use ethanol.
• Forgetting to classify: the class decides the mechanism (SN1 or SN2).
• Ignoring toxicity: many haloalkanes and all cyanides are hazardous.