Chapter 15: Hydrocarbons

Hydrocarbons , focusing on hydrocarbons, establishes crude oil as the primary source of these compounds, which are molecules composed solely of carbon and hydrogen, serving as both fuels and key industrial starting materials. The discussion begins with alkanes, which are characterized as saturated hydrocarbons defined by the general formula CnH2n+2, featuring only carbon-carbon single covalent bonds and sp3 hybridization. Alkanes are generally unreactive due to their non-polar nature, resulting from the minimal difference in electronegativity between carbon and hydrogen, making them resistant to attack by nucleophiles or electrophiles. Alkanes are widely used as fuels. When burned in abundant oxygen, they undergo complete combustion to produce carbon dioxide and water. However, in limited oxygen, they undergo incomplete combustion, yielding toxic carbon monoxide gas. Burning hydrocarbon fuels in vehicles also releases acidic nitrogen oxides (NO and NO2), formed at high engine temperatures, and unburnt volatile organic compounds (VOCs), which contribute to acid rain and photochemical smog. These pollutants are mitigated by catalytic converters, which oxidize carbon monoxide and unburnt hydrocarbons to carbon dioxide and water, and reduce nitrogen oxides to nitrogen gas, though they do nothing to reduce the output of carbon dioxide. Alkanes can also undergo free-radical substitution reactions with halogens like chlorine or bromine when exposed to ultraviolet light. This mechanism proceeds through three steps: initiation (homolytic fission of the halogen bond creating free radicals), propagation (a chain reaction generating new products and regenerating free radicals), and termination (free radicals combining). This substitution process is often unsuitable for preparing specific halogenoalkanes due to the resulting mixture of products. The chapter then shifts to alkenes, unsaturated hydrocarbons containing a carbon-carbon double bond, represented by the general formula CnH2n. To meet the high demand for smaller, more useful hydrocarbons like gasoline, the process of cracking converts larger, less useful hydrocarbon molecules into smaller alkanes and reactive alkenes using high heat and a catalyst like aluminum oxide. Alkenes are significantly more reactive than alkanes because the double bond contains an electron-rich pi bond, making them susceptible to attack by electrophiles. The characteristic reaction of alkenes is addition across the double bond. Key addition reactions include hydrogenation (adding hydrogen over a platinum or nickel catalyst to form an alkane), the addition of halogens (like bromine water, which is decolorized, serving as a test for unsaturation), the addition of hydrogen halides, and the addition of steam (using concentrated phosphoric acid catalyst to form alcohols). The mechanism for these additions is electrophilic addition. When hydrogen halides add to asymmetric alkenes, the major product is determined by the inductive effects of alkyl groups, which are electron-donating, stabilizing the positive charge of the secondary carbocation intermediate (making it more energetically stable than a primary carbocation). Alkenes can also be oxidized: cold, dilute acidified potassium manganate(VII) solution (KMnO4) results in a diol, while hot, concentrated acidified KMnO4 solution completely breaks the C=C bond to yield products like ketones or carboxylic acids, a technique used to determine the position of the original double bond. Finally, alkenes undergo addition polymerisation, where unsaturated monomers (like ethene) react to form long-chain polymers (like poly(ethene)). The disposal of poly(alkene)s is difficult because they are chemically inert and non-biodegradable. While burning them can conserve fossil fuels, it also releases carbon dioxide, contributing to enhanced global warming, and if waste poly(chloroethene) is burned, toxic compounds like hydrogen chloride gas and dioxins can be released.