Chapter 9 — Hydrocarbons

What are hydrocarbons?
Hydrocarbons are organic molecules that contain only carbon and hydrogen atoms. They form the simplest class of organic compounds and are the starting point for learning organic chemistry. Carbon atoms can link together in straight chains, branched chains, or ring structures. The variety comes from how many carbons are present, how they are connected and whether the bonding includes single, double or triple bonds.

Main classes
Hydrocarbons are classified by the types of carbon–carbon bonds they contain:

• Alkanes: contain only single bonds (C–C) and are saturated; general formula CnH2n+2 for open chains.
• Alkenes: contain at least one carbon–carbon double bond (C=C) and are unsaturated; general formula CnH2n for simple acyclic alkenes.
• Alkynes: contain at least one carbon–carbon triple bond (C≡C); general formula CnH2n-2 for simple acyclic alkynes.
• Aromatic hydrocarbons: contain conjugated ring systems with special stability, benzene (C6H6) being the prototypical example.

Structural representations
Several ways are used to show structures. Molecular formula lists element counts (e.g., C3H8). Condensed structural formulas compress bonded atoms (e.g., CH3CH2CH3). Full structural formulas show every bond. Skeletal (line) formulas are convenient for larger molecules: carbon atoms are ends or junctions of lines and hydrogens bonded to carbons are usually omitted. Practice converting between representations — this skill helps naming and predicting reactivity.

Homologous series
Members of a homologous series differ by a CH2 unit and show regular changes in physical properties. For example, the alkanes: methane, ethane, propane, butane, etc., form an homologous series. Regular patterns help predict boiling points, melting points and physical state at room temperature.

Why classification matters
Classification groups compounds with similar chemical behaviour. Saturated alkanes are relatively inert and undergo substitution; unsaturated alkenes and alkynes are more reactive in addition reactions; aromatic compounds prefer substitution that preserves delocalised π-electron systems. Recognising class quickly guides which reactions and tests to consider.

Carbon valence and the need for hybridisation
Carbon has four valence electrons and forms four bonds to complete its octet. To explain observed molecular geometries, atomic orbitals mix to form hybrid orbitals. Hybridisation describes how s and p orbitals combine to form new equivalent orbitals that form sigma bonds.

sp3 hybridisation (alkanes)
In alkanes each carbon is sp3 hybridised: one s and three p orbitals mix to form four sp3 hybrid orbitals arranged tetrahedrally. Each sp3 orbital overlaps with another atom’s orbital to form a sigma (σ) bond. The ideal bond angle is 109.5°. sp3 bonding accounts for the single-bond framework of saturated hydrocarbons and allows free rotation about single bonds.

sp2 hybridisation (alkenes)
In alkenes the carbons of the C=C double bond are sp2 hybridised: one s and two p orbitals mix to give three sp2 orbitals in a plane at 120°; the remaining unhybridised p orbital is perpendicular to this plane and participates in π bonding. A C=C double bond therefore consists of one σ bond formed by sp2–sp2 overlap and one π bond from side-on overlap of unhybridised p orbitals. The π bond restricts rotation and concentrates electron density above and below the plane, making alkenes more reactive toward electrophiles.

sp hybridisation (alkynes)
Alkyne carbons involved in a C≡C triple bond are sp hybridised: one s and one p orbital mix to form two linear sp orbitals at 180°; the two remaining p orbitals form two π bonds perpendicular to each other. The triple bond is linear and much shorter than single bonds. Terminal alkynes bear an sp C–H bond that is relatively acidic compared with sp2 or sp3 C–H.

Sigma and pi bond properties
σ bonds are strong, formed by head-on overlap and allow rotation (unless sterically hindered). π bonds are weaker individually and formed by side-to-side overlap; they create regions of high electron density susceptible to electrophilic attack. Bond lengths decrease and bond energies increase as bond order increases: single < double < triple.

Consequences for reactivity
Hybridisation affects acidity (sp > sp2 > sp3), bond strengths and types of reactions. Understanding hybridisation helps predict molecular shape, bond angles and sites of reactivity and is therefore fundamental to organic chemistry problems and mechanisms.

Purpose and basic steps
IUPAC nomenclature gives each compound a unique, systematic name based on structure. For simple hydrocarbons the name is built from the longest carbon chain (parent), numbering that chain to give lowest locants for substituents and unsaturations, naming substituents and combining these parts in a set format. Practising the steps prevents common mistakes.

Naming alkanes
1) Find the longest continuous carbon chain; this determines the parent name (meth-, eth-, prop-, but-, pent- etc.). 2) Number the chain from the end that gives substituents the lowest possible numbers. 3) Identify and name alkyl substituents (methyl, ethyl, propyl, isopropyl, etc.). 4) If substituents repeat, use prefixes di-, tri-, tetra- and list substituents alphabetically (ignore prefixes when alphabetising). 5) Use hyphens to separate numbers from words and commas to separate numbers. Examples: CH3CH2CH2CH3 → butane; CH3CH(CH3)CH2CH3 → 2-methylbutane.

Naming alkenes and alkynes
Double and triple bonds are indicated by suffixes -ene and -yne. The parent chain must include the multiple bond and numbering should give the lowest possible number to the multiple bond. For alkenes, give the position of the double bond by the number of the first carbon in the double bond (e.g., but-2-ene). For molecules with both double and triple bonds, include both suffixes in the order required by IUPAC and use locants to minimise numbers; practice with specific examples.

Cycloalkanes and substituted benzene
For rings, use the prefix cyclo- (e.g., cyclohexane). When rings have substituents, number the ring to give lowest set of locants to substituents; if the ring is part of a larger chain, decide whether the chain or ring is the parent by the number of carbons and functional groups. Simple substituted benzenes follow substituent naming rules (e.g., chlorobenzene, nitrobenzene).

Tips and pitfalls
Always check for the longest chain — a mistaken parent yields wrong locants. For branched substituents, use parentheses when needed (e.g., 4-(tert-butyl)heptane). Remember that alphabetising ignores prefixes di-, tri- etc. and common multiplicative prefixes. Practice by converting skeletal formulas to names and vice versa until the process becomes reliable.

Definition and general idea
Isomers are compounds with the same molecular formula but different arrangements of atoms. In hydrocarbons, isomerism explains why molecules with identical numbers of C and H can have different properties and reactivities. Two main categories are constitutional (structural) isomers and stereoisomers. Understanding both is essential to naming, drawing and predicting behaviour of organic molecules.

Constitutional (structural) isomers
Constitutional isomers differ in the order in which atoms are connected. For hydrocarbons this means different carbon chains, positions of multiple bonds, and ring versus chain structures. For example, C4H10 has two constitutional isomers: n-butane (a straight chain) and 2-methylpropane (a branched chain). As number of carbons increases the count of constitutional isomers grows rapidly because more distinct connectivities become possible. Systematic drawing, using symmetry to avoid duplicates and checking all possible branching patterns, helps enumerate isomers for small formulas.

Positional isomerism
A subtype of structural isomerism is positional isomerism where a functional feature (like a double bond or substituent) is located at different positions on the same carbon skeleton. For example, 1-butene and 2-butene are positional isomers: both C4H8 but the double bond is at different positions. Positional isomers often have different chemical reactivity and different physical properties such as boiling point and density.

Stereoisomerism in hydrocarbons
Stereoisomers have the same connectivity but different spatial arrangements. In hydrocarbons the most common stereoisomerism is geometric (cis/trans or E/Z) about C=C double bonds where rotation is restricted. Geometric isomerism requires that each carbon of the double bond has two different substituents. cis (or Z) places higher-priority groups on the same side; trans (or E) places them on opposite sides. Geometric isomers show different dipole moments, melting/boiling points and sometimes different chemical behaviour.

Counting isomers: method and cautions
To count isomers correctly, follow these steps: list all possible carbon skeletons (straight and branched), place multiple bonds in every allowable position and check for symmetry to avoid duplicates, identify stereoisomers where geometric constraints apply, and include cyclic isomers when appropriate. Note that some formulas permit both acyclic and cyclic isomers (e.g., C4H8 includes 1-butene, 2-butene (cis/trans), 2-methylpropene, cyclobutane and methylcyclopropane). Always state whether cyclic isomers are included in the count.

Optical isomerism and limitations in simple hydrocarbons
Optical isomerism (chirality) occurs when a carbon has four different groups. Simple saturated hydrocarbons rarely show chirality unless they have substituted carbon frameworks creating asymmetric centres. For class 11 focus mainly on constitutional and geometric isomerism, but be aware that chirality becomes important in substituted hydrocarbon derivatives and later organic chemistry topics.

General physical behaviour
Hydrocarbons are largely non-polar molecules because carbon and hydrogen have similar electronegativities. This makes them insoluble in water but miscible with non-polar organic solvents. Physical properties such as boiling point, melting point and density depend primarily on molecular weight (size), shape (branching) and the presence of unsaturation or rings.

Effect of molecular size
As the number of carbons increases in a homologous series, the surface area and London dispersion forces increase, raising boiling and melting points. For example, methane, ethane and propane are gases at room temperature, whereas octane is a liquid. The rise is approximately steady for straight-chain alkanes but can be irregular due to molecular packing and symmetry affecting melting points.

Effect of branching and shape
Branching lowers boiling point for isomers of the same formula because it reduces surface area available for intermolecular contact and weakens dispersion forces. For example, n-pentane boils higher than 2-methylbutane and much higher than 2,2-dimethylpropane. Symmetry also affects melting points: more symmetrical molecules pack well and tend to have higher melting points.

Unsaturation and aromatic rings
Double and triple bonds change molecular geometry and electron distribution. Alkenes and alkynes generally have boiling points comparable to alkanes of similar molar mass but differ in reactivity and sometimes polarity. Aromatic compounds like benzene have characteristic densities (often slightly greater than alkanes of similar mass) and relatively high boiling points compared with small alkenes due to delocalised π systems and stronger intermolecular interactions like π–π stacking.

Practical implications and safety
Physical properties determine uses: light, volatile hydrocarbons are useful as fuels and solvents but pose evaporation and flammability risks; heavier hydrocarbons serve as lubricants. Volatility affects inhalation risk and flammability. Density below water means most hydrocarbons float on water, relevant for spill response. Always handle hydrocarbons with appropriate ventilation and ignition-source control.

Preparation methods
Alkanes are obtained naturally from petroleum and natural gas by refining. In the laboratory or synthesis, alkanes can be prepared by reduction of alkyl halides (e.g., using hydrogen with Pd or by metal-acid reduction), by the Wurtz reaction (coupling of alkyl halides using sodium in dry ether) to give symmetrical alkanes, and by Kolbe electrolysis of carboxylate salts which gives coupled products. Decarboxylation of sodium carboxylates with soda-lime removes a carbon as CO2 producing an alkane one carbon shorter.

Wurtz reaction and limitations
Wurtz reaction: 2 R–X + 2 Na → R–R + 2 NaX in dry ether. It is simple for making symmetrical alkanes but gives mixtures when two different alkyl halides are used and is not suitable for alkyl halides prone to elimination or rearrangement. Yields may be low and side reactions common.

Combustion of alkanes
Alkanes burn in oxygen producing CO2 and H2O when oxygen is ample. Combustion is highly exothermic: CxHy + (x + y/4) O2 → x CO2 + (y/2) H2O. In limited oxygen incomplete combustion yields CO and carbon (soot), which are hazardous. Combustion properties make alkanes important fuels; the heat of combustion increases with chain length but per-carbon values converge.

Free radical halogenation
Under UV light or heat, alkanes react with halogens via free radical chain mechanisms. Steps: initiation (X2 → 2 X•), propagation (X• extracts H from alkane giving HX and alkyl radical; alkyl radical reacts with X2 to give alkyl halide and X•), and termination (radicals combine). Reactivity depends on bond dissociation energies; tertiary hydrogens are more easily abstracted than secondary or primary, so tertiary substitution predominates in selective bromination.

Practical and safety considerations
Alkanes are flammable and volatile (gases or liquids). In lab work avoid open flames when handling volatile hydrocarbons and ensure good ventilation. Proper disposal and storage reduce fire and health risks.

Overview of elimination methods
Alkenes and alkynes are commonly prepared by removing atoms from saturated precursors: elimination of H and X (dehydrohalogenation) from alkyl halides, or removal of H and OH (dehydration) from alcohols, produces alkenes. Alkynes arise by double dehydrohalogenation of dihalides or by alkylation of acetylide ions. Understanding how conditions influence regiochemistry (which alkene forms) and stereochemistry (cis/trans / E/Z) is essential for synthesis planning.

Dehydration of alcohols to alkenes
Acid-catalysed dehydration converts alcohols to alkenes: the alcohol is protonated, water leaves to give a carbocation (for secondary and tertiary alcohols), and deprotonation yields the alkene. Zaitsev's rule often applies: the more substituted alkene is generally favoured because it is more thermodynamically stable. However, if a bulky base or steric hindrance is present, the Hofmann product (less substituted alkene) may predominate. Carbocation intermediates can undergo hydride or alkyl shifts, producing rearranged alkenes; anticipating these rearrangements is part of predicting products.

Dehydrohalogenation of alkyl halides
Alkyl halides treated with strong bases (e.g., KOH or NaOEt in ethanol) eliminate HX to form alkenes. Mechanism depends on substrate and conditions: E2 is a concerted, bimolecular elimination that requires an anti-periplanar arrangement of the β-H and leaving group and gives stereospecific products; E1 proceeds via carbocation and may give mixtures. Using bulky non-nucleophilic bases (e.g., t-BuOK) favours elimination over substitution and may favour Hofmann product due to steric factors.

Formation of alkynes
Alkynes can be prepared by double dehydrohalogenation of vicinal or geminal dihalides using strong base (e.g., alcoholic KOH at high temperature or NaNH2). For terminal alkynes, formation from dihalides followed by careful deprotonation is standard. Another powerful method is alkylation of acetylide ions: deprotonate a terminal alkyne with NaNH2 to give the acetylide ion, then perform SN2 reaction with a primary alkyl halide to form a new C–C bond and a substituted alkyne. Remember SN2 is best with primary halides to avoid competing elimination or poor yields.

Stereochemical control and practical tips
E2 eliminations have stereochemical constraints: anti-periplanar geometry leads to preferred products and can influence whether cis or trans alkenes form from cyclic substrates. To avoid rearrangements, use conditions that favour concerted E2 over carbocation-forming E1. Choose bases, temperature and solvent to steer the reaction. For alkylation of acetylides, use dry aprotic solvents and primary halides to maximise SN2 reactivity. Understand these principles to plan multi-step syntheses that give the desired alkene or alkyne selectively.

Why alkenes undergo addition
Alkenes contain a π bond that is electron rich compared with σ bonds. The π electrons are more exposed and can interact with electrophiles. When an electrophile approaches an alkene, it accepts electron density from the π bond and sets up a positively charged intermediate; a nucleophile then attaches, completing the addition and converting the C=C into two new σ bonds. This general behaviour explains many common reactions of alkenes.

Two-step ionic pathway
Typical ionic electrophilic addition follows two main steps: first, electrophilic attack on the π bond forms either a carbocation intermediate or a three-membered bridged ion (such as a halonium ion). Second, a nucleophile attacks that intermediate to give the final product. The nature of the intermediate (open carbocation or bridged ion) depends on the electrophile: H+ additions commonly give carbocations, while halogen additions often give bridged halonium ions which influence stereochemistry of attack.

Regioselectivity and Markovnikov's rule
When adding HX (e.g., HBr) to an unsymmetrical alkene, the more stable carbocation intermediate is preferred. Markovnikov's rule summarises this: hydrogen attaches to the carbon with more hydrogens, and the halide attaches to the more substituted carbon. This rule follows from carbocation stability (tertiary > secondary > primary) and guides prediction of major products. Exceptions occur under radical conditions where different mechanisms operate (e.g., peroxide-initiated HBr addition gives anti-Markovnikov product).

Stereochemistry: syn vs anti addition
Stereochemical outcome depends on mechanism. If a bridged halonium ion forms (e.g., bromination), nucleophilic attack occurs from the side opposite the bridged ion, leading to anti addition and trans stereochemistry in cyclic systems. Catalytic hydrogenation adds H2 syn across the double bond because both hydrogens are delivered from the same catalyst surface. For additions involving carbocations, subsequent attack can occur from either side, leading to mixtures or racemisation at stereogenic centres formed during reaction.

Other practical points and exceptions
Reagent choice and reaction conditions influence regiochemistry and stereochemistry. Peroxides and radical initiators change mechanism for HBr (anti-Markovnikov). Bulky substituents and solvents can alter intermediate stability and product distribution. For advanced problems, consider rearrangements of carbocations that give thermodynamically more stable final products. For exam problems, draw the mechanism steps clearly to justify regiochemical assignments.

General reactivity of alkynes
Alkynes are hydrocarbons containing a carbon–carbon triple bond composed of one σ and two π bonds. The triple bond has higher electron density and distinct geometry from alkenes: the bonded carbons are sp hybridised and linear. Alkynes undergo addition reactions similar to alkenes but often in two steps: initial addition converts an alkyne to an alkene, then further addition converts it to an alkane if conditions allow.

Hydrogenation and stereochemical control
Hydrogenation of alkynes can be controlled to stop at the alkene stage. Using Lindlar's catalyst (a poisoned Pd catalyst) gives syn hydrogenation to yield cis-alkenes selectively. Alternatively, dissolving metal reduction (Na/NH3) provides trans-alkenes by an anti addition pathway. Complete hydrogenation with Pd/C converts alkynes fully to alkanes. Choice of catalyst and conditions therefore determines product stereochemistry.

Acidity of terminal alkynes and nucleophilic acetylide formation
Terminal alkynes (RC≡CH) have a relatively acidic hydrogen (pKa ≈ 25) because the sp-hybridised carbon holds electron density more tightly. Strong bases like NaNH2 deprotonate terminal alkynes to give acetylide ions (RC≡C−), which are good nucleophiles. Acetylide ions react with primary alkyl halides in SN2 reactions to form new C–C bonds and extend carbon chains. For successful alkylation avoid secondary or tertiary halides which favour elimination or give poor yields.

Hydrohalogenation and halogenation patterns
Addition of HX to alkynes follows regiochemistry similar to alkenes for the first addition; however, excess HX can add again leading to geminal dihalides (both halogens on the same carbon). Halogenation with X2 first yields a dihaloalkene then a tetrahalide if excess halogen is used. Control of stoichiometry and reaction conditions allows isolation of vinyl halides or further products as desired.

Oxidation and cleavage
Strong oxidants like hot KMnO4 or ozone can cleave triple bonds, yielding carboxylic acids from internal alkynes and CO2 or carboxylates from terminal alkynes under vigorous conditions. Mild oxidation can transform alkynes into diketones. In planning syntheses, avoid harsh oxidants if the alkyne is a necessary functional handle, or use them intentionally when oxidative cleavage is the goal.

Practical considerations
Alkyne chemistry is valuable for forming C–C bonds and building complexity in molecules. Key lab skills include handling strong bases (NaNH2), controlling stoichiometry for partial versus full hydrogenation or halogenation, and selecting primary halides for SN2 alkylations. Safety: many alkynes and reagents are flammable and some are toxic; use proper protective measures and inert atmospheres where required.

Benzene and the nature of aromaticity
Benzene, C6H6, is the archetypal aromatic compound. Its six carbon atoms form a regular hexagonal ring; each carbon is sp2 hybridised, contributing one p orbital with a single electron to a system of six delocalised π electrons above and below the plane. Rather than behaving like alternating single and double bonds, benzene shows bond equalisation: all C–C bonds have identical lengths intermediate between single and double bonds. This delocalisation gives benzene extra stabilisation relative to hypothetical localized structures and underlies the concept of aromaticity.

Hückel's rule and requirements
A planar, cyclic, conjugated π system is aromatic if it contains (4n + 2) π electrons, where n is an integer (Hückel's rule). For benzene n = 1, so 6 π electrons satisfy the rule and account for its special stability. Aromatic compounds resist reactions that would destroy this conjugation and instead undergo reactions that preserve the aromatic system, typically by substituting on the ring rather than adding across π bonds.

Electrophilic aromatic substitution (EAS) mechanism
EAS reactions proceed in two main stages. First, a strong electrophile is generated (for nitration, the nitronium ion NO2+ is formed from HNO3/H2SO4). The aromatic ring attacks the electrophile to form a non-aromatic σ-complex (arenium ion) where one carbon is sp3 hybridised and the ring's delocalisation is temporarily lost. In the second step a base removes a proton from the σ-complex, restoring the aromatic π system. The overall process replaces a ring hydrogen with an electrophile while maintaining aromaticity in the final product.

Directing and activating/deactivating effects
Substituents already on the ring affect both reactivity and regioselectivity. Electron-donating groups (EDGs) increase electron density at ortho and para positions, activating the ring and directing new substitution to those positions. Examples include –OH, –OR and alkyl groups (via hyperconjugation). Electron-withdrawing groups (EWGs) withdraw electron density and typically direct incoming electrophiles to the meta position, as ortho/para σ-complexes place a positive charge adjacent to the withdrawing group and are destabilised. Strength of activation or deactivation influences both rate and position of substitution.

Practical synthetic considerations
Control of regioselectivity is central in synthesis. When multiple substituents are present, predict the major product by considering the dominant directing influence and steric hindrance. Protecting or blocking groups, choice of reaction order, and use of catalysts or alternative methods can achieve selective substitution. Finally, many aromatic compounds and reagents are hazardous (e.g., benzene is carcinogenic); labs must follow safety protocols for handling and disposal.

Why mechanisms are used
Reaction mechanisms provide a stepwise description of how bonds are broken and formed during chemical transformations. They help explain regiochemistry, stereochemistry and reactivity trends. In hydrocarbon chemistry three broad mechanistic categories are most important at this stage: radical, ionic and concerted (pericyclic-like) processes. Recognising which type applies allows you to predict products and conditions needed.

Radical mechanisms
Radical reactions centre on species with unpaired electrons. A classic radical chain reaction includes three phases: initiation (generation of radicals, for example by homolytic cleavage of Cl2 under UV light), propagation (radicals react to form products and regenerate radicals, e.g., Cl• abstracts H from RH to give R• and HCl; R• reacts with Cl2 to form RCl and Cl•) and termination (radicals combine to form stable molecules, stopping the chain). Radical polymerisation and radical halogenation follow this pattern. Radical reactivity favors formation of more stable radicals (tertiary > secondary > primary) which determines site selectivity.

Ionic mechanisms
Ionic mechanisms involve charged intermediates: carbocations (positively charged) in electrophilic additions and SN1 substitutions, and carbanions/nucleophiles (negatively charged) in SN2 or nucleophilic additions. SN2 is a concerted, bimolecular nucleophilic displacement with inversion of configuration at a chiral centre and rate dependent on both substrate and nucleophile. SN1 involves a slow ionisation to form a carbocation followed by fast nucleophilic attack; the rate depends only on substrate concentration. E1 and E2 eliminations follow similar logic: E1 via carbocation intermediate, E2 concerted with anti-periplanar geometry influencing stereochemical outcome.

Introductory view of pericyclic reactions
Pericyclic reactions are concerted processes where bonding changes occur via cyclic movement of electrons in a single transition state. While full treatment requires orbital symmetry considerations, at this level students should recognise that some rearrangements and cycloadditions can proceed without discrete ionic or radical intermediates and that such concerted reactions often have stereospecific outcomes. For example, certain electrocyclisations or concerted additions in more advanced chemistry are pericyclic in nature.

Applying mechanistic reasoning
When faced with a reaction, identify possible intermediates (radical, carbocation, carbanion) by considering reagents and conditions (light/heat for radicals, strong acids for carbocations, strong bases for carbanions). Use stability trends to predict which pathway is favoured and draw curved-arrow mechanisms to visualise electron flow. Mechanistic thinking builds transferable skills for synthetic planning and problem solving in organic chemistry.

Polymer formation from alkenes
Many useful polymers are produced by polymerising simple hydrocarbon monomers. Addition (chain-growth) polymerisation of alkenes like ethene and propene gives polyethylene and polypropylene, respectively. The basic chain polymerisation steps are initiation (creation of an active centre such as a radical), propagation (monomer units add to the growing chain), and termination (active chains stop growing). Control of initiation and chain transfer determines molecular weight and branching.

Types of polymerisation and catalysts
Free-radical polymerisation is widely used for simple vinyl monomers; coordination catalysts such as Ziegler–Natta or metallocene catalysts give better control of stereochemistry and produce high-density or isotactic polymers. Radical polymerisation is more tolerant of impurities but provides less control over stereoregularity. Coordination polymerisation inserts monomers into a metal–carbon bond on the catalyst, allowing precise control over chain structure and tacticity, which affects crystallinity and mechanical strength.

Industrial hydrocarbon processing
Crude oil is separated by fractional distillation into gas, petrol, kerosene, diesel and heavy residues. Processes such as catalytic cracking, hydrocracking and reforming convert heavy fractions into lighter, more valuable products like petrol and olefins. Steam cracking of naphtha or ethane is a major source of ethene and propene used as feedstocks for plastics. Hydrogenation, desulfurisation and catalytic reforming tune product compositions to meet fuel specifications and environmental standards.

Polymer properties and uses
Polymer properties depend on monomer chemistry, chain length, branching and crystallinity. Low-density polyethylene (LDPE) is branched and flexible, while high-density polyethylene (HDPE) is linear and crystalline with higher strength. Polypropylene’s stereoregularity (isotactic vs atactic) affects its melting point and mechanical properties. Applications range from packaging films to pipes, fibres and engineering plastics.

Environmental and sustainability concerns
Polymers from hydrocarbons are useful but create waste management challenges. Recycling, development of biodegradable alternatives, and reducing single-use plastics are important. Process improvements to increase efficiency, reduce emissions and recycle monomers are active industrial priorities. Students should understand both the chemistry and the environmental context when considering hydrocarbon-based materials.

Preparation routes
Alkyl halides are prepared by halogenation of hydrocarbons under radical conditions, or from alcohols using halogenating reagents. Alcohols react with SOCl2, PCl5 or with HBr/HCl (often in presence of ZnCl2 for Lucas reaction) to form alkyl halides. Benzylic and allylic hydrogens are particularly susceptible to radical halogenation, allowing selective functionalisation at those positions.

Nucleophilic substitution mechanisms
Alkyl halides undergo nucleophilic substitution by SN2 (concerted bimolecular) or SN1 (unimolecular via carbocation) pathways. SN2 is favoured by primary substrates and strong nucleophiles and gives inversion of configuration at stereocentres. SN1 is favoured by tertiary substrates and polar protic solvents, because carbocations are stabilised; SN1 leads to racemisation at chiral centres. Predict which mechanism applies by assessing substrate structure, nucleophile strength and solvent.

Elimination reactions and competition
Under basic conditions alkyl halides can eliminate HX to form alkenes. E2 is a concerted bimolecular elimination requiring an anti-periplanar geometry between β-H and leaving group; E1 proceeds via carbocation and may accompany SN1 depending on conditions. Temperature, base strength and substrate determine whether substitution or elimination is dominant. Bulky bases and elevated temperature usually favour elimination.

Synthetic utility and precautions
Alkyl halides are versatile intermediates: they undergo organometallic formation (Grignard reagents) giving strong nucleophiles for carbon–carbon bond formation, and they can be transformed into alcohols, ethers, nitriles, etc., through substitution. However, many halogenated compounds are toxic or environmentally persistent; handle with care and dispose appropriately. Choose reagents like SOCl2 for clean conversion of alcohols to chlorides when rearrangements must be avoided.

Practical tips
For SN2 alkylations prefer primary halides and polar aprotic solvents. For converting alcohols to halides without rearrangement use thionyl chloride (SOCl2) with pyridine or similar conditions. For benzylic functionalisation, radical bromination using NBS provides selectivity for benzylic positions under controlled radical conditions.

Combustion chemistry
Combustion is an oxidation reaction where hydrocarbons react with oxygen to form CO2 and H2O under complete combustion. The balanced stoichiometry is CxHy + (x + y/4) O2 → x CO2 + (y/2) H2O. Incomplete combustion due to limited oxygen yields CO, soot (carbon) and various unburnt hydrocarbons, which are harmful pollutants. Combustion efficiency and emission control are important in engines and industry.

Controlled oxidation in synthesis
Mild oxidation can convert hydrocarbons into functionalised products: for example, oxidation of alkyl side chains on aromatic rings with KMnO4 converts benzylic methyl groups to benzoic acid if a benzylic hydrogen is present. Ozonolysis or strong oxidants can cleave multiple bonds, producing carbonyl compounds or carboxylic acids depending on substitution. Choose oxidants and conditions carefully to achieve desired transformations without over-oxidation.

Environmental consequences of hydrocarbon use
Combustion of fossil hydrocarbons releases CO2, a greenhouse gas contributing to climate change. Emissions of CO, NOx, SOx and particulates affect air quality and human health. Hydrocarbon spills on land or water harm ecosystems and can persist in sediments. Some hydrocarbons, such as benzene, are toxic and carcinogenic; occupational exposure limits and monitoring are required in industry and laboratories.

Mitigation strategies and cleaner technologies
Measures to reduce environmental impact include improving combustion efficiency, catalytic converters to reduce CO and NOx emissions, desulfurisation of fuels to lower SOx formation and development of low-emission fuels (biofuels, hydrogen). For polymers and plastics, recycling and designing for degradability are important. Process intensification and green chemistry approaches aim to reduce waste, energy use and pollution.

Laboratory and practical safety
When working with oxidants and combustibles in the lab, follow safety rules: use fume hoods, keep oxidants away from organic materials that could ignite, and store flammable solvents in appropriate containers. Proper waste segregation, spill response plans and environmental awareness are essential parts of chemical practice.

Simple chemical tests for unsaturation
Laboratory tests allow quick classification of hydrocarbons. The bromine test uses brown Br2 in an inert solvent (e.g., CCl4): an alkene or alkyne will add bromine and decolourise the solution, indicating unsaturation. The Baeyer test uses cold, dilute KMnO4: unsaturated compounds oxidise KMnO4 (purple) to MnO2 (brown precipitate), indicating presence of C=C. These tests are qualitative and must be carried out with care, using small amounts and proper waste handling.

Separation and purification methods
Fractional distillation separates hydrocarbon mixtures based on boiling points and is central to petroleum refining. Simple distillation works for components with widely separated boiling points. For laboratory-scale purification, column chromatography on silica or alumina using non-polar eluents can separate hydrocarbons by polarity and size. Drying agents such as anhydrous CaCl2 or Na2SO4 remove residual water from hydrocarbon solvents; allow sufficient time for drying and filter off the drying agent before use.

Introductory spectroscopy
Infrared (IR) spectroscopy detects bond vibrations: aliphatic C–H stretches appear near 2850–2960 cm−1; sp2 C–H (alkenes and aromatics) around 3000–3100 cm−1; sp C–H (terminal alkynes) near 3300 cm−1; C=C stretches appear near 1600–1680 cm−1; aromatic rings show characteristic bands in the 1500–1600 cm−1 region. Proton NMR (1H NMR) gives information on different hydrogen environments: aliphatic protons resonate at 0.5–2.5 ppm, vinylic protons at 4.5–6.5 ppm, and aromatic protons around 7–8 ppm. Splitting patterns (n+1 rule) show neighbouring protons, and integration gives relative proton counts.

Combining tests and spectra
No single test gives full structure; combine chemical tests with spectroscopic data and elemental analysis for confident identification. For example, bromine test positive and IR C=C stretch suggests an alkene; 1H NMR with vinylic signals confirms the substitution pattern. Use chromatographic separation to isolate components from mixtures before spectroscopic analysis when possible.

Safety, disposal and good practice
Perform all tests in a fume hood, wear gloves and goggles, and use minimal amounts. Dispose of organic wastes and halogenated solvent residues according to institutional rules — never pour them down the drain. Keep oxidants and reactive reagents properly labelled and stored. Good lab technique ensures reliable results and protects health and environment.