Hydrocarbons, petroleum and fuels
Hydrocarbons and structural families
A hydrocarbon contains only carbon and hydrogen. Alkanes have only carbon–carbon single bonds and are saturated in the introductory sense. An acyclic alkane has formula CₙH₂ₙ₊₂. Acyclic monoalkenes have one carbon–carbon double bond and formula CₙH₂ₙ, while acyclic monoalkynes have one triple bond and formula CₙH₂ₙ₋₂. These formulas depend on the acyclic and bond-count assumptions; a ring can change hydrogen count without adding a double bond. Aromatic hydrocarbons such as benzene have a different delocalised structure and characteristic chemistry. Always inspect bonding or structural information before classifying an unknown solely from one empirical ratio.
Naming and drawing isomers
Use methane, ethane, propane and butane as the first four unbranched alkane names, then pentane and hexane. Branches receive names such as methyl and suitable locants. The longest continuous parent chain may turn a corner in the drawing, so follow bonds rather than the page's horizontal direction. Alkenes and alkynes require the relevant multiple bond in the parent and its location where needed. But-1-ene and but-2-ene illustrate position isomerism; butane and 2-methylpropane illustrate chain isomerism. A drawing flipped or rotated may represent the same , so check connectivity before counting a supposed new isomer.
Alkanes and substitution
Alkanes are relatively unreactive under many mild conditions but undergo combustion and suitable substitution reactions. A simplified chlorination example is under suitable light or initiation. Further substitutions can occur, so the actual mixture may contain several products rather than only one neatly selected . The reaction replaces hydrogen with chlorine rather than adding across a double bond. Conditions and product control matter in industrial chemistry. Do not infer that alkanes are completely inert or safe to breathe because they do not rapidly decolourise bromine water in an ordinary controlled unsaturation test.
Alkenes and addition reactions
An alkene's double bond permits addition that makes new single bonds. Ethene can add hydrogen to form ethane with a suitable , add bromine to form a dibromo compound or react with steam under appropriate catalytic conditions to form ethanol. Addition polymerisation joins many ethene units into polyethene without removing a small in the ideal equation. Bromine-water decolourisation under suitable conditions supports reactive unsaturation, but does not unrestrictedly identify every possible organic compound. Geometric isomerism requires two different substituents on each double-bond carbon; ethene lacks that requirement even though it contains a double bond.
Alkynes and aromatic behaviour
Ethyne has a carbon–carbon triple bond and formula C₂H₂. Its addition reactions and combustion differ in detail from those of an alkane. School descriptions may discuss preparation from a suitable carbide and water under controlled apparatus, but ethyne handling requires specialist safety controls. Benzene is not accurately described as a simple rapidly alternating cyclohexatriene. Its delocalised electron arrangement helps explain why it favours substitution in many familiar reactions rather than behaving like an ordinary alkene in a mild bromine test. Suitable catalysts and conditions are required for aromatic substitutions. Similarity in a line drawing does not make all carbon–carbon unsaturation equally reactive.
Crude petroleum and fractional
Crude oil is a complex mixture of hydrocarbons and other substances. Fractional separates portions with overlapping boiling ranges rather than producing one pure compound in each fraction. In a simplified column, hotter lower regions collect less volatile, larger- fractions while cooler upper regions collect more volatile fractions. Fractions can refinery gases, petrol components, kerosene, diesel, lubricating materials and heavy residues under the refinery's design. Boiling behaviour, viscosity and flammability vary across the mixture. The process is mainly physical separation; it does not by itself break every large hydrocarbon into small ones. Product names and exact ranges depend on specifications and processing.
Cracking, reforming and petrochemicals
Cracking breaks larger hydrocarbon molecules into smaller products, often including an alkane and alkene in simplified examples. Thermal and catalytic routes have different conditions and product patterns. Balance atoms when proposing a possible cracking equation; several products may be possible, so one balanced equation is not the only real outcome. Reforming rearranges molecular structures to improve useful properties or produce chemical feedstocks. Petrochemicals provide starting materials for polymers, solvents, detergents and many other products. Distinguish chemical conversion from fractionation. The fact that a product originated from petroleum does not determine final properties without its structure and processing history.
Fuel combustion and octane number
Complete hydrocarbon combustion forms carbon dioxide and water under sufficient oxygen and suitable mixing; incomplete combustion can form carbon monoxide and soot. Balance carbon and hydrogen first, then oxygen. Octane rating concerns to knocking in the defined spark-ignition fuel comparison, not simply total per litre or the percentage of one arbitrary eight-carbon . Engine design and the stated fuel specification determine the required rating. Diesel ignition quality uses a different comparison concept. Increasing octane number does not automatically make every engine more efficient. Good explanations identify measured fuel behaviour rather than treating a quality number as a universal ranking of all properties.
Environmental and practical trade-offs
Fuel production and use can involve greenhouse emissions, air pollutants, spills and ecological effects. Sulfur impurities can contribute to sulfur-oxide emissions; incomplete combustion and poor maintenance can increase carbon monoxide and particulates. Renewable or alternative fuels still require evaluation of feedstock production, land use, processing and emissions. Hydrogenation or refining may improve one property while introducing cost and requirements. State comparison boundaries before declaring a fuel clean or efficient. Organic liquids can be flammable and vapours hazardous; laboratory tests need controlled quantities, suitable heating methods and ventilation rather than open-ended household experimentation.
Self-check with equations
Complete butane combustion is. One simplified cracking equation is C₁₀H₂₂ → C₈H₁₈ + C₂H₄, conserving carbon and hydrogen. Ethene hydrogenation is C₂H₄ with an appropriate . These represent combustion, cracking and addition respectively. A refinery fraction containing many similar-boiling compounds is still a mixture, even with one commercial name. Before answering a hydrocarbon question, identify the structural family, process type, conditions and whether the required formula is molecular, structural or a balanced reaction. Check all carbon valencies in a structure and all counts in an equation.