Organic Chemistry Basics Cheat Sheet

A printable reference covering hydrocarbons, functional groups, isomers, naming rules, reactions, and organic formulas for grades 11-12.

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Organic chemistry studies carbon-containing compounds and the patterns that control their structures, names, properties, and reactions. Students need this cheat sheet because organic formulas can look complex unless they are organized by bonding, functional groups, and naming rules. It gives a compact reference for identifying molecules, drawing structures, and predicting common reaction types. It is designed for quick review before problem sets, labs, quizzes, and exams. The core ideas are that carbon usually forms four covalent bonds, functional groups control reactivity, and structure affects physical properties. Hydrocarbons include alkanes, alkenes, alkynes, and aromatic compounds, with general formulas such as $\mathrm{C}_{n}\mathrm{H}_{2n+2}$ for acyclic alkanes. Important reactions include combustion, substitution, addition, elimination, esterification, and polymerization. IUPAC naming depends on finding the longest carbon chain, numbering to give the lowest locants, and naming substituents and functional groups correctly.

Key Facts

Carbon is tetravalent, so a neutral carbon atom in most organic molecules forms four bonds, as in $\mathrm{CH}_{4}$ .
Acyclic alkanes have the general formula $\mathrm{C}_{n}\mathrm{H}_{2n+2}$ , alkenes with one double bond have $\mathrm{C}_{n}\mathrm{H}_{2n}$ , and alkynes with one triple bond have $\mathrm{C}_{n}\mathrm{H}_{2n-2}$ .
Complete combustion of a hydrocarbon forms carbon dioxide and water, such as $\mathrm{CH}_{4} + 2\mathrm{O}_{2} \rightarrow \mathrm{CO}_{2} + 2\mathrm{H}_{2}\mathrm{O}$ .
In IUPAC naming, choose the longest continuous carbon chain as the parent and number it to give the lowest possible locants to multiple bonds, substituents, and functional groups.
Alcohols contain the hydroxyl functional group $\mathrm{-OH}$ , carboxylic acids contain $\mathrm{-COOH}$ , aldehydes contain $\mathrm{-CHO}$ , and ketones contain $\mathrm{C{=}O}$ within the chain.
Structural isomers have the same molecular formula but different connectivity, such as $\mathrm{C}_{4}\mathrm{H}_{10}$ forming butane and 2-methylpropane.
Addition reactions occur across multiple bonds, such as hydrogenation of ethene: $\mathrm{C}_{2}\mathrm{H}_{4} + \mathrm{H}_{2} \rightarrow \mathrm{C}_{2}\mathrm{H}_{6}$ .
Esterification combines a carboxylic acid and an alcohol to form an ester and water, written generally as $\mathrm{RCOOH} + \mathrm{R'OH} \rightleftharpoons \mathrm{RCOOR'} + \mathrm{H}_{2}\mathrm{O}$ .

Vocabulary

Hydrocarbon: A compound made only of carbon and hydrogen atoms, such as an alkane, alkene, alkyne, or aromatic compound.
Functional group: A specific atom or group of atoms, such as $\mathrm{-OH}$ or $\mathrm{-COOH}$ , that gives an organic molecule characteristic properties and reactions.
Homologous series: A family of organic compounds with the same functional group and a pattern in which neighboring members differ by $\mathrm{CH}_{2}$ .
Isomer: One of two or more compounds with the same molecular formula but different structures or spatial arrangements.
Saturated compound: An organic compound with only single carbon-carbon bonds, meaning it has the maximum number of hydrogen atoms for its carbon skeleton.
Substituent: An atom or group of atoms attached to the parent carbon chain, such as methyl $\mathrm{-CH}_{3}$ or chloro $\mathrm{-Cl}$ .

Common Mistakes to Avoid

Forgetting carbon's four-bond rule is wrong because most neutral organic structures must give each carbon exactly four bonds, counting a double bond as two and a triple bond as three.
Choosing a shorter parent chain is wrong because IUPAC names are based on the longest continuous carbon chain that includes the highest-priority functional group or multiple bond when required.
Numbering from the wrong end is wrong because locants must be as low as possible for functional groups, double bonds, triple bonds, and substituents according to priority rules.
Confusing molecular and structural formulas is wrong because $\mathrm{C}_{4}\mathrm{H}_{10}$ gives atom counts only, while a structural formula shows whether the compound is butane or 2-methylpropane.
Writing unbalanced combustion equations is wrong because atoms must be conserved, so both carbon and hydrogen must be balanced before oxygen in reactions such as $\mathrm{C}_{x}\mathrm{H}_{y} + \mathrm{O}_{2} \rightarrow \mathrm{CO}_{2} + \mathrm{H}_{2}\mathrm{O}$ .

Practice Questions

1 Find the molecular formula of an acyclic alkane with $n = 7$ carbon atoms using $\mathrm{C}_{n}\mathrm{H}_{2n+2}$ .
2 Balance the complete combustion equation for propane: $\mathrm{C}_{3}\mathrm{H}_{8} + \mathrm{O}_{2} \rightarrow \mathrm{CO}_{2} + \mathrm{H}_{2}\mathrm{O}$ .
3 Name the compound $\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}(\mathrm{CH}_{3})\mathrm{CH}_{3}$ using IUPAC rules.
4 Explain why ethanol, $\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}$ , is more soluble in water than ethane, $\mathrm{CH}_{3}\mathrm{CH}_{3}$ , even though both contain two carbon atoms.

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