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 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 .
- Acyclic alkanes have the general formula , alkenes with one double bond have , and alkynes with one triple bond have .
- Complete combustion of a hydrocarbon forms carbon dioxide and water, such as .
- 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 , carboxylic acids contain , aldehydes contain , and ketones contain within the chain.
- Structural isomers have the same molecular formula but different connectivity, such as forming butane and 2-methylpropane.
- Addition reactions occur across multiple bonds, such as hydrogenation of ethene: .
- Esterification combines a carboxylic acid and an alcohol to form an ester and water, written generally as .
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 or , 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 .
- 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 or chloro .
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 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 .
Practice Questions
- 1 Find the molecular formula of an acyclic alkane with carbon atoms using .
- 2 Balance the complete combustion equation for propane: .
- 3 Name the compound using IUPAC rules.
- 4 Explain why ethanol, , is more soluble in water than ethane, , even though both contain two carbon atoms.
Understanding Organic Chemistry Basics
Organic molecules are often drawn in several ways, and each drawing shows different information. A molecular formula gives only the number of each atom. A displayed formula shows every bond.
A condensed formula groups atoms to save space. A skeletal formula uses lines for carbon bonds, with carbon atoms understood at line ends and corners. Hydrogens attached to carbon are usually left out in skeletal drawings.
This can feel confusing at first, so count carefully. Every line end or corner represents a carbon unless another element is written. Then add enough hidden hydrogens so each carbon has its usual total of four bonds.
Bond type affects shape, movement, and reactivity. A single carbon to carbon bond can rotate, which gives flexible chains many possible shapes. A double bond cannot rotate freely.
This creates fixed arrangements that can have different properties. In some alkenes, groups lie on the same side or opposite sides of the double bond. These forms are geometric isomers.
Molecules can also be mirror images that do not fit exactly on top of each other. Such molecules matter in medicines because living cells may respond strongly to one mirror image but not the other.
Structure is therefore more than a drawing choice. It can change boiling point, smell, biological effect, and reaction behavior.
Functional groups change how molecules interact with each other. Hydrocarbons are mostly nonpolar, so they do not mix well with water. They tend to dissolve better in nonpolar substances such as oils.
Groups containing oxygen often make a molecule more polar. Alcohol molecules can form hydrogen bonds, which usually raises their boiling points compared with similar hydrocarbons. Small alcohols can mix with water, while long carbon chains reduce this solubility.
Carboxylic acids can donate hydrogen ions in water and often have sharp odors. Esters are common in flavorings and fragrances. Fats, soaps, fuels, plastics, food molecules, medicines, and cleaning products all depend on these links between structure and properties.
When predicting a reaction, first locate the part of the molecule most likely to change. Multiple bonds contain electron density that can attract reacting particles, so they are common sites for addition. A substitution replaces one atom or group with another.
An elimination removes atoms or groups and often creates a multiple bond. Reaction conditions matter. Heat, light, a catalyst, concentration, and solvent can change the main product or whether a reaction happens at all.
In school problems, track atoms before and after the reaction. Check that the same number of each kind of atom appears on both sides.
In practical work, remember that many organic liquids are flammable, volatile, or irritating. Use small amounts, avoid open flames unless instructed, and work with good ventilation.
A reliable naming method prevents most mistakes. Identify the highest priority functional group first because it controls the ending of the name and often must be included in the parent chain. Next choose a parent chain that contains that group and any important multiple bond.
Number from the end that gives the earliest important feature the lowest number. Only after this should you identify branches, alphabetize their names, and use prefixes for repeated branches. Draw the name back into a structure as a final check.
This two way practice is valuable because exams often switch between names, formulas, and line drawings. Pay close attention to small details such as bond positions, branch locations, and the difference between a terminal group and one within a chain.