Organic Chemistry Basics
Grade 11 · Science · Study Guide
Discover carbon-based compounds — from simple hydrocarbons to functional groups — and the reactions that build drugs, fuels, and living organisms.
Organic chemistry is the study of carbon-containing compounds. Carbon is uniquely versatile — it has four valence electrons, can form four covalent bonds, and can chain to itself in rings, branches, and long straight lines. This structural diversity accounts for over 10 million known organic compounds. The simplest class is hydrocarbons, containing only carbon and hydrogen. Alkanes contain only single C-C bonds (CₙH₂ₙ₊₂) and are called saturated because they hold the maximum number of hydrogens. Alkenes have at least one C=C double bond (CₙH₂ₙ) and alkynes have at least one C≡C triple bond (CₙH₂ₙ₋₂). These are unsaturated because they can accept more hydrogen across the multiple bond.
Functional groups are specific atom groupings that determine a molecule's chemical behavior. An alcohol (−OH) is polar and hydrogen-bonds readily, making many alcohols water-soluble. Carboxylic acids (−COOH) donate protons and are weakly acidic — vinegar is acetic acid. Amines (−NH₂) act as weak bases and are found in amino acids. Esters (−COO−) form from acids and alcohols and are responsible for many fruit aromas. Aldehydes (−CHO) and ketones (>C=O) both contain carbonyl groups and are important in metabolic chemistry.
Isomers are compounds with the same molecular formula but different structural arrangements. Structural isomers differ in the connectivity of atoms (e.g., butane vs. isobutane, both C₄H₁₀). Geometric isomers (cis/trans) differ in arrangement around a double bond. Optical isomers (enantiomers) are non-superimposable mirror images — critically important in drug chemistry because the two forms can have completely different biological effects.
Key reaction types in organic chemistry: addition reactions add atoms across a double or triple bond; substitution reactions replace one atom or group with another; elimination reactions remove atoms to form a new pi bond; condensation reactions join two molecules with loss of a small molecule like water (this is how proteins and polysaccharides are built). Hydrolysis is the reverse — breaking a bond with water.
| Functional Group | Structure | Example Compound | Key Property |
|---|---|---|---|
| Alcohol | −OH | Ethanol (C₂H₅OH) | Polar, hydrogen bonding, water-soluble |
| Carboxylic acid | −COOH | Acetic acid (CH₃COOH) | Weakly acidic, proton donor |
| Amine | −NH₂ | Methylamine (CH₃NH₂) | Weakly basic, accepts protons |
| Ester | −COO− | Ethyl acetate | Fruity odor, formed from acid + alcohol |
| Aldehyde | −CHO | Formaldehyde (HCHO) | Reactive carbonyl, oxidized to acid |
| Ketone | >C=O | Acetone (CH₃COCH₃) | Good solvent, less reactive than aldehyde |
| Class | Bond Type | Formula | Example |
|---|---|---|---|
| Alkane | Single C-C only | CₙH₂ₙ₊₂ | Methane (CH₄), Ethane (C₂H₆) |
| Alkene | At least one C=C | CₙH₂ₙ | Ethene (C₂H₄), Propene (C₃H₆) |
| Alkyne | At least one C≡C | CₙH₂ₙ₋₂ | Ethyne/Acetylene (C₂H₂) |
| Aromatic | Delocalized ring | Varies | Benzene (C₆H₆) |
| Reaction Type | What Happens | Example |
|---|---|---|
| Addition | Atoms add across a double/triple bond | Ethene + H₂ → Ethane |
| Substitution | One atom/group replaced by another | CH₄ + Cl₂ → CH₃Cl + HCl |
| Elimination | Atoms removed; new double bond forms | C₂H₅OH → C₂H₄ + H₂O |
| Condensation | Two molecules join; small molecule lost | Amino acids → Peptide + H₂O |
| Hydrolysis | Bond broken using water | Ester + H₂O → Acid + Alcohol |
Key terms
- Hydrocarbon
- Organic compound containing only carbon and hydrogen.
- Alkane
- Saturated hydrocarbon with only single C-C bonds; general formula CₙH₂ₙ₊₂.
- Alkene
- Hydrocarbon with at least one C=C double bond; CₙH₂ₙ.
- Functional Group
- Specific atom or group that determines characteristic chemical behavior of a molecule.
- Isomer
- Compounds with the same molecular formula but different structural arrangements.
- Condensation Reaction
- Two molecules join with the release of a small molecule (usually water).
- Addition Reaction
- Atoms add across a double or triple bond, converting it to a single (or double) bond.
- Polymer
- Large macromolecule built from repeating monomer units linked by covalent bonds.
Practice questions
What is the general formula for alkanes?
- CₙH₂ₙ
- CₙH₂ₙ₊₂
- CₙH₂ₙ₋₂
- CₙHₙ
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Alkanes are fully saturated — each carbon forms 4 single bonds. The formula CₙH₂ₙ₊₂ maximizes the hydrogen count.
Which functional group is characteristic of carboxylic acids?
- −OH
- −NH₂
- −COOH
- −CHO
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The carboxyl group (−COOH) contains both a carbonyl (C=O) and hydroxyl (−OH), enabling proton donation (weak acid behavior).
Compounds with the same molecular formula but different structural arrangements are called ______.
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Isomers share molecular formula but differ in atom connectivity or spatial arrangement, giving them different physical and chemical properties.
In an addition reaction with an alkene, what happens to the double bond?
- It gains a third bond
- Atoms add across it, converting it to a single bond
- It is removed entirely
- It shifts to an adjacent carbon
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Addition reactions add atoms (e.g., H₂, Br₂, H₂O) across a C=C double bond, saturating it to a C-C single bond.
How many covalent bonds does a carbon atom form?
- 2
- 3
- 4
- 6
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Carbon has 4 valence electrons and forms 4 covalent bonds — this tetravalency is the foundation of organic chemistry's structural diversity.
Fun facts
- There are over 10 million known organic compounds — more than all inorganic compounds combined — yet they all follow the same bonding rules with just a handful of elements.
- The smell of bananas comes from isoamyl acetate, an ester. The smell of vinegar is acetic acid. Organic chemistry literally defines the scent landscape of everyday life.
- Thalidomide was prescribed in the 1950s as a sedative. One optical isomer (enantiomer) was therapeutic; the other caused severe birth defects. Both were present in the drug — a tragic lesson in the importance of chirality in pharmaceutical chemistry.
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