1. Why carbon is special
Carbon has four valence electrons. To get a full octet, it forms four covalent bonds. That ability, combined with the way carbon bonds easily to other carbons (forming long chains, branches, and rings), is responsible for the millions of organic compounds in existence: fuels, plastics, sugars, fats, proteins, DNA.
All organic compounds contain carbon. (Note: a few simple carbon-containing molecules like CO, CO₂, and carbonates are traditionally considered inorganic.)
2. Hydrocarbon series
Hydrocarbons contain only carbon and hydrogen. They fall into three families based on the type of carbon-to-carbon bond. See Reference Table Q.
| Series | Bond type | Saturation | General formula | Suffix | Example (C₂) |
|---|---|---|---|---|---|
| Alkanes | All single bonds | Saturated | CnH2n+2 | -ane | Ethane (C₂H₆) |
| Alkenes | At least one C=C double bond | Unsaturated | CnH2n | -ene | Ethene (C₂H₄) |
| Alkynes | At least one C≡C triple bond | Unsaturated | CnH2n−2 | -yne | Ethyne (C₂H₂) |
Saturated vs unsaturated
A saturated hydrocarbon (alkane) has the maximum number of hydrogens it can hold. Every carbon-to-carbon bond is a single bond. An unsaturated hydrocarbon (alkene or alkyne) has at least one double or triple bond, meaning fewer hydrogens than the saturated version. The double or triple bond can "open up" to add more atoms; that is why unsaturated hydrocarbons are more reactive.
Prefixes (Reference Table P)
| Carbon count | Prefix | Alkane | Alkene | Alkyne |
|---|---|---|---|---|
| 1 | meth- | methane CH₄ | — | — |
| 2 | eth- | ethane C₂H₆ | ethene C₂H₄ | ethyne C₂H₂ |
| 3 | prop- | propane C₃H₈ | propene C₃H₆ | propyne C₃H₄ |
| 4 | but- | butane C₄H₁₀ | butene C₄H₈ | butyne C₄H₆ |
| 5 | pent- | pentane C₅H₁₂ | pentene C₅H₁₀ | pentyne C₅H₈ |
| 6 | hex- | hexane C₆H₁₄ | hexene C₆H₁₂ | hexyne C₆H₁₀ |
| 7 | hept- | heptane C₇H₁₆ | — | — |
| 8 | oct- | octane C₈H₁₈ | — | — |
| 9 | non- | nonane C₉H₂₀ | — | — |
| 10 | dec- | decane C₁₀H₂₂ | — | — |
Prefix mnemonic
First four (1-2-3-4): Meth-Eth-Prop-But. After that, the prefixes go Greek and predictable: pent (5), hex (6), hept (7), oct (8), non (9), dec (10). You've seen "oct" (octopus, octagon) and "dec" (decade, decimal) before.
3. IUPAC naming
IUPAC (International Union of Pure and Applied Chemistry) gives each organic compound a systematic name based on its structure.
Steps for naming a hydrocarbon
- Find the longest continuous carbon chain. This is the parent chain.
- Choose the prefix for that number of carbons (Table P).
- Choose the suffix based on the bonds: -ane (single), -ene (double), -yne (triple).
- Number the chain from the end closest to the multiple bond or branch.
- Indicate the position of the double or triple bond with a number. For example, "2-butene" means the double bond starts at carbon 2.
- Name any branches (substituents) with the prefix from Table P and the suffix "-yl" (methyl, ethyl, propyl). Indicate position with a number.
Worked example
Name CH₃—CH₂—CH(CH₃)—CH₃.
- Longest chain: 4 carbons. Prefix = but-.
- All single bonds. Suffix = -ane.
- Number from the right (closer to the branch). The CH₃ branch is on carbon 2.
- The branch is a methyl group.
Name: 2-methylbutane.
4. Functional groups (Reference Table R)
A functional group is a specific arrangement of atoms in a molecule that determines its chemical behavior. Table R lists nine of them. Memorize all nine.
| Class | Functional group | General formula | Example |
|---|---|---|---|
| Halide | —X (F, Cl, Br, I) | R—X | Chloroethane, CH₃CH₂Cl |
| Alcohol | —OH | R—OH | Ethanol, CH₃CH₂OH |
| Ether | —O— | R—O—R' | Dimethyl ether, CH₃OCH₃ |
| Aldehyde | —CHO (C=O on the end) | R—CHO | Methanal (formaldehyde), HCHO |
| Ketone | —CO— (C=O in middle) | R—CO—R' | Propanone (acetone), CH₃COCH₃ |
| Organic acid | —COOH | R—COOH | Ethanoic acid (vinegar), CH₃COOH |
| Ester | —COO— | R—COO—R' | Methyl ethanoate, CH₃COOCH₃ |
| Amine | —NH₂ | R—NH₂ | Methylamine, CH₃NH₂ |
| Amide | —CONH₂ | R—CONH₂ | Ethanamide, CH₃CONH₂ |
The big four to recognize instantly
- Alcohol (—OH): ethanol, the drinking alcohol, is a classic. Suffix -ol.
- Organic acid (—COOH): vinegar (ethanoic acid). Suffix -oic acid.
- Amine (—NH₂): compounds like methylamine; relatives of ammonia.
- Ester (—COO—): the smells of fruits and the products of organic acid + alcohol reactions.
5. Isomers
Isomers are different compounds that have the same molecular formula but different structural formulas. The atoms are arranged differently.
Classic example: C₄H₁₀
Two isomers of butane share the formula C₄H₁₀:
- n-Butane: CH₃—CH₂—CH₂—CH₃ (straight chain)
- 2-Methylpropane (isobutane): CH₃—CH(CH₃)—CH₃ (branched, 3-carbon main chain with a methyl branch)
Same molecular formula, same molar mass, but different physical properties: n-butane boils at −0.5 °C while 2-methylpropane boils at −12 °C. Different shapes mean different IMFs mean different properties.
What is NOT an isomer
Same compound drawn differently (rotated, mirrored, written in a different order) is not an isomer of itself. Isomers must be genuinely different molecules.
6. Seven organic reactions
The Regents focuses on seven specific reaction types. Know what each one looks like and what identifies it.
(1) Combustion
A hydrocarbon (or any organic compound) reacts with O₂ to produce CO₂ + H₂O + energy.
CH₄ + 2 O₂ → CO₂ + 2 H₂O + heat
Identifier: hydrocarbon + O₂ → CO₂ + H₂O. The "burning" reaction.
(2) Substitution (alkanes)
A halogen atom (Cl, Br) replaces a hydrogen on a saturated hydrocarbon. Requires energy (heat or light).
CH₄ + Cl₂ → CH₃Cl + HCl
Identifier: alkane + diatomic halogen → halogenated product + HX.
(3) Addition (alkenes and alkynes)
A double or triple bond "opens up" and atoms add directly across it. No byproducts.
C₂H₄ + Br₂ → C₂H₄Br₂
Identifier: unsaturated hydrocarbon + something → one larger product. Only one product. The double bond becomes a single bond.
Substitution vs addition
The clean way to tell them apart: substitution produces two products (often the organic product + HCl or HBr). Addition produces one product (the alkene/alkyne plus the added atoms all in one molecule).
(4) Esterification
An organic acid + an alcohol → an ester + water. Often catalyzed by H₂SO₄. Esters smell fruity (banana, apple, pineapple).
CH₃COOH + CH₃OH → CH₃COOCH₃ + H₂O
(ethanoic acid + methanol → methyl ethanoate + water)
Identifier: acid + alcohol → ester + water.
(5) Saponification
A fat (an ester) + a strong base (NaOH or KOH) → glycerol + soap (a sodium salt of a fatty acid). This is how soap is made.
Identifier: fat + base → soap + glycerol.
(6) Fermentation
A sugar (glucose) is converted by enzymes in yeast into ethanol and CO₂. Anaerobic (no oxygen needed).
C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂
Identifier: sugar + yeast → alcohol + CO₂. (How bread rises and how beer and wine are made.)
(7) Polymerization
Small molecules (monomers) join to form a long chain (polymer). Two types:
- Addition polymerization: monomers with double bonds open up and add to each other with no byproducts. Polyethylene from ethene is the classic example.
- Condensation polymerization: monomers join with the loss of a small molecule (usually water) at each link. Nylon, polyester, and biological polymers (proteins, starch, cellulose) work this way.
Natural polymers vs synthetic polymers
- Natural: starch, cellulose, proteins, DNA, natural rubber.
- Synthetic: polyethylene, PVC, polystyrene, nylon, polyester.
7. Properties of organic compounds
Mostly nonpolar
Most hydrocarbons are nonpolar (C and H have similar electronegativities), so they don't dissolve in water. They do dissolve in nonpolar solvents (gasoline, ether).
Low melting/boiling points
Held together by weak London dispersion forces. Compare to ionic compounds (held by strong electrostatic attractions).
Don't conduct electricity
Most organic compounds are molecular covalent and don't ionize. No free charge carriers.
Slower reactions
Covalent bonds in organic compounds are strong and directional. Most organic reactions are slower than typical ionic reactions in solution.
Key terms
| Organic compound | Compound containing carbon (with a few exceptions like CO, CO₂, carbonates). |
| Hydrocarbon | Organic compound made of only carbon and hydrogen. |
| Saturated | Hydrocarbon with all single C–C bonds (alkane). |
| Unsaturated | Hydrocarbon with at least one C=C or C≡C bond. |
| Alkane | Saturated hydrocarbon, suffix -ane. General formula CnH2n+2. |
| Alkene | Hydrocarbon with one double bond, suffix -ene. General formula CnH2n. |
| Alkyne | Hydrocarbon with one triple bond, suffix -yne. General formula CnH2n−2. |
| Isomer | Compound with same molecular formula but different structural formula. |
| Functional group | Specific atom arrangement that gives a molecule characteristic properties. |
| Monomer | Small molecule that joins with others to form a polymer. |
| Polymer | Long chain of repeating monomer units. |
| Ester | Compound with —COO— linkage, formed from an organic acid + alcohol. |
Practice questions
Q1. Which compound is an unsaturated hydrocarbon?
Answer: (4) C₂H₄. The general formula for an alkane is CnH2n+2. The first three all fit that formula and are alkanes (saturated). C₂H₄ fits CnH2n, the alkene formula, so it has a double bond and is unsaturated (this is ethene).
Q2. Which functional group is present in an organic acid?
Answer: (3) —COOH. Organic acids (carboxylic acids) have the —COOH carboxyl group, like ethanoic acid (CH₃COOH, vinegar). —OH alone is an alcohol, —CHO is an aldehyde, and —NH₂ is an amine.
Q3. Which type of reaction produces an ester?
Answer: (3) esterification. Esters are formed by the reaction of an organic (carboxylic) acid with an alcohol, producing an ester and water. The name of the reaction matches the name of the product. Combustion produces CO₂ + H₂O, addition makes a single new larger molecule across a double bond, and fermentation produces ethanol + CO₂.
Q4. (Part B-2) Two compounds have the same molecular formula C₅H₁₂ but different physical properties. Explain what this means in terms of structural arrangement, and give one specific physical property that could differ between them.
Sample full-credit response: The two compounds are isomers: they have the same molecular formula (same number and type of atoms) but different structural arrangements. C₅H₁₂ has three isomers: n-pentane (straight chain), 2-methylbutane (branched), and 2,2-dimethylpropane (more branched). The branched isomers have lower boiling points than the straight-chain isomer because branching produces more compact molecules with less surface area for London dispersion forces to act on. For example, n-pentane boils at 36 °C while 2,2-dimethylpropane boils at only 9.5 °C. Same formula, same molar mass, different shape, different boiling point.
Q5. (Part C) Ethanol can be produced by fermentation of glucose. (a) Write the balanced equation for the fermentation of glucose. (b) Identify the type of organic compound ethanol is and name its functional group. (c) Ethanol can react with ethanoic acid (CH₃COOH) to form an ester. Write the products of this reaction and name the reaction type.
Sample full-credit response:
(a) Fermentation equation:
C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂
(Glucose → ethanol + carbon dioxide)
(b) Type of compound: Ethanol (C₂H₅OH or CH₃CH₂OH) is an alcohol.
Its functional group is the hydroxyl group, —OH. The name ends in "-ol" to
indicate the alcohol class.
(c) Reaction with ethanoic acid: Ethanoic acid + ethanol →
ethyl ethanoate (an ester) + water.
CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O
The reaction type is esterification. The —OH of the alcohol combines with the
—OH of the acid to release water, leaving an ester linkage (—COO—) between the two
original carbon chains.