So many carbon compounds
Optional · not on the AP exam: this lesson is optional; AP Chemistry doesn’t test organic chemistry on its own. Molecules like ethanol and acetic acid do show up in AP questions, though, so it helps to recognize them.
Methane in natural gas, ethanol in drinks, acetic acid in vinegar, plastics, proteins: all contain carbon, and carbon forms far more compounds than any other element. The reason is how carbon bonds. With 4 valence electrons it forms 4 covalent bonds (lesson 2.4.2), and carbon atoms can link into long chains and rings with single, double, or triple bonds.
Carbon compounds are generally called organic compounds有机化合物. CO, CO₂, carbonic acid, and carbonates behave like inorganic substances and are usually classed as inorganic (lesson 1.8.1).
Hydrocarbons
Compounds of only carbon and hydrogen are hydrocarbons烃. By the bonds between carbons, the three simplest families are:
| Family | Structure | General formula | Examples |
|---|---|---|---|
| Alkane烷烃 | only single C–C bonds | CₙH₂ₙ₊₂ | methane CH₄, ethane C₂H₆ |
| Alkene烯烃 | one C=C double bond | CₙH₂ₙ | ethene C₂H₄ |
| Alkyne炔烃 | one C≡C triple bond | CₙH₂ₙ₋₂ | ethyne C₂H₂ |
Alkanes are named by the number of carbons with the prefixes meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec- plus the ending -ane. Alkenes and alkynes swap -ane for -ene or -yne; from 4 carbons up, a number gives the position of the double or triple bond, as in 1-butene.
Compounds with the same kind of structure that differ by one or more CH₂ units form a homologous series同系物: methane, ethane, propane, and so on.
There are several ways to write organic molecules: the molecular formula (C₂H₆), the structural formula (every bond drawn), and the condensed formula (CH₃CH₃, C–H bonds left out). Compounds with the same molecular formula but different structures are isomers同分异构体; C₄H₁₀, for example, is both butane, CH₃CH₂CH₂CH₃, and isobutane (2-methylpropane), CH₃CH(CH₃)CH₃.
Choose a number of carbons and a family below to see the name, molecular formula, and condensed formula. Up to 4 carbons it also draws the structural formula; count the bonds on each carbon to check there are exactly 4:
Functional groups
An atom or group of atoms that gives an organic compound its characteristic properties is a functional group官能团. Compounds with the same functional group behave alike, so organic compounds are classified by their functional groups:
| Family | Functional group | Example |
|---|---|---|
| alcohol | hydroxyl, −OH | ethanol, CH₃CH₂OH |
| carboxylic acid | carboxyl, −COOH | acetic (ethanoic) acid, CH₃COOH |
| ester | ester, −COO− | ethyl acetate, CH₃COOCH₂CH₃ |
| amine | amino, −NH₂ | methylamine, CH₃NH₂ |
Carbon–carbon double and triple bonds count as functional groups too. Acetic acid is the weak acid of lesson 2.12.2: the carboxyl group releases H⁺. Ethanol has an −OH but is not a base, because that O–H is covalently bonded to carbon and doesn’t release OH⁻.
Heating acetic acid and ethanol with concentrated sulfuric acid makes the sweet-smelling ester ethyl acetate. This esterification酯化反应 is reversible (lesson 2.11.3):
Below, one molecule appears at a time. Decide which family it belongs to, then look at its functional group, shown in red:
How Chinese textbooks differ
- Chinese textbooks name hydrocarbons with the characters 甲、乙、丙、丁、戊、己、庚、辛、壬、癸 for 1 to 10 carbons, plus 烷 (alkane), 烯 (alkene), or 炔 (alkyne): 甲烷 is methane and 1-丁烯 is 1-butene.
- The compulsory Chinese course covers methane, ethene, ethanol, acetic acid, and esters; alkynes and amines come in the elective organic chemistry course. US Honors chemistry often surveys the main functional groups in a final chapter.
- Chinese names usually follow the systematic pattern (乙酸, “ethanoic acid”), while US textbooks often use common names: ethylene, acetylene, acetic acid, ethyl acetate.
Example 1: naming and writing formulas
(a) What is the alkane with 5 carbons called, and what is its molecular formula? (b) What is the hydrocarbon with 3 carbons and one C=C double bond called, and what is its formula?
- Naming rules. The prefixes for 1 to 10 carbons are meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, and dec-. Only single bonds gives -ane; one C=C double bond gives -ene.
- (a) Name. Five carbons is pent-, so the alkane is pentane.
- (a) Formula. Alkanes are CₙH₂ₙ₊₂; with n = 5 there are 2 × 5 + 2 = 12 H: C₅H₁₂.
- (b) Name. Three carbons is prop-, so the alkene is propene. With only 3 carbons the double bond has one possible position, so no number is needed.
- (b) Formula. Alkenes with one double bond are CₙH₂ₙ; with n = 3 there are 2 × 3 = 6 H: C₃H₆, condensed formula CH₂=CHCH₃.
- Check. Each carbon must form 4 bonds. In CH₂=CHCH₃, the left C has 2 C–H bonds plus a double bond (counting as 2) = 4; the middle C has 1 C–H, the double bond, and 1 C–C = 4; the right C has 3 C–H and 1 C–C = 4.
- A common mistake. Writing propene as C₃H₈. That is propane; adding a double bond removes 2 H.
Example 2: functional groups in lactic acid
Lactic acid, found in yogurt, has the condensed formula CH₃CH(OH)COOH. Which functional groups does it contain, and which families’ properties does it show?
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Spot the groups. Read left to right: CH₃ is a methyl group; the middle carbon carries an −OH (the parentheses mean it is attached to that carbon); the right end is −COOH.
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Name them. −OH is a hydroxyl group and −COOH is a carboxyl group.
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Carboxylic acid properties. The carboxyl group makes lactic acid acidic, so it reacts with NaOH and NaHCO₃. With NaHCO₃, only the carboxyl group reacts; the hydroxyl group doesn’t:
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Alcohol properties. The hydroxyl group gives it alcohol properties too: for example, it reacts with sodium metal to release hydrogen and can form an ester with a carboxylic acid.
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Think about it. One molecule can contain several functional groups and show the properties of each family.
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A common mistake. Counting the −OH inside −COOH as a separate hydroxyl group. The carboxyl group is one unit, so lactic acid has one hydroxyl and one carboxyl.
- C₃H₈
It fits CₙH₂ₙ₊₂ with n = 3: propane.
- CH₂=CHCH₃
It has a C=C double bond: propene.
- HC≡CH
It has a C≡C triple bond: ethyne (acetylene).
- CH₃OH
It has a hydroxyl group, −OH: methanol.
- HCOOH
It has a carboxyl group, −COOH: methanoic (formic) acid.
- CH₃COOCH₃
It has an ester group, −COO−: methyl acetate.
- CH₃CH₂NH₂
It has an amino group, −NH₂: ethylamine.
- C₄H₁₀
It fits CₙH₂ₙ₊₂ with n = 4: butane.
- CH₃CH₂CH₂OH
It has a hydroxyl group: 1-propanol.
Common mistakes
Mistake 1: “Every carbon compound is organic.” CO, CO₂, H₂CO₃, and carbonates such as CaCO₃ contain carbon but behave like inorganic substances and are classed as inorganic.
Mistake 2: “Anything with the formula CₙH₂ₙ is an alkene.” Ring-shaped alkanes such as cyclohexane, C₆H₁₂, also fit CₙH₂ₙ but have only single bonds. The formula alone isn’t enough; you need the structure.
Mistake 3: “Ethanol has −OH, so it’s a base.” The −OH in ethanol is covalently bonded to carbon and doesn’t release OH⁻, so ethanol solutions are neutral. Only in compounds like NaOH is OH⁻ an ion.
Quick check
Which of these is an alkane?
Alkanes fit CₙH₂ₙ₊₂; with n = 6 that is C₆H₁₄ (hexane). C₆H₁₂ has two fewer H and could be an alkene or a cycloalkane; C₂H₅OH and CH₃COOH contain oxygen and aren’t hydrocarbons.
Which functional group does ethyl acetate (CH₃COOCH₂CH₃) contain?
Ethyl acetate forms from acetic acid and ethanol and contains an ester group, −COO−. The acid’s carboxyl group and the alcohol’s hydroxyl group are gone.
Summary
- Hydrocarbons contain only C and H: alkanes (single bonds, CₙH₂ₙ₊₂), alkenes (C=C, CₙH₂ₙ), and alkynes (C≡C, CₙH₂ₙ₋₂), named with meth-, eth-, prop-, but-, and so on.
- Members of a homologous series differ by CH₂ units; isomers share a molecular formula but differ in structure.
- Functional groups give organic compounds their characteristic properties: alcohols (−OH), carboxylic acids (−COOH), esters (−COO−), and amines (−NH₂). A carboxylic acid and an alcohol react to form an ester.
On to AP (for later)
AP doesn’t test organic naming or organic reactions, but it uses organic molecules to test other ideas: ethanol’s hydrogen bonding and high boiling point (Unit 3), carboxylic acids as weak acids (Unit 8), and balancing combustion reactions and their enthalpy changes (Units 4 and 6). Recognizing the functional groups is all you need.