Organic Chemistry: Introduction, Structure, Isomerism and Nomenclature — Notes

Organic chemistry is the chemistry of carbon compounds (other than oxides of carbon, carbonates and a few others). Over ten million are known, because of two properties of carbon:

• Tetravalency: carbon (2, 4) cannot easily lose or gain four electrons, so it shares them and forms four covalent bonds. In methane the four bonds point to the corners of a tetrahedron.
• Catenation: carbon atoms link to one another to form long chains, branched chains and rings, because the C–C bond is strong and the carbon atom is small.

Two carbon atoms can share one, two or three pairs of electrons: a single (C–C), double (C=C) or triple (C≡C) bond. Every carbon still has four bonds in total.

• Straight (open, unbranched) chain: CH₃–CH₂–CH₂–CH₃ (butane).
• Branched chain: a side group on the main chain, CH₃–CH(CH₃)–CH₃ (2-methylpropane). A carbon joined to three or four other carbons marks a branch.
• Closed chain (cyclic): the chain closes into a ring.

Benzene, C₆H₆: a flat hexagonal ring of six carbon atoms with alternate single and double bonds (Kekulé structure); each carbon carries one hydrogen atom. It is the only ring compound in this chapter.

Hydrocarbons contain carbon and hydrogen only.

• Saturated (alkanes): only C–C single bonds.
• Unsaturated: at least one C=C (alkenes) or C≡C (alkynes).

A molecular formula counts the atoms; a structural formula shows how they are joined; a condensed formula writes each carbon with its hydrogens (CH₃–CH=CH–CH₃). To draw one: chain first, multiple bond next, then hydrogens until every carbon has four bonds. Each double bond removes two hydrogen atoms from the alkane formula; a triple bond removes four.

Isomers: compounds with the same molecular formula but different structural formulae (and so different properties).

• Chain isomers differ in the carbon skeleton. C₄H₁₀: butane and 2-methylpropane. C₅H₁₂: pentane, 2-methylbutane and 2,2-dimethylpropane. More branching lowers the boiling point.
• Position isomers have the same skeleton but the multiple bond or functional group in a different position: but-1-ene and but-2-ene; propan-1-ol and propan-2-ol; 1-chloropropane and 2-chloropropane.

Methane, ethane, propane, ethene, propene and propyne have no chain or position isomers. Before calling two drawings isomers, number each chain from both ends — they may be the same compound.

A homologous series is a group of organic compounds with the same functional group, in which successive members differ by a –CH₂– unit. Characteristics:

1. All members have the same general formula and the same functional group.
2. Successive members differ by CH₂, i.e. by 14 u in molecular mass.
3. They have similar chemical properties.
4. Physical properties show gradation: melting point, boiling point and density increase with molecular mass (C₁–C₄ alkanes are gases, C₅ onwards liquids).
5. They can be prepared by similar general methods.

IUPAC name = (position + prefix) + word root + primary suffix + (position + secondary suffix).

• Word root (longest chain containing the functional group): meth 1, eth 2, prop 3, but 4, pent 5.
• Numbering: from the end nearer the functional group, then the multiple bond, then the branch — lowest number.
• Primary suffix: -ane, -ene, -yne. Prefixes: methyl-, chloro-, bromo-, iodo-, methoxy-.
• Secondary suffix: -ol (alcohol), -al (aldehyde), -one (ketone), -oic acid (carboxylic acid). The final "e" of -ane is dropped before a vowel: ethanol, propanal.
• Ethers: alkoxyalkane (smaller group as alkoxy): CH₃OC₂H₅ is methoxyethane. Esters: alkyl alkanoate: CH₃COOC₂H₅ is ethyl ethanoate.

Trivial names to know: methyl alcohol (methanol), ethyl alcohol (ethanol), formaldehyde (methanal), acetaldehyde (ethanal), acetone (propanone), formic acid (methanoic acid), acetic acid (ethanoic acid), ethylene (ethene), acetylene (ethyne), methyl chloride (chloromethane).