S3.2 Functional groups: Classification of organic compoundsIB Chemistry SL: Revision notes

Organic compounds can be shown in several ways. The empirical formula is the simplest whole-number ratio of atoms (C₃H₆O); the molecular formula gives the actual numbers (C₆H₁₂O₂). A full structural formula shows every atom and bond; a condensed structural formula writes each carbon with its attached atoms (CH₃CH₂CH₂COOCH₂CH₃). A skeletal formula shows the carbon chain as a zig-zag of lines, with carbons at the ends and corners, hydrogens on carbon omitted, and functional groups drawn in. Stereochemical formulas show the 3D arrangement using wedges and dashes. Building 3D models helps you see that a chain can rotate about single bonds.

A functional group is the part of a molecule that gives it its characteristic chemical and physical properties. Groups to know:

• halogeno –X (Cl, Br, I)
• hydroxyl –OH
• carbonyl C=O (aldehydes –CHO, ketones –CO–)
• carboxyl –COOH
• alkoxy –OR (ethers)
• amino –NH₂
• amido –CONH–
• ester –COO–
• phenyl –C₆H₅

A homologous series is a family with the same functional group and general formula, where successive members differ by CH₂. Series to know: alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alkynes CₙH₂ₙ₋₂, halogenoalkanes, alcohols CₙH₂ₙ₊₁OH, aldehydes, ketones, carboxylic acids, ethers, amines, amides and esters.

Trend in physical properties: boiling and melting points rise with chain length because more electrons and a larger surface area give stronger London (dispersion) forces. Branched isomers have lower boiling points than straight-chain ones (less surface contact). Series with –OH (alcohols, acids) boil much higher than alkanes of similar mass because of hydrogen bonding.

Name compounds with up to six carbons in the parent chain and one type of halogeno, hydroxyl, carbonyl or carboxyl group.

1. Find the longest chain containing the functional group: meth-, eth-, prop-, but-, pent-, hex-.
2. Add the suffix: -ane, -ene, -ol, -al (aldehyde), -one (ketone), -oic acid; halogens are prefixes (chloro-, bromo-).
3. Number from the end that gives the functional group (or C=C) the lowest locant.
4. Add alkyl branches as prefixes with locants (2-methyl).

Examples: (CH₃)₂CHCH₂OH is 2-methylpropan-1-ol; CH₃CH=CHCH₃ is but-2-ene; CH₃COCH₃ is propanone; CH₃CH₂COOH is propanoic acid.

Structural isomers have the same molecular formula but different connectivities. Types:

• Chain (branched) isomers: butan-1-ol and 2-methylpropan-1-ol.
• Position isomers: same group on a different carbon, e.g. butan-1-ol and butan-2-ol.
• Functional group isomers: different functional groups, e.g. alcohol and ether (C₄H₁₀O), aldehyde and ketone (C₃H₆O), carboxylic acid and ester (C₃H₆O₂).