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

Know empirical, molecular, full and condensed structural, stereochemical and skeletal formulas, and convert between them. Identify the functional groups halogeno, hydroxyl, carbonyl, carboxyl, alkoxy, amino (–NH₂), amido (–CONH–), ester and phenyl, and the homologous series alkanes, alkenes, alkynes, halogenoalkanes, alcohols, aldehydes, ketones, carboxylic acids, ethers, amines, amides and esters. Successive members differ by CH₂; boiling points rise with chain length because London forces increase.

Name compounds up to C6 with one type of halogeno, hydroxyl, carbonyl or carboxyl group: longest chain containing the group, correct suffix or prefix, lowest locants, branches as alkyl prefixes (CH₃CH₂CH(CH₃)CH₂Cl is 1-chloro-2-methylbutane). Structural isomers share a molecular formula but differ in connectivity: chain (branched vs straight), position (group on a different carbon) and functional group isomers (alcohol/ether, aldehyde/ketone, acid/ester).

Stereoisomers have the same constitution (atoms, connectivity and bond multiplicities) but different spatial arrangements. Cis–trans isomerism needs (1) restricted rotation, from a C=C bond or a small ring, and (2) two different groups on each of the two carbons involved. But-2-ene has cis and trans forms; but-1-ene does not (C1 has two H). In C3 and C4 rings such as 1,2-dichlorocyclopropane, the substituents can be on the same side (cis) or opposite sides (trans) of the ring. E–Z naming is not assessed.

A chiral carbon has four different groups arranged tetrahedrally (the C2 of 2-bromobutane: H, Br, CH₃, C₂H₅). It gives two enantiomers: non-superimposable mirror images, drawn with wedge-dash (tapered) bonds. Enantiomers are optically active: they rotate plane-polarised light by equal angles in opposite directions. A racemic mixture (equal amounts) shows no net rotation. Enantiomers have the same physical properties otherwise, but behave differently in chiral environments (e.g. with enzymes or receptors in the body).

In MS the molecular ion M⁺ gives the relative molecular mass; the molecule also fragments, and the fragments (listed in the data booklet) reveal parts of the structure: 15 CH₃⁺, 29 C₂H₅⁺ or CHO⁺, 43 CH₃CO⁺ or C₃H₇⁺, 45 COOH⁺, 57 C₂H₅CO⁺ or C₄H₉⁺. Differences from M⁺ show what was lost (M − 17 = OH).

IR absorptions identify bonds in the functional group region: C=O 1700–1750 cm⁻¹; O–H in alcohols 3200–3600 cm⁻¹ (broad); O–H in carboxylic acids 2500–3000 cm⁻¹ (very broad); C–O 1050–1410 cm⁻¹. Use the data booklet table.

¹H NMR shows hydrogen chemical environments:

• number of signals = number of environments
• chemical shift (ppm, compared with the data booklet) = what the H is near (–O–CH₂– about 4, CH₃–CO– about 2)
• integration (relative areas) = ratio of H atoms in each environment
• splitting by the n + 1 rule: n H on adjacent carbons give n + 1 peaks (singlet, doublet, triplet, quartet). A triplet (3 H) plus a quartet (2 H) signals an ethyl group.

Combine techniques: MS gives Mr and fragments, IR the bonds present, NMR the carbon–hydrogen framework. IR cannot distinguish isomers with the same bonds (ethyl ethanoate and methyl propanoate), but NMR and MS can.