Functional groups, naming and isomers

The ten functional groups

• A functional group is the part of a molecule that reacts; the rest is a hydrocarbon chain that mostly goes along for the ride.
• Level 3 uses ten groups. Learn each one's structure, its suffix or prefix, and whether it makes the molecule polar or able to hydrogen bond.
• Alcohols are primary, secondary or tertiary by how many carbons are bonded to the C carrying the OH; haloalkanes are classed the same way, and amines by how many carbons are bonded to the N.

IUPAC names, step by step

• Find the longest carbon chain that contains the main functional group; its length gives the stem (meth, eth, prop, but, pent, hex, hept, oct).
• The highest-priority group gives the suffix: carboxylic acid, then acyl chloride and ester, then amide, aldehyde, ketone, alcohol, amine. Any other group becomes a prefix, such as hydroxy- for OH, oxo- for C=O, amino- for NH₂ and methyl- for a branch.
• Number from the end that gives the suffix group the lowest number. The carbon of -COOH, -CHO, -COCl and -CONH₂ is always carbon 1, so it needs no number.
• Esters are named in two words: the alkyl group from the alcohol first, then the acid part ending in -oate. CH₃COOCH₂CH₃ is ethyl ethanoate.
• On an amide or amine, a group on the nitrogen is shown with N-, so CH₃CONHCH₃ is N-methylethanamide.

Try it: build and name a molecule

Constitutional isomers

• Isomers have the same molecular formula but different structures. Constitutional (structural) isomers have their atoms joined in a different order.
• Chain isomers have a different carbon skeleton: butan-1-ol and 2-methylpropan-1-ol.
• Position isomers have the same group in a different place: propan-1-ol and propan-2-ol.
• Functional group isomers belong to different families: propanal and propanone are both C₃H₆O, and so are an acid and an ester of the same size, such as propanoic acid and methyl ethanoate (C₃H₆O₂).
• Because the groups differ, functional group isomers have different chemical reactions as well as different physical properties.

Geometric (cis-trans) isomers

• A C=C double bond cannot rotate, so groups on each end are locked on one side or the other.
• Geometric isomers need two different groups on each carbon of the C=C. But-2-ene (CH₃CH=CHCH₃) qualifies; but-1-ene does not, because one carbon carries two H atoms.
• In the cis isomer the matching groups are on the same side; in the trans isomer they are on opposite sides.
• Cis and trans isomers have slightly different boiling and melting points. Cis-but-2-ene is slightly polar and boils at about 4 °C, while trans-but-2-ene, whose bond dipoles cancel, boils at about 1 °C.

Optical isomers (enantiomers)

• A carbon bonded to four different groups is an asymmetric (chiral) carbon, marked with an asterisk. A molecule with one has two forms that are mirror images and cannot be superimposed, like left and right hands.
• Draw enantiomers in 3D: two bonds in the plane, one wedge coming out and one dashed going back, then reflect the whole molecule in a mirror line.
• Enantiomers have identical boiling points, solubility and reactions with ordinary reagents. The one difference is that they rotate plane-polarised light by the same angle in opposite directions.
• A 50:50 mixture of the two (a racemic mixture) shows no rotation, because the two effects cancel.
• Living things often treat enantiomers differently: one carvone enantiomer smells of spearmint and the other of caraway, because our smell receptors are chiral too.

Try it: which kind of isomerism?

Physical properties come from the groups

• Molecules of similar size have similar dispersion forces, so differences in boiling point come mostly from the functional group.
• Alcohols, carboxylic acids, primary amines and amides have O-H or N-H bonds, so their molecules hydrogen bond to each other and boil higher. Carboxylic acids form pairs held by two hydrogen bonds; amides hydrogen bond through both N-H and C=O, so propanamide is a solid at room temperature.
• Aldehydes, ketones, esters, acyl chlorides and haloalkanes are polar but have no O-H or N-H, so only permanent dipole attractions add to their dispersion forces.
• Small molecules that can hydrogen bond with water dissolve in it; as the hydrocarbon chain gets longer, solubility falls because the non-polar part cannot.
• Acyl chlorides react with water rather than dissolving, giving off steamy HCl fumes.

What Achieved, Merit and Excellence look like

• Achieved names or draws correctly and states a property. Merit explains the property from the structure. Excellence compares the compounds, links each structure to its forces or its isomerism, and justifies every conclusion.