Mission 1: How can C₂H₆O be both a liquid and a gas at room temperature?

Five kinds of formula

• Empirical: simplest whole-number ratio of atoms. Hexane: C₃H₇.
• Molecular: actual number of each atom. Hexane: C₆H₁₄.
• Full structural: every atom and bond. Condensed: CH₃CH₂CH₂CH₂CH₂CH₃, or CH₃(CH₂)₄CH₃.
• Skeletal: a zig-zag of bonds; carbons and their hydrogens are implied.
• Stereochemical: wedges and dashes show the 3D shape. Draw one only when asked.
• Build it as a 3D model (real or virtual): each carbon is tetrahedral, so chains zig-zag.

Reading a skeletal formula

• Every line end and corner is a carbon, unless a line ends in a label (OH, Cl): that end is the labelled atom.
• A group drawn at a corner, or on a short line off it, is attached to that carbon.
• H on carbon is not drawn: give each carbon enough H for four bonds. Other atoms, and H on them, are always drawn.
• Lines are bonds: count the points, not the lines.
• Worked example: five zig-zag lines, the last ending in OH. That is 5 C and 1 O.
• H: 3 + 2 + 2 + 2 + 2 + 1 (on O) = 12. So C₅H₁₂O, CH₃CH₂CH₂CH₂CH₂OH.

Functional groups

• A functional group gives a compound its typical reactions and decides its class.

Telling look-alikes apart

• Carboxyl: the C=O carbon also carries –OH. It counts as one group.
• Ester: the single-bonded O is bonded to a second carbon, not to H.
• Amido: N bonded to the C=O carbon. Amino: N on a carbon with no C=O.
• Alkoxy: C–O–C with no C=O.
• Saturated: only single C–C bonds. Unsaturated: at least one C=C or C≡C.
• Cyclohexane, C₆H₁₂, is saturated: its ring, not a double bond, lowers the H count.

Mission 2: Petrol and candle wax are both mostly alkanes. Why is one a runny liquid and the other a solid?

Homologous series

• A homologous series is a family with the same functional group.
• Each member differs from the next by CH₂.
• One general formula fits every member.
• Members have similar chemical properties; physical properties change gradually.

The series, part 1

The series, part 2

• Shared general formulae: aldehydes and ketones; acids and esters; alcohols and ethers. The functional group decides the series.

Why boiling point rises along a series

• Boiling separates whole molecules. No covalent bonds break.
• Each extra CH₂ adds electrons and contact area: stronger London (dispersion) forces.
• More energy is needed to separate the molecules, so the boiling point rises.
• In the alcohols, each molecule still has one –OH: its hydrogen bonding stays about the same.
• Melting points rise too, but less regularly: how molecules pack in the solid also matters.

Mission 3: Could you draw a molecule from its name alone?

The parts of a name

• Stem = carbons in the parent chain: meth 1, eth 2, prop 3, but 4, pent 5, hex 6.
• -an- only single C–C bonds; -en- one C=C; -yn- one C≡C.
• Suffix for the main group: -ol (hydroxyl), -al (aldehyde), -one (ketone), -oic acid (carboxyl). Alkanes end in -e.
• Prefixes for side groups: methyl, ethyl; fluoro, chloro, bromo, iodo.
• Locants are carbon numbers: 2-methylpentan-3-ol.

Four steps to a name

• 1. Find the longest chain that contains the functional-group carbon. It may bend through a branch.
• 2. Number from the end that gives the suffix group the lowest locant.
• 3. Then the lowest locant to the C=C or C≡C, then to the prefixes.
• 4. Prefixes in alphabetical order. Repeats: di-, tri- (not used for alphabetical order). Commas between numbers; hyphens between numbers and letters.

Worked examples: prefixes only

• CH₃CH₂CH(CH₃)CH(CH₃)CH₃: longest chain 5 → pentane.
• From the right, methyls on C2 and C3; from the left, C3 and C4. Lower set wins.
• Name: 2,3-dimethylpentane.
• CH₃CH(CH₃)CHBrCH₂CH₃: from the left {2, 3}; from the right {3, 4}.
• Name: 3-bromo-2-methylpentane. Prefixes go in alphabetical order (b before m), not number order.

Groups that take a suffix

• -ol: CH₃CH₂CH₂CH(OH)CH₃ is pentan-2-ol, not pentan-4-ol.
• -one: CH₃COCH₂CH₂CH₃ is pentan-2-one. Propanone needs no number.
• -al and -oic acid: that carbon is always C1, so no number: CH₃CH₂CH₂COOH is butanoic acid.
• With a branch: (CH₃)₂CHCOOH is 2-methylpropanoic acid.
• -ene, -yne: number the multiple bond by its lower carbon: CH₂=CHCH₂CH₂CH₃ is pent-1-ene; CH₃C≡CCH₃ is but-2-yne.

A C=C plus a suffix group

• The suffix group gets the lowest locant first; the C=C locant goes before -en-.
• CH₂=CHCH₂CH(OH)CH₃: from the CH₃ end, OH is on C2 and the C=C starts at C4: pent-4-en-2-ol.
• Not pent-1-en-4-ol: the suffix group outranks the double bond.
• Branch plus suffix: CH₃CH(CH₃)CH₂CH₂OH is 3-methylbutan-1-ol.

Mission 4: How many different molecules can you build from C₅H₁₂?

Structural isomers

• Structural isomers: same molecular formula, atoms connected differently.
• Chain (branched-chain) isomers: different skeleton. Hexane and 2-methylpentane, both C₆H₁₄.
• Position isomers: same group, different place. Propan-1-ol and propan-2-ol.
• Functional group isomers: different series. Propanal and propanone, both C₃H₆O.
• Rotating, flipping or bending a drawing does not make a new isomer.

Primary, secondary, tertiary

• Alcohols and halogenoalkanes: count the carbons bonded to the carbon carrying –OH or –X.
• One → primary; two → secondary; three → tertiary.
• Propan-1-ol is primary, propan-2-ol secondary, 2-methylpropan-2-ol tertiary. (CH₃)₃CBr is a tertiary halogenoalkane.
• Amines: count the carbons bonded to N. CH₃CH₂NH₂ 1°, (CH₃)₂NH 2°, (CH₃)₃N 3°.
• The class is not the locant: hexan-3-ol is secondary.

The big ideas

• Formulas show more and more: empirical → molecular → structural → skeletal → stereochemical.
• The functional group sets the class; a homologous series adds CH₂ each time.
• Boiling points rise along a series: stronger London forces, same functional-group force.
• IUPAC: longest chain with the group; lowest locant to the suffix; prefixes in alphabetical order.
• Structural isomers: same formula, different connections. 1°, 2°, 3°: count carbons on that C (on N for amines).