3.3.1 Introduction to Organic Chemistry

Keep the formula types distinct: empirical is the simplest ratio, molecular gives actual atom numbers, structural shows connectivity, displayed shows every bond and skeletal omits C/H labels in the carbon framework.

Match the representation to the question. A general formula such as CₙH₂ₙ₊₂ describes a homologous series; a skeletal formula is a drawing of one particular molecule. They are not interchangeable just because both are compact.

When you identify a homologous series, start with the functional group because that is what gives the characteristic reactions. The members then follow a systematic structural pattern and successive members differ by CH₂.

Members share a functional group and general formula, show similar chemical reactions and display a gradual trend in physical properties as chain length changes. Successive members differ by CH₂.

For a name-from-structure question, find the principal functional group first, choose the correct parent chain or ring, then number it so the important group and substituents have the required locants. Build the name from substituent prefixes, parent and suffix.

Check the name back against the structure before moving on: carbon count, functional-group position and every substituent should all match. A neat-looking name with the wrong parent chain is still the wrong compound.

Translate representations by preserving connectivity. In a skeletal formula every vertex/end is a carbon unless labelled otherwise, and hydrogens on carbon are omitted but heteroatoms and their hydrogens are shown.

When you convert a displayed structure to skeletal form, check the carbon count first, then keep every heteroatom and any H attached to it visible. Carbonyl C=O bonds and multiple C–C bonds must still be drawn explicitly.

Within the course naming scope, work systematically: identify the parent chain or ring, number it from the correct end, then add substituent locants and the functional-group ending. Chains or rings are no more than six carbons in each relevant unit.

Do not choose a shorter parent just because it gives simpler substituent names. The parent must satisfy the functional-group and longest-chain rules before you worry about the rest of the name.

When a name is given, draw the parent skeleton first and number it mentally. Put the principal functional group at its locant, then add substituents and any multiple bond in the stated positions.

Finish with a valency check. Every carbon should have four bonds, and the number of carbons in the drawing should agree with the parent plus any carbon-containing substituents in the name.

A mechanism explains a reaction by showing what the electrons do. In polar mechanisms, full curly arrows track electron pairs; in radical mechanisms, dots identify unpaired electrons and the chain steps are written as balanced radical equations.

Every bond made or broken must follow from the electron movement you show. Keep the mechanism short if necessary, but make every arrow and charge chemically meaningful.

A radical has an unpaired electron, shown by a single dot. For the free-radical mechanisms required here, identify the radical species correctly and write the initiation, propagation and termination equations without trying to use full curly arrows.

Keep a radical dot and an ionic charge conceptually separate. Cl• and Cl⁻ are different species with different electron counts and different chemistry.

Initiation generates radicals, propagation uses a radical but produces another radical, and termination removes radicals by combining them. The propagation steps are what keep the chain reaction going.

Check each equation for both atoms and radical bookkeeping. A propagation equation that consumes a radical without making one has accidentally become a termination step.

For a polar mechanism, I want the curly arrow to start exactly where the electron pair starts: on a lone pair or on the covalent bond that breaks. Point the arrowhead to the atom or bond that receives that pair.

For heterolytic bond breaking, the arrow starts on the bond and ends on the atom taking both electrons. Starting an arrow in empty space loses the electron-accounting logic of the mechanism.

Draw every reacting species and show each electron-pair movement with a full curly arrow. After the arrows, the next structure must display the bonds and formal charges those movements create.

A useful self-check is to read the mechanism one arrow at a time: where did this pair start, where did it finish, and does the following structure show exactly that change?

Structural isomers have the same molecular formula but different connectivity. That can arise from a different carbon skeleton, a functional group or substituent in a different position, or a different functional group altogether.

When you generate examples, write the molecular formula beside them and check it has not changed. Redrawing the same connectivity in a different orientation does not create another structural isomer.

For stereoisomers, keep the connectivity fixed. The atoms are joined in the same order, but the three-dimensional arrangement differs; E/Z and optical isomerism are the two course contexts you meet.

The phrase “same molecular formula” is not enough here because structural isomers also share a molecular formula. The defining extra point is the same structural formula/connectivity.

To check whether E/Z isomerism is possible, ask whether each carbon of the C=C has two different groups attached. Restricted rotation matters, but it only gives an E/Z pair when that attachment condition is met.

Do the eligibility check before assigning E or Z. Restricted rotation by itself does not guarantee that two stereoisomers exist.

To assign E/Z, apply the Cahn–Ingold–Prelog priority rule separately at each carbon of the C=C bond. The directly attached atom with the higher atomic number has higher priority; if those atoms tie, compare the next set of attached atoms until a difference appears.

After you have identified the higher-priority group on each alkene carbon, compare their positions: same side gives Z; opposite sides give E. Do the priority decision before looking at the overall drawing.

For the definition, I’d learn this accurately: structural isomers are compounds with the same molecular formula but different structural formulae. In practical terms, their atoms are connected differently.

Do not define them as compounds that merely “look different”. Two drawings can look different on the page while representing exactly the same connectivity.

For chain isomerism, change the carbon skeleton; for positional isomerism, keep the skeleton and move a substituent, multiple bond or functional group; for functional-group isomerism, keep the molecular formula but change the functional group.

Use a molecular-formula check after each drawing. It is very easy to make a plausible new structure by accidentally adding or losing a carbon or hydrogen.

Stereoisomers have the same structural formula but a different arrangement of atoms in space. Keep that distinction separate from structural isomerism, where the atoms are connected in a different order.

If the connectivity changes, stop: you are looking at structural isomerism. Stereoisomers keep the same atom-to-atom connections and differ only in their three-dimensional arrangement.

To draw a pair of E/Z isomers, keep the same structural formula and the same groups attached to each alkene carbon, but reverse their spatial arrangement across the C=C bond. E has the higher-priority groups on opposite sides; Z has them on the same side.

Draw the same connectivity twice and place the higher-priority groups on the same side for Z and opposite sides for E. Do not rotate the whole page and mistake the same isomer for its partner.

When you assign E or Z, give CIP priority independently on the two alkene carbons. Compare the directly attached atoms first; higher atomic number wins, and you only move outward if those atoms tie.

Once the higher-priority group on each carbon is identified, Z has those groups on the same side and E has them on opposite sides. Do not use group size or mass by eye as the priority rule.