Introduction to organic chemistry

Isomers have the same molecular formula and a different arrangement of atoms. The syllabus splits them two ways.

“How many isomers have the formula X?” is the most common shape of question in this topic, and the options are always the answers you get by missing one. Work to a fixed order rather than by inspection:

1. Longest chain first. Draw the straight chain, then move the functional group along it, one position at a time. Stop when you start repeating by symmetry.
2. Shorten the chain by one, add a branch. Move the branch through every distinct position, then move the functional group through every distinct position for each.
3. Keep shortening. Repeat until the chain is too short to branch.
4. Then change the functional group — if the formula allows an ether as well as an alcohol, or a ketone as well as an aldehyde, those count too.
5. Finally check each $\mathrm{C}=\mathrm{C}$ for cis-trans.

A general formula covers a whole homologous series: alkanes are ${\mathrm{C}}_n{\mathrm{H}}_{2n+2}$, alkenes with one double bond ${\mathrm{C}}_n{\mathrm{H}}_{2n}$, alcohols ${\mathrm{C}}_n{\mathrm{H}}_{2n+1}\mathrm{O}\mathrm{H}$. A homologous series shares a general formula and functional group, and its members differ by $\mathrm{C}{\mathrm{H}}_2$ — which is why their boiling points climb steadily as the chain lengthens.

Names are built in three parts: a stem from the longest carbon chain containing the functional group (meth-, eth-, prop-, but-, pent-, hex-), a suffix for the group, and prefixes with numbers for everything else. Number from whichever end gives the functional group the lowest locant.

Nearly a third of this topic’s questions ask you to classify a reaction rather than to predict one. Six types cover the AS course:

• Addition — two molecules become one; the double bond opens. Alkenes do this.
• Substitution — one atom or group is swapped for another. Alkanes with halogens, halogenoalkanes with nucleophiles.
• Elimination — a small molecule is removed and a double bond forms. The reverse of addition.
• Oxidation — adds oxygen or removes hydrogen. Primary alcohol → aldehyde → carboxylic acid.
• Reduction — adds hydrogen or removes oxygen. The reverse.
• Hydrolysis — split by water, often acid- or alkali-catalysed. Esters and amides.

Addition and elimination are opposites; oxidation and reduction are opposites. Questions exploit that symmetry, so read which direction the arrow points before classifying.

A covalent bond can break two ways, and which one decides everything that follows:

• Homolytic fission — the pair splits, one electron to each atom, giving two free radicals (species with an unpaired electron, written with a dot). Caused by UV light. Shown with a half-headed curly arrow.
• Heterolytic fission — both electrons go to one atom, giving a positive and a negative ion. Shown with a full-headed curly arrow.

Two words the exam will not let you swap:

• A nucleophile is an electron-pair donor — it has a lone pair and attacks positive centres. $\mathrm{O}{\mathrm{H}}^{-}$, $\mathrm{C}{\mathrm{N}}^{-}$, $\mathrm{N}{\mathrm{H}}_3$.
• An electrophile is an electron-pair acceptor — electron-deficient, attacks regions of high electron density such as a $\mathrm{C}=\mathrm{C}$.

A curly arrow always shows a pair of electrons moving (or one electron, if half-headed), and it starts from where the electrons are — a lone pair or a bond — never from a positive charge. That single rule is the most common reason a mechanism loses marks in the A2 papers this topic prepares you for.

• A σ bond is end-on overlap of orbitals, with the electron density directly between the two nuclei. It can rotate freely. Every single bond is one σ bond.
• A π bond is sideways overlap of p orbitals, above and below the plane. It cannot rotate — which is exactly why cis-trans isomerism exists. A double bond is one σ plus one π; a triple bond is one σ plus two π.

Shapes follow from the number of regions of electron density around each carbon: four single bonds gives tetrahedral, 109.5°; a double bond and two singles gives trigonal planar, 120°; a triple bond and a single gives linear, 180°. The same counting as in chemical bonding.

1. 1.Claiming cis-trans isomerism without checking both conditions

   A $\mathrm{C}=\mathrm{C}$ is not enough. Each carbon of the double bond needs two different groups. But-1-ene fails; but-2-ene passes.

2. 2.Counting the same isomer twice

   A chain drawn left-to-right and right-to-left is one compound. Number from the end that gives the lowest locant and the duplicates disappear.

3. 3.Nucleophile and electrophile swapped

   Nucleophile = electron-pair donor, attacks positive. Electrophile = electron-pair acceptor, attacks electron-rich. The name says what it likes, not what it is.

4. 4.Curly arrows starting at a charge

   They start where the electrons are — a lone pair or a bond — and point to where they go. An arrow from a $+$ sign is backwards.

5. 5.Forgetting functional group isomers

   ${\mathrm{C}}_3{\mathrm{H}}_6\mathrm{O}$ is propanal and propanone. When a formula could support a second functional group, it usually does, and the question is counting on you to miss it.