Part 2: Structural isomers and electron movement

Distinguish a new connectivity from a rotated drawing, then learn the arrow and radical conventions used throughout organic chemistry.

Same molecular formula, different connectivity

Structural isomers have the same molecular formula but different structural formulae: atoms are connected differently. Changing the carbon skeleton, functional-group position or functional-group type can create them. OCR treats these together as structural isomerism; the category labels are helpful organisation, not a substitute for correct structures.

C₄H₁₀ gives CH₃CH₂CH₂CH₃ and (CH₃)₃CH. C₃H₈O gives propan-1-ol, propan-2-ol and methoxyethane (an ether); the last demonstrates different functional-group connectivity. Rotating or reversing a drawing of the same connected atoms does not make a new isomer.

Work systematically: longest unbranched skeleton first, then shorter skeletons with branches, then permitted group positions. Check formula, valency and duplicates. Stereoisomers instead retain connectivity but differ in spatial arrangement; E/Z examples belong in Alkenes.

Two ways to split a covalent bond

Homolytic fission gives one bonding electron to each atom, producing radicals. A radical has an unpaired electron, shown by a dot: Cl•. Heterolytic fission gives both bonding electrons to one atom, often producing positive and negative ions. A radical dot and a negative charge mean different things.

A reaction mechanism shows the steps and electron movements, not just the overall equation. OCR AS radical mechanisms can be written with balanced equations and correctly placed radical dots; half-headed curly arrows are not required for those radical steps.

A full curly arrow moves an electron pair

Start a full curly arrow at the electron pair: a bond, a lone pair or a clearly indicated negative charge. End it at the atom or bond receiving those electrons. A bond-breaking arrow starts in the bond and ends on the atom retaining the pair. Do not begin an arrow at a bare positive charge.

A nucleophile donates an electron pair; an electrophile accepts one. In OH⁻ attacking CH₃Br, the oxygen lone pair points to the δ+ carbon while the C–Br bond pair points to bromine. Products CH₃OH and Br⁻ preserve total charge −1.

Mechanism drawings must show connectivity and all relevant charges. If the intermediate is a carbocation, put a full positive charge on its carbon, not just δ+. Trace atoms from one step to the next before naming the mechanism.

Worked enumeration: the four acyclic alcohols with C₄H₁₀O

Keep the requested family fixed: this task asks for alcohols, so do not add ethers even though some have the same molecular formula. On the unbranched four-carbon skeleton, OH can be on carbon 1 or 2. Positions 4 and 3 reproduce those structures by reversing the chain. These give butan-1-ol and butan-2-ol.

Now use the branched 2-methylpropane skeleton. Its three outer carbon atoms are equivalent, so placing OH on an outer carbon gives just one structure, 2-methylpropan-1-ol. Placing OH on the central carbon gives 2-methylpropan-2-ol. This produces four different alcohol connectivities in total.

Check each has four carbons, ten hydrogens and one oxygen, then compare connected neighbours rather than the shape of the drawing. Counting different orientations as new structures is the common source of overcounting.

A charge, a radical dot and a partial charge say different things

Cl• is a chlorine radical with an unpaired electron. Cl⁻ is a chloride ion with a full negative charge. The Clδ− end of a polar covalent bond has an uneven share of electron density but is still part of that molecule. These symbols cannot be exchanged to make a mechanism look complete.

A carbocation has a full positive charge on carbon and typically three bonds at that carbon in the simple intermediates studied here. A saturated carbon labelled δ+ in a haloalkane still has four bonds before substitution. Drawing five ordinary bonds on the final carbon is not repaired by adding a charge.

Many simple radicals in the AS reactions are neutral, but the definition is the unpaired electron rather than neutrality: a molecular ion in mass spectrometry can be both charged and a radical. Match the notation to the species actually being represented.

Read each curly arrow as a sentence about electrons

For hydroxide and bromoethane, the first sentence is “the oxygen lone pair supplies the new C–O bond”. The arrow therefore starts at that pair and ends at the carbon bonded to Br. The second is “the old C–Br bonding pair moves onto Br”. That arrow begins in C–Br and ends on Br, accounting for Br⁻.

Check the total charge before and after: neutral haloalkane plus OH⁻ totals −1, and neutral alcohol plus Br⁻ totals −1. Check the carbon skeleton is unchanged. Then name the process from what happened: a nucleophile substituted for a leaving group.

For homolysis, each atom instead receives one of the two bonding electrons, producing radical dots rather than the pair movement represented by a full curly arrow. Equations with radical dots are sufficient for the AS radical mechanism; practise full curly arrows for the specified ionic mechanisms.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources