Why carbon needs so many representations

Organic chemistry is the branch of chemistry that studies carbon-containing compounds, especially those with carbon–carbon and carbon–hydrogen bonding. Carbon does so much chemistry because a carbon atom has four valence electrons, forms four covalent bonds, and readily bonds to other carbon atoms. Catenation is the covalent bonding of atoms of the same element into chains or rings; in carbon chemistry, it produces straight-chain, branched-chain and cyclic molecules.

No single drawing style shows everything. A molecular formula is quick and gives atom counts clearly, but it hides connectivity. A 3D model shows shape well, but it takes too long to draw in an exam. Good chemists switch between representations depending on the question.

Formula types

An empirical formula is a chemical formula that shows the simplest whole-number ratio of atoms of each element in a compound. For example, C₆H₁₂O₆ has empirical formula CH₂O. Some information is lost here: many compounds can have the same empirical formula.

A molecular formula is a chemical formula that shows the actual number of atoms of each element in one molecule. C₂H₆O tells you there are two carbon atoms, six hydrogen atoms and one oxygen atom, but not whether the compound is ethanol or methoxymethane.

A structural formula is a representation that shows how atoms are connected in a molecule. Three versions matter here:

• A full structural formula is a two-dimensional structural formula that shows all atoms and all covalent bonds.
• A condensed structural formula is a structural formula that shows atom order and grouping while omitting some or all bond lines, for example CH₃CH₂OH.
• A skeletal formula is a line-angle structural formula in which carbon atoms are represented by line ends and vertices, most carbon-bound hydrogen atoms are omitted, and heteroatoms or functional groups are shown explicitly.

A stereochemical formula is a structural representation that shows the three-dimensional arrangement of atoms, usually with wedge and dash bonds when stereochemistry matters. You are not expected to draw stereochemical formulas in this topic, except where a later statement specifically asks you to do so.

In structural drawings, one line represents a single covalent bond, two lines represent a double bond, and three lines represent a triple bond. In skeletal formulas, don’t forget the invisible hydrogens: each carbon is assumed to have enough hydrogen atoms to make four bonds unless another atom or multiple bond is drawn.

Interconverting formulas

To change a full or condensed formula into a skeletal formula, first find the carbon chain or ring. Draw the carbon skeleton as a zig-zag line, put any double or triple bonds in the correct positions, then add atoms that are not carbon or carbon-bound hydrogen: halogens, O, N, and groups such as OH, CHO or COOH.

To work from a skeletal formula back to a molecular or structural formula, count every vertex and line end as a carbon atom. Then add enough hydrogens to each carbon to give carbon four bonds. Multiple bonds count as two or three bonds, as usual.

Using models wisely

A model is a simplified representation of a system that highlights selected features while leaving out others. Real or virtual 3D molecular models are especially helpful for seeing tetrahedral geometry, restricted rotation, and non-superimposable mirror images. The drawback is that they can hide the bookkeeping: formulas and names still work better for atom counts, connectivity and concise communication.

The link back to Structure 2.2 matters: carbon forms more compounds than all other elements combined largely because it forms strong covalent bonds to itself and to many other elements, can make single, double and triple bonds, and can build stable chains, branches and rings.

Functional groups classify organic molecules

A functional group is an atom or group of atoms in an organic molecule that gives the molecule characteristic physical and chemical properties. That idea does a lot of work in this topic: if you can spot the functional group, you can often predict solubility, boiling point trends and likely reactions before you know every detail of the molecule.

An organic class is a family of organic compounds defined by a shared functional group. Alcohols, for example, contain a hydroxy group. Aldehydes contain a terminal carbonyl group, while carboxylic acids contain a carboxyl group.

In general formulas, R and R′ stand for carbon-containing groups, usually alkyl groups. The prime simply shows that the two groups may be different. An alkyl group is a hydrocarbon substituent made by removing one hydrogen atom from an alkane, such as –CH₃ or –C₂H₅.

Recognition summary for common organic functional groups and their naming clues.

Functional groups you must recognise

A halogeno group is a functional group in which a halogen atom, F, Cl, Br or I, is bonded to carbon. When the rest of the molecule is alkane-like, compounds containing this group are often called halogenoalkanes.

A hydroxy group is a functional group consisting of –OH bonded to carbon. Compounds with this as the principal group are alcohols.

A carbonyl group is a functional group consisting of a carbon atom double-bonded to oxygen, C=O. When the carbonyl carbon sits at the end of a carbon chain and is bonded to hydrogen, the compound is an aldehyde. When the carbonyl carbon is bonded to two carbon groups, the compound is a ketone.

A carboxyl group is a functional group consisting of –C(=O)OH. Compounds with this as the principal group are carboxylic acids.

An alkoxy group is a functional group in which an oxygen atom is bonded between two carbon groups, R–O–R′. Compounds with this linkage are ethers.

An amino group is a nitrogen-containing functional group, commonly –NH₂ when primary. Compounds with amino groups are amines. In Structure 2.4, two amino acids join to form a dipeptide by a condensation reaction: an amine group and a carboxyl group form an amide link while water is eliminated.

An amido group is a functional group consisting of –C(=O)NH₂ or a substituted version of this unit. Compounds containing it are amides.

An ester group is a functional group consisting of –C(=O)O– between carbon groups. Esters are often formed from carboxylic acids and alcohols.

A phenyl group is a substituent consisting of a benzene ring attached to another part of a molecule.

Saturated and unsaturated compounds

A saturated compound is an organic compound in which all carbon–carbon bonds are single bonds. Alkanes are saturated hydrocarbons.

An unsaturated compound is an organic compound containing at least one carbon–carbon double bond or carbon–carbon triple bond. Alkenes and alkynes are unsaturated hydrocarbons.

Functional group reactivity lets us plan pathways. To convert ethene into ethanoic acid, for instance, a chemist recognises that an alkene can be changed into an alcohol, and that a primary alcohol can be oxidised through an aldehyde to a carboxylic acid. The details sit mainly in Reactivity 3, but the thinking starts here: classify first, then predict.

What a homologous series tells you

A homologous series is a family of organic compounds. Each next member differs by the same structural unit, usually –CH₂–, and the compounds share a common functional group and general formula. Members of a homologous series usually react in similar ways, while their physical properties change gradually.

A general formula is an algebraic chemical formula that represents every member of a homologous series. In formulas such as CₙH₂ₙ₊₂, n gives the number of carbon atoms in one molecule (dimensionless). For example, when n = 4, CₙH₂ₙ₊₂ gives C₄H₁₀.

Selected homologous series, their general formulas, and the +CH₂ step between successive members.

You need to be able to identify these series:

Don’t treat general formulas as magic spells. They work because each extra –CH₂– unit adds one carbon and two hydrogens, while the functional group pattern stays the same.

3D models and the invisible

Real and virtual models help because molecules aren’t flat. In this part of the course, they let you see chain shape, branching and where functional groups sit. Later, they become essential for recognising enantiomers. Models still make choices, though: a ball-and-stick model exaggerates atom spacing, while a space-filling model shows molecular volume well but can hide bonds. That is the usual bargain in science — a good model clarifies one feature by simplifying another.

Why boiling points usually rise

As you move along a homologous series, the boiling point usually increases as the carbon chain gets longer. A boiling point is the temperature at which a liquid’s vapour pressure equals the external pressure, so particles can escape throughout the liquid. Longer molecules contain more electrons and have a larger surface area, which makes the London dispersion forces between molecules stronger. Separating the molecules then takes more energy.

This connects directly to intermolecular forces from Structure 2.2. Increasing chain length strengthens London dispersion forces. Branching usually lowers boiling point because branched molecules have less surface contact. Functional groups can introduce permanent dipole–dipole attractions or hydrogen bonding. For example, alcohols and carboxylic acids have much higher boiling points than similar-sized alkanes because they can form hydrogen bonds.

Melting points tend to rise with molar mass too, although the pattern is less smooth than it is for boiling points. A melting point is the temperature at which solid and liquid phases coexist at a given pressure. Crystal packing matters, so symmetry and shape can disrupt the trend.

Measuring and modelling the trend

You can measure the boiling points of liquids in a homologous series using simple distillation, with a thermometer or temperature probe placed near the outlet to the condenser. Heat the liquid, let the vapour condense in the condenser, and use the steady temperature during distillation as the boiling point of the collected fraction.

A simple graph is often the easiest practical model for testing the trend. Choose a homologous series, collect reliable boiling point data from a database, plot boiling point against number of carbon atoms, and add a suitable best-fit line or curve. Interpolation within your data range is usually safer than extrapolation well beyond it. If you use software, the coefficient of determination R², where R² is a dimensionless measure of how well the model accounts for variation in the data, can help you compare models — but chemical sense still comes first.

Why systematic names matter

IUPAC nomenclature is a rule-based naming system developed by the International Union of Pure and Applied Chemistry. It gives systematic names to chemical compounds. In this topic, you use it for saturated or mono-unsaturated compounds with up to six carbon atoms in the parent chain and one type of functional group from halogeno, hydroxy, carbonyl or carboxyl.

A parent chain is the longest continuous carbon chain that contains the principal functional group or multiple bond required by the name. A substituent is an atom or group attached to that chain, but not counted as part of it. A locant is the number in a systematic name that shows where a substituent, multiple bond or functional group is positioned.

Roots, prefixes and multipliers

For chains up to six carbons, the root names are meth-, eth-, prop-, but-, pent- and hex-. Saturated parent chains use the suffix -ane. Mono-unsaturated chains with one C=C use -ene, with the double-bond locant placed before -ene, as in hex-2-ene.

Alkyl substituents end in -yl: methyl, ethyl, propyl and so on. Halogeno substituents use the prefixes fluoro-, chloro-, bromo- and iodo-. When identical substituents repeat, use numeric prefixes: mono, di, tri, tetra, penta and hexa. Commas separate numbers; hyphens separate numbers from words.

Naming alkanes and branched chains

For alkanes, choose the longest chain, then number it so the substituents get the lowest possible locants. List substituents alphabetically, and end with the parent alkane. For example, a six-carbon parent chain with methyl groups on carbons 2 and 3 is 2,3-dimethylhexane.

Naming alkenes

For alkenes, include the C=C bond in the parent chain. The double bond gets the lowest possible locant, even before alkyl or halogeno substituents. A compound with a six-carbon chain, a double bond after carbon 2, and a methyl group on carbon 4 is 4-methylhex-2-ene. The E–Z system is not assessed here.

Naming halogeno compounds

Name halogeno groups as prefixes. A single halogen on methane or ethane does not need a locant, since every position is equivalent: chloroethane, not 1-chloroethane. With several halogens, use locants and list different halogens alphabetically, ignoring multiplier prefixes when alphabetising.

Naming alcohols

Alcohols take the suffix -ol. The carbon bearing the hydroxy group gets the lowest possible locant, and in the examples required here this takes priority over carbon–carbon double bonds and substituents. Methanol and ethanol need no hydroxy locant; propan-1-ol and propan-2-ol do.

Naming carbonyl compounds and carboxylic acids

Aldehydes use the suffix -al. The carbonyl carbon is always carbon 1 and sits at the end of the chain, so no carbonyl locant is needed: butanal, 3-methylbutanal.

Ketones use the suffix -one. The carbonyl carbon lies within the chain and gets the lowest possible locant when needed: pentan-2-one, hexan-3-one.

Carboxylic acids use the suffix -oic acid. The carboxyl carbon is included in the parent chain and is carbon 1, so no locant is needed for the carboxyl group: ethanoic acid, 3-chlorobutanoic acid.

Systematic names give chemists a shared language. Common names still exist, but IUPAC names let chemists in different countries draw the same structure from the same name.

Same formula, different connectivity

Structural isomers are molecules with the same molecular formula but different atom-to-atom connectivity. The older name, constitutional isomers, refers to the same idea. Since the atoms join up in different ways, structural isomers can have different boiling points, solubilities and chemical reactions.

A straight-chain isomer is a structural isomer where the carbon atoms form one unbranched chain. In a branched-chain isomer, at least one carbon group is attached as a side chain to the main carbon chain.

A chain isomer is a structural isomer that differs from another isomer in the arrangement of the carbon skeleton. Butane and methylpropane are the classic pair: same C₄H₁₀, different skeleton.

A position isomer is a structural isomer that differs from another isomer because a functional group, substituent or multiple bond sits in a different position on the same carbon skeleton. 1-bromopropane and 2-bromopropane are position isomers.

A functional group isomer is a structural isomer that differs from another isomer by having a different functional group. Ethanol and methoxymethane both have molecular formula C₂H₆O, but one is an alcohol and the other is an ether. Propanal and propanone both have formula C₃H₆O; one is an aldehyde, the other a ketone.

Primary, secondary and tertiary compounds

A primary carbon atom is a carbon atom bonded directly to one other carbon atom. A secondary carbon atom is bonded directly to two other carbon atoms. A tertiary carbon atom is bonded directly to three other carbon atoms.

A primary alcohol is an alcohol in which the carbon bonded to the hydroxy group is bonded to one other carbon atom, except methanol which is often treated separately. In a secondary alcohol, the hydroxy-bearing carbon is bonded to two other carbon atoms. In a tertiary alcohol, the hydroxy-bearing carbon is bonded to three other carbon atoms.

Halogenoalkanes are classified in the same way: count the carbon atoms attached to the carbon bearing the halogen. This classification matters because primary, secondary and tertiary halogenoalkanes can favour different substitution or elimination pathways in Reactivity 3.

Amines are classified differently. A primary amine is an amine in which nitrogen is bonded to one carbon group. A secondary amine has nitrogen bonded to two carbon groups. A tertiary amine has nitrogen bonded to three carbon groups. Here, you count carbon groups attached to nitrogen, not carbons attached to a functional-group carbon.

Benzene’s structure links neatly to Structure 2.2: there are only three dibromobenzene isomers because all six C–C bonds in benzene are equivalent in the delocalised model. If benzene had alternating single and double bonds fixed in place, chemists would expect more distinct disubstituted arrangements.

Same connectivity, different space

Stereoisomers are molecules with the same atom identities, connectivities and bond multiplicities, but with different three-dimensional arrangements of atoms. Flat structural formulas can only take you so far: they show what is bonded to what, but not always which way groups point in space.

A conformational isomer is a stereoisomer formed by rotation about single bonds without breaking bonds. Ethane, for example, has staggered and eclipsed conformations. They interconvert so rapidly that we do not usually isolate them.

A configurational isomer is a stereoisomer that can be converted into another only by breaking and reforming bonds. In this syllabus, the key configurational isomers are cis–trans isomers and optical isomers.

Cis–trans isomerism in alkenes and cycloalkanes

Cis–trans isomerism is configurational stereoisomerism caused by restricted rotation, where groups can sit on the same side or on opposite sides of a reference plane. In non-cyclic alkenes, the C=C double bond restricts rotation. For an alkene to show cis–trans isomerism, each carbon atom of the double bond must be attached to two different groups.

A cis isomer is a stereoisomer in which two identical or corresponding groups lie on the same side of the reference plane. A trans isomer is a stereoisomer in which those groups lie on opposite sides of the reference plane. But-2-ene shows cis–trans isomerism; propene does not, because one double-bond carbon has two identical hydrogen atoms.

Cis–trans isomerism can also occur in disubstituted cycloalkanes, since the ring restricts rotation. For C₃ and C₄ cycloalkanes, use the plane of the ring as the reference plane. If the two substituents are on the same face of the ring, the isomer is cis; if they are on opposite faces, it is trans.

The E–Z naming system is not assessed here, so don’t wander into priority rules unless a teacher asks for enrichment.

Chirality and optical isomerism

A chiral carbon atom is a tetrahedral carbon atom bonded to four different atoms or groups of atoms. It is also called a stereocentre or asymmetric carbon. A molecule with one chiral carbon usually has two possible spatial arrangements.

Optical isomerism is stereoisomerism in which isomers differ in their effect on plane-polarized light. Plane-polarized light is light whose electric field oscillates in one plane. Optical activity is the ability of a substance to rotate the plane of plane-polarized light.

An enantiomer is one of a pair of stereoisomers that are non-superimposable mirror images of each other. Non-superimposable means you cannot rotate one molecule in 3D space and make every group line up with the other. Real or virtual 3D models help a lot here: build the mirror image, then try to overlay it.

A racemic mixture is a 1:1 mixture of two enantiomers of the same compound. It does not rotate plane-polarized light overall, because the two enantiomers rotate the plane by equal amounts in opposite directions.

To draw enantiomers, use wedge–dash representations. A wedge bond is a tapered bond showing a bond coming out of the page towards the viewer. A dash bond is a tapered or hashed bond showing a bond going behind the page away from the viewer. Ordinary line bonds lie in the plane of the page. Around a chiral carbon, make the tetrahedral arrangement clear: two bonds in the plane, one wedge and one dash.

Enantiomers have identical chemical properties in non-chiral environments, but they can behave differently in chiral environments. Biological systems contain many chiral molecules, so different enantiomers can have different smells, tastes or physiological effects.

What fragmentation tells you

Mass spectrometry is an analytical technique that ionises particles and then separates the ions by their mass-to-charge ratio. In organic mass spectrometry, molecules may break into smaller ions. That bond breaking is called fragmentation, and it produces characteristic charged pieces of the original molecule.

A mass spectrum plots relative intensity against mass-to-charge ratio, m/z, where m is the mass of an ion (kg) and z is its charge number (dimensionless). For many school-level organic spectra, most ions carry a single positive charge, so m/z values can be treated like relative masses when interpreting the spectrum.

The molecular ion forms when the intact molecule loses one electron. The molecular ion peak is the peak from this intact molecular ion, often found at the highest significant m/z value. If the ion has a +1 charge, this peak gives the relative molecular mass.

A fragmentation pattern is the set of peaks in a mass spectrum produced by the different fragment ions. The data booklet gives data for specific fragments, so the task is to match the evidence to the structure rather than memorise a long list.

Using a mass spectrum

A sensible interpretation sequence is:

1. Find the molecular ion peak to estimate the molecular mass.
2. Use the molecular formula, if given or deduced, to limit possible structures.
3. Match strong fragment peaks with likely structural units using the data booklet.
4. Check that the proposed structure can reasonably lose those fragments.

For example, an alcohol may show peaks for fragments such as CH₂OH⁺ or C₂H₅⁺. A carboxylic acid may show a COOH⁺ fragment. Isomers can have the same molecular ion peak but different fragment peaks, so MS is useful for distinguishing structural features, not just measuring mass.

Bonds absorb infrared radiation

Infrared spectroscopy identifies bonds or functional groups by measuring how they absorb infrared radiation, which makes molecules vibrate. A bond may stretch, compress or bend; the frequency absorbed depends on the bond strength and on the masses of the atoms involved.

Stronger bonds usually absorb at higher frequency. Heavier atoms usually vibrate at lower frequency. That’s why IR spectra help identify functional groups such as O–H, C=O and N–H.

The relationship between wave properties is c = λf, where c is the speed of light in vacuum (m s⁻¹), λ is wavelength (m), and f is frequency (s⁻¹). IR spectra usually use wavenumber: ṽ = 1/λ, where ṽ is wavenumber (m⁻¹, commonly reported as cm⁻¹). A higher wavenumber means a shorter wavelength and a higher frequency.

Simplified IR correlation table for diagnostic functional-group absorptions.

IR active vibrations

An IR active vibration is a molecular vibration that changes the dipole moment of the molecule, so infrared radiation can be absorbed. Homonuclear diatomic molecules such as H₂, O₂ and Cl₂ are IR inactive because stretching them does not produce a changing dipole. Heteronuclear molecules such as HF are IR active because stretching changes the dipole.

Polyatomic molecules can have several vibrational modes. Water and carbon dioxide can absorb IR radiation because some of their vibrations change the molecular dipole, even though carbon dioxide has no permanent dipole overall.

This links directly to greenhouse gases. They absorb outgoing infrared radiation from Earth when their vibrations are IR active. Their global warming potential depends on factors such as how strongly they absorb IR, which wavelengths they absorb, how long they remain in the atmosphere, and their atmospheric concentration.

Interpreting the functional group region

An IR spectrum is a graph of absorption or transmittance against wavenumber. In school spectra, the y-axis is often transmittance, so absorptions show up as downward dips. The functional group region, roughly above 1500 cm⁻¹, contains many diagnostic absorptions.

Use the data booklet, not memory alone. Typical diagnostic ideas include:

• a broad O–H absorption for alcohols or carboxylic acids;
• a strong C=O absorption for aldehydes, ketones, carboxylic acids, esters and amides;
• N–H absorptions for amines and amides;
• C–H absorptions whose exact position depends on bonding environment.

IR is very good for saying “this functional group is present” or “this functional group is absent”. On its own, it usually cannot prove a complete structure, because many isomers contain the same functional group.

What proton NMR measures

Proton nuclear magnetic resonance spectroscopy is an analytical technique that uses the behaviour of hydrogen-1 nuclei in a magnetic field to identify different hydrogen environments in an organic molecule. Hydrogen-1 is written ¹H and has one proton in its nucleus.

Place ¹H nuclei in a magnetic field and they can sit in different spin energy states. The energy gap between those states changes slightly with the electronic environment around the proton, so protons in different chemical environments absorb radio-frequency radiation at different positions in the spectrum.

A chemical environment means the neighbouring atoms, plus any electron-withdrawing or electron-donating effects, experienced by a particular atom in a molecule. Equivalent hydrogen atoms produce one signal. Non-equivalent hydrogen atoms produce different signals.

A chemical shift is the position of an NMR signal relative to a reference compound. It is represented by δ, where δ is chemical shift (ppm). The reference is usually tetramethylsilane, TMS, assigned δ = 0 ppm.

Reading low-resolution ¹H NMR

A ¹H NMR spectrum is a graph of signal intensity against chemical shift. Count the signals to find the number of different hydrogen environments. Watch for symmetry: two CH₃ groups may give one signal if they are chemically equivalent.

An integration trace is a stepped line or numerical value showing the relative area under each NMR signal. That relative area is proportional to the number of hydrogen atoms causing the signal. If two signals integrate in a 3:2 ratio, the environments contain hydrogens in that ratio — perhaps CH₃ and CH₂.

Example low-resolution ¹H NMR signals linked to hydrogen environments and integrals.

Use three pieces of evidence together:

• number of signals → number of hydrogen environments;
• chemical shifts → likely nearby functional groups, using the data booklet;
• integration → relative number of hydrogens in each environment.

For example, chloroethane has two hydrogen environments: CH₃ and CH₂Cl. It therefore gives two signals, with integration ratio 3:2. In a symmetrical molecule such as propanone, there is only one hydrogen environment because both CH₃ groups are equivalent.

The data booklet gives chemical shift ranges, so the skill is not memorising every value. It’s matching a proposed structure to the observed pattern.

Why signals split

In high-resolution ¹H NMR, a single signal may appear as a small cluster of peaks rather than one line. Spin–spin coupling is the interaction between nearby non-equivalent hydrogen nuclei that causes NMR signal splitting. From that splitting, you can work out how many hydrogens sit on neighbouring carbon atoms.

A multiplicity is the number of peaks in a split NMR signal. For the cases required here, use the N + 1 rule, where N is the number of equivalent neighbouring hydrogen atoms on adjacent carbon atoms (dimensionless). A signal is split into N + 1 peaks.

¹H NMR splitting patterns from the N + 1 rule for 0–3 neighbouring hydrogens.

The required splitting patterns are:

The relative peak heights come from Pascal’s triangle. Equivalent hydrogens in the same carbon environment don't split each other; focus on neighbouring, non-equivalent hydrogens.

Using splitting to build structure

Splitting gives connectivity detail. A triplet and quartet with integration 3:2 is a classic clue for an ethyl group, –CH₂CH₃. The CH₃ signal is split into a triplet by two neighbouring CH₂ hydrogens, while the CH₂ signal is split into a quartet by three neighbouring CH₃ hydrogens.

When interpreting a spectrum, combine splitting with the evidence from Structure 3.2.10. The number of signals gives the environments, integration gives relative hydrogen counts, chemical shift points to nearby groups, and splitting shows neighbouring hydrogens. The mathematics is simple, but its value is chemical: it turns a spectrum into a map of neighbouring atoms.