Why carbon needs so many representations

Organic chemistry is a branch of chemistry that studies carbon-containing compounds, especially compounds with carbon–carbon and carbon–hydrogen bonding. Carbon does so much 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 gives the whole picture. A molecular formula is quick and clear about atom counts, 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, $C_6H_{12}O_6$ has empirical formula $CH_2O$. Some information has disappeared 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. $C_2H_6O$ tells you there are two carbon atoms, six hydrogen atoms and one oxygen atom, but it does not tell you 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 $CH_3CH_2OH$.
• 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 using 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, remember 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 convert a full or condensed formula into a skeletal formula, start by identifying 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 convert a skeletal formula back into 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 useful for visualising tetrahedral geometry, restricted rotation, and non-superimposable mirror images. The drawback is bookkeeping: models can make formulas and names feel less visible, even though these are still better for atom counts, connectivity and concise communication.

The link back to Structure 2.2 matters here: 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 sits at the centre of this topic. Once 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; carboxylic acids contain a carboxyl group.

In general formulas, $R$ and $R'$ represent carbon-containing groups, usually alkyl groups. The prime simply shows that the two groups may be different. An alkyl group is a hydrocarbon substituent formed by removing one hydrogen atom from an alkane, such as –$CH_3$ or –$C_2H_5$.

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. Compounds that contain it are often called halogenoalkanes when the rest of the molecule is alkane-like.

A hydroxy group is a functional group consisting of –$OH$ bonded to carbon. When this is the principal group, the compounds are alcohols.

A carbonyl group is a functional group consisting of a carbon atom double-bonded to oxygen, $C=O$. If the carbonyl carbon is at the end of a carbon chain and bonded to hydrogen, the compound is an aldehyde. If that 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 containing it 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 –$NH_2$ 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 –$C(=O)NH_2$ 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 helps chemists 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 belong 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 where each member differs from the next by the same structural unit, usually $-CH_2-$. The compounds share a common functional group and general formula. Because of that, 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 can represent every member of a homologous series. In a formula such as

For example, when $n = 4$, $C_nH_{2n+2}$ gives $C_4H_{10}$.

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

The series you must be able to identify are:

Don’t treat general formulas as magic spells. They work because each extra $-CH_2-$ 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. At this stage of the course, they let you see chain shape, branching, and where functional groups sit. Later, they become essential for recognising enantiomers.

Still, every model chooses what to show. A ball-and-stick model exaggerates atom spacing. A space-filling model shows molecular volume well, but it can hide bonds. That’s the usual bargain in science: a good model makes one feature clearer by simplifying another.

Why boiling points usually rise

In a homologous series, each successive member usually has a higher boiling point 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 links back 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, but the pattern is less regular 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 here, so symmetry and shape can disrupt the trend.

Measuring and modelling the trend

To measure boiling points of liquids in a homologous series, you can use simple distillation with a thermometer or temperature probe placed near the outlet to the condenser. The liquid is heated, the vapour condenses in the condenser, and the steady temperature during distillation gives the boiling point of the collected fraction.

A simple graph is often the most practical way to test the trend. Choose a homologous series, collect reliable boiling point data from a database, plot boiling point against number of carbon atoms, and draw a suitable best-fit line or curve. Interpolation within your data range is usually safer than extrapolation far beyond it. If you use software, the coefficient of determination $R^2$, where $R^2$ is a dimensionless measure of how well the model accounts for variation in the data, can help 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 to give 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 joined to the parent chain but not counted as part of it. A locant is a number in a systematic name that shows the position of a substituent, multiple bond or functional group.

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 are written as prefixes: fluoro-, chloro-, bromo- and iodo-. When identical substituents appear more than once, use numeric prefixes: mono, di, tri, tetra, penta and hexa. Use commas between numbers; use hyphens between numbers and words.

Naming alkanes and branched chains

For alkanes, choose the longest chain. Number it so the substituents get the lowest possible locants, then list substituents alphabetically and finish 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, the C=C bond must sit in the parent chain and 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

Halogeno groups are named as prefixes. A single halogen on methane or ethane doesn't need a locant because every position is equivalent: chloroethane, not 1-chloroethane. With several halogens, include locants and list different halogens alphabetically, ignoring multiplier prefixes when alphabetising.

Naming alcohols

Alcohols use the suffix -ol. Give the carbon bearing the hydroxy group the lowest possible locant; 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 is terminal, so no carbonyl locant is needed: butanal, 3-methylbutanal.

Ketones use the suffix -one. The carbonyl carbon sits within the chain and receives 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 term constitutional isomers means the same thing. Since the atoms join up in different ways, these isomers can have different boiling points, solubilities and chemical reactions.

A straight-chain isomer is a structural isomer in which the carbon atoms form one unbranched chain. A branched-chain isomer is a structural isomer in which 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 standard example: same $C_4H_{10}$, different skeleton.

A position isomer is a structural isomer that differs from another isomer in the position of a functional group, substituent or multiple bond on the same carbon skeleton. For example, 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 $C_2H_6O$, but ethanol is an alcohol while methoxymethane is an ether. Propanal and propanone both have formula $C_3H_6O$; 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 use the same classification: count the carbon atoms attached to the carbon bearing the halogen. This matters because primary, secondary and tertiary halogenoalkanes can favour different substitution or elimination pathways in Reactivity 3.

Amines are classified in a different way. 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. For amines, count the carbon groups attached to nitrogen, not the carbons attached to a functional-group carbon.

Benzene’s structure gives a useful link 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, more distinct disubstituted arrangements would be expected.

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 only go so far: they show what is bonded to what, but not always how those groups sit in space.

A conformational isomer is a stereoisomer formed by rotation about single bonds without breaking bonds. Ethane has staggered and eclipsed conformations. They interconvert so rapidly, though, 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 causes the restricted 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.

Disubstituted cycloalkanes can also show cis–trans isomerism because the ring restricts rotation. For $C_3$ and $C_4$ 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 containing 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 to make every group line up with the other. Real or virtual 3D models help a lot here: build the mirror image and 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, then separates the ions formed according to their mass-to-charge ratio. In organic mass spectrometry, molecules may break into smaller ions. This bond breaking is called fragmentation, and it produces characteristic charged pieces of the original molecule.

A mass spectrum is a graph of 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). In 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 skill is matching evidence to structure rather than memorising a long list.

Using a mass spectrum

A sensible way to interpret a spectrum 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 from fragments such as $CH_2OH^+$ or $C_2H_5^+$. A carboxylic acid may show a $COOH^+$ fragment. Isomers can have the same molecular ion peak but different fragment peaks, so MS helps distinguish structural features rather than merely measuring mass.

Bonds absorb infrared radiation

Infrared spectroscopy is an analytical technique used to identify bonds or functional groups by measuring absorption of infrared radiation, which causes molecular vibrations. Bonds can stretch, compress or bend, and the frequency absorbed depends on bond strength and the masses of the atoms involved.

Stronger bonds usually absorb at higher frequency. Heavier atoms tend to vibrate at lower frequency. That pattern makes IR spectra useful for spotting functional groups such as O–H, C=O and N–H.

The relationship between wave properties is

IR spectra usually use wavenumber:

A higher wavenumber corresponds to 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 $H_2$, $O_2$ and $Cl_2$ are IR inactive, since stretching the bond does not produce a changing dipole. Heteronuclear molecules such as HF are IR active because stretching changes the dipole.

Polyatomic molecules may 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. Greenhouse gases absorb outgoing infrared radiation from Earth when their vibrations are IR active. Their global warming potential depends on factors such as the strength of IR absorption, the wavelengths absorbed, 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 $1500\ \mathrm{cm}^{-1}$, contains many diagnostic absorptions.

Use the data booklet rather than relying on 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 excellent for deciding whether a functional group is present or absent. On its own, though, it is usually not enough to 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 identifies different hydrogen environments in an organic molecule by using the behaviour of hydrogen-1 nuclei in a magnetic field. Hydrogen-1 is written $^1$H and has one proton in its nucleus.

When placed in a magnetic field, $^1$H nuclei can occupy different spin energy states. The energy gap between these 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 is the set of neighbouring atoms and 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 shown by

. The reference is usually tetramethylsilane, TMS, which is assigned $\delta = 0$ ppm.

Reading low-resolution $^1$H NMR

A $^1$H NMR spectrum is a graph of signal intensity against chemical shift. Count the signals to find the number of different hydrogen environments. Symmetry can change this: two $CH_3$ groups may give one signal if they are chemically equivalent.

An integration trace is a stepped line or numerical value that shows the relative area under each NMR signal. This area is proportional to the number of hydrogen atoms causing that signal. If two signals integrate in a $3:2$ ratio, the environments contain hydrogens in that same ratio — perhaps $CH_3$ and $CH_2$.

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: $CH_3$ and $CH_2Cl$. 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 $CH_3$ groups are equivalent.

The data booklet gives chemical shift ranges, so the skill is matching a proposed structure to the observed pattern rather than memorising every value.

Why signals split

In high-resolution $^1$H NMR, a single signal can 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 needed here, use the $N + 1$ rule, where $N$ is the number of equivalent neighbouring hydrogen atoms on adjacent carbon atoms (dimensionless). So 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:

Pascal’s triangle gives the relative peak heights. Equivalent hydrogens in the same carbon environment do not split each other; focus on neighbouring, non-equivalent hydrogens.

Using splitting to build structure

Splitting adds connectivity detail. A triplet and quartet with integration 3:2 is a classic clue for an ethyl group, –$CH_2CH_3$: the $CH_3$ signal is split into a triplet by two neighbouring $CH_2$ hydrogens, while the $CH_2$ signal is split into a quartet by three neighbouring $CH_3$ hydrogens.

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

Why one spectrum is rarely enough

Structural analysis means using experimental evidence to work out how atoms are connected and arranged in a molecule. In practice, a single technique usually gives only one piece of the answer. The structure becomes convincing when several pieces of evidence point the same way.

Typical evidence includes:

• combustion analysis, to find an empirical formula;
• mass spectrometry, to find molecular mass and fragment clues;
• IR spectroscopy, to identify functional groups;
• $^{1}\text{H}$ NMR spectroscopy, to identify hydrogen environments, relative numbers of hydrogens, chemical shifts and splitting;
• reaction information, such as whether a compound behaves as an alkene, alcohol, aldehyde or carboxylic acid;
• data booklet tables for IR, MS and NMR interpretation.

A reliable workflow

Start with the formula if you have it. The empirical formula gives atom ratios, and the molecular ion peak can turn this into a molecular formula. Then estimate the degree of unsaturation by asking which combinations of rings, $\text{C=C}$ bonds, $\text{C=O}$ bonds or aromatic rings fit the formula. Don’t make this more complicated than the question needs.

Use IR to identify functional groups, or to rule them out. A strong $\text{C=O}$ absorption narrows the possibilities to aldehydes, ketones, carboxylic acids, esters or amides. A broad O–H absorption changes the set of possible structures again.

Use MS fragments to test possible carbon skeletons or functional groups. After that, use $^{1}\text{H}$ NMR to choose between isomers. The number of NMR signals checks symmetry; integration checks hydrogen counts; chemical shifts check nearby electronegative atoms or carbonyl groups; splitting checks neighbouring hydrogen atoms.

Worked data set showing how formula, IR, MS and ¹H NMR evidence agree for propanone.

A good final answer is more than a structure. It gives the reasons: “The molecular ion gives this molecular mass; IR shows a carbonyl but no broad O–H; NMR has these environments with this integration and splitting; therefore this isomer fits and the alternatives do not.” At that point, structural analysis is chemistry, not just pattern matching.