Chemists identify unknown compounds without a single reaction. Three spectra each give a different clue: the mass, the bonds and the hydrogen environments.
⚖️ Mass spectrometry and infrared
Mass spectrometry: the peak with the highest m/z is usually the molecular ion M⁺, which gives the relative molecular mass. Other peaks are fragments. A compound with one chlorine atom shows an M and an M+2 peak in a 3 : 1 ratio (³⁵Cl and ³⁷Cl); one bromine gives 1 : 1.
Infrared (IR) shows which bonds are present from absorption wavenumbers.
C=O about 1700 cm⁻¹ (strong, sharp)
O–H alcohol 3200–3550 cm⁻¹ (broad)
O–H acid 2500–3300 cm⁻¹ (very broad)
C–H about 2850–3100 cm⁻¹
ethanol C₂H₅OH: M⁺ at m/z = 46
🖼️ See it
Match each strong absorption in the spectrum to a bond to find the functional groups.Three environments give three signals in the ratio 3 : 2 : 1. The n + 1 rule gives the splitting: CH₃ sees 2 neighbours (triplet), CH₂ sees 3 (quartet).
🧲 Proton NMR
¹H NMR shows each set of chemically equivalent hydrogens as a separate signal. The number of signals is the number of different environments; the integration gives the ratio of hydrogens; the splitting follows the n + 1 rule (a signal is split into n + 1 peaks by n neighbouring hydrogens). The chemical shift is measured relative to TMS (0 ppm).
ethanol CH₃CH₂OH: 3 signals
ratio CH₃ : CH₂ : OH = 3 : 2 : 1
CH₃ next to CH₂ (2 H): 2 + 1 = 3 peaks (triplet)
CH₂ next to CH₃ (3 H): 3 + 1 = 4 peaks (quartet)
propane CH₃CH₂CH₃: 2 signals
Exchangeable O–H protons usually appear as a singlet because they are not split by neighbours.
✏️ Quick check · 1 of 8
Mass spectrum
The molecular ion peak in a mass spectrum gives the compound’s:
Highest m/z.
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IR
A strong, sharp IR absorption at about 1700 cm⁻¹ indicates:
A carbonyl.
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Chlorine
A compound with one chlorine atom shows M and M+2 peaks in the ratio:
Natural abundance.
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NMR splitting
A CH₃ group next to a CH₂ group gives a signal that is a:
n + 1 rule.
✍️ Type your answer · 5 of 8
Ethanol, C₂H₅OH (Ar: C = 12, H = 1, O = 16).
What is the m/z of its molecular ion?
Enter an exact answer, without rounding.
Add up the relative atomic masses.
✍️ Type your answer · 6 of 8
Ethanol, CH₃CH₂OH.
How many signals does its ¹H NMR spectrum show?
Enter an exact answer, without rounding.
Count the environments.
✍️ Type your answer · 7 of 8
In ethanol the CH₂ hydrogens have 3 neighbouring hydrogens on the CH₃.
How many peaks does the CH₂ signal split into?
Enter an exact answer, without rounding.
n + 1 rule.
✍️ Type your answer · 8 of 8
Propane, CH₃CH₂CH₃.
How many ¹H NMR signals does it show?
Enter an exact answer, without rounding.
Count the environments.
📝 Lesson quiz · 5 questions
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Re-read the lesson, then give it another go.
Question 1 of 5
The n + 1 rule says a signal is split into:
Question 2 of 5
The reference compound for NMR chemical shift is:
Question 3 of 5
A broad IR absorption at 3200–3550 cm⁻¹ indicates an:
Question 4 of 5
The ratio of integration for CH₃CH₂OH is:
Question 5 of 5
Fragmentation in a mass spectrum produces:
Lesson 2 · Organic chemistry
⚗️ Acyl Chlorides, Amines & Azo Dyes
Acyl chlorides are among the most reactive organic compounds. Azo dyes give many clothes their bright colours.
🧪 Acyl chlorides
Acyl chlorides (RCOCl) react quickly with water, alcohols, phenol and amines by nucleophilic addition–elimination. They give off HCl. They are far more reactive than carboxylic acids because the C=O carbon is very electron-poor.
Phenylamine is made by reducing nitrobenzene with tin and HCl. With HNO₂ below 10 °C, it forms a diazonium salt, which couples with phenol to give a brightly coloured azo dye containing –N=N–. Phenylamine is a weaker base than ethylamine, as the ring draws in the nitrogen’s lone pair.
Compared with carboxylic acids, acyl chlorides are:
They fume in moist air.
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Product
An acyl chloride with an alcohol gives:
Sweet-smelling.
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Mechanism
Reaction of an acyl chloride with water is:
Add, then lose Cl.
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Azo
An azo dye contains the group:
Two nitrogens.
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Temperature
Diazonium salts are made below 10 °C because they:
They are unstable.
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CH₃COCl has C = 12, H = 1, O = 16 and Cl = 35.5.
What is its relative molecular mass?
Enter an exact answer, without rounding.
Add all the atoms.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Which reacts most readily with water?
Question 2 of 5
Phenylamine is made from nitrobenzene using:
Question 3 of 5
Phenylamine is a weaker base than ethylamine because:
Question 4 of 5
Ethanoyl chloride with ammonia gives:
Question 5 of 5
A phenol coupling with a diazonium salt is done in:
Lesson 3 · Inorganic and physical chemistry
🌈 Transition Metal Complexes & Partition
Why is copper sulfate blue, and why does it turn deep blue with ammonia? The answer lies in the d-orbitals.
💠 Complexes
A transition element forms at least one ion with a partly filled d-subshell. A ligand donates a lone pair to the metal to form a complex ion. Ligands split the d-orbitals; electrons absorb light to jump the gap, and we see the complementary colour. A larger stability constant (Kstab) means a more stable complex.
[Cu(H₂O)₆]²⁺: pale blue
[Cu(NH₃)₄(H₂O)₂]²⁺: deep blue
[CuCl₄]²⁻: yellow
coordination number: number of bonds from ligands to the metal
bidentate: 2 bonds from each ligand, such as ethane-1,2-diamine
⚖️ Partition coefficients
A solute shared between two immiscible solvents reaches a constant ratio at equilibrium: the partition coefficient Kpc = [X]₁ / [X]₂. Thin-layer and gas–liquid chromatography rely on the same idea.
Iodine: 0.40 mol/dm³ in hexane, 0.02 mol/dm³ in water
Kpc = 0.40 ÷ 0.02 = 20
Rf = distance moved by spot ÷ distance moved by solvent
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Definition
A ligand is a species that:
A lone pair donor.
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Colour
A complex appears coloured because it:
Think complementary.
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Scandium
Sc³⁺ is not coloured because it has:
Count d electrons.
✏️ Quick check · 4 of 6
Kstab
A larger stability constant means the complex is:
More stable.
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A complex has six water ligands, each forming one bond.
What is its coordination number?
Enter an exact answer, without rounding.
Count the bonds.
✍️ Type your answer · 6 of 6
A solute is 0.40 mol/dm³ in hexane and 0.02 mol/dm³ in water.
What is the partition coefficient, hexane to water?
Enter an exact answer, without rounding.
Divide the concentrations.
📝 Lesson quiz · 5 questions
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Re-read the lesson, then give it another go.
Question 1 of 5
Adding excess ammonia to [Cu(H₂O)₆]²⁺ gives a:
Question 2 of 5
Ethane-1,2-diamine is:
Question 3 of 5
Why can transition metals have variable oxidation states?
Question 4 of 5
In TLC, the Rf value is:
Question 5 of 5
A partition coefficient applies to two liquids that are:
Final Challenge
🏆 12-Question Challenge Quiz
Questions from every topic in this course. Pick an answer for each one, then hit Check My Answers.
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💪 Keep practicing — go back through the lessons and retake the quiz!
Q1 · Spectroscopy: Mass Spectrometry, IR & NMR
Which technique tells you which functional groups are present?
Q2 · Spectroscopy: Mass Spectrometry, IR & NMR
In mass spectrometry, the molecular ion peak gives the:
Q3 · Spectroscopy: Mass Spectrometry, IR & NMR
A very broad IR absorption at 2500–3300 cm⁻¹ shows:
Q4 · Spectroscopy: Mass Spectrometry, IR & NMR
A strong, sharp IR peak near 1700 cm⁻¹ shows:
Q5 · Spectroscopy: Mass Spectrometry, IR & NMR
In ¹H NMR, the number of peaks shows:
Q6 · Spectroscopy: Mass Spectrometry, IR & NMR
The standard reference compound in NMR is:
Q7 · Spectroscopy: Mass Spectrometry, IR & NMR
By the n + 1 rule, a hydrogen with 2 hydrogens on the neighbouring carbon gives a:
Q8 · Spectroscopy: Mass Spectrometry, IR & NMR
The small M+1 peak in a mass spectrum is caused by:
Q9 · Acyl Chlorides, Amines & Azo Dyes
Acyl chlorides have the functional group:
Q10 · Acyl Chlorides, Amines & Azo Dyes
The colour of azo dyes comes from:
Q11 · Transition Metal Complexes & Partition
The d-orbitals in a complex are split by the:
Q12 · Transition Metal Complexes & Partition
Kpc = 8 and the concentration in water is 0.05 mol/dm³. The concentration in the organic layer is: