Functional groups: Classification of organic compounds: notes and practice questions
- This topic covers representing, classifying, and naming organic compounds based on their structure and functional groups.
- Organic compounds use empirical, molecular, structural (full/condensed), stereochemical, and skeletal formulas.
- Identify common functional groups (e.g., halogeno, hydroxy, carbonyl, carboxyl) and distinguish saturated/unsaturated compounds.
- Homologous series share a general formula (e.g., CH₂) and show predictable trends in physical properties.
- Apply IUPAC nomenclature to saturated/mono-unsaturated compounds (up to six carbons) with one functional group.
- Recognize straight-chain, branched-chain, position, and functional group isomers.
How it is examined
The densest single-mark region of Paper 2. May 2025 HL Paper 2 TZ1 ran a chain on an unknown X: state the functional group name [1], deduce the systematic IUPAC name [1], interpret its ¹H NMR spectrum [2], draw an isomer belonging to a different homologous series [1]. Separately it asked for the structural formula of cis-1-chlorobut-2-ene [1] and why the cis isomer is polar [1], and for a drawing showing optical isomerism with the chiral carbon marked with an asterisk [2]. Structure elucidation from combined spectra is the classic HL extended part at 3 to 6 marks.
Characteristic IR absorption ranges, ¹H NMR chemical shift ranges, and common MS fragment losses. That means a question asking a student to recall that a carbonyl absorbs near 1700 cm⁻¹, or that an aldehyde proton sits near 9-10 ppm, is testing nothing. What is recall: functional group names and structures, homologous series general formulas, IUPAC rules, and how to count chemical environments.
- 3.2.1 Organic compounds can be represented by different types of formulas: empirical, molecular, structural (full and condensed), stereochemical and skeletal. Students identify different formulas and interconvert molecular, skeletal and structural formulas, and construct 3D models (real or virtual).
- 3.2.2 Functional groups give characteristic physical and chemical properties to a compound. Organic compounds are divided into classes according to the functional groups present. Students identify these functional groups by name and structure: halogeno, hydroxy, carbonyl, carboxyl, alkoxy, amino, amido, ester, phenyl.
- 3.2.3 A homologous series is a family of compounds in which successive members differ by a common structural unit, typically CH₂, and each series can be described by a general formula. Students identify these homologous series: alkanes, alkenes, alkynes, halogenoalkanes, alcohols, aldehydes, ketones, carboxylic acids, ethers, amines, amides and esters.
- 3.2.4 Successive members of a homologous series show a trend in physical properties. Students describe and explain the trend in melting and boiling points.
- Stereochemical formulas are not expected to be drawn, except where specifically indicated.
- HL: nomenclature using the E-Z system will not be assessed. Use cis-trans.
- 3.2.7 Stereoisomers have the same constitution (atom identities, connectivities and bond multiplicities) but different spatial arrangements of atoms. Students describe and explain the features that give rise to cis-trans isomerism and recognize it in non-cyclic alkenes and C3 and C4 cycloalkanes; draw stereochemical formulas showing the tetrahedral arrangement around a chiral carbon; describe and explain a chiral carbon atom giving rise to stereoisomers with different optical properties; and recognize a pair of enantiomers as non-superimposable mirror images from 3D modelling.
- 3.2.8 Mass spectrometry (MS) of organic compounds can cause fragmentation of molecules. Students deduce information about the structural features of a compound from specific MS fragmentation patterns.
- 3.2.9 Infrared (IR) spectra can be used to identify the type of bond present in a molecule. Students interpret the functional group region of an IR spectrum using a table of characteristic frequencies (wavenumber / cm⁻¹).
- 3.2.10 Proton nuclear magnetic resonance spectroscopy (¹H NMR) gives information on the different chemical environments of hydrogen atoms in a molecule. Students interpret ¹H NMR spectra to deduce structures from the number of signals, the chemical shifts, and the relative areas under signals (integration traces).
Guiding questions
- How does the classification of organic molecules help us to predict their properties?
Linking questions
- Structure 2.2 What is unique about carbon that enables it to form more compounds than the sum of all the other elements' compounds? What is the influence of the carbon chain length, branching and the nature of the functional groups on intermolecular forces? What features of a molecule determine whether it is IR active or not?
- Nature of science, Structure 2.2 What are the advantages and disadvantages of different depictions of an organic compound?
- Nature of science, Tool 2 How useful are 3D models (real or virtual) to visualize the invisible?
- Nature of science, Reactivity 3.2, 3.4 How can functional group reactivity be used to determine a reaction pathway between compounds, for example converting ethene into ethanoic acid?
- Structure 2.2 (HL) How does the fact that there are only 3 isomers of dibromobenzene support the current model of benzene's structure?
- Structure 2.4 (HL) What is the nature of the reaction that occurs when two amino acids form a dipeptide?
- Reactivity 1.3 What properties of a greenhouse gas determine its "global warming potential"?
Practice questions
24 questions · 9 easy · 15 mediumQuestion 1
EasyPaper 2 · calculator1 markEthanol () is often blended with gasoline as a biofuel. What are the sole products of the complete combustion of ethanol?
A. Carbon monoxide and water
B. Carbon and water
C. Carbon dioxide and hydrogen
D. Carbon dioxide and water
Complete combustion involves reacting a substance with an excess of oxygen. Think about the most stable oxides formed from the elements carbon and hydrogen.
Question 2
MediumPaper 2 · calculator6 marksA nitrogenous base, compound Y, extracted from a plant species, was found to have the following percentage composition by mass:
C = 60.94%
H = 15.37%
N = 23.69%
(a) Determine the empirical formula of compound Y.
(b) A solution was prepared by dissolving 1.18 g of Y in deionized water to make a 50.00 cm³ solution. A 25.00 cm³ aliquot of this solution was titrated with 0.500 mol dm⁻³ hydrochloric acid, HCl(aq). Complete neutralization required 20.00 cm³ of the HCl solution. Determine the molar mass (M) of Y.
(c) State the molecular formula of Y.
(d) Compound Y is a weak base that reacts with water according to the equation:
Y(aq) + H₂O(l) ⇌ YH⁺(aq) + OH⁻(aq)
Identify the functional group present in Y.
Start by assuming a 100 g sample of the compound. Then, convert the mass of each element to moles by dividing by its atomic mass. Finally, find the simplest whole number ratio of the moles.
First, calculate the moles of HCl that reacted. Since this is a monobasic compound, the moles of HCl will equal the moles of Y in the 25.00 cm³ sample. Then, use this to find the total moles in the original 50.00 cm³ solution and calculate the molar mass.
Calculate the mass of the empirical formula you found in part (a). Compare this to the molar mass you calculated in part (b) to find the relationship between the empirical and molecular formulas.
The compound is described as a nitrogenous base. What is the name of the functional group containing nitrogen that gives organic compounds basic properties?
Question 3
EasyPaper 1A · calculator1 markWhich molecule exhibits cis-trans isomerism?
A.
B.
C.
D.
For cis-trans isomerism to occur in an alkene, each carbon atom of the carbon-carbon double bond must be bonded to two different atoms or groups. Check each option against this rule.
Question 4
MediumPaper 1A · calculator1 markWhich alcohol and conditions would produce the highest yield of butanal?
Alcohol
Conditions
A. Butan-1-ol
Reflux
B. Butan-2-ol
Reflux
C. Butan-1-ol
Distillation
D. 2-methylpropan-2-ol
Distillation
Consider the type of functional group in butanal. What class of alcohol is oxidized to form this functional group? What specific reaction conditions are needed to ensure this is the major product and prevent further oxidation?
Question 5
EasyPaper 2 · calculator1 markAll of the following compounds exhibit cis–trans isomerism except:
A. 1-chloro-2-fluoroethene
B. 1,2-dichlorocyclobutane
C. but-1-ene
D. pent-2-ene
For cis-trans isomerism to occur in an alkene, each carbon atom in the double bond must be bonded to two different groups. For a cycloalkane, at least two carbon atoms in the ring must each be bonded to two different groups.
Question 6
MediumPaper 2 · calculator1 markWhich structural isomer of hexane, , will form exactly two different structural isomers of monochlorohexane, , upon free-radical substitution?
A. Hexane
B. 2-Methylpentane
C. 2,2-Dimethylbutane
D. 2,3-Dimethylbutane
Draw the structure of each isomer of hexane listed. Identify the number of unique environments for hydrogen atoms in each molecule. A unique environment will lead to a different substitution product.
Question 7
EasyPaper 1A · calculator1 markWhich molecule contains a carboxyl functional group?
A.
B.
C.
D.
The carboxyl functional group consists of a carbon atom double-bonded to one oxygen atom and single-bonded to a hydroxyl group. Its general formula is R-COOH.
Question 8
MediumPaper 2 · calculator7 marksA student investigates the rate of hydrolysis of two structural isomers of a chloroalkane with the formula : 1-chlorobutane and 2-chloro-2-methylpropane. The reaction is carried out by warming each chloroalkane with aqueous potassium hydroxide.
(a) For each isomer, state whether it is a primary, secondary or tertiary haloalkane.
(b) Predict, with a reason, which isomer will react faster with aqueous potassium hydroxide.
(c) Explain your prediction in part (b), referring to the mechanisms of nucleophilic substitution.
(d) State the IUPAC name of the organic product formed from the reaction of 2-chloro-2-methylpropane with aqueous potassium hydroxide.
Recall the definitions of primary, secondary, and tertiary haloalkanes. This is based on the number of carbon atoms directly bonded to the carbon atom that is bonded to the halogen.
Consider the stability of the carbocation intermediate that can be formed from each isomer. How does this relate to the reaction mechanism and rate?
Compare the mechanism, which is favoured by tertiary haloalkanes, with the mechanism. Key factors to discuss are the stability of intermediates and steric hindrance.
The reaction is a nucleophilic substitution where the chlorine atom is replaced by a hydroxyl group (). Name the resulting alcohol.
Question 9
EasyPaper 1A · calculator1 markWhich process for producing a halogenoalkane has an atom economy of 100%?
A.
B.
C.
D.
Consider the definition of atom economy. A reaction with 100% atom economy converts all reactant atoms into the desired product, with no waste products. Examine the products of each reaction.
Question 10
MediumPaper 2 · calculator6 marksButan-2-ol, , is a secondary alcohol that can be oxidized using acidified potassium dichromate(VI) solution to produce an organic compound, X.
(a) Identify the organic compound X and state the colour change observed for the oxidizing agent.
(b) The structure of a compound can be confirmed using ¹H NMR spectroscopy. Predict the number of signals and the ratio of the areas under the signals for the ¹H NMR spectrum of butan-2-ol.
(c) Predict the number of signals and the ratio of the areas under the signals for the ¹H NMR spectrum of compound X.
Consider the product formed when a secondary alcohol is oxidized. Recall the characteristic colours of the dichromate(VI) ion and the chromium(III) ion formed upon reduction.
Identify the number of unique proton environments in the butan-2-ol molecule. The ratio of the areas under the signals corresponds to the ratio of the number of protons in each of these unique environments.
First, draw the structure of compound X (butanone). Then, identify the non-equivalent proton environments and count the number of protons in each environment to determine the integration ratio.
Question 11
EasyPaper 1A · calculator1 markWhich compound is produced by the oxidation of propan-2-ol with acidified potassium dichromate(VI)?
A. Propanal
B. Propanoic acid
C. Propanone
D. Propene
Identify propan-2-ol as a primary, secondary, or tertiary alcohol. Recall the product formed from the oxidation of this class of alcohol.
Question 12
MediumPaper 1A · calculator1 markWhich compound(s) will decolorize warm, acidified potassium manganate(VII) solution?
I.
II.
III.
A. I only
B. I and III only
C. II and III only
D. I, II and III
Consider the classification of each alcohol as primary, secondary, or tertiary. Recall how each class of alcohol reacts with a strong oxidizing agent like acidified potassium manganate(VII).
Question 13
EasyPaper 1A · calculator1 markWhich formula represents an ester?
A.
B.
C.
D.
Recall the general structure of an ester functional group. It contains a carbonyl group () bonded to an oxygen atom, which is then bonded to another alkyl group. The general formula is .
Question 14
MediumPaper 1A · calculator1 markWhat is the half-equation for the oxidation of ethanol, , to ethanoic acid, , in acidic solution?
A.
B.
C.
D.
Follow the systematic steps for balancing a half-equation in acidic medium. First, balance atoms other than O and H. Then, balance O atoms using . Next, balance H atoms using . Finally, balance the charge using electrons. Remember that for oxidation, electrons are products.
Question 15
EasyPaper 1A · calculator1 markWhich haloalkane has the highest boiling point?
A.
B.
C.
D.
Consider how the identity of the halogen atom affects the strength of the intermolecular forces present. Which type of intermolecular force is most significant in determining the boiling point trend for this series of compounds?
Question 16
MediumPaper 1A · calculator1 markPropene, , is an important feedstock in the chemical industry.
Which row correctly identifies the major organic product formed when propene reacts with the given reagent?
A. Reagent: (acid catalyst), Product: Propan-1-ol
B. Reagent: , Product: 2-bromopropane
C. Reagent: , Product: 1,1-dibromopropane
D. Reagent: (Ni catalyst), Product: Prop-1-ene
Recall the addition reactions of alkenes. For unsymmetrical alkenes like propene, consider Markovnikov's rule to predict the major product in reactions with reagents like and . Remember what products are formed during halogenation and hydrogenation.
Question 17
EasyPaper 1A · calculator1 markWhich species acts as the nucleophile in the reaction below?
A.
B.
C.
D.
A nucleophile is a species that donates a pair of electrons to form a new covalent bond. Look for a species with a lone pair of electrons or a negative charge that attacks an electron-deficient centre.
Question 18
MediumPaper 1A · calculator1 markWhich compound has the lowest boiling point?
A.
B.
C.
D.
Consider the factors that affect the strength of intermolecular forces, such as molar mass and molecular shape (branching).
Question 19
MediumPaper 1A · calculator1 markWhich reaction represents the reduction of a functional group?
A.
B.
C.
D.
Reduction in organic chemistry often involves an increase in the number of carbon-hydrogen bonds or a decrease in the number of carbon-oxygen bonds. Analyse each reaction to see which one fits this description.
Question 20
MediumPaper 1A · calculator1 markWhich alcohol(s) will decolorize acidified potassium permanganate(VII) solution upon warming?
I.
II.
III.
A. I only
B. III only
C. I and III only
D. I, II and III
Consider the classification of each alcohol as primary, secondary, or tertiary. Recall which classes of alcohols can be oxidized by common oxidizing agents like acidified potassium permanganate(VII).
No question on this page matches those filters. Try another difficulty or paper.
4 more Functional groups: Classification of organic compounds questions in the app
Every answer is marked mark by mark, IB-style, and the AI tutor helps when you are stuck.
Where marks are lost
- Reaching for "human error" or "only one trial." A source of error has to be a specific step in the method, not a general apology for the result.
- Joining the dots instead of drawing a curve.
- Naming a chemical instead of the property that distinguishes it, or vice versa. Answering with the nearest fact that comes to mind rather than the fact the command term and stem jointly ask for is a recurring way to answer a question that was not, quite, the one asked.
