Free Fundamental Principles of Organic Chemistry MCQs with Answers

1. Organic compounds are built around carbon because a carbon atom

• A. forms four covalent bonds and can catenate into long chains and rings
• B. readily forms ions of charge four
• C. has a very large atomic radius
• D. always forms only single bonds

Explanation: With four valence electrons and a small radius, carbon forms four strong covalent bonds and bonds to itself almost as strongly as to other elements, so chains, branches and rings of unlimited length are possible. It also forms stable double and triple bonds, which silicon below it cannot do. Carbon rarely forms simple ions because removing or adding four electrons would cost far too much energy.

2. A homologous series is a family of compounds in which each member

• A. has the same molecular formula
• B. differs from the next by a CH2 group and shares the same general formula and functional group
• C. has the same physical properties
• D. contains the same number of carbon atoms

Explanation: Members share a general formula and a functional group, so their chemical properties are similar, while physical properties such as boiling point change gradually as the chain lengthens. The alkanes, with the general formula CnH2n+2, are the simplest example. Compounds with the same molecular formula but different structures are isomers, not members of a series.

3. Which of the following is a heterocyclic compound?

• A. Benzene
• B. Cyclohexane
• C. Pyridine
• D. Naphthalene

Explanation: A heterocyclic compound has a ring containing at least one atom other than carbon, and in pyridine one of the six ring positions is occupied by nitrogen. Benzene, cyclohexane and naphthalene are all carbocyclic, since their rings are made only of carbon. Furan, thiophene and pyrrole are the other common five membered heterocycles.

4. The functional group present in an aldehyde is

• A. a hydroxyl group
• B. a carbonyl group at the end of the chain, written CHO
• C. a carboxyl group
• D. a carbonyl group between two carbon atoms

Explanation: In an aldehyde the carbonyl carbon carries at least one hydrogen and so must be terminal, whereas in a ketone the same carbonyl group lies between two carbon atoms. This structural difference is why aldehydes are readily oxidised to carboxylic acids and give a positive test with Tollens reagent while ketones do not. The carboxyl group of an acid combines a carbonyl and a hydroxyl on the same carbon.

5. The general formula CnH2n could represent

• A. an alkane
• B. an alkyne
• C. an alcohol
• D. an alkene or a cycloalkane

Explanation: Both an alkene, with one double bond, and a cycloalkane, with one ring, have two hydrogens fewer than the corresponding alkane, so they share this general formula and are functional isomers of each other. Alkanes are CnH2n+2 and alkynes CnH2n-2. This is why a general formula alone can never identify a compound with certainty.

6. The functional group of a carboxylic acid is

• A. COOH
• B. CHO
• C. OH
• D. CO

Explanation: The carboxyl group combines a carbonyl and a hydroxyl on the same carbon atom, and this arrangement makes the hydrogen far more acidic than in an alcohol, because the resulting carboxylate ion is stabilised by delocalisation. CHO is the aldehyde group and CO between two carbons is a ketone. Acids therefore liberate carbon dioxide from carbonates, which alcohols do not.

7. Compounds with the same molecular formula but different structural formulae are called

• A. isotopes
• B. isomers
• C. homologues
• D. allotropes

Explanation: Isomers share the molecular formula but differ in how the atoms are connected or arranged in space, so they are genuinely different substances with different properties. Butane and 2-methylpropane both have the formula C4H10 yet boil at different temperatures. Homologues differ by CH2 and so have different molecular formulae.

8. Butan-1-ol and butan-2-ol are examples of

• A. chain isomers
• B. functional group isomers
• C. position isomers
• D. geometric isomers

Explanation: The carbon skeleton and the functional group are the same in both, and only the position of the hydroxyl group along the chain differs, which defines position isomerism. Chain isomers differ in the branching of the skeleton, and functional group isomers carry different groups altogether, as an alcohol and an ether do. Position isomerism is why butan-1-ol oxidises to an aldehyde while butan-2-ol gives a ketone.

9. Ethanol and dimethyl ether both have the molecular formula C2H6O. They are

• A. position isomers
• B. chain isomers
• C. optical isomers
• D. functional group isomers

Explanation: Ethanol contains a hydroxyl group while dimethyl ether contains an oxygen bridging two carbons, so the functional group itself differs and the two compounds behave completely differently, ethanol boiling at 78 degrees Celsius against minus 25 for the ether. That large difference is due to hydrogen bonding, which the ether cannot do. Aldehydes with ketones and acids with esters are the other standard pairs.

10. Geometric or cis trans isomerism arises in an alkene because

• A. the double bond prevents free rotation, fixing the groups on either side
• B. the molecule contains a chiral carbon
• C. the double bond rotates freely
• D. alkenes contain a ring

Explanation: The pi bond locks the two carbons in a fixed relative orientation, so substituents are held either on the same side, cis, or on opposite sides, trans, and the two forms cannot interconvert without breaking the pi bond. Each doubly bonded carbon must also carry two different groups, otherwise the two arrangements are identical. Single bonds rotate freely, so alkanes show no such isomerism.