Free Classification of Organic Compounds 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. In the IUPAC name 2-methylbutane, the number 2 indicates

• A. the number of methyl groups present
• B. the total number of carbon atoms
• C. the position on the main chain where the methyl group is attached
• D. the number of double bonds

Explanation: The parent chain is numbered from the end that gives substituents the lowest possible locants, so the methyl group here sits on the second carbon of a four carbon chain. If two methyl groups were present the prefix would be dimethyl with both positions listed. Systematic naming exists precisely so that one formula corresponds to exactly one name.

5. Which of the following is a saturated hydrocarbon?

• A. Ethene
• B. Ethyne
• C. Benzene
• D. Ethane

Explanation: A saturated hydrocarbon contains only single carbon to carbon bonds and therefore holds the maximum possible number of hydrogen atoms, which is true of ethane and of the alkanes generally. Ethene has a double bond, ethyne a triple bond and benzene a delocalised system, so all three are unsaturated. Unsaturation is detected in the laboratory by the decolourising of bromine water.

6. An electrophile is a species that

• A. donates a pair of electrons
• B. accepts a pair of electrons and is attracted to electron rich centres
• C. always carries a negative charge
• D. contains an unpaired electron

Explanation: Electrophile means electron loving, so such a species is electron deficient and attacks regions of high electron density such as a double bond, examples being NO2+ and the polarised bromine molecule. A nucleophile does the opposite, donating a lone pair to an electron poor carbon. A species with an unpaired electron is a free radical, a third category altogether.

7. Heterolytic fission of a covalent bond produces

• A. two free radicals
• B. two neutral atoms
• C. a positive and a negative ion, because both electrons go to one atom
• D. no change in the molecule

Explanation: In heterolytic fission the shared pair leaves with the more electronegative atom, giving oppositely charged ions such as a carbocation and a halide, and this is typical of polar solvents and ionic mechanisms. Homolytic fission splits the pair evenly and gives two free radicals, which is what happens under ultraviolet light or high temperature. Recognising which type a reaction uses is the key to predicting its mechanism.

8. A nucleophile attacks

• A. a region of high electron density such as a double bond
• B. another nucleophile
• C. an electron deficient carbon atom, donating a lone pair to it
• D. only free radicals

Explanation: Nucleophile means nucleus loving, so these electron rich species, such as hydroxide, cyanide and ammonia, seek out carbon atoms made positive by an attached electronegative group. This is why alkyl halides undergo nucleophilic substitution while alkenes undergo electrophilic addition. Every nucleophile is a Lewis base, since it donates an electron pair.