Free Fundamental Principles of Organic Chemistry MCQs with Answers

11. A carbon atom that gives rise to optical isomerism must be attached to

• A. two identical groups and two different ones
• B. four different atoms or groups
• C. at least one double bond
• D. three hydrogen atoms

Explanation: A carbon with four different substituents has no plane of symmetry, so its mirror image cannot be superimposed on it and the two forms are optical isomers that rotate plane polarised light in opposite directions. Such a carbon is described as chiral or asymmetric. If any two of the groups are the same, the mirror image is identical to the original and the molecule is achiral.

12. The number of structural isomers of C4H10 is

• A. one
• B. two
• C. three
• D. four

Explanation: Butane has an unbranched chain of four carbons and 2-methylpropane has a three carbon chain with a methyl branch, and no other arrangement is possible, so there are two. Pentane, C5H12, has three isomers, which is why three is offered here as a distractor. The number of isomers rises very steeply as the chain lengthens.

13. 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.

14. 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.

15. 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.

16. 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.

17. The functional group present in an ester is

• A. COOR, formed by replacing the acidic hydrogen of a carboxylic acid
• B. OH attached to a benzene ring
• C. NH2 attached to a carbon chain
• D. a carbon to carbon triple bond

Explanation: An ester is formed when a carboxylic acid reacts with an alcohol, water being eliminated, and the resulting group is responsible for the pleasant fruity smells used in flavourings and perfumes. A hydroxyl on a benzene ring makes a phenol and an NH2 group makes an amine. Esterification is reversible and is catalysed by concentrated sulphuric acid.

18. Which pair of compounds are functional group isomers?

• A. Propan-1-ol and propan-2-ol
• B. Butane and 2-methylpropane
• C. Propanal and propanone
• D. Ethane and propane

Explanation: Propanal is an aldehyde and propanone a ketone, yet both have the molecular formula C3H6O, so they differ in functional group. The two propanols differ only in the position of the same hydroxyl group, making them position isomers, and butane with 2-methylpropane are chain isomers. Ethane and propane are homologues, not isomers, since their formulae differ.

19. 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.

20. Which compound would show optical isomerism?

• A. CH3CH2OH
• B. CH3CHClCH2CH3
• C. CH3CH2CH3
• D. CH2Cl2

Explanation: In CH3CHClCH2CH3 the second carbon carries a methyl group, a chlorine, a hydrogen and an ethyl group, four different substituents, so it is chiral and the molecule exists as two non superimposable mirror images. In every other option at least two of the four groups on each carbon are identical. Checking each carbon for four different groups is the whole test.