All Free Chemistry MCQs with Answers

231. 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.

232. 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.

233. 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.

234. 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.

235. 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.

236. 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.

237. 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.

238. 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.

239. 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.

240. 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.