Which of the following statements relating to carbon and its compounds are correct? 1. 1-Propyne has three double bonds. 2. The boiling point of acetic acid is higher than methane. 3. Ethanol reacts with sodium metal and produces hydrogen gas. 4. Esters give effervescence on reaction with sodium bicarbonate. Select the answer using the code given below.
- (a)1 and 3
- (b)2 and 3 only
- (c)1 and 2
- (d)2, 3 and 4
Correct — B, 2 and 3 only. Statement 2 is right: acetic acid boils at about 391 K (118 degrees Celsius) while methane boils at about 112 K, because acetic acid molecules hydrogen-bond to one another and even pair up as dimers, whereas methane has only weak dispersion forces. Statement 3 is right too: ethanol reacts with sodium metal to give sodium ethoxide and hydrogen gas, which is the standard laboratory test that distinguishes an alcohol from a hydrocarbon. Statement 1 is false, because the suffix -yne means a triple bond, so 1-propyne has one triple bond and no double bonds at all. Statement 4 is false, because effervescence with sodium bicarbonate is the test for a carboxylic acid; an ester is neutral and gives no gas.
- (a)1 and 3 — Keeps the true statement 3 but brings in statement 1. Propyne is CH3-C(triple bond)CH; the name ends in -yne and the compound has a single triple bond, so 'three double bonds' is wrong twice over.
- (c)1 and 2 — Pairs a false statement with a true one. Statement 2 about boiling points is sound, but statement 1 misreads the -yne suffix, and no combination containing statement 1 can be correct.
- (d)2, 3 and 4 — Correct as far as statements 2 and 3 go, then adds statement 4. Esters are neutral and do not react with sodium hydrogencarbonate; it is ethanoic acid and other carboxylic acids that fizz, releasing carbon dioxide.
Four separate ideas from the carbon chapter are being tested at once. Nomenclature: -ane, -ene and -yne mark single, double and triple bonds, so propyne must contain a triple bond. Physical properties: hydrogen bonding raises boiling point steeply, which is why a small carboxylic acid boils far above a hydrocarbon of similar mass. Reactions of alcohols: ethanol has an acidic hydrogen weak enough to be displaced by sodium, giving hydrogen gas. Functional-group tests: the carbonate and hydrogencarbonate test identifies carboxylic acids by effervescence, and esters, being neutral, fail it.
This item is best worked statement by statement rather than option by option, because two of the four statements can be settled instantly from naming rules alone. Once statement 1 is rejected on the -yne suffix, options (a) and (c) both fall, and once statement 4 is rejected on the effervescence test, option (d) falls with them. That leaves only one surviving option, and it is worth noticing that this route never needed the boiling-point figures at all. This card marks a different letter from the one we have on file for this question; the statement-by-statement working above is the reason.
- The suffix -yne denotes a carbon-carbon triple bond; 1-propyne, CH3-C(triple bond)CH, has one triple bond.
- Acetic acid boils at about 391 K against methane's 112 K, largely because of hydrogen bonding.
- Ethanol reacts with sodium to give sodium ethoxide and hydrogen gas.
- Effervescence with sodium hydrogencarbonate identifies a carboxylic acid, not an ester.
- Esters are neutral, sweet-smelling compounds that undergo hydrolysis, and saponification with alkali.
Rejecting statement 1 removes two options and rejecting statement 4 removes a third, so the item can be closed without touching the boiling-point numbers.
- Reading -yne as a double bond; the vowel order a, e, y follows the bond order one, two, three.
- Assuming any organic compound will fizz with a carbonate — only acidic ones do.
- Comparing boiling points by molecular mass alone and forgetting hydrogen bonding.
A four-statement code item that mixes nomenclature, physical properties and functional-group tests, so it can be closed quickly by elimination if the naming rule is secure.
What is the action of litmus on ethanol ?
- (a) Litmus is neutral towards ethanol
- (b) Ethanol turns blue litmus to red; confirming acidic nature of ethanol
- (c) Ethanol turns red litmus to blue; confirming basic nature of ethanol
- (d) Ethanol decolorizes litmus through bleaching action
Answer(a) Litmus is neutral towards ethanol
Fills in the other half of statement 3. Ethanol is not acidic enough to turn litmus, yet it still gives up its hydroxyl hydrogen to sodium metal — the two facts sit together.
C₄H₈ belongs to the homologous series of
- (a) alkanes
- (b) alkenes
- (c) alkynes
- (d) cycloalkanes
Answer(b) alkenes
The general-formula habit that kills statement 1 here. Reading a name or a formula for its bond order is the quickest single skill in this chapter.
- practice — not a real PYQ
Effervescence on adding sodium hydrogencarbonate indicates the presence of which functional group?
- (a)Alcohol
- (b)Aldehyde
- (c)Carboxylic acid
- (d)Ester
Answer(c) Carboxylic acid — carbon dioxide is released, and neither an alcohol nor an ester gives this reaction.
- practice — not a real PYQ
How many carbon-carbon triple bonds are present in one molecule of propyne?
- (a)None
- (b)One
- (c)Two
- (d)Three
Answer(b) One — propyne is CH3-C(triple bond)CH, and the -yne suffix marks that single triple bond.