IMO Practice Test — Organic Chemistry: Some Basic Principles and Techniques
12 Questions • 15 min • Olympiad level
15:00
Question 1 of 12
The number of $\sigma$ and $\pi$ bonds in propyne $CH_3−C{\equiv}CH$ is:
6 $\sigma$, 2 $\pi$
5 $\sigma$, 2 $\pi$
6 $\sigma$, 1 $\pi$
7 $\sigma$, 2 $\pi$
Explanation: C−H bonds (4) + C−C (1) + the $\sigma$ of the triple bond (1) = 6 $\sigma$; the triple bond adds 2 $\pi$.
Question 2 of 12
The IUPAC name of $CH_3CH(CH_3)CH(C_2H_5)CH_2CH_3$ is:
2-ethyl-3-methylpentane
3-ethyl-2-methylpentane
3,4-dimethylhexane
2-methyl-3-ethylpentane
Explanation: Longest chain is 5 C (pentane); ethyl on C-3, methyl on C-2; alphabetised: 3-ethyl-2-methylpentane.
Question 3 of 12
How many structural isomers (chain + position skeleton) exist for $C_4H_9Cl$?
2
3
4
5
Explanation: 1-chloro-, 2-chlorobutane, 1-chloro-2-methylpropane and 2-chloro-2-methylpropane — four isomers.
Question 4 of 12
Which carbocation is the MOST stable?
$CH_2=CH−CH_2^+$ (allyl)
$CH_3CH_2^+$
$CH_3^+$
$CH_2=CH^+$ (vinyl)
Explanation: The allyl cation is resonance-stabilised; the vinyl cation is especially unstable ($sp$ carbon).
Question 5 of 12
Among $−F$, $−Cl$, $−Br$, $−I$, the strongest $−I$ (inductive) effect is shown by:
$−I$
$−Br$
$−Cl$
$−F$
Explanation: The inductive effect tracks electronegativity, which is greatest for fluorine.
Question 6 of 12
The number of $\alpha$-hydrogens available for hyperconjugation in the tert-butyl cation $(CH_3)_3C^+$ is:
3
6
9
1
Explanation: Three methyl groups, each with three C−H bonds, give nine $\alpha$-hydrogens.
Question 7 of 12
Which species is both a Brønsted base and a Lewis base?
$BF_3$
$NH_3$
$H^+$
$CO_2$
Explanation: $NH_3$ accepts a proton (Brønsted base) and donates its lone pair (Lewis base).
Question 8 of 12
The empirical formula of a compound with $40\%$ C, $6.7\%$ H and $53.3\%$ O (by mass) is:
$CHO$
$CH_2O$
$C_2H_4O$
$CH_2O_2$
Explanation: Moles: C $=40/12=3.33$, H $=6.7/1=6.7$, O $=53.3/16=3.33$; ratio $1:2:1$ → $CH_2O$.
Question 9 of 12
A Carius estimation of $0.20\ \text{g}$ of an organochlorine compound gave $0.287\ \text{g}$ of AgCl. The $\%Cl$ is about (AgCl = 143.5, Cl = 35.5):
$18\%$
$25\%$
$35.5\%$
$71\%$
Explanation: $\%Cl=\dfrac{35.5}{143.5}\times\dfrac{0.287}{0.20}\times100=\dfrac{35.5}{143.5}\times143.5=35.5\%$.
Question 10 of 12
In Kjeldahl's method, $0.25\ \text{g}$ of a compound liberated ammonia that needed $25\ \text{mL}$ of $0.25\ \text{N}$ acid. The $\%N$ is:
$3.5\%$
$35\%$
$8.75\%$
$17.5\%$
Explanation: $\%N=\dfrac{1.4\times0.25\times25}{0.25}=\dfrac{8.75}{0.25}=35\%$.
Question 11 of 12
Which pair are metamers (same formula, different alkyl groups on the same functional group)?
$CH_3OCH_3$ and $CH_3CH_2OH$
$CH_3OC_3H_7$ and $C_2H_5OC_2H_5$
but-1-ene and but-2-ene
propanal and propanone
Explanation: Methyl propyl ether and diethyl ether ($C_4H_{10}O$) differ in the alkyl groups around oxygen — metamerism.
Question 12 of 12
Steam distillation purifies a compound that is:
water-soluble and high boiling
steam-volatile and immiscible with water
a non-volatile solid
decomposed by water
Explanation: Steam distillation works for compounds volatile in steam and immiscible with water (e.g. aniline), distilling below $100^\circ$C.