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Practice each question, then open the answer or worked solution.
Which of the following ions is more stable? Use resonance to explain your answer.
The stability of carbocation is decided by inductive effect, hyperconjugation, resonance etc.
As structure A has a carbocation, which is primary and allylic carbocation, whereas the carbocation in structure B has secondary and allylic carbocation.
Hence, structure B is more stable.
The structure of triphenylmethylcation is given below. This is very stable, and some of its salts can be stored for months. Explain the cause of the high stability of this cation.
Triphenylcarbocation is tertiary carbocation and the positive charge is on the carbon atom, which is stabilised by the three phenyl group by resonance. Due to the resonance, the stability increases.
Write structures of various carbocation that can be obtained from 2-methyl butane. Arrange these carbocations in the order of increasing stability.
the stability order is found to be:
(III) > (II) > (I) > (IV)
This is because (III) is tertiary carbocation, (II) is secondary carbocation, (I) and (IV) are primary,
Three students, Manish, Ramesh and Rajni were determining the extra
elements present in an organic compound given by their teacher. They
prepared the Lassaigne’s extract (L.E.) independently by the fusion of the
compound with sodium metal. Then they added solid FeSO4 and dilute
sulphuric acid to a part of Lassaigne’s extract. Manish and Rajni obtained
Prussian blue colour, but Ramesh got a red colour. Ramesh repeated the test
with the same Lassaigne’s extract but again got red colour only. They were
surprised and went to their teacher and told him about their observation.
A teacher asked them to think over the reason for this. Can you help them by
giving the reason for this observation? Also, write the chemical equations to
explain the formation of compounds of different colours
3NaCNS + FeSO4 + dilute sulphuric→ Fe(CNS)3 (Blood red colour) + 3Na+
Ramesh has Nitrogen as well as Sulphur present in his organic compound, so he observed the Blood-red colour of Fe(CNS)3 and Manish and Rajni had only Nitrogen in their compound; hence they got Prussian blue colour.
Name the compounds whose line formulae are given below:
(i) The name of the compound is 3-ethyl-4-methyl-5-heptane-2-one.
(ii) The name of the compound is; 1-nitro-cyclohexane-2-ene.
Write structural formulae for compounds named as-
(a) 1-Bromoheptane
(b) 5-Bromoheptanoic acid
(a)
(b)
Draw the resonance structures of the following compounds;
Identify the most stable species in the following set of ions giving reasons:
(i) C+H3 will be more stable as the bromine atom destabilizes the positive charge on a carbon atom. Bromine atom is an electron-withdrawing group and has a lone pair of electrons.
(ii) CCl3 will be most stable because chlorine is more electron-withdrawing atom and the negative charge on carbon will be stabilized by the chlorine atom. As the number of chlorine atom attached to carbocation increases the stability also increases.
Give three points of differences between the inductive effect and resonance effect.
1. In the inductive effect, the transmission of the electron is only from sigma bond whereas in resonance effect the transmission occurs from both sigma and pi bond.
2. It is possible when the polarization of bond takes place whereas the resonance effect is possible only if the system is conjugated.
3. Electronegativity plays role in inductive effect whereas in resonance effect more number of an alternative double bond will lead to more resonating structure and hence more stability.
Which of the following compounds will not exist as a resonance hybrid. Give the reason for your answer:
Conjugation is only possible when an atom has any of the charge or alternative position of the double bond. Here CH3OH will not exist as resonance hybrid because of the reason.