Question

Explain the following observations in a concise manner. Use equations, mechanisms, diagrams and/or resonance structures to illustrate your answer. i. The unusual molecule 1,6-methano[10]annulene ( ) exhibits two sets of signals in the 'H-NMR spectrum at 8 = 7.10 (8H) and -0.50 (2H). (4 marks] Compound C is more acidic that compound D. с base [4 marks) Propose a reasonable mechanism for the following reaction of benzene with 1,5-dibromopentane in the presence of a Lewis acid catalyst.

Answer 1

Answer :

d.

(i)

♡♡H 1,6- methano [10 Jannulene. In this molecule the two me the lene the bridged CH2.) is just above the plane of the conjugaled IT-system. The planar Ti-system contains 10 st ei le it follow (4m+2) é.(n=2). That means it is a dia tropie molecule. In diatropie molecule. the 17-electrons in the conjugated ring ; ereates a ring ewygient by the movement of the mes in the ring In dieutropie molecules, this movement of the ping current is such that, the region above and below the aromatic ring, in are became shielded but outside of the peripheri of the ring became deshilded. This is due .. (

to the generation of a induced magnetic field by the movement of the ring current and the direction of the induced magnetic field is opposite to that of the NMR- static magnetic field. . If we just consider the planer ring then - Pind . nt at System * This effect is known as Magnetic Anisotropy- that is why the H-atoms which are in the plan of the ring, are indeshielded region and have a chemical shift greater than expected. But the H-atums which lie just above or below the 4n t?) in e ring are shiled, and their chemical shift is decreases and goes to

negative values In 1,6-methano [10] annulene there are 8-17 atom in the plane of the cromatic ring, so this 8. H. are deshielded and their Chemical shift value in 4H-NMR spectra 15 high poround «7.10 ppm But the datt in Hebridged eta group is just above the planer ring and so they are highly shidded by the ring "ews spent. 56, their chemical shift reduces very highly and become negative, around a -0.5 ppm. at Highly shielded $=-045 ppm 11. s . and Sat 7.10 ppm

(ii)

Acidity of a compound is measured by the stability of its conjugate base( i.e. the species after removal of a proton). If the conjugate base is highly stable then its acidity is high.

Between two acidic molecule, the stronger acid will be that one whose conjugate base is more stable.

Base conjugate Acid base of (c) ANCHE a . n this ring.eu has oite de. .( 4n+2) I; n = 1 That means it is aromatic and highly stable * eyelie conjugation will (4672) Makes it aromatic L G 5o, after losing a proton (H) from the (E); it become aromatic. that means comfugare base of (c) is dighly stable aromatie..

conjugate base of (D). a I 5o, the conjugate base of (o) is only stabilised by resonance . But it is not a aromatic compound because although it has one line 4n+ 2 ;n=1) it is een not a eyelic com sugated system... : so, its stability is less than the com jug ate base of ce) That is why. (e) is more aùdic tham (D)

(Answered)