Skip to main content

Mesomeric effect and Resonance


Mesomeric  effect


         It is special type of resonance of in which lone pair of electrons are delocalized over the alternative double bond and lone pair always present on same heteroatom in function group.
Heteroatom=> 
                                       
These types of functional group are called +M Group .same function groups are present on which lone pairs are not present but these functional groups show mesomeric effect and these types of functional groups are called –M group
+M group = O- >NR2 >NHR,>NH2 >HO>NHCOR>-OCOR and alky |& ary |groups.
-M group = -NO2>-CN>-CHO>COR>COR>-COOH>-COOP>CONH2 etc
+M group =ortho-para directing and activating groups.
In this example negative charge observed at ortho and para position to the –NH2 group and
negative charge means extra electron is present in the ring and ring is activated and electrophile   attacks on ortho & para positions.
-M group => meta directing and deactivating groups.
e.g.
In this example positive charge observed at ortho and para position to the –NO2 group and positive charge means electron density decreases in  ring at ortho and para positions results deactivation  of the ring and electrophile attacks on the meta position.
Resonance = It is definel as the delocalisation  of electrons in conjugation electron moves from higher electron density to lower electron conjugation is the alternation of double bond with single bond ,negative charge ,positive to charge delocalization over the bonds.
Resonance effect tells about the stability of most of intermediates.
(A)   Stability of carbocation=
When resonance increases in any organic compound, then the stability of corbocation increases.
e.g
(A)  



(B)
(C)
In above three examples ( C) is most stable carbocation  because positive change delocalized over the three example  (A) is the least stable than  (C) & (D) because the positive charge delocalized over only one double bond .
Thus the stability order is
((B)   Stability of corbanions =>
When resonance increases in any organic compound than the stability of corbanion also increases
(A)


(B)
In above example
(A) is more stable than(B) because in (A) three three double bond are present and in (B) Only one double bond present
                                                

Note= Halogens are exception which are ortho & para directing groups



1 . According to resonance halogens are ortho & para directing groups
X=F,CL,Br,I



In this example lone pair present on halogen resonace with the double bond in benzene ring and give the position on which electropile are attack .(i.e inductive effect tell about orientation)



2 . Acording to inductive effect halogens are deactivators because halogen are higher electronegative atoms and they pull the electrons from electrons from benzene ring (i.e. Inductive effect about nature of halogens)
Thus the halogens are ortho,para directing and deactivating group
Electonagative order/inducative effect =F>CI>Br>I
Resonance effect order=F>CI>Br>I  (Because ‘F’is very small halogen and better overlapped with carbon offering while  I is very bigger halogen
Resonance effect generally observed higher electron density to lower electron density.




(A) Nagative charge to double ()
(B)   Lone pair to  double bond     ()
              
(C) Double bond to positive charge ()


Comments

Post a Comment

Popular posts from this blog

Why is tan90⁰ not defined?

In the series of our posts after our last post titled How to prepare for 10th boards, 12th boards, IIT and NEET Exams We at Alchemy Learning bring you a very basic but valuable concept which can bring a change in the way you prepare for your exams Trigonometric ratios are defined as the ratios of sides of a right angle triangle till class 10 th but trigonometric ratios are not constrained to a right angle triangle. In earlier classes tan 𝛉   is defined as the ratio of perpendicular to base in a right angle triangle and tan90 o is written as ∞   in trigonometric tables. We see in trigonometric table there is one value of tan 𝛉   written as “NOT DEFINED”. But we never ask ourselves WHY? Considering  𝛉  = 0 o Let us consider a right angle triangle ABC which has right angle (90 o ) at B and consider angle A =  𝛉 , AC = hypotenuse, BC = perpendicular and AB = base. When we consider values of  𝛉 to be 30 o , 45 o and 6...
2 comments
Read more

Graphical Representation of Motion

Concept of motion and it's graphical representation has always been a jargon and a pain to students of class 9th,10th,11th and 12th, be it RBSE or CBSE. Even for cracking IIT and NEET this concept has to be done well. We at Alchemy Learning always insist on getting the concept right. Distance – Time Graph: When an object is moving with uniform velocity, the slope of graph is always a straight line. In other words, slope of straight line of a distance-time graph shows that object is moving with uniform speed. In the above graph, straight slope line shows that object is moving with uniform velocity. Slope OA shows the speed of the object. Calculation of Speed using distance-time graph: To calculate the SPEED, let take two points A and B on the slope OB. Draw a line parallel to y-axis and another parallel to x-axis from B. Again draw a line parallel to y-axis and another parallel to x-axis from point A. Let, line parallel to x-axis from point B cut ...
Post a Comment
Read more

How to prepare for 10th boards, 12th boards, IIT and NEET Exams

Students preparing for exams of any kind, board level (10 th board & 12 th board), college entrance level ( IIT JEE Mains , NEET & AIIMS ), feel the pressure of performance and it causes them a lot of stress. The pressure they feel is the main reason of their anxiety and stress, which leads to depression. These exams are known for their fear among the students. But if you prepare with a strategy and prepare well, these exams will turn out to be less stressful. Today, in the days of competition, it is important that you study in smart ways. The goal of studying smartly is to convert your efforts into your better performance. In the starting of your school curriculum, it is important that you reconsider your strategy for preparation of these exams. It is high time. . . . . . . You must have heard a lot of tips from your parents, relatives, teachers, friends etc. but it must be hard for you to understand whose strategy is the best for you to imbibe in your routine. From...
2 comments
Read more