Chapter 2 - Benzene and Aromaticity PDF

Lecture no. 2 Introduction and aromaticity 25 © 2011 Pearson lectureEducation, part 1 Inc.  Benzene and Aromatic Compounds Selected drugs that contain benzene ring Introduction and aromaticity 26 © 2011 Pearson...

Lecture no. 2 Introduction and aromaticity 25 © 2011 Pearson lectureEducation, part 1 Inc.  Benzene and Aromatic Compounds Selected drugs that contain benzene ring Introduction and aromaticity 26 © 2011 Pearson lectureEducation, part 1 Inc.  Aromaticity A property of some unusually stable organic molecules such as benzene. The term “aromaticity” actually has nothing to do with smell. We saw that aromatic molecules: a] Have an extremely high resonance b] have delocalized pi-electrons c] Energy (36 kcal/mol for benzene). d] undergo substitution rather than addition reactions. N.B:- a- Benzene is not the only aromatic compounds b- Smell has nothing to do with aromaticity. c- Aromatic compounds might be organic or inorganic d- Aromatic feature might be inclusive.. in neutral, anionic or cationic species. Introduction and aromaticity © 2011 Pearson Education, Inc. lecture part 1 27 What are conditions that are responsible for aromaticity?!!! There turn out to be 3 conditions a molecule must meet in order for it to be aromatic. It’s all or nothing. If any of these conditions are violated, no aromaticity is possible. 1- It must be cyclic. 2- The molecule must have [4n+2] pi electrons we’ll 3- Every atom in the ring must be conjugated(Every atom in the ring must be able to participate in resonance). Flat molecule.. Introduction and aromaticity 28 © 2011 Pearson lectureEducation, part 1 Inc. Benzene and Aromatic Compounds The Criteria for Aromaticity—Hückel’s Rule Considering aromaticity, a compound can be classified in one of three ways: 1. Aromatic:- A cyclic, planar, completely conjugated compound with 4n + 2  electrons. 2. Antiaromatic:- A cyclic, planar, completely conjugated compound with 4n  electrons. 3. Nonaromatic:- A compound that lacks one (or more) of the following requirements for aromaticity:”cyclic, planar, and completely conjugated”. Introduction and aromaticity 29 © 2011 Pearson lectureEducation, part 1 Inc.  Introduction and aromaticity 30 © 2011 Pearson lectureEducation, part 1 Inc.  Aromatic compounds may be organic or inorganic Borazine is aromatic as there are 3 nitrogen atoms in the structure of borazine that donate two electrons each. Boron doesn't donate any, but it has an empty p-orbital available through which the electrons can delocalize. Hence, in total it has 6 π-electrons. Though the delocalisation is lesser than that in benzene, it still is aromatic. Introduction and aromaticity 31 © 2011 Pearson lectureEducation, part 1 Inc. Aromaticity and resonance contributors of cyclopreopenyl ions and radical Resonance contributors of cyclopropenyl anion Resonance hybrid Resonance Resonance contributors of cyclopropenyl radical hybrid Resonance contributors of cyclopropenyl cation Resonance hybrid Introduction and. aromaticity 32 32 © 2011 Pearson lectureEducation, part 1 Inc. Aromaticity and resonance contributors of cyclopentadienyl and its ions and radical Introduction and. aromaticity 33 © 2011 Pearson lectureEducation, part 1 Inc.  Aromaticity and resonance contributors of cycloheptatrienyl cation and the not aromatic cycloheptrienyl anion Introduction and aromaticity 34 © 2011 Pearson lectureEducation, part 1 Inc. The most common cationic and anionic aromatic compounds © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc.  Some well-known Hetero-aromatic compounds There is a sort of similarity between the above 5 compounds and cyclopentadienyl anion Introduction and. aromaticity 37 © 2011 Pearson lectureEducation, part 1 Inc.  Introduction and. aromaticity 38 © 2011 Pearson lectureEducation, part 1 Inc. Imidazole ring and aromaticity Imidazole, thiazole and oxazole have the same situation of aromaticity interpritation Introduction and. aromaticity 39 © 2011 Pearson lectureEducation, part 1 Inc. The above three heterocycic compounds are aromatic; because the three requirements are available 1- cyclic 2- 4n+2= 6 3- The six electrons resonate over the whole ring and each atom participate in this resonance Introduction and aromaticity 40 © 2011 Pearson lectureEducation, part 1 Inc. Lecture no. 3 Introduction and aromaticity 41 © 2011 Pearson lectureEducation, part 1 Inc. Effect of aromaticity on basicity The Rule :- if the lone pair on nitrogen is a part of the Huckle electrons that are required to get aromaticity achieved; this case indicates that aromaticity is at the Expense of basicity. For example; in Pyrrole ; aromaticity affects negatively on nitrogen basicity due to the participation of nitrogen lone pair in creating aromaticity ( this is called sixtet system). While in pyridine; the lone pair of nitrogen is localized on nitrogen atom; no need to enter the ring to create aromaticity since it is already aromatic having the sixtet system and no need otherwise electrons number becomes 8 which means destroying aromaticity is expected; while having aromaticity means Introduction and stability.. aromaticity 42 © 2011 Pearson lectureEducation, part 1 Inc. How to make sure that your interpretation is correct? Or what is the proof of Pyrrole to be less basic than pyridine? What is reason responsible for different pKa values? + Introduction and. aromaticity 43 © 2011 Pearson lectureEducation, part 1 Inc.  !!?? Which is the weakest base? Introduction and. aromaticity 3744 © 2011 Pearson lectureEducation, part 1 Inc.  Next Identify which of the following compounds is expected to be a stronger base. a b Introduction and aromaticity 45 © 2011 Pearson. part 1 Inc. lectureEducation, 45 Effect of aromaticity on acidity Easier ; because it creates an aromatic ion; Notice the value of pKa = 16 easier Difficult ; because it doesnt create an aromatic ion; notice the value of pKa = 36 difficult Introduction and aromaticity 46 © 2011 Pearson lectureEducation, part 1 Inc. All compounds try to be aromatic; they all seek for aromaticity O O O H H H Base H Easier Loosing the acidic proton create a structure O in which upon resonace; an aromatic ring is O formed H H Base H This resonance form is not aromatic 40 © 2011 Pearson Education, Inc. Effect of aromaticity on polarity N O S H H 1.81 D 0.70 D 0.51D Cyclopentadienyl Pyrrole Furan Thiophene anion 48. © 2011 Pearson Education, Inc. 41 Indicate the polarity direction in the following compound N H 42 © 2011 Pearson Education, Inc. Lecture no. 4 © 2011 Pearson Education, Inc.  Inductive and mesomeric effects in benzene ring 1-Inductive effect:- In the organic compound molecule, due to the influence of different electronegativity substituents (atoms or atomic groups), the density of the bonding electron cloud in the whole molecule is shifted in a certain direction, and the effect of polarization of the molecule is called an Induction effect. If it is away from the ring it leads to minimize electron density in the ring and called (-I effect) or leads to maximize the electron density inside the ring and then it is called   + I effect. A A     - I effect +I effect atoms group 5,6 Introduction andMainly; aromaticityalkyl group 51 © 2011 Pearson lectureEducation, part 1 Inc. and 7 (N,P,O,S,F,Cl...etc) O O CO2H - I effect +I effect Introduction and aromaticity 52 © 2011 Pearson lectureEducation, part 1 Inc.  2- Mesomeric “resonance” effect M Effect It involves the delocalization of the π or n electrons in a conjugated orbital system. We will focus only on two types of resonance cases:- a] Case 1 “n-π”:- (lone pair, single bond and π bond are delocalized to double bond, single bond and lone pair with two opposite charges on each terminal... A A.. B B b] Case 2 “π-π”:- (π bond, single bond and π bond “conjugated system” are delocalized to a π bond in between with two opposite charges on each terminal OR OR Introduction and aromaticity 53 © 2011 Pearson lectureEducation, part 1 Inc.  A is an atom from the group 5,6 or 7.. all these have lone pair that starts the case of resonance A A + M effect.. B B..O....O Free electrons (lone pair) move towards the ring; thus the amount of elecrons increases inside the ring at the expenss of the part outside the ring that push electrons; as a result it is a + M effect Introduction and aromaticity 54 © 2011 Pearson lectureEducation, part 1 Inc. O O resonance does not occur in this way X O O Introduction and aromaticity 55 © 2011 Pearson lectureEducation, part 1 Inc.  The M-effect is more effective than the I effect. Why ? Discuss both I and m effect for each of the following. H N H H N O O O H O Introduction and aromaticity 56 © 2011 Pearson lectureEducation, part 1 Inc. Benzoic acid O O OH -M effect OH  O O is more EN than C OH  electrons flow out the ring hence; it is -I effect Introduction and aromaticity 57 © 2011 Pearson lectureEducation, part 1 Inc. Benzonitrile  N C N is more EN than C electrons flow out the ring  hence; it is -I effect N N C C -M effect Introduction and aromaticity 58 © 2011 Pearson lectureEducation, part 1 Inc. Discussion Introduction and aromaticity 59 © 2011 Pearson lectureEducation, part 1 Inc. Indicate both I and M-effect for each of the following; 1- Benzoic acid (- I & - M) 2- Phenol (- I & + M) 3- Acetophenone (- I & - M) 4- Benzaldehyde (- I & - M) 5- Nitrobenzene (- I & - M) 6- Acetanilide (- I & + M) 7- Benzonitrile (- I & - M) 8- Aniline (- I & + M) 9- Anisol (- I & + M) 10- Benzene sulphonic acid (- I & - M) 11- Toluene (+ I & M effect is N A) Introduction and aromaticity 60 © 2011 Pearson lectureEducation, part 1 Inc. Aniline H N  N is more EN than C H  electrons flow out the ring hence; it is -I effect H H N N.. H +M effect H Introduction and aromaticity 61 © 2011 Pearson lectureEducation, part 1 Inc.  Introduction and aromaticity 62 © 2011 Pearson lectureEducation, part 1 Inc.  Introduction and aromaticity 63 © 2011 Pearson lectureEducation, part 1 Inc. Q1] Discuss aromaticity feature for the following compounds N O O Q2] Indicate the polarity direction for the following compound O N CH3 O X Y Z A Introduction and aromaticity 64 © 2011 Pearson lectureEducation, part 1 Inc. Q3] Arrange the following compounds according to the increase in their basicity N N N H H H X Y Z Q4] Arrange the following according to the increase in their basicity N N N N H H H 1 2 3 4 Introduction and aromaticity 65 © 2011 Pearson lectureEducation, part 1 Inc.  Q5] Which is more basic; Thiazole or Oxazole? Explain S O N N Thiazole Oxazole Q6] Discuss the acidity for the following N N H H Introduction and aromaticity 66 © 2011 Pearson lectureEducation, part 1 Inc. Q7] Which of the following two salts is drawn in a wrong way. Tell why Q8] What is expected salt structure of mono, di, tri and tetra salt NH N NH NH Introduction and aromaticity 67 © 2011 Pearson lectureEducation, part 1 Inc. Q9] Discuss Oxygen nucleophilicity in the following two compounds O O 1 2 Q10] Which is the most Nucleophilic center in the following compound Introduction and aromaticity 68 © 2011 Pearson lectureEducation, part 1 Inc.