P Organic 1 Lecture 7 PDF

Nahda University Faculty of Pharmacy Pharmaceutical Chemistry Department Pharmaceutical Organic Chemistry I POC1101 (Pharm D) Lecture (7) Aromaticity...

Nahda University Faculty of Pharmacy Pharmaceutical Chemistry Department Pharmaceutical Organic Chemistry I POC1101 (Pharm D) Lecture (7) Aromaticity 1  Monocyclic Aromatic Hydrocarbons Aromatic hydrocarbons are compounds such as benzene and related compounds that similar in their chemical behavior with benzene. As we have seen the aliphatic hydrocarbons undergo addition to the multiple bonds (C=C, C≡ C) and free radical substitution. In contrast, the aromatic hydrocarbons are characterized by tendency to undergo substitution.  Aromaticity  What is an aromatic compound? Benzene is one member of a large class of aromatic compounds, compounds that are substantially stabilized by π-electron delocalization. The resonance energy of an aromatic compound is a measure of its gain in stability, e.g., benzene is 36 kcal/mol and naphthalene is 61 kcal/mol.  Benzene and Aromatic Compounds Benzene (C6H6) is the simplest aromatic hydrocarbon (or arene). The resonance description of benzene consists of two equivalent Lewis structures, each with three double bonds that alternate with three single bonds. The true structure of benzene is a resonance hybrid of the two Lewis structures, with the dashed lines of the hybrid indicating the position of the  bonds. 4  Benzene and Aromatic Compounds Because each  bond has two electrons, benzene has six  electrons. 5  In benzene, the actual bond length (1.39 Å) is intermediate between the carbon—carbon single bond (1.53 Å) and the carbon—carbon double bond (1.34 Å). 6  Stability of Benzene Consider the heats of hydrogenation of cyclohexene, 1,3- cyclohexadiene and benzene, all of which give cyclohexane when treated with excess hydrogen in the presence of a metal catalyst. 7  Stability of Benzene The low heat of hydrogenation of benzene means that benzene is especially stable. This unusual stability is characteristic of aromatic compounds. Benzene’s unusual behavior is not limited to hydrogenation. Benzene does not undergo addition reactions. Benzene does not react with Br2 to yield an addition product. 8  Nomenclature of Benzene Derivatives To name a benzene ring with one substituent, name the substituent and add the word benzene. Many monosubstituted benzenes have common names which you must also learn. 9  Nomenclature of Benzene Derivatives There are three different ways that two groups can be attached to a benzene ring, so a prefix—ortho, meta, or para—can be used to designate the relative position of the two substituents. ortho-dibromobenzene meta-dibromobenzene para-dibromobenzene or or or o-dibromobenzene m-dibromobenzene p-dibromobenzene or 1,2-dibromobenzene or 1,3-dibromobenzene or 1,4-dibromobenzene 10  If the two groups on the benzene ring are different, alphabetize the names of the substituent preceding the word benzene. If one substituent is part of a common root, name the molecule as a derivative of that monosubstituted benzene. 11 The disubstituted benzene has 3 isomers, for example dibromobenzene can be: Trisubstituted benzene  A benzene substituent is called a phenyl group, and it can be abbreviated in a structure as “Ph-”. Therefore, benzene can be represented as PhH, and phenol would be PhOH. The benzyl group, another common substituent that contains a benzene ring, differs from a phenyl group. 14  The Criteria for Aromaticity—Hückel’s Rule Four structural criteria must be satisfied for a compound to be aromatic. A molecule must be cyclic. To be aromatic, each p orbital must overlap with p orbitals on adjacent atoms. A molecule must be planar. 15  A molecule must be completely conjugated. Aromatic compounds must have a p orbital on every atom. A molecule must satisfy Hückel’s rule, and contain a particular number of  electrons. Hückel's rule: An aromatic compound must contain 4n+2π electrons [n (integer no.) = 0, 1, 2, and so…] Benzene is aromatic and especially stable because it contains 6  electrons. Cyclobutadiene is non aromatic and especially unstable because it contains 4  electrons. 16  Benzene Cyclobutadiene An aromatic compound non-aromatic compound 4n+2= 6 e 4n+2=4 e 4n=6 - 2=4 4n= 4 - 2 =2 n=4/4=1 n=2/4 = 1.5 aromatic non-aromatic Note that Hückel’s rule refers to the number of  electrons, not the number of atoms in a particular ring. 17  Cycloheptatrienyl (tropyllium) cation  Cycloheptatriene has six π electrons. However, the six π electrons of cycloheptatriene can not be fully delocalized because of the presence of the CH2 group, a group that does not have an available P orbital.  When cycloheptatriene is treated with a reagent that can abstract a hydride ion, it is converted to the cycloheptatrienyl (tropyllium) cation. The loss of hydride ion from cycloheptatriene occurs with unexpected ease and the cation is found to be unusually stable. The cycloheptatrienyl cation is an aromatic cation  Cyclopropenyl cation  It is usually stable. In contrast, the cyclopropenyl free radical and anion are not usually stable, indeed, the anion seems to be practicularly unstable. The cation has the Huckel number of two π electrons (n=0) and is aromatic.  - Cylopropenyl cation 2 π electrons (aromatic).  - Cylopropenyl anion 4 π electrons (antiaromatic).  Cyclopentadienyl anion Thus, although five resonance structures can also be drawn for the cyclopentadienyl cation and radical, only the cyclopentadienyl anion has 6  electrons, a number that satisfies Hückel’s rule. 20  Annulenes Completely conjugated rings larger than benzene are also aromatic if they are planar and have 4n + 2  electrons. Hydrocarbons containing a single ring with alternating double and single bonds are called annulenes. To name an annulene, indicate the number of atoms in the ring in brackets and add the word annulene. 21 Indicate which of the following are aromatic and non-aromatic? B A D C 22  Q1: Give the IUPAC nomenclature?! ………………………………………………………. ………………………………………………………. ………………………………………………………. ………………………………………………………. ………………………………………………………. 23 Q2:Choose the correct answer 1- Which of the following compounds is not aromatic? 24 Q2:Choose the correct answer 2- Which of the following compounds is not aromatic? 25 26