Organic Chemistry - II Unit 1 Notes PDF

## Organic Chemistry - II ### Unit 1 Notes ### Benzene - Kekule's structure - Evidences of benzene - Electrophilic substitution reactions of benzene - Effect of substitution - Benzene derivatives ### Benzene - Benzene is an organic, colorless chemical compound. - It is a highly flammable liquid...

## Organic Chemistry - II ### Unit 1 Notes ### Benzene - Kekule's structure - Evidences of benzene - Electrophilic substitution reactions of benzene - Effect of substitution - Benzene derivatives ### Benzene - Benzene is an organic, colorless chemical compound. - It is a highly flammable liquid with a sweet smell and high melting point. - The chemical formula of benzene is $C_6H_6$, so it contains 6 carbon & 6 hydrogens, also known as hydrocarbon. - It has a boiling point of 80°C and is lighter than water. - Natural sources of benzene includes volcanoes and forest fires - It is also a natural part of crude oils. - Benzene and all those compounds which resemble benzene in their chemical behavior are known as aromatic compounds. - Benzene is a highly toxic carcinogenic chemical. ### Structure of Benzene - The molecular formula of benzene ($C_6H_6$) shows a high degree of unsaturation. - Sir August Kekule was the first scientist who introduced the chemical structure of benzene as a 6 membered ring structure. - He also proposed the presence of 3 alternate double bonds in benzene ring. - According to Sir Kekule the 3 alternate double bonds in benzene also changes their position rapidly to satisfy tetravalency of carbon. - A diagram of the Kekule structure of benzene is shown. ### Kekule Structure of benzene - Although Kekule structure of benzene satisfies the structural features of benzene and also explain the equivalent nature of hydrogen, but yet this structure fails to explain the unusual behavior of benzene. - Since this structure of benzene is a great discovery and significant approach by sir Kekule, hence , it is still in use. - Following are the given characteristics that are not explained by Kekule's model. ### Chemical Reaction - Benzene undergoes substitution reaction rather than addition reaction like other unsaturated compounds. ### C-C Bond Length - According to Sir Kekule, two kinds of bond present in benzene, single & double bond - C-C: Bond length (1.54 Å) - C=C: Bond Length (1.34 Å) - But experimentally it is seen that bond length between all carbon atoms is 1.39 Å. ### Stability - Kekule's structure could not explain the stability of benzene towards oxidizing agents (All unsaturated compounds undergoes oxidation but benzene does not). ### Evidences in the Structure of Benzene There are many analytical, synthetic and other evidences are given in the derivation of structure of benzene as follows: ### Analytical Evidence - From its elemental composition and molecular weight determination it is found that benzene contains 6-C & 6-H atoms. - Benzene has molecular formula C6H6 compared to Hexane C6H14, that confirms benzene as a highly unsaturated compound compare to hexane (C6H14). - So it was concluded that benzene contains =/= bonds in its structure. ### Synthetic Evidence - Since it was confirmed that benzene has unsaturation, so it could be constructed either as straight chain or in a ring. - The possible straight chain structure for benzene could have been: - CH2=CH-C≡C-CH=CH2 - CH3-C≡C-C=C - CH3 - CH = C-CH2-CH2-C = CH - But all the above structures were ruled out because benzene doesn't give the usual reactions of Alkenes, Alkynes. - So it was concluded that benzene is a highly unsaturated closed ring structure. - BENZENE + Br2 /CCl4 → No Reaction - BENZENE + Dilute Cold KMNO4 → No Reaction - BENZENE + H2O/H+ → No Reaction ### Other Evidences - Benzene reacts with bromine in the presence of AlCl3 (catalyst) to form Bromobenzene. - Now only one (mono) bromobenzene is obtained that shows all 6 hydrogen atom in benzene were identical. - This becomes possible only if benzene has a cyclic structure of six C atom and each carbon atom is attached with one H atom. ### Resonance Structure of Benzene - To explain all the limitations of Kekule's Structure, it has been proposed that benzene is a resonance hybrid of two Kekule Structure. (Ie Ⅱ) - Resonance Hybrid (A) is a very stable structure and this is why benzene undergoes substitution reaction very easily rather than addition reaction. - Resonance Structure of Benzene explains all the facts like unusual stability and equality of C-C bond length. - A diagram of the resonance structure of Benzene is shown. ### Molecular Orbital Structure of Benzene - Molecular orbital theory states that all carbon atoms of benzene are sp2 hybridized. - Each carbon atom forms two C-C sigma bonds and one C-H sigma bond. - Carbon atoms are still left with unhybridized P- orbitals that overlaps to form π bonds. - A diagram showing the molecular orbital picture of benzene. ### Chemical Structure of Benzene - All the sigma bonds in benzene lies in the same plane - All bond angle = - All C-C bond length = 1.40 Å - All C-H bond length = 1.09 Å - Chemical Formula = C6H6 - Molecular Weight = 78 g/mol - A diagram showing the basic structure of benzene (bonds, angles, and lengths). ### Aromatic Character (Huckle's Rule) - Benzene and compounds that resemble benzene in their chemical behavior are known as aromatic compounds. - They undergo substitution reaction rather than addition reaction. - Aromaticity is basically decided by Huckel's Rule. ### Huckel's Rule In 1931, a German scientist Huckel gave certain rules for defining aromaticity of organic compounds as follows: - Compound should be Cyclic - There should be a conjugation (delocalized electrons) - Compound should be planer - Total number of π electrons should be (4n+2), where n = 0,1,2.... ### Chemical Reactions of Benzene - Most commonly benzene undergoes electrophilic substitution reaction (Electrophilic Aromatic Substitution Reaction). - In these reactions hydrogen atom of the aromatic ring is replaced by an electrophile. ### Types of Electrophilic Aromatic Substitution Reaction Benzene mainly undergoes 5 types of Electrophilic Aromatic Substitution Reaction. 1. Halogenation 2. Nitration 3. Sulfonation 4. Friedel Craft's Alkylation 5. Friedel Craft's Acylation ### Halogenation of Benzene In Halogenation, benzene reacts with Halogens (Br2, Cl2, I2) in the presence of Lewis acid catalyst (AICl3) to give Bromobenzene or Chlorobenzene or Jodobenzene. - A chemical diagram is provided showing the process of the reaction. ### Mechanism of Halogenation (Chlorination) - STEP-I: Formation of Electrophile (Chloronium Ion) - STEP-II: Attack of Electrophile on Benzene to form Carbonium Ion - Now the above carbonium ion is resonance stabilized and forms a σ-complex. - STEP-III: Loss of proton and formation of product (Chlorobenzene) ### Nitration In Nitration, Benzene reacts with concentrated HNO3 in the presence of concentrated H2SO4 to form Nitrobenzene. - A chemical diagram is provided showing the process of the reaction. ### Mechanism - STEP-I: Formation of Electrophile (Nitronium Ion) - STEP-II: Attack of Electrophile to form carbocation intermediate - STEP-III: Loss of proton leads to the formation of Nitrobenzene ### Sulfonation In Sulfonation, Benzene reacts with concentrated or fuming sulphuric acid to form Benzene Sulphonic Acid. - A chemical diagram is provided showing the process of the reaction. ### Mechanism -STEP-I: Formation of Electrophile (Sulpher Trioxide/SO3) - STEP-II: Attack of Electrophile on Benzene to form Carbocation - STEP-III: Loss of proton leads to formation of Benzene Sulphonic Acid. ### Friedel-Crafts Alkylation In Friedel - Crafts Alkylation (named after a French chemist, Charles Friedel and an american chemist James Crafts) Benzene reacts with Alkyl Group to form Alkyl Benzene. - A chemical diagram is provided showing the process of the reaction. ### Mechanism - STEP-I: Formation of Electrophile - STEP-II: Attack of Electrophile to form Carbocation Intermediate - STEP-III: Loss of proton leads to the formation of Methyl Benzene (Product). ### Friedel- Crafts Acylation In Friedel crafts Acylation benzene reacts with Acyl Group to form Aromatic ketones (Acetophenone) in the presence of AlCl3. - A chemical diagram is provided showing the process of the reaction. ### Mechanism - STEP-I: Formation of Electrophile (Acylium Ion) - STEP-II: Attack of Electrophile to form Carbocation Intermediate - STEP-III: Loss of proton leads to the formation of Acetophenone (Product) ### Effect of Substituents - Benzene ring contains 6 Hydrogens and all 6 hydrogen of benzene are identical in nature. - Now when 1 Hydrogen of benzene is substituted by a group then it becomes monosubstituted Benzene. - Now this Monosubstituted benzene contains 3 positions: ortho, meta, para. - A diagram showing the positions of the substituents on the benzene ring is provided. - Now, here in the above structure X can be any substituent. - Also the nature of X (substituent) decides the reactivity of benzene towards other electrophiles and also their positions. ### Types of Substituents/Groups There are mainly two types of Groups: - Ring Activating Groups - Ring Deactivating Groups ### Ring Activating Groups - Groups that increases the der reactivity of benzene towards Electrophilic Substitution Reaction are known as Ring Activating Groups - These groups increases the e- density on benzene ring. - They are also known as Ortho- Para Directing Groups. - These Groups can be further classified into 3 types: - Strongly Activating: - NH2, NR2, -OH etc. - Moderately Activating: -NH-CH3 , -CH3, -OR etc. - Weakly Activating: CH3, -C6H5 etc. #### Effect on Reactivity - Since Ring Activating Groups have the tendency to donate e-, hence they increases e- density on benzene ring. - Now due to this increased e-, benzene ring becomes more reactive towards electrophilic substitution reaction. #### Effect on Orientation - Ring Activating Groups guide the second substituents towards Ortho & Para Positions. - Given below is the mechanism of Ortho & Para directions. - A diagram showing the resonance structures that explain ortho/para directing groups is provided. ### Ring Deactivating Groups - Groups that decreases the reactivity of benzene towards Electrophilic Substitution Reaction are known as Ring Deactivating Groups. - These groups decreases the e- density on benzene ring. - They are also known as Meta directing Groups. - They can be further divided into 3 types: - Strongly Deactivating: - NO2, -C=N, SO3H etc. - Moderately Deactivating: COOH, COCH3 etc. - Weakly Deactivating: Halogens (Br,CI, F) #### Effect on Reactivity - Since Ring Deactivating Groups have the tendency to accept/withdraw electrons, hence they decreases e- density on benzene ring. - Now due to this, benzene ring becomes less reactive towards electrophilic substitution reaction. #### Effect on Orientation - Ring Deactivating Groups guide the second substituents towards Meta Position. - Here is the mechanism of Meta direction. - A diagram showing resonance structures that explain meta directing groups and their effect on e- density. ### Benzene Derivatives - When Hydrogen atoms of benzene are replaced by new groups then, the new structures are known as Benzene Derivatives. - Here, We have to study about structure and uses of following 4 benzene derivatives: - DDT - Saccharin - BHC - Chloramine - T ### DDT (Dichloro Diphenyl Trichloro Ethane) - A diagram is provided showing the structure of DDT. #### Uses - It is a highly toxic contact poison for wide variety of insects and works by disorganizing their nervous system. - It is applied as dusting powder or its aqueous solution is sprayed on the target site. ### Saccharin - A diagram showing the structure of saccharin is provided. #### Uses - Saccharin is used as an artificial sweetening agent. - Saccharin provides products with increased stability, improved taste and lower production cost. - It is also added in various vitamin supplements and medicines. - Its soluble salt is known as Saccharin Sodium. ### BHC (Benzene Hexachloride) - A diagram showing the structure of BHC is provided. #### Uses - It is used as insecticides in crops farming. - It is also used in various pharmaceuticals. ### Chloramine - T - A diagram showing the structure of chloramine-T is provided. #### Uses - It is mainly used as disinfectant. - It is also used in treatment of bums and wounds. - It is used as an Oral Mouthwash.