Benzene

By students of Sir Dr. M.S. GOSAVI COLLEGE OF PHARMACEUTICAL EDUCATION AND RESEARCH, NASHIK.

1. Continuous Assessment 2 Group 2 Roll no. 09 to 16 PHARMACEUTICAL ORGANIC CHEMISTRY II Sir Dr. M.S. Gosavi College of Pharmaceutical Education and Research College in Nashik, Maharashtra

2. Roll No 9 to 16 9) Pooja Dilipkumar bora 10) Siddhesh Gajendra Borade 11) Tanmay Sunil Bothe 12) Shruti Umesh Chaudhari 13) Payal Himmat Chavan 14) Apurva Mukund Chothave 15) Simantini Pundlik Dabhade 16) Nikhil Rajendra Datir

3. Benzene

4. ☆ Benzene is one of the most important organic compounds with the chemical formula C 6 H 6 . Benzene is the parent compound of the various aromatic compound. ☆ Benzene is the simplest organic,  aromatic hydrocarbon . Benzene is one of the elementary petrochemicals and a natural constituent of crude oil. It has a gasoline-like odour and is a colourless liquid. Benzene is highly toxic and carcinogenic in nature. It is primarily used in the production of polystyrene. ☆ Benzene is a naturally occurring substance produced by volcanoes and forest fires and present in many plants and animals, but benzene is also a major industrial chemical made from coal and oil. As a pure chemical, benzene is a clear, colourless liquid. In industry benzene is used to make other chemicals as well as some types of plastics, detergents, and pesticides. It is also a component of gasoline. INTRODUCTION

5. Structure of benzene ☆KEKULE STRUCTURE = In 1865, Kekule suggested a ring structure for benzene which consisted of a  cyclic planar structure of six carbons having alternate double and single bonds . Each of the six carbons was attached to one hydrogen.

6. ☆ CHEMICAL STRUCTURE = Benzene, C 6 H 6 , is a planar molecule  containing a ring of six carbon atoms, each with a hydrogen atom attached . The six carbon atoms form a perfectly regular hexagon. All of the carbon-carbon bonds have exactly the same lengths - somewhere between single and double bonds. Because of the aromaticity of benzene, the resulting molecule is planar in shape with each C-C bond being 1.39 Å in length and each bond angle being 120°. ☆ RESONANCE STRUCTURE = In Benzene There Are Continuous delocalisation of pi bond, so it form Resonance and also show resonance hybrid structure

7. MOLECULAR ORBITAL STRUCTURE OF BENZENE

8. Rules For AromaticiTY In 1931, German chemist and physicist Erich Hückel proposed a theory to help determine if a planar ring molecule would have aromatic properties The molecule is cyclic (a ring of atoms) The molecule is planar (all atoms in the molecule lie in the same plane) The molecule is fully  conjugated  (p orbitals at every atom in the ring) The molecule has  4n+2 π4 n+2 π   electrons ( n=0n=0  or any positive integer Example = Benzene

9. Resonance in benzene A resonance form is another way of drawing a Lewis dot structure for a given compound. Equivalent Lewis structures are called resonance forms. They are used when there is more than one way to place double bonds and lone pairs on atoms. Resonance structures arise when there are more than one way to draw a Lewis dot diagram that satisfies the  octet rule Benzene is commonly seen in Organic Chemistry and it has a resonance form. Benzene has two resonance structures, showing the placements of the bonds. Another example of resonance is ozone. Ozone is represent by two different Lewis structures. The difference between the two structures is the location of double bond.

10. Nitration ☆ Nitration happens when one (or more) of the hydrogen atoms on the benzene ring is replaced by a nitro group, NO 2 . Benzene is treated with a mixture of concentrated nitric acid and concentrated sulfuric acid at a temperature not exceeding 50°C. The mixture is held at this temperature for about half an hour. Yellow oily nitrobenzene is forrmed.

11. The concentrated sulfuric acid is acting as a catalyst and so is not written into the equations . At higher temperatures there is a greater chance of getting more than one nitro group substituted onto the ring. You will get a certain amount of 1,3-dinitrobenzene formed even at 50°C. Some of the nitrobenzene formed reacts with the nitrating mixture of concentrated acids.

12. Sulphonation Sulfonation is a reversible reaction that produces benzenesulfonic acid by adding sulfur trioxide and fuming sulfuric acid. The reaction is reversed by adding hot aqueous acid to benzenesulfonic acid to produce benzene.

13. Mechanism (sulphonation) To produce benzenesulfonic acid from benzene, fuming sulfuric acid and sulfur trioxide are added. Fuming sulfuric acid, also refered to as  oleum , is a concentrated solution of dissolved sulfur trioxide in sulfuric acid. The sulfur in sulfur trioxide is electrophilic because the oxygens pull electrons away from it because oxygen is very electronegative. The benzene attacks the sulfur (and subsequent proton transfers occur) to produce benzenesulfonic acid.

14. Halogenation Halogenation is an example of electrophillic aromatic substitution. In electrophilic aromatic substitutions, a benzene is attacked by an electrophile which results in substition of hydrogens. However, halogens are not electrophillic enough to break the aromaticity of benzenes, which require a catalyst to activate. Benzene reacts with chlorine or bromine in the presence of a catalyst, replacing one of the hydrogen atoms on the ring by a chlorine or bromine atom. The reactions happen at room temperature. The catalyst is either aluminum chloride (or aluminum bromide if you are reacting benzene with bromine) or iron.

15. The reaction with chlorine The reaction between benzene and chlorine in the presence of either aluminum chloride or iron gives chlorobenzene. or, written more compactly C 6 H 6+ Cl 2→ C 6 H 5 Cl + HCl

16. The reaction with bromine The reaction between benzene and bromine in the presence of either aluminum bromide or iron gives bromobenzene. Iron is usually used because it is cheaper and is more readily available. Or C 6 H 6+ Br 2→ C 6 H 5 Br + HBr

17. Addition reactions In the presence of ultraviolet light (but without a catalyst present), hot benzene will also undergo an addition reaction with chlorine or bromine. The ring delocalization is permanently broken and a chlorine or bromine atom adds o adds n to each carbon atom. For example, if you bubble chlorine gas through hot benzene exposed to UV light for an hour, you get 1,2,3,4,5,6-hexachlorocyclohexane.

18. Friedel Craft’s alkylation An alkyl group can be added to a benzene molecule by an electrophile aromatic substitution reaction called the  Friedel‐Crafts alkylation reaction.  One example is the addition of a methyl group to a benzene ring. One example is the addition of a methyl group to a benzene ring.

19. 1 . 2. The electrophile attacks the π electron system of the benzene ring to form a nonaromatic carbocation. The mechanism for this reaction begins with the generation of a methyl carbocation from methyl chloride . The carbocation then reacts with the π electron system of the benzene to form a nonaromatic carbocation that loses a proton to reestablish the aromaticity of the system.

20. 3. The positive charge on the carbocation that is formed is delocalized throughout the molecule.

21. 4. The aromaticity is restored by the loss of a proton from the atom to which the methyl group has bonded. 5. Finally, the proton reacts with the AlCl  4   −  to regenerate the AlCl  3  catalyst and form the product HCl.

22. Limitations of Friedel Craft’s alkylation There are possibilities of carbocation rearrangements when you are trying to add a carbon chain greater than two carbons. The rearrangements occur due to hydride shifts and methyl shifts. For example, the product of a Friedel-Crafts Alkylation will show an iso rearrangement when adding a three carbon chain as a substituent. Also, the reaction will only work if the ring you are adding a substituent to is not deactivated. For a look at substituents that activating or deactivating Benzene Rings. ☆ The three key limitations of Friedel-Crafts alkylation are: Carbocation Rearrangement - Only certain alkylbenzenes can be made due to the tendency of cations to rearrange. Compound Limitations - Friedel-Crafts fails when used with compounds such as nitrobenzene and other strong deactivating systems. Polyalkylation  - Products of Friedel-Crafts are even more reactive than starting material. Alkyl groups produced in Friedel-Crafts Alkylation are electron-donating substituents meaning that the products are more susceptible to electrophilic attack than what we began with. For synthetic purposes, this is a big dissapointment

23. Friedel Craft’s acylation The Friedel–Crafts acylation is the reaction of an arene with acyl chlorides or anhydrides using a strong Lewis acid catalyst The very first step involves the formation of the acylium ion which will later react with benzene :.

24. The second step involves the attack of the acylium ion on benzene as a new electrophile to form one complex: The third step involves the departure of the proton in order for aromaticity to return to benzene:

25. During the third step, AlCl 4  returns to remove a proton from the benzene ring, which enables the ring to return to aromaticity. In doing so, the original AlCl 3  is regenerated for use again, along with HCl. Most importantly, we have the first part of the final product of the reaction, which is a ketone. Thie first part of the product is the complex with aluminum chloride as shown:

26. The final step involves the addition of water to liberate the final product as the acylbenzene :

27. Limitations of Friedel Crafts acylation T he acylium ion (as was shown in step one) is stabilized by resonance, no rearrangement occurs. B ecause of of the deactivation of the product, it is no longer susceptible to electrophilic attack and hence no longer goes into further reactions. Friedel-Crafts Acylation fails with strong deactivating rings.

28. our special aknowledgement to M r s.S.M.Saudagar ma ’A m......... Thank you !