B.Sc. Sem-II Unit-III (B) Aryl halides by Dr Pramod R Padole

Aryl Halides: Synthesis chlorobenzene from benzene, phenol, and benzene diazonium chloride, Synthesis of benzyl chloride from toluene and benzyl alcohol, Reactions of both with aqueous KOH, NH3, and sodium ethoxide, Comparison of the reactivity of chlorobenzene and benzyl chloride. Benzyne intermediate mechanism.

1. 1 B.Sc. First year Students Semester – II Unit-III B) Aryl Halides by Dr Pramod R Padole

2. Our Inspiration 2

3. Organic Chemisrtry Unit-III A) Alkenyl Halides: B) Aryl Halides: C) Alcohols:

4. III B) Aryl Halides: First Synthesis Chlorobenzene from benzene, phenol and benzene diazonium chloride Second Synthesis of Benzyl chloride from toluene and benzyl alcohol Third Reactions of both with aqueous KOH, NH3 and sodium ethoxide, Comparison of reactivity of chlorobenzene and benzyl chloride. Fourth Benzyne intermediate mechanism

5. Unit-III B) Aryl Halides: Aryl Halides: Synthesis chlorobenzene from benzene, phenol and benzene diazonium chloride, Synthesis of benzyl chloride from toluene and benzyl alcohol, Reactions of both with aqueous KOH, NH 3 and sodium ethoxide, Comparison of reactivity of chlorobenzene and benzyl chloride. Benzyne intermediate mechanism.

6. Unit-III B) Aryl Halides : Study of Benzyl Chloride (C 6 H 5 -CH 2 Cl): Or α-chlorotoluene or Chloromethyl benzene: Q.1) Benzyl chloride is also known as α-chlorotoluene . (S-16, ½ Mark) (a) α-chlorotoluene (b) Benzal chloride (c) Chlorobenzene (d) None of the above   B e n z y l c h l or i d e i s a r y l s u b s t i t u a t e d a l k y l h a l i d e i . e . s i d e c ha i n a r y l h a li d e (i.e. Benzyl chloride is a phenyl substituted methyl chloride). I t i s m ono c h l o r o de r i v a t i v e o f t o l uen e i n w h ic h c h l o ri n e i s a t t a c h e d t o t h e s i d e c h a i n . I t i s re g arde d a s a r y l s ub s t i tu t e d m eth y l c h l o ri de . Q.1) Aryl halides are less reactive towards nucleophilic substitution reactions as compared to alkyl halides due to Resonance effect . (W-12, ½ Mark)

7. Study of Benzyl Chloride (C 6 H 5 -CH 2 Cl): B e n z y l c h l or i d e i s a r y l s u b s t i t u a t e d a l k y l h a l i d e i . e . s i d e c ha i n a r y l h a li d e (i.e. Benzyl chloride is a phenyl substituted methyl chloride).

8. By Dr. Pramod R. Padole Method of Preparation of Benzyl Chloride: From Toluene: Method Of Preparation Of Benzyl Chloride From Benzyl alcohol :

9. Preparation of Benzyl Chloride from Toluene: Action of Sulphuryl Chloride (SO 2 Cl 2 ) in presence of Benzoyl Peroxide: From Toluene Action of Cl 2 in presence of light: (i.e. Side chain Halogenation)

10. Preparation of Benzyl Chloride from Toluene with Sulphuryl Chloride: When toluene is treated with sulphuryl chloride (SO 2 Cl 2 ) in presence of Benzoyl Peroxide; to form benzyl chloride . Q.1) How will you prepare / obtain Benzyl Chloride from Toluene? (S-04, W-04, W-05, S-12, W-12, W-17 & S-19, 2 Mark)

11. Preparation of Benzyl Chloride from Toluene with Chlorine: Action of Cl 2 in presence of light: (i.e. Side chain Halogenation) When toluene is treated with chlorine (Cl 2 ) in presence of sun light (U.V. light) or heat at high temperature or in absence of Halogen carrier, it undergoes side chain halogenation; to form benzyl chloride Q.1) How will you prepare / obtain Benzyl Chloride from Toluene? (S-04, W-04, W-05, S-11, W-11, S-12, W-12, S-16 & W-16, 2 Mark) Q.2) Complete the following reaction: (W-13 & S-17, 2 Mark)

12. [email protected] By Dr Pramod R Padole Preparation of Benzyl chloride from Benzyl alcohol: Reaction with PCl 5 : From Benzyl alcohol: Reaction with Thionyl chloride ( SOCl 2 ):

13. From Benzyl alcohol with PCl 5 : a) Reaction with PCl 5 : When ben z y l a l c oho l is t r eate d or reacted w i t h PCl 5 o r SOCl 2 ; to form ben z y l c h l o ri de . b) Reaction with Thionyl chloride ( SOCl 2 ): [email protected] By Dr Pramod R Padole Q.1) How will you prepare / obtain Benzyl Chloride from Benzyl alcohol? (W-04, W-08, W-11, W-13, S-16, W-17, S-19 & W-19, 2 Mark)

14. By Dr Pramod R Padole Chemical Reaction of Benzyl Chloride (C 6 H 5 -CH 2 Cl) : Common reactions shown by benzyl chloride:

15. Nucleophilic Substitution Reaction: Reaction with aqueous KOH or NaOH: Reaction with Ammonia (NH 3 ): Reaction with alco. Sodium Ethoxide: C 6 H 5 -CH 2 Cl

16. Reaction with aqueous KOH or NaOH: R ea c t i o n w i t h a q u e o u s K O H: When benzyl chloride is treated or reacted or heated w ith a q ueou s s o l u ti o n o f s od i u m o r pot a ssi u m h y d r o x i d e (aq. NaOH or KOH); to form ben z y l a lc o ho l . Q.1) How will you obtain Benzyl alcohol from Benzyl Chloride? (W-04, S-05 & W-10, 2 Mark) Q.2) Complete the following reaction: (S-13, 2 Mark) Benzyl chloride + aq. NaOH ? Q.3) Alkaline hydrolysis of benzyl chloride gives Benzyl alcohol . (W-13, ½ Mark) Q.4) What happen when: Benzyl chloride is reacted with aq. KOH? (W-15, 2 Mark)

17. Reaction with aqueous KOH or NaOH: R ea c t i o n w i t h a q u e o u s Na O H: When benzyl chloride is treated or reacted or heated w ith a q ueou s s o l u ti o n o f s od i u m o r pot a ssi u m h y d r o x i d e (aq. NaOH or KOH); to form ben z y l a lc o ho l . Q.1) How will you obtain Benzyl alcohol from Benzyl Chloride? (W-04, S-05 & W-10, 2 Mark) Q.2) Complete the following reaction: (S-13, 2 Mark) Benzyl chloride + aq. NaOH ? Q.3) Alkaline hydrolysis of benzyl chloride gives Benzyl alcohol . (W-13, ½ Mark) Q.4) What happen when: Benzyl chloride is reacted with aq. KOH? (W-15, 2 Mark)

18. Reaction with Ammonia (NH 3 ): R eac t i o n w i t h a mmon i a: When Ben zy l c hl o r i de is treated or reacted or heated w ith alcoholic solution of a mm o n i a ; to form be n zy l a m i n e . Q.1) How will you convert: Benzyl Chloride to Benzyl amine? (S-08, S-14, S-15 & W-19, 2 Mark) Q.2) Complete the reaction.

19. Reaction with alco. Sodium Ethoxide: R e act i o n w i t h s o d i u m e th o x i d e: When Ben zy l c hl o r i de is treated or reacted or heated w ith alcoholic solution of sodium ethoxide ; to form B e n zy l e t h y l eth e r, b y th e r ep l a c e m en t o f c h l o r i n e ato m w i t h etho x i d e g r o up . Q.1) How will you convert :- i ) Benzyl Chloride to Benzyl ethyl ether?

20. Study of Chlorobenzene (C 6 H 5 -Cl): Chlorobenzene is an example of Aryl halide

21. Study of Chlorobenzene (C 6 H 5 -Cl): Chlorobenzene i s a r o m at i c ha l o g e n de riv at i v e i n w h ic h th e c h l o ri n e ato m i s d ir e c t l y atta c he d t o th e ben z en e nu cl eu s . Chlorobenzene is the compound obtaining by replacing one of the hydrogens of the benzene. In chlorobenzene, chlorine atom is bonded to sp 2 hybridized carbon atom of the benzene ring . Q.1) Chlorobenzene is an example of ____________. (S-17 & W-19, ½ Mark) (a) alkyl halide (b) Alkenyl halide (c) Aryl halide (d) Haloalkane Q.2) In chlorobenzene, chlorine atom is bonded to sp 2 hybridized carbon atom of the benzene ring. (W-17 & S-19, ½ Mark)

22. Method of Preparation of Chlorobenzene: Preparation Of Chlorobenzene Benzene Phenol BDC

23. Preparation of Chlorobenzene from Benzene: From Benzene: The replacement of hydrogen atom of benzene ring (nucleus) by halogen is called nuclear halogenation . When benzene is reacted with Chlorine in presence of a catalyst, such as, iron(Fe) / iron chloride(FeCl 3 ) / FeBr 3 / AlCl 3 / AlBr 3 or iodine (is called as halogen carrier ) or in absence of sunlight ; to form chlorobenzene . Q.1) How will you obtain / synthesise / prepare Chlorobenzene from Benzene? (S-05, S-11, S-12, W-15 & W-19, 2 Mark) Q.2) What happens when, Benzene reacts with Cl 2 in presence of anhydrous AlCl 3 ? (S-13, 2 Mark)

24. Preparation of Chlorobenzene from Phenol: From Phenol: When pheno l is t r ea t ed or reacted w ith p h o s ph o rou s p e n t a c h l o ri d e ( PCl 5 ) ; to form c h l oro b en z e n e . Q.1) How will you obtain / synthesise /prepare Chlorobenzene from Phenol? (S-05, S-09, W-09 & S-12, 2 Mark) Q.2) How will you convert :- ii) Phenol into Chlorobenzene? (S-06 & W-11, 2 Mark) Q.3) How will you prepare Chlorobenzene from Phenol? (W-14 & W-18, 2 Mark) Q.4) Complete the following reaction; (S-17, 2 Mark)

25. [email protected] By Dr Pramod R Padole From BDC: Or From Benzene Diazonium Chloride ( C 6 H 5 -N=N-Cl ): From BDC: When b e n z e n e d i a z o n i u m c h l or i d e ( BDC ) is t r e a t ed wi t h c up r ou s c h l o r i d e (Cu 2 Cl 2 ) in HCl ; to form c h l o r o b en z e n e . Q.1) How will you obtain / prepare Chlorobenzene from Benzene Diazonium Chloride (BDC)? ( S-04, S-07, W-08, S-09 & W-15, 2 Mark) Q.2) Complete the following. (S-14, 2 Mark)

26. Chemical Reaction of Chlorobenzene: Chemical Reaction of Chlorobenzene (C 6 H 5 -Cl): I n c h l o r ob e n z e n e, c h l o r i n e a t o m i s f i r m l y h e l d b y t h e ben z en e nu c l eu s . H e n c e i t c a nno t b e ea s i l y r ep l a c e d b y t h e nu cl eoph ile unde r o r d i na r y c o nd i t i on s o f t e m p e r a t ur e a n d pre s s u r e . H o w e v e r , u n de r d r a s t i c c o nd i t i o n s c h l o ri n e a t o m o f c h l o r ob e n z en e c a n b e r ep l a c e d b y nu c l eoph il e .

27. Chemical Reaction of Chlorobenzene: C 6 H 5 -Cl Reaction with aqueous NaOH or KOH: Dow’s Process: (Preparation of Phenol) Reaction with Sodium Methoxide or Sodium Ethoxide: Nucleophilic Substitution Reaction: Reaction with aq. Ammonia (NH 3 ):

28. Reaction with aqueous NaOH or KOH: Dow’s Process: (Preparation of Phenol) Reaction with aq. NaOH: When c h l o r ob e n z e n e is h e at ed w i t h 1 % NaOH s o l u t i o n a t abo u t 6 2 3 t o 63 3 K & 3 a t m o s p he r i c p r e ss u r e ; to form s od i u m p hen o x i d e , w h i c h o n a c i d i f ic a t i o n b y d il u t e HC l ; to form phenol as a final product . Q.1) How will you bring about the change Chlorobenzene to Phenol? (W-04, 2 Mark) Q.2) How will you obtain / synthesise / prepare Phenol from Chlorobenzene? (W-05, 2 Mark) Q.3) Starting from Chlorobenzene how will you obtain Phenol ? ( S-06, 2 Mark)

29. [email protected] By Dr Pramod R Padole Reaction with aq. Ammonia (NH 3 ): R e a c t i o n w i t h aq. a mm o n i a & Cu 2 O at 473 K : When c hl o r obe n z en e is heat i n g w i t h a qu eou s solution of a mm on i a i n p r e s e n c e o f c u p r o u s o x i d e a t 4 7 3 K & u n de r pre s s u r e ; to form a n ili n e . Q.1) How will you convert :- i ) Chlorobenzene to Aniline. (S-05, 2 Mark) Q.2) Starting from Chlorobenzene how will you obtain Aniline. (S-06, 2 Mark) Q.3) How can you bring the following conversions? (W-06 & S-07, 2 Mark) Q.4) How will you prepare Aniline from chlorobenzene? (W-15 & S-18, 2 Mark) Or

30. Reaction with Sodium Methoxide: Reaction with alco. Sodium methoxide: When c h l o r oben z e n e is treated or h e a t ed w i t h alcoholic solution of s od i u m me t h o x i d e a t 473- 49 3 K and under pressure ; to form phe n y l -m et h y l eth e r (methoxy benzene) . Q.1) What happens when: Chlorobenzene is treated sodium methoxide at 493 K? Q.2) Complete the following reaction? (S-12, 2 Mark)

31. Reaction with Sodium Ethoxide: Reaction with alco. Sodium ethoxide: When c h l o r oben z e n e is treated or h e a t ed w i t h alcoholic solution of s od i u m e t h o x i d e a t 473- 49 3 K and under pressure ; to form phe n y l - et h y l eth e r (ethoxy benzene) . Q.1) How will you convert Chlorobenzene to ethoxy benzene? (W-14, 2 Mark) Q.2) What happens when: Chlorobenzene is heated with sodium ethoxide at 473 K? (S-18, 2 Mark)

32. Q.1) Explain benzyne intermediate mechanism. (S-14, S-17 & S-18, 4 Mark) Q.2) Complete the following: (S-15, 2 Mark) Q.3) Explain the Elimination-Addition mechanism of aromatic nucleophilic substitution in Chlorobenzene. (S-15, 4 Mark) Q.4) What is Benzyne? (W-15, 1 Mark) Q.5) Explain Benzyne intermediate mechanism (Elimination-Addition mechanism) of aromatic nucleophilic substitution. (W-15 & W-16, 4 Mark) Q.6) Explain the Benzyne intermediate mechanism of aromatic nucleophilic substitution in Chlorobenzene. (W-18, 4 Mark)   Elimination-Addition Mechanism of Nucleophilic Aromatic Substitution Reaction: or Benzyne Mechanism:

33. The Elimination-Addition Mec h a n i s m o f Nu c l e oph il i c A r o m at i c S u b st i t u t i o n R e a ct i o n : or B e n z y n e Mec h a ni sm : When chlorobenzene is reacted with sodium amide (NaNH 2 ) or potassium amide (KNH 2 ) in presence of liquid ammonia (as a solvent); to form aniline.

34. Benzyne Mechanism: Step – 3) Step – 2 ) Step – 1) Final Product - Aniline Addition Step Elimination Step Abstraction of a proton from solvent:

35. Benzyne Mechanism: M ec h a n i s m: S t e p - 1) E li m i n a t i o n s t e p: A m i d e (NH 2 - ) i s v e r y st r o n g ba s e . I t b ri ng s a b ou t de h y d r o ha l og en a t i o n o f c h l o r obe n z en e b y a b s t ra c t i n g a p r o t o n f ro m c a r b o n a t o m a d j a c e n t t o c h l o r o g r ou p o f c h l o ro ben z en e . T h e pr o d u c t o f t h is st e p is a n un st ab le i n t e r m ed i a te c a ll e d b e n z y n e . T r i p l e b o n d o f b e n z y n e i s s o m e w h a t d i f f e r e n t f r o m u s u a l t r i p l e bo n d o f a n a l ky n e .

36. Benzyne Mechanism: M ec h a n i s m: F o ll o wi n g st ep s a r e i n v o l v e d i n ben z y n e m e c h a n is m . S t e p - 2) Addition s t e p: A m i d e i o n a c t s a s n u c l e o p h i l e . I t a d d s t o o n e o f t h e c a r b o n a t om s o f t ri p l e bon d; t o form c a r ban i on .

37. Benzyne Mechanism: M ec h a n i s m: F o ll o wi n g st ep s a r e i n v o l v e d i n ben z y n e m e c h a n is m . S t e p - 3 ) Abstraction of a proton from solvent: F i na l l y t h e c a r b a n i o n a b s t r a c t s a p r o t o n f r o m s o l v en t, like liquid a mm on i a; t o form final product as a n ili ne .

38. Benzyne Mechanism: Note that: In benzyne intermediate, one π bond is normal it is just part of the aromatic system and the other π bond is abnormal and is formed by overlap of two sp 2 orbitals outside the ring. This external π bond is very weak and benzyne is a very unstable intermediate.

39. Comparison of Reactivity of Chlorobenzene & Benzyl Chloride:

40. Comparison of Reactivity of Chlorobenzene & Benzyl Chloride: Q.1) Explain why Cl 2 in chlorobenzene is less reactive than in benzyl chloride, with the help of Resonance Structure. (W-04, W-05, S-06,W-06, W-07, S-08, W-08, W-09 & W-10, 2- 4 Mark) Q.2) In chlorobenzene the chlorine atom is attached / bonded to sp 2 hybridized carbon atom. (S-11 & S-15, ½ Mark) Q.3) Compare the reactivity of chlorobenzene with that of benzyl chloride. (W-11, 4 Mark) Q.4) Explain why benzyl chloride is more reactive than chlorobenzene. (S-13, 4 Mark) Q.5) Explain: Why Benzyl chloride is more reactive than chlorobenzene towards nucleophilic substitution reactions? (S-14, 4 Mark) Q.6) Compare the reactivity of Chlorobenzene and Benzyl chloride. (W-15, W-17 & W-18, 4 Mark) Q.7) Why Chlorobenzene is less reactive towards nucleophilic substitution than that in Benzyl chloride? (S-17, 4 Mark) Q.8) In chlorobenzene, the chlorine atom is bonded to sp 2 hybridized carbon atom of the benzene ring. (W-17, ½ Mark) Q.9) Hybridization state of carbon to which chlorine attached in benzyl chloride is sp 3 . (W-18, ½ Mark) Q.10) Compare the reactivity of Chlorobenzene and Benzyl chloride towards the nucleophilic substitution reaction. (S-19, 4 Mark)

41. Co m p a ri s o n o f R ea c t i v i t y o f C h l o ro b e n z e n e a n d B e n z y l c hl o ri d e: The reason for the more reactivity of benzyl chloride than chlorobenzene is due to the following facts: R e a c t i v i t y o f C h l o r o b e n z e n e: i ) I n c h l o r ob e n z e ne , t h e c h l o r i n e a t o m i s d i r e c t l y a t t a c h e d t o be n z en e ring ( nu c l eu s) . T h e c a r bo n a t o m t o w h i c h chlorine atom i s a tt a c h e d i n c h l o r o be n z e n e i s p r e s e n t in sp 2 - h y b ri d i s a t i o n st a t e an d p o ss e s 33 . 3 % ‘ s’ c ha r a ct e r . H en ce C-Cl bon d is a st r o n g er b o nd .

42. Co m p a ri s o n o f R ea c t i v i t y o f C h l o ro b e n z e n e a n d B e n z y l c hl o ri d e: R e a c t i v i t y o f C h l o r o b e n z e n e: ii ) I n c h l o r oben z ene , th e lone pa i r e l e c t r o ns o f c h l o ri n e a t o m g e t delocalized (positive resonance effect) o v e r en t ir e b e n z e n e r i n g an d resonance stabilizes c h l o r o b e n z e n e a n d t h e d o u b l e b o n d c h a r a c t e r o f C-Cl bon d m a ke it stronger. T h e c h l o r i n e ato m i n resonating s t r u c tu r e s c a rr i e s p o s i t i v e c h a r ge . H e n c e re p l a c e m e n t o f c h l o r i n e b y n u c l e o ph i l e s un d e r ord i n ar y c on d i t i o n s is n o t po ssi b l e . H en ce unde r d r a stic c ond iti o n t h e nu c l eo p h il e c a n r e p l a c e c h l o r i n e a t o m .

43. Co m p a ri s o n o f R ea c t i v i t y o f C h l o ro b e n z e n e a n d B e n z y l c hl o ri d e: R e a c t i v i t y o f B enzyl chloride: I n ben z y l c h l o r i de , t h e c h l o r i n e a t o m i s a t t a c h e d t o t h e s i d e c h a i n carbon a t o m . T h e s i d e c h a i n c a r b o n a t o m i s p r e s e n t i n s p 3 h y b r i d i s a ti o n st a t e an d po ss e s 25 % o f ' s ' c ha r a ct e r. S o C-C l bon d i n ben zy l c h l o ri d e i s c o m pa r a t i v e l y w ea ker than chlorobenzene .

44. Co m p a ri s o n o f R ea c t i v i t y o f C h l o ro b e n z e n e a n d B e n z y l c hl o ri d e: i i ) I n b en z y l c h l o r i de , th e lone pair e l e c t r o ns o f c h l o r i n e a t o m d o no t p a r t ic i pa t e i n r e s o n an c e . T he r e i s ne i t he r do u b l e bon d c ha r a ct e r t o C-Cl bon d no r t h e po siti v e c ha rg e o n t h e c h l o r i n e a t o m .

45. Co m p a ri s o n o f R ea c t i v i t y o f C h l o ro b e n z e n e a n d B e n z y l c hl o ri d e: A f t e r he t e r o l y s i s o f C- C l bon d in be n zy l c h l o ri de , b e n zy l c a r bon i u m i o n i s o b ta i ne d w h ic h g e t s t a b i l i s e d b y r e s o n a n c e . Fr o m abo v e d isc u ssi o n i t i s cl ea r t h a t C-Cl b o n d i n b e n z y l c h l o ri d e i s w e a k e r t h a n C- C l bo n d i n c h l o r ob e n z e n e . H en c e ben z y l c h l o r i d e e a s i l y u n d e r goe s n u c l e o p h i l i c s u b s ti t u t i o n . Hence, Chlorine in chlorobenzene is less reactive than in benzyl chloride

46. Dr. Pramod R. Padole Professor Department of Chemistry Shri Shivaji Science College, Amravati Mobile: [phone] Email: [email protected]

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