Unit 3 msc chemistry thesis inorganic nano.pptx

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1. Unit 3 The chemistry of reactive intermediates A reaction intermediate is a short lived chemical species formed from the reactants and reacts further to give the products of a chemical reaction. Main carbon reactive intermediates include Carbocations and their stabilized equivalents such as oxonium ions Carbanions and their stabilized equivalents such as enolates Free radicals Carbenes Reactive intermediates are, in general, not isolable but can be detected by spectroscopic methods or trapped chemically or their presence is confirmed by indirect evidence. Another reactive intermediate that we will discuss is nitrene 1

2. A carbocation is any positively charged carbon atom 3.1 Carbocations Two groups of carbocations : carbenium ions (trivalent positive species, common place) e.g. Me 3 C + , carbonium ions ( pentavalent positive species, rare) e.g. H 5 C + . and Historically, a structure with a positively charged carbon atom was called a carbonium ion Carbocation is a planar species exhibiting sp2 hybridization Structure: 2

3. 3.1.1 Generation of Carbocation A) Heterolytic fission of neutral species Carbocations (stable or unstable) may be generated in various ways. removal of an electronegative atom or group along with its pair of electrons attached to carbon Ionization by using a highly polar (high ɛ), powerful ion solvating medium Ionization by adding a Lewis acid The work of Olah with SbF 5 as a Lewis acid and with super acid, SbF 5 /FSO 3 H, leads to the formation of simple alkyl carbocations 3

4. B) Addition of cation to neutral species Carbocations may also be formed by protonation of unsaturated linkages or lone pairs Lewis acids may also be used and other cations , e.g. NO 2 + 4

5. C) From other cations Carbocations may be obtained from the decomposition of other cations and also by the use of a readily available carbocation 3.1.2 Carbocations Stability A. Simple alkyl carbocations Stability sequence: This is due to inductive effect and hyperconjugation (donation of C-H σ-bond (or C-C σ-bond) electrons into empty p orbital) greater number of C-H (or C-C) σ-bonds the greater the extent of hyperconjugation and the greater stabilisation 5 Tropylium cation T rityl cation

6. The most stable of all alkyl cations is the tert -butyl cation which is shown by the fact that butane, in superacid , gave only the tert -butyl cation . An essential requirement for such stabilization is that the carbocation should be planar, because it is only in this configuration that effective delocalization can occur. This is why Ph 3 CCl in liquid SO 2 is ionized readily whereas analogous bridged 1-bromotriptycene does not ionize in SO 2 . 6

7. B) allylic and benzylic carbocations Allyl cation and benzyl cation are more stable than most other carbocations . Molecules which can form allyl or benzyl carbocation are especially reactive. Stabilization, through delocalization, can also occur through aromatization. e.g., 1-bromocyclohepta-2,4,6-triene ( tropylium bromide) is a crystalline solid (mp 208 ◦C) that is highly soluble in water, giving bromide ions in solution, that is, it is an ion pair. 7

8. Stabilization can also occur , by delocalization, through the operation of a neighbouring group effect resulting in the formation of a bridged carbocation . Another structural feature that increases carbocation stability is the presence of an adjacent heteroatom bearing lone pair of electrons 8

9. 3.1.3 Reactions of carbocations Carbocations are found to undergo four basic types of reactions a) Combination with a nucleophile b) Elimination of a proton c) Addition to an unsaturated linkage d) Rearrangement of their structure 9

10. Most of these possibilities are neatly illustrated in the reaction of 1-aminopropane with sodium nitrite and dilute hydrochloric acid 10

11. Treatment of an alkene with formaldehyde in the presence of an acid gives a 1,3-diol together with an α,β -unsaturated alcohol or cyclic acetal ( Prins reaction) (c) 11

12. Rearrangements of Carbocations without change in carbon skeleton: 1,2-hydride shift and allylic rearrangements + 12

13. With change in carbon skeleton: Neopentyl rearrangements Changes in carbon skeleton involving carbocations are known collectively as Wagner- Meerwein rearrangements 13

14. Rearrangement of hydrocarbons Rearrangement of alkenes takes place readily in the presence of acids Pinacol / Pinacolone rearrangements the pinacol rearrangement is driven by formation of a strong C=O bond Migration of a group to a cationic carbon atom occurs in the acid catalyzed rearrangement of 1,2-diols, e.g , pinacol , to ketones , e.g. pinacolone . pinacol pinacolone (CC) 14

15. Semi- Pinacol rearrangements Similar to pinacol rearrangement occurs with compounds capable of forming the crucial carbocation (CC) , e.g 1,2-bromohydrin and 1,2-aminoalcohol 1,2-bromohydrin 1,2-aminoalcohol 15

16. Structures of Carbanions 3.2 Carbanions A carbanion is an anion in which carbon has an unshared pair of electrons and bears a negative charge usually with three substituents for a total of eight valence electrons. Formally a carbanion is the conjugate base of a carbon acid. The negatively charged saturated carbon atom is sp3 hybridized in carbanion pyramidal inversion is generally fast for sp3 hybridised carbanions (they are isoelectronic with NH 3 ) and hence chiral carbanions generally undergo rapid racemisation. vinyl anions and cyclopropyl anions are the exceptions and are generally considered configurationally stable sp3 sp3 sp2 16

17. 3.2.2 Generation of carbanions 1) An atom or a group attached to carbon leaves without its electron pair. The most common leaving group is X = H, though other leaving groups are also known, for example: CO 2 or Cl - Decarboxylation Reduction of σ bonds Deprotonation - very basic reagents can deprotonate C-H, e.g. n- BuLi , MeLi , PhLi - like any acid / base chemistry, it’s an equilibrium dictated by pKa - many hydrocarbons can’t be easily deprotonated due to kinetics 17

18. Deprotonation of hydrocarbons is made possible in the presence of structural features that promote the acidity of H atoms on carbon or that stabilize the carbanion with respect to the undissociated alkane A Table of some pka values for carbon acids 18

19. 3.2.3 Stability of Carbanions The stability of the carbanion is directly related to the strength of the conjugate acid. The weaker is the acid, the greater is the base strength and the lower is the stability of the carbanion . Carbanions vary widely in stability, depending on the hybridization of the carbon atom and the ability of substituent groups to stabilize negative charge. Factors determining the stability and reactivity of a carbanion : a) Hybridization of the carbanion carbon. The greater the s character of the carbanion carbon, the more stable the anion; 2) Addition of a negative ion to a carbon-carbon double or triple bond. Thus the order of stability is: 19

20. b) Electron-withdrawing inductive effect. Electronegative atoms adjacent to the carbanion carbon will stabilize the negative charge c) Conjugation of the carbanion lone pair with an unsaturated bond: However, alkyl substituents decrease carbanion stability in the order: Methyl > 1 o > 2 o > 3 o . The order of effectiveness of various groups for stabilizing the carbanion is: Example: greater acidity of HCF 3 ( pKa = 28) and HC(CF 3 ) 3 ( pKa = 11) than CH 4 and stability of the resultant carbanion 20

21. e) Stabilization through aromatization: d) Stabilization by sulfur or phosphorus: Due to delocalization of negative charge of carbanion by a vacant d-orbital ( pπ–dπ bonding) of phosphorus and sulfur Sulfonium ylide Phosphonium ylide The much greater acidity of cyclopentadiene ( pKa = 16) relative to simple alkene ( pKa = 37) reflects the aromatic stabilization of the resultant cyclopentadienyl anion cyclopentadiene cyclooctatetraene Cyclooctatetraenyl anion 21

22. 3.2.4 Reactions of Carbanions Carbanions can take part in most of the main reaction types such as addition, elimination , displacement, rearrangement , etc. 1) Addition There are large group of reactions in which carbanions add to the C=O group (works best when Y = H, OR, NR 2 , X; R = 1 o , allylic, sp2, sp) 1,2 additions to carbonyl functions 1,4-additions such as Michael reactions 22

23. 2) Elimination decarboxylation of a carboxylic acid takes place through the carboxylate anion, from which the group R departs along with its bonding electron pair. Carbanions involve as intermediates in elimination reactions that proceed by the E1cB mechanism. Dehydration of aldol is the most common E1cB reaction. Y = poor LG Decarboxylation Another useful reaction is addition to the very weak electrophile CO 2 to form the corresponding carboxylate anion, and is known as carbonation R = alkyl, aryl or acetylide M = Li, K Useful method for the preparation of acetylenic acids 23

24. C=O can also act like NO 2 , and the anions of β - keto acids are decarboxylated very readily β - keto acid itself readily decarboxylate , probably due to incipient proton transfer to C=O through hydrogen bonding Β , γ -unsaturated acids probably also decarboxylate by a similar pathway EWGs in R promote decarboxylation by stabilizing the carbanion intermediate by delocalization of the negative charge, e.g. NO 2 substituted carboxylate anion 24

25. 3) D isplacement /Substitution Carbanions are involved in a variety of displacement reactions, either as intermediates or as attacking nucleophiles S E 1 – Subsitution Electrophilic Unimolecular - formally related to a carbanion as S N 1 is to a carbocation α- halogenation of ketones Pyridine-2-carboxylic acid, free acid, undergoes ready decarboxylation 25

26. 26 S N 2 alkylation reactions Useful carbanions are those derived from reactive methylenes such as β - diesters , β - ketoesters , β - diketones , α - cyanoesters , nitroalkanes acetylide anion is a powerful nucleophile as well as a strong base The alkyls of more electropositive metals may react with alkyl halides, The carbanion can also act as a base and promote elimination Wurtz reaction Disproportionation - alkane + alkene - side reaction to the normal Wurtz reaction

27. 27 Intramolecular displacement between carbanion of α - haloester and carbonyl compound to yield α -epoxy ester, Darzens reaction . carbanion α - chloroester α , β - epoxyester Reimer- Tiemann Reaction A reaction used for the ortho-formylation of phenols with the simplest example being the conversion of phenol to salicylaldehyde .

28. 28 Kolbe-Schmidt Reaction the nucleophile addition of a phenolate to CO 2 to give the salicylate 4) Rearrangements of Carbanions 1,2-alkyl shifts in carbanions are much less common than similar 1,2-alkyl shifts in carbocations Carbocation T.S. Carbanion T.S . h ighly unstable

29. 29 1,2-aryl shifts transition state for 1,2-shift of carbanions has 4 electrons cyclically conjugated in a ring and is anti-aromatic

30. 30 Favorskii rearrangement overall in the Favorskii rearrangement an alkyl group ( R) moves from one side of the carbonyl group to the other organic reaction used to convert an α- haloketone to a rearranged acid or ester using a strong base (hydroxide or alkoxide )

31. 31 3.3 Free Radicals Radicals are neutral species with an unpaired electron. C-centred radicals are generally very reactive The first organic free radical identified was triphenylmethyl radical, by Moses Gomberg (the founder of radical chemistry) in 1900.

32. 32 3.3.1 Generation of Free Radicals

33. 33 3.3 Carbenes neutral, highly reactive species containing a divalent carbon atom with six valence electrons and having the general formula RR’C: structure singlet carbenes have three electron pairs and an empty orbital to place around the central atom - a similar case to carbocations ( carbenium ions) singlet carbenes are bent with the bond pairs and lone pairs in sp2 hybridised orbitals , along with a vacant p-orbital typical bond angle range for singlet carbenes are 100° - 110° the total spin = 0 bond angle = 102 o for single methylene the total spin = 1; paramagnetic, may be observed by electron spin resonance spectroscopy singlet triplet triplet triplet carbenes are bent with a typical bond angle range of 130° - 150°

34. 34 methylene (CH 2 ) has a triplet ground state – the singlet state is ca. 38 kJmol-1 higher in energy triplet singlet dialkylcarbenes , arylalkylcarbenes and some diarylcarbenes have triplet ground states heteroatom substituted carbenes have singlet ground states due to delocalisation increasing carbene stability

35. 35