reaction mechanism (part-2)
TRANSCRIPT
PowerPoint PresentationPaper: 1C (Organic Chemistry) Topic:
Reaction mechanism(Part-2)
Faculty Name: Dr. Rupali Gupta College Affiliation: M. M. Mahila College, Ara
Date: 14/05/2020
TYPES OF REAGENTS
• All through the series on understanding where electrons are, and how they flow, we’ve been talking about how the basis of chemistry is that opposite charges attract and like charges repel, and that in reactions, electrons flow from “electron rich” areas to “electron poor” areas
• Today, we’ll officially give a name to the types of species that
are considered “electron rich“ and “electron poor”. They’re called respectively known as:
(i) Nucleophiles (ii) Electrophiles • Electrophile and nucleophile are the chemical species
that donate or accept electrons to form a new chemical bond.
• During a polar organic reaction, a nucleophile attacks an electrophilic centre of the substrate which is that specific atom or part of the electrophile that is electron deficient.
• Similarly, the electrophiles attack at nucleophilic centre, which is the electron rich centre of the substrate.
• Thus, the electrophiles receive electron pair from nucleophile when the two undergo bonding interaction.
• A curved-arrow notation is used to show the movement of an electron pair from the nucleophile to the electrophile.
• Examples of nucleophiles are the negatively charged ions with lone pair of electrons such as hydroxide (HO– ), cyanide (NC–) ions and carbanions (R3C:–).
• Examples of electrophiles include carbocations (CH3 +).
NUCLEOPHILE
Definition: A nucleophile is a reagent comprising an unparalleled or lone pair of electron. As a nucleophile is wealthy in electron, it looks for electron deficient locations, i.e. nucleus means loving nucleus. Its important properties are as follows: • Nucleophiles act as Lewis bases, according to Lewis concept of acids
and bases. • The term nucleophile can be split into “nucleo” derived from the
nucleus and “phile” which means loving. • They are electron rich and hence nucleus loving. They are negatively
charged or neutrally charged. • They donate electrons. • Movement of electrons depends on the density. • They move from low-density area to high-density area. • They undergo nucleophilic addition and substitution reactions. • A nucleophile is also called as Lewis base. • Examples: OH-, CN-, S-, Cl-
ELECTROPHILE
Definition: Positively loaded or neutral species are called electrophiles that are deficient in electrons and can accept a couple of electrons. These are also called species that love electron (philic). Its important properties are as follows: • The term electrophile can be split into “electro” derived from
electron and “phile” which means loving. • They are electron deficient and hence electrons loving. • They are positively charged or neutrally charged. • They attract electrons. Movement of electrons depends on the
density. • They move from high-density area to low density area. • They undergo electrophilic addition and electrophilic substitution
reactions. • An electrophile is also called as Lewis acid. • Examples: CH3
+, BF3, AlCl3
ELECTROPHILE NUCLEOPHILE
Also called as Lewis acid Also called as Lewis base
They are positively charged / neutral
They are negatively charged / neutral
They undergo electrophilic addition and electrophilic substitution reactions
They undergo nucleophilic addition and nucleophilic substitution reactions
Electron-deficient Electron-rich
It accepts a pair of an electron to form a covalent bond
It donates pair of an electron to form a covalent bond
All carbocations. All carbanions.
REACTION INTERMEDIATE
Reaction intermediate, are short lived and their importance lies in the assignment of reaction mechanisms on the pathway from the starting substrate to stable products. These reactive intermediates are not isolated, but are detected by spectroscopic methods, or trapped chemically or their presence is confirmed by indirect evidence. There are following types of reaction intermediates which will be discussed one by one: (i) Carbocations (ii) Carbanions (iii) Free radicals (iv) Carbenes (v) Benzyne
1. CARBOCATIONS • Carbocations are formed from the heterolytic cleavage of a carbon-
heteroatom (meaning a non carbon atom in general) bond where the other atom is more electronegative than carbon like a C-O, C-N, C-X (X can be Cl, Br, I, etc) bond.
• This is quite logical as after the cleavage if a carbocation is to be
formed the two electrons of the bond must go to the other atom. And this is favored if that other atom is electronegative.
• Carbocations are the key intermediates in several reactions and
particularly in nucleophilic substitution reactions.
• Generally, in the carbocations the positively charged carbon atom is bonded to three other atoms and has no nonbonding electrons.
• These carbocations have trigonal planar shape with positively charged carbon being sp2 hybridised.
• It is sp2 hybridized with a planar structure and bond angles of about 120°.
• There is a vacant unhybridized p orbital which in the case of CH3
+ lies perpendicular to the plane of C—H bonds.
STRUCTURE OF CARBOCATIONS
• There is an increase in carbocation stability with additional alkyl substitution.
• Thus one finds that addition of HX to three typical olefins decreases
in the order (CH3)2C=CH2>CH3—CH = CH2 > CH2 = CH2 • This is due to the relative stabilities of the carbocations formed in
the rate determining step which in turn follows from the fact that the stability is increased by the electron releasing methyl group (+I), three such groups being more effective than two, and two more effective than one.
• Stability of carbocations 3°> 2° > 1° > CH3 +
• Electron release : Disperses charge, stabilizes ion
STABILITY OF CARBOCATIONS
• Further, any structural feature which tends to reduce the electron deficiency at the tricoordinate carbon stabilizes the carbocation.
• Thus when the positive carbon is in conjugation with a double bond, the stability is more. This is so, because due to resonance the positive charge is spread over two atoms instead of being concentrated on only one.
• This explains the stability associated with the allylic carbocation. The benzylic cations are stable, since one can draw canonical forms as for allylic cations.
Figure 2: Canonical forms of benzylic carbocation
• The benzyl cation stability is affected by the presence of substituents on the ring.
• Electron donating p-methoxy and p-amino groups stabilize the carbocation by 14 and 26 kcal/mole, respectively.
• The electron withdrawing groups like p-nitro destabilize by 20 kcal/mol.
2. CARBANIONS
• These are intermediates also formed as a result of heterolysis, but here the electron pair from the bond is kept by the carbon atom.
• From what we saw earlier the more electronegative atom keeps the electrons, so in this case carbon must be the more electronegative of the two atoms making up the bond.
• Now there are only a few atoms (non-metals; metals are not usually part of organic chemistry) which are less electronegative, so the most common bond cleavage which yields carbanions is the C-H bond.
STRUCTURE OF CARBANIONS
• A carbanion possesses an unshared pair of electron and thus represents a base.
• The best likely description is that the central carbon atom is sp3 hybridized with the unshared pair occupying one apex of the tetrahedron.
• Carbanions would thus have trigonal pyramidal structures similar to those of amines.
• It is believed that carbanions undergo a rapid interconversion between two pyramidal forms.
• According to the VSEPR theory, carbanion is isostructural with NH3.
Figure 3: Shape of carbanion.
• Factors which can stabilize or disperse the negative charge on carbon will stabilize a carbanion.
• The stability of carbanion depends on the following factors: (i) Inductive effect (ii) Extent of conjugation of the anion (iii) Hybridization of the charge-bearing atom (iv) Aromaticity • Stability of carbanions order: CH3
- >1°> 2° > 3°
• The alkyl groups are electron releasing in nature due to inductive effect (+I). More the number of alkyl groups attached lesser will be the stability. Carbanions prefer a lesser degree of alkyl substitution.
• Electron donation : accumulates negative charge, destabilizes ion
STABILITY OF CARBANIONS
• If negatively charged carbon is in conjugation with a double bond the resonance effects will stabilize the anion by spreading out the charge by rearranging the electron pairs. Eg. Benzyl anion:
• Stability of anion will depend upon the s character of carbanion i.e. more the s character, higher will be the stability of anion. The percentage s character in the hybrid orbitals is as follows:
sp (50%)> sp2 (33%)>sp3 (25%)
3. FREE RADICALS
• These are neutral intermediates, formed due to homolytic cleavage of a single bond.
• Some common bonds which cleave to give free radicals in organic
chemistry are shown: C-O, C-Cl, C-Br, C-I, C-C, C-H. Carbon free radicals are mainly generated by:
(i) Photolysis (action of light) like acetone alpha cleavage (ii) Other radical initiator like allylic bromination by N- Bromosuccinimide (NBS) • A free radical is a species which has one or more unpaired
electrons. In radicals, however, since there are one or more unpaired electrons, there is a net magnetic moment and the radicals as a result are paramagnetic.
STRUCTURE OF FREE RADICALS
• There has been a certain degree of debate as to what the shape and geometry of a free radical is like. Revisiting the theory of hybridization, there can be two basic shapes of these radicals.
• If the centre carbon atom of the radical is sp3 hybridized (remember the one which was made of one s and three orbitals as in CH4), the geometry will be tetrahedral.
• But in the case of a radical there are only three groups attached to the sp3 hybridized carbon atom so they we will have a shape of what resembles a pyramid—it’s a tetrahedron with its head cut off.
• So sp3 hybridized radicals are pyramidal in shape. The single electron of the radical would then be housed in a sp3 orbital. The other option is sp2 hybridization.
• In that case the C atom is sp2 hybridized, so as discussed previously the shape would be planar with the single electron in the unhybridized p-orbital with the three substituents having sp2 hybridized bonds.
Figure 4: Two different geometries of free radicals. The single electrons are shown as black dots.
STABILITY OF FREE RADICALS
• As in the case of carbocation, the stability order of free radicals is:
tertiary > secondary > primary • This is explained on the basis of hyperconjugation.
• The stabilizing effects in allylic radicals and benzyl radicals is due to
vinyl and phenyl groups in terms of resonance structures.
• The triphenyl methyl type radicals are no doubt stabilized by resonance, however, the major cause of their stability is the steric hindrance to dimerization.
• Bond dissociation energies shown that 19 kcal/mol less energy is needed to form the benzyl radical from toluene than the formation of methyl radical from methane.
4. CARBENES
• Carbenes are neutral intermediates having bivalent carbon, in which a carbon atom is covalently bonded to two other groups and has two valency electrons distributed between two non bonding orbitals.
• When the two electrons are spin paired the carbene is a singlet, if the spins of the electrons are parallel it is a triplet.
STRUCTURE OF CARBENES
• A singlet carbene is thought to possess a bent sp2 hybrid structure in which the paired electrons occupy the vacant sp2 orbital.
• A triplet carbene can be either bent sp2 hybrid with an electron in each unoccupied orbital, or a linear sp hybrid with an electron in each of the unoccupied p-orbital.
• It has however, been shown that several carbenes are in a non- linear triplet ground state.
• However, the dihalogenocarbenes and carbenes with oxygen, nitrogen and sulphur atoms attached to the bivalent carbon, exist probably as singlets.
• The singlet and triplet state of a carbene display different chemical behavior. Thus addition of singlet carbenes to olefinic double bond to form cyclopropane derivatives is much more stereoselective than addition of triplet carbenes.
Figure 5: Different types of carbenes
Generation of Carbenes: Carbenes are obtained by thermal or photochemical decomposition of diazoalkanes. These can also be obtained by a-elimination of a hydrogen halide from a haloform with base, or of a halogen from a gem dihalide with a metal.
Reactions of Carbenes: These add to carbon double bonds and also to aromatic systems and in the later case the initial product rearranges to give ring enlargement products (a car-benoids -organometallic or complexed intermediates which, while not free carbenes afford products expected from carbenes are usually called carbenoids).
5. BENZYNE
• Benzynes also known as aryne are highly reactive species derived from an aromatic ring by removal of two substituents.
• The most common arynes are ortho but meta- and para-arynes are also known.
• The alkyne representation of benzyne is the most widely encountered. o-Arynes, or 1,2-didehydroarenes, are usually described as having a strained triple bond.
• Geometric constraints on the triple bond in ortho-benzyne result in diminished overlap of in-plane p-orbitals, and thus weaker triple bond.
• The vibrational frequency of the triple bond in benzyne was assigned by Radziszewski to be 1846 cm−1, indicating a weaker triple bond than in unstrained alkyne with vibrational frequency of approximately 2150 cm−1.
• Nevertheless, ortho-benzyne is more like a strained alkyne than a biradical, as seen from the large singlet–triplet gap and alkyne-like reactivity.
REFERENCES
Faculty Name: Dr. Rupali Gupta College Affiliation: M. M. Mahila College, Ara
Date: 14/05/2020
TYPES OF REAGENTS
• All through the series on understanding where electrons are, and how they flow, we’ve been talking about how the basis of chemistry is that opposite charges attract and like charges repel, and that in reactions, electrons flow from “electron rich” areas to “electron poor” areas
• Today, we’ll officially give a name to the types of species that
are considered “electron rich“ and “electron poor”. They’re called respectively known as:
(i) Nucleophiles (ii) Electrophiles • Electrophile and nucleophile are the chemical species
that donate or accept electrons to form a new chemical bond.
• During a polar organic reaction, a nucleophile attacks an electrophilic centre of the substrate which is that specific atom or part of the electrophile that is electron deficient.
• Similarly, the electrophiles attack at nucleophilic centre, which is the electron rich centre of the substrate.
• Thus, the electrophiles receive electron pair from nucleophile when the two undergo bonding interaction.
• A curved-arrow notation is used to show the movement of an electron pair from the nucleophile to the electrophile.
• Examples of nucleophiles are the negatively charged ions with lone pair of electrons such as hydroxide (HO– ), cyanide (NC–) ions and carbanions (R3C:–).
• Examples of electrophiles include carbocations (CH3 +).
NUCLEOPHILE
Definition: A nucleophile is a reagent comprising an unparalleled or lone pair of electron. As a nucleophile is wealthy in electron, it looks for electron deficient locations, i.e. nucleus means loving nucleus. Its important properties are as follows: • Nucleophiles act as Lewis bases, according to Lewis concept of acids
and bases. • The term nucleophile can be split into “nucleo” derived from the
nucleus and “phile” which means loving. • They are electron rich and hence nucleus loving. They are negatively
charged or neutrally charged. • They donate electrons. • Movement of electrons depends on the density. • They move from low-density area to high-density area. • They undergo nucleophilic addition and substitution reactions. • A nucleophile is also called as Lewis base. • Examples: OH-, CN-, S-, Cl-
ELECTROPHILE
Definition: Positively loaded or neutral species are called electrophiles that are deficient in electrons and can accept a couple of electrons. These are also called species that love electron (philic). Its important properties are as follows: • The term electrophile can be split into “electro” derived from
electron and “phile” which means loving. • They are electron deficient and hence electrons loving. • They are positively charged or neutrally charged. • They attract electrons. Movement of electrons depends on the
density. • They move from high-density area to low density area. • They undergo electrophilic addition and electrophilic substitution
reactions. • An electrophile is also called as Lewis acid. • Examples: CH3
+, BF3, AlCl3
ELECTROPHILE NUCLEOPHILE
Also called as Lewis acid Also called as Lewis base
They are positively charged / neutral
They are negatively charged / neutral
They undergo electrophilic addition and electrophilic substitution reactions
They undergo nucleophilic addition and nucleophilic substitution reactions
Electron-deficient Electron-rich
It accepts a pair of an electron to form a covalent bond
It donates pair of an electron to form a covalent bond
All carbocations. All carbanions.
REACTION INTERMEDIATE
Reaction intermediate, are short lived and their importance lies in the assignment of reaction mechanisms on the pathway from the starting substrate to stable products. These reactive intermediates are not isolated, but are detected by spectroscopic methods, or trapped chemically or their presence is confirmed by indirect evidence. There are following types of reaction intermediates which will be discussed one by one: (i) Carbocations (ii) Carbanions (iii) Free radicals (iv) Carbenes (v) Benzyne
1. CARBOCATIONS • Carbocations are formed from the heterolytic cleavage of a carbon-
heteroatom (meaning a non carbon atom in general) bond where the other atom is more electronegative than carbon like a C-O, C-N, C-X (X can be Cl, Br, I, etc) bond.
• This is quite logical as after the cleavage if a carbocation is to be
formed the two electrons of the bond must go to the other atom. And this is favored if that other atom is electronegative.
• Carbocations are the key intermediates in several reactions and
particularly in nucleophilic substitution reactions.
• Generally, in the carbocations the positively charged carbon atom is bonded to three other atoms and has no nonbonding electrons.
• These carbocations have trigonal planar shape with positively charged carbon being sp2 hybridised.
• It is sp2 hybridized with a planar structure and bond angles of about 120°.
• There is a vacant unhybridized p orbital which in the case of CH3
+ lies perpendicular to the plane of C—H bonds.
STRUCTURE OF CARBOCATIONS
• There is an increase in carbocation stability with additional alkyl substitution.
• Thus one finds that addition of HX to three typical olefins decreases
in the order (CH3)2C=CH2>CH3—CH = CH2 > CH2 = CH2 • This is due to the relative stabilities of the carbocations formed in
the rate determining step which in turn follows from the fact that the stability is increased by the electron releasing methyl group (+I), three such groups being more effective than two, and two more effective than one.
• Stability of carbocations 3°> 2° > 1° > CH3 +
• Electron release : Disperses charge, stabilizes ion
STABILITY OF CARBOCATIONS
• Further, any structural feature which tends to reduce the electron deficiency at the tricoordinate carbon stabilizes the carbocation.
• Thus when the positive carbon is in conjugation with a double bond, the stability is more. This is so, because due to resonance the positive charge is spread over two atoms instead of being concentrated on only one.
• This explains the stability associated with the allylic carbocation. The benzylic cations are stable, since one can draw canonical forms as for allylic cations.
Figure 2: Canonical forms of benzylic carbocation
• The benzyl cation stability is affected by the presence of substituents on the ring.
• Electron donating p-methoxy and p-amino groups stabilize the carbocation by 14 and 26 kcal/mole, respectively.
• The electron withdrawing groups like p-nitro destabilize by 20 kcal/mol.
2. CARBANIONS
• These are intermediates also formed as a result of heterolysis, but here the electron pair from the bond is kept by the carbon atom.
• From what we saw earlier the more electronegative atom keeps the electrons, so in this case carbon must be the more electronegative of the two atoms making up the bond.
• Now there are only a few atoms (non-metals; metals are not usually part of organic chemistry) which are less electronegative, so the most common bond cleavage which yields carbanions is the C-H bond.
STRUCTURE OF CARBANIONS
• A carbanion possesses an unshared pair of electron and thus represents a base.
• The best likely description is that the central carbon atom is sp3 hybridized with the unshared pair occupying one apex of the tetrahedron.
• Carbanions would thus have trigonal pyramidal structures similar to those of amines.
• It is believed that carbanions undergo a rapid interconversion between two pyramidal forms.
• According to the VSEPR theory, carbanion is isostructural with NH3.
Figure 3: Shape of carbanion.
• Factors which can stabilize or disperse the negative charge on carbon will stabilize a carbanion.
• The stability of carbanion depends on the following factors: (i) Inductive effect (ii) Extent of conjugation of the anion (iii) Hybridization of the charge-bearing atom (iv) Aromaticity • Stability of carbanions order: CH3
- >1°> 2° > 3°
• The alkyl groups are electron releasing in nature due to inductive effect (+I). More the number of alkyl groups attached lesser will be the stability. Carbanions prefer a lesser degree of alkyl substitution.
• Electron donation : accumulates negative charge, destabilizes ion
STABILITY OF CARBANIONS
• If negatively charged carbon is in conjugation with a double bond the resonance effects will stabilize the anion by spreading out the charge by rearranging the electron pairs. Eg. Benzyl anion:
• Stability of anion will depend upon the s character of carbanion i.e. more the s character, higher will be the stability of anion. The percentage s character in the hybrid orbitals is as follows:
sp (50%)> sp2 (33%)>sp3 (25%)
3. FREE RADICALS
• These are neutral intermediates, formed due to homolytic cleavage of a single bond.
• Some common bonds which cleave to give free radicals in organic
chemistry are shown: C-O, C-Cl, C-Br, C-I, C-C, C-H. Carbon free radicals are mainly generated by:
(i) Photolysis (action of light) like acetone alpha cleavage (ii) Other radical initiator like allylic bromination by N- Bromosuccinimide (NBS) • A free radical is a species which has one or more unpaired
electrons. In radicals, however, since there are one or more unpaired electrons, there is a net magnetic moment and the radicals as a result are paramagnetic.
STRUCTURE OF FREE RADICALS
• There has been a certain degree of debate as to what the shape and geometry of a free radical is like. Revisiting the theory of hybridization, there can be two basic shapes of these radicals.
• If the centre carbon atom of the radical is sp3 hybridized (remember the one which was made of one s and three orbitals as in CH4), the geometry will be tetrahedral.
• But in the case of a radical there are only three groups attached to the sp3 hybridized carbon atom so they we will have a shape of what resembles a pyramid—it’s a tetrahedron with its head cut off.
• So sp3 hybridized radicals are pyramidal in shape. The single electron of the radical would then be housed in a sp3 orbital. The other option is sp2 hybridization.
• In that case the C atom is sp2 hybridized, so as discussed previously the shape would be planar with the single electron in the unhybridized p-orbital with the three substituents having sp2 hybridized bonds.
Figure 4: Two different geometries of free radicals. The single electrons are shown as black dots.
STABILITY OF FREE RADICALS
• As in the case of carbocation, the stability order of free radicals is:
tertiary > secondary > primary • This is explained on the basis of hyperconjugation.
• The stabilizing effects in allylic radicals and benzyl radicals is due to
vinyl and phenyl groups in terms of resonance structures.
• The triphenyl methyl type radicals are no doubt stabilized by resonance, however, the major cause of their stability is the steric hindrance to dimerization.
• Bond dissociation energies shown that 19 kcal/mol less energy is needed to form the benzyl radical from toluene than the formation of methyl radical from methane.
4. CARBENES
• Carbenes are neutral intermediates having bivalent carbon, in which a carbon atom is covalently bonded to two other groups and has two valency electrons distributed between two non bonding orbitals.
• When the two electrons are spin paired the carbene is a singlet, if the spins of the electrons are parallel it is a triplet.
STRUCTURE OF CARBENES
• A singlet carbene is thought to possess a bent sp2 hybrid structure in which the paired electrons occupy the vacant sp2 orbital.
• A triplet carbene can be either bent sp2 hybrid with an electron in each unoccupied orbital, or a linear sp hybrid with an electron in each of the unoccupied p-orbital.
• It has however, been shown that several carbenes are in a non- linear triplet ground state.
• However, the dihalogenocarbenes and carbenes with oxygen, nitrogen and sulphur atoms attached to the bivalent carbon, exist probably as singlets.
• The singlet and triplet state of a carbene display different chemical behavior. Thus addition of singlet carbenes to olefinic double bond to form cyclopropane derivatives is much more stereoselective than addition of triplet carbenes.
Figure 5: Different types of carbenes
Generation of Carbenes: Carbenes are obtained by thermal or photochemical decomposition of diazoalkanes. These can also be obtained by a-elimination of a hydrogen halide from a haloform with base, or of a halogen from a gem dihalide with a metal.
Reactions of Carbenes: These add to carbon double bonds and also to aromatic systems and in the later case the initial product rearranges to give ring enlargement products (a car-benoids -organometallic or complexed intermediates which, while not free carbenes afford products expected from carbenes are usually called carbenoids).
5. BENZYNE
• Benzynes also known as aryne are highly reactive species derived from an aromatic ring by removal of two substituents.
• The most common arynes are ortho but meta- and para-arynes are also known.
• The alkyne representation of benzyne is the most widely encountered. o-Arynes, or 1,2-didehydroarenes, are usually described as having a strained triple bond.
• Geometric constraints on the triple bond in ortho-benzyne result in diminished overlap of in-plane p-orbitals, and thus weaker triple bond.
• The vibrational frequency of the triple bond in benzyne was assigned by Radziszewski to be 1846 cm−1, indicating a weaker triple bond than in unstrained alkyne with vibrational frequency of approximately 2150 cm−1.
• Nevertheless, ortho-benzyne is more like a strained alkyne than a biradical, as seen from the large singlet–triplet gap and alkyne-like reactivity.
REFERENCES