MECHANISM OF ORGANIC REACTIONS
Conversion of a chemical compound into another chemical
compound is known as a chemical reaction. Organic reactions usually involve the
fission and formation of covalent bonds. Organic reactions take place in
accordance with general fundamental concepts of thermodynamics. Usually, the sequence
and timing of these bonds breaking and bond forming processes are important in
the study of these reactions. These processes may occur separately, for
example, bond breaking may proceed bond formation or vice-versa resulting
in a step-wise reaction.
ARROW NOTATION (FORMALISM)
Organic reactions usually involve the fission and formation
of covalent bonds. The covalent bond is often represented as a dash (-) and the
movement of a pair of electrons is shown by a curved arrow in fact,
arrow formalism (notation) is very important for organic reactions. Arrows in
chemical drawing have specific meanings. Just as it is important to learn the
structural representation and names of molecules, it is important to learn the
language of arrow formalism in organic chemistry.
- The curved arrows are used to show the movement of electrons in reactions and in resonance structures. Therefore, curved arrows always start at the initial position of electrons and end at their final position.
It is crucial to remember that it represents the formal flow
of electrons, not the flow of atoms.
Straight arrows with half-heads are commonly used in pairs to
indicate that reaction is reversible.
A double-headed straight arrow between two structures
indicates that they are resonance structures. Such an arrow does not indicate
the occurrence of a chemical reaction.
As mentioned earlier, in most reactions of organic compounds one or more covalent bonds are broken. Organic reaction mechanism thus may be divided into three basic types, depending on how the bonds are broken.
BREAKING (FISSION) OF COVALENT BONDS
There are following two types of covalent bond breaking or fission:
(i) Heterolytic bond fission or heterolysis: If a
covalent bond is broken in such a way that both bonding pair of electrons are
taken by only one fragment then the breaking of bond is called heterolytic bond
fission or heterolysis.
The covalent bond is represented by dash (-) and the
movement of pair of electrons is shown by (curved arrow).
As a result of heterolysis ions are formed. It is the more
electronegative atom which takes both the bonding pair of electrons.
Heterolysis usually takes place in polar compounds and in polar solvents.
(ii) Homolytic bond fission or homolysis: If a covalent bond is broken in such a way that each fragment takes one of the bonding pair of electrons then the breaking of bond is called homolytic bond fission or homolysis.
As a result of homolysis free radicals are formed which have
unpaired electron(s). Homolysis is usually brought about by heat, light or
organic peroxides. It is the most common mode of bond fission in the vapour
phase.
It would seem that all bonds must break in one of the two ways described above to give ions or free radicals but there is a third type of mechanism in which electrons move in a close ring. There are no intermediates, ions or free radicals, and it is impossible to say whether the migrating electrons are paired or unpaired. Reactions with this type of mechanism are called Pericyclic reactions.
TYPES OF REAGENTS
Heterolysis of organic compounds is usually brought about by
certain reagents. For a large number of reactions, it is convenient to call one
reactant as substrate (which supplies carbon to the new bond) and the other as attacking
reagent. In heterolytic reactions the attacking reagent generally brings a
pair of electrons to the substrate or takes a pair of electrons from the
substrate. A reagent (reagent means attacking reagent) that brings an electron
pair is called a nucleophile and the reaction is known is nucleophilic
reaction. For example,
A reagent that takes an electron pair is called electrophile
and the reaction is known as electrophilic reaction. For example:
In a reaction in which the substrate molecule becomes cleaved, the part that does not contain carbon is usually called leaving group. The leaving group that carries electron pair is called nucleofuge. If the leaving group comes away without the electron pair, it is called electrofuge. For Example:
Nucleophilic reagents or Nucleophiles
Reagents having an unshared pair of electrons are known as
nucleophilic reagents or nucleophiles because they have a tendency to share
this pair of electrons with electron deficient substrate. Thus, nucleus loving
species are known as nucleophiles. Nucleophiles can be classified into three
groups:
(i) Neutral Nucleophiles: Electron rich species due to the presence of non-bonding electrons are known as neutral nucleophiles. Central atom of such species should have complete octet. They are not charged and are electrically neutral. For example:
Note: Organic compounds having C-C multiple bond(s)
also behave as neutral nucleophiles because such species have ⊼ electron clouds above and below the plane of
the molecule.
The addition of neutral nucleophile to a positively charged substrate will give a positively charged product.
Negative Nucleophiles: Negative nucleophiles are those which carry an electron pair and are negatively charged because of one extra electron. For example:
Addition of a negative nucleophile to a positively charged substrate results in a neutral molecule.
Ambident Nucleophiles: The nucleophiles which can attack through two or more atoms are called ambident nucleophiles. For example, CN⊝ can attack through N or C to give cyanide (RCN) or isocyanide (RNC). Other examples of ambident nucleophiles are:
Electrophilic Reagents or Electrophiles
An electrophile (electron loving) is a reagent that is
electron deficient and the deficiency accounts for its affinity for electrons.
Electrophiles can also be of two types :
(i) Neutral Electrophiles: The neutral electrophiles, though electron deficient, do not carry positive charge. They have incomplete valence shells. The examples are AICI3, BF3, CH2 (carbene), etc. They are electron deficient due to incomplete outermost shell.
(ii) Positive Electrophiles: The positive electrophiles are those which carry a positive charge on central atom and have incomplete octet.
The positive electrophile attacks the substrate (negative nuc1eophile) and accepts an electron pair for sharing, thus forming a neutral molecule. On the other hand, a neutral electrophile will attack an electron rich substrate (negative nucleophile) to produce a negatively charged molecule.
REACTION INTERMEDIATES
Homolytic and heterolytic bond fission results in the
formation of short-lived species called reaction intermediates. These
short-lived intermediates are very reactive and are quickly converted to more
stable molecules. However, under certain circumstances they are of sufficient
stability to be isolated and studied. The four most common kinds of reaction
intermediates with fewer than four covalent bonds at carbon are :
Carbocations
All the cations in which the positive charge is carried by a
carbon are called carbocations or carbonium ions. These are
reactive intermediates in a large number of organic reactions. The reason for
their reactivity is that the carbon has only six electrons in the valency shell
and has a great tendency to complete its octet.
Structure of Carbocations
The carbon atom of carbocation is sp2 hybridised in which the p orbital is devoid of any electron. The three bonding orbitals form three Cf bonds, the remaining p orbital is vacant. This vacant p orbital makes the carbon atom electron deficient and gives it a positive charge. The carbocation thus has a planar (flat) structure having all the three Cf bonds in one plane with the bond angles of 120° between them.
Stability of carbocations
The cycloheptatrienyl cation is an exceptionally stable
carbocation which can be isolated and studied. At the other extreme is the
methyl carbocation, which is so unstable that it has never even been detected
as an intermediate in any ordinary chemical reaction. In between these two
extremes there exist a whole spectrum of carbocations; some can be isolated,
some are detectable but not isolable and others can be generated only under
extreme conditions.
Some carbocations are listed below in order of their decreasing relative stability (in solution):
The relative stabilities of the first five cations are
easily understood on the basis of resonance. When the positively charged carbon
atom is in conjugation with 1t-bond, the stability is greater because of
increased delocalization due to resonance 'and because positive charge is
spread over the whole molecule instead of being concentrated on one atom.
The allyl carbocation has only two resonating forms, but
both resonance forms are equivalent. Because of this, this ion has a stability
similar to benzyl cation.
The order of stability of alkyl carbocations is as follows :
Tertiary > Secondary > Primary
The stability order of these carbocations can be explained by hyperconjugation and by field effect (Inductive effect).
Formation of Carbocations
Carbocations can be generated in a variety of ways. Some of the reactions forming carbocations are given below:
(i)
Direct ionisation:
(ii) Protonation of alkenes and carbonyl compounds
(iii) Protonation of alcohols followed by dehydration
(iv) Deamination of amines with nitrous acid
Reactions of Carbocations
Although a carbocation undergoes a variety of reactions, the common goal of all these reactions is to provide a pair of electrons to complete the octet of the positively charged carbon. A carbocation may react in anyone of the following ways:
(a) Combination with a nucleophile
(b) Elimination of a proton
(c) Rearrangement to form a more stable carbocation: carbocations also formed a stable form by the rearrangement of reaction.
Carbanions
All the anions in which the negative charge is carried by a
carbon are called carbanions. For example:
Structure of Carbanions
A carbanion possesses an unshared pair of electrons and three pairs of bonding electrons around the central carbon atom which is sp3 hybridised. The shape of this anion is pyramidal, similar to that of ammonia and amine molecules. Out of four sp3 hybrid orbitals, three form pi bonds and the fourth is a nonbonding orbital containing two paired electron.
Formation of Carbanions
The carbanions are obtained by heterolysis of a covalent bond in which both the bonding electrons are taken by the carbon. Usually departing constituent is a proton. The heterolytic cleavage of alkanes are very difficult process. As there is very little difference between electronegativities of carbon and hydrogen, the polarity of C-H bond is very small. In other words, hydrogen atom bonded to sp3 carbon atom shows negligible acidity. Carbanions are formed only from those compounds which contain a functional group; capable of weakening a nearby C-H bond. Thus hydrogens on the carbon atom alpha to nitro, cyano, carboxylic, carbonyl and triple bond have acidic character (weak C-H bond) and can be removed as protons leaving resonance stabilised anions. For example, nitromethane is sufficiently acidic to react with aqueous alkali because the anion formed has a high degree of stability.
The stability of this carbanion is largely a result of the
electronegativity of the oxygen atoms of the nitro group. There is stabilizing
-I inductive effect as well as a powerful resonance effect, both of which
withdraw negative charge from the carbon into the nitro group. Electronegative
atoms or groups, depending on their ability to withdraw electrons will lead to
more or less stable carbanions. The groups are listed in order of decreasing
electron withdrawing ability.
Stability of Carbanions
Electron-withdrawing atoms or groups stabilise a carbanion
by dispersing (delocalising) its negative charge. The following is the order of
stability of simple alkyl carbanions:
methyl > primary > secondary > tertiary
This stability order can be attributed to +1 effect
of the alkyl groups. The electron-donating alkyl groups increase the electron
density on the carbanionic carbon; hence its negative charge is intensified and
the carbanion is destabilised. This destabilisation increases as the number of
alkyl groups attached to the carbanionic carbon increases. This explains the
above order of stability of carbanions. Many carbanions are far more stable
than the simple kind of carbanions mentioned above. Their increased stability
is due to certain structural features. Such types of carbanions are generally
stabilised by electron-withdrawing substituents and resonance or only by
resonance.
(i) Resonance: Stability of allylic, benzylic and aromatic carbanions can be explained by resonance.
In these cases, a multiple (double or triple) bond is located alpha to carbanionic carbon, the ion is stabilised by resonance in which the unshared pair overlaps with the 1t electrons of the double bonds. This factor is responsible for the stabilisation of allylic and benzylic type of carbanions.
(ii)Resonance and -1 effect: When the carbanionic carbon is
conjugated with a carbon-oxygen or carbon-nitrogen multiple bond (see formation
of carbanions) the stability of carbanion is greater than that of benzyl and
allylic type carbanions. In these cases, there is a sizable inductive effect as
well as a powerful resonance effect, both of which withdraw negative charge
from carbon into more electronegative atoms. These electronegative atoms are
better capable of bearing a negative charge than carbon.
(iii) The stability of carbanions increases with increasing s character of the carbanionic carbon. Thus, the order of stability is:
Increase s character means
that the electrons are closer to the nucleus and hence are of lower energy.
Reactions of Carbanions
The most common reaction of carbanions is combination with a positive species (electrophile) usually a proton.
Carbanions frequently add to the carbonyl double bond (e.g., aldol condensation)
Free Radicals
Chemical species having one or more unpaired electrons are called free radicals. As mentioned earlier homolytic bond fission leads to the formation of free radicals. Thus free radicals are odd electron molecules, e.g.,
and are highly reactive. Free radicals are paramagnetic, i.e., possess
a small permanent magnetic moment, due to the presence of unpaired electron.
This property is used for detection of the presence of free radicals.
Structure of Free Radicals
Similar to carbanions, carbon containing an unpaired electron(s) in free radicals also may either be in sp2 hybrid state in which case the structure would be planar with odd electron in the p orbital or it could be sp3 hybridised which would make the structure pyramidal (non-planar) and the odd electron will be in one of the sp3 orbitals.
Physical and chemical evidences
(E.S.R and stereochemistry) point to a planar configuration at the radical
centre for simple alkyl radicals but not for fluorinated derivatives (e.g., CF3)
which prefer pyramidal shape. In resonance stabilised free radicals the radical
carbon is certainly in sp2 hybrid state.
Formation of Free Radicals
Free radicals are formed by
homolytic fission of a covalent bond, when a molecule is supplied with
sufficient energy--thermal or photochemical.
(i) Thermal cleavage: Most of the covalent bonds are thermally stable up to a temperature of 200°C, used in solution chemistry. However, there are a few group of compounds (peroxy and azo) which undergo homolytic cleavage at temperatures below 200°C. These are designated as initiators.
(ii) Photochemical cleavage: A second general method of obtaining radical is through irradiation with either UV or visible light. The energy transferred to the molecule by the interaction must be of the order of bond dissociation energy or greater to produce homolysis.
(iii) Chemical method
Stability of Free Radicals
Most simple free radicals, such as methyl and t-butyl radicals, are highly reactive species. Even if they are kept out of contact with various other substances they cannot be obtained in any appreciable concentration because they under go dimerization and disproportionation:
These two reactions are nearly
instantaneous and irreversible for a free radical to have a longer lifetime, it
would seem necessary that the stability of the radical be at least comparable
with that of the covalent compound it would form on dimerization.
The stability of alkyl free
radicals is of the following order
Tertiary >Secondary> Primary> Methyl
The stability order of free
radicals can be explained by hyperconjugation.
pi electrons of a alpha C-H
bond can delocalise with the half-filled p-orbital of the carbon atom containing
odd electron, thus spreading the odd electron over all such bonds and thereby
stabilising the radical to some extent.
Reactions of Free Radicals
The most common reactions of free radicals are the
substitution and addition, for example, the halogenation of alkanes in the
presence of light and the addition of HBr in the presence of a peroxide.
Halogenation of alkanes:
Free radical addition of HBr :
Free radicals also undergo combinations of same or different radicals, or disproportionation giving a saturated and an unsaturated molecule.
Carbenes
Carbenes are neutral, divalent, highly reactive carbon
intermediates containing two single bonds and two non-bonding electrons around
a carbon (X-C-Y).
The simplest carbene is methylene (CH2). Other carbenes are simply named as substituted derivatives of methylene. For example:
Structure of Carbenes
The carbene carbon is usually considered to be sp2 hybridised. There are two bonding electrons (both in sp2 orbitals) and two non-bonding electrons associated with the carbene carbon. Since the non-bonding electrons can have their spins paired or parallel, there is possibility of two electronic arrangements or spin states. Depending on this there are two kinds of carbenes: the singlet carbene and the triplet carbene. The singlet state carbene has the spins of its non-bonding electrons paired (spin multiplicity M = 1)*. This non-bonding electron pair is in a sp2 orbital leaving a vacant p-orbital.
In the triplet carbene the two non-bonding electrons have parallel spins (spin multiplicity M = 3) and both the sp2 and p-orbitals contain one electron each.
Hence triplet carbenes with their unpaired electrons exhibit
properties of diradicals, and can be detected by ESR (electron spin resonance)
spectroscopy.
Some carbenes have linear structure. In such cases, the carbene carbon will be in sp hybrid state. Two p orbitals remain unhybridized, and the two non-bonded electrons go into each with spin parallel according to Hund's rule because these p orbitals would be degenerate. This carbene will be sp triplet.
Stability of Carbenes
Generally, a triplet carbene (e.g., CH2) is 7.7-10 kcal/mole more stable than the singlet carbene (CH2). The triplet carbene has a lower energy because with two electrons in different orbitals there is less electron-electron repulsion than that when both are in the same orbital. However, the nature of substituents has an important effect on the stability of carbenes. For example, as the substituents on the carbene carbon become better pi electron donors, the stability of the singlet state increases and the singlet state carbene becomes more stable than the triplet state carbene. Thus, halocarbenes are more stable in the singlet state than in the triplet state. This is because a p-orbital of the halogen (X) with a lone pair of electrons can overlap laterally with the vacant p-orbital of the singlet carbet1f.', thereby stabilising the singlet state. This stabilisation is not possible in the triplet state whose p-orbitals are not vacant. The resonating structures are:
Amongst dihalocarbenes, the most stable is the singlet
difluorocarbene. This can be explained as follows: Since fluorine and carbon
are in the same period, their p-orbitals are of about the same size permitting
more efficient overlapping. Furthermore, of the C-X bonds, the C-F bond length
is the shortest which again provides far more extensive overlap of the
respective p-orbitals. On this basis
it can be concluded that the singlet CF2 is more stable than
CCl2 which is more stable than CBr2.
Formation of Carbenes
Carbenes are usually generated from precursors by the loss of small, stable molecules.
(i) From diazo compounds: Diazo compounds easily decompose thermally and photochemically to give carbenes.
The most common diazo compound is diazomethane.
(ii) From ketones
Reactions of Carbenes
Carbenes are electron deficient intermediates because the
carbene carbon has only six electrons in its valency shell. Therefore, in
general, carbenes are highly electrophilic in their reactions. -I group present
on carbene carbon increases electron deficiency and hence increases the
electrophilic character of the carbene.
Carbenes undergo two principal reactions, viz.,
cycloaddition with alkenes, and insertion into a sigma-bond.
Cycloaddition to Alkenes
Addition of carbenes to alkenes gives cyclopropane derivatives. A concerted mechanism is possible for singlet carbenes, and they add stereo specifically to alkenes. As a result, the stereochemistry present in the alkene is retained in the cyclopropane.
Rearrangements of Carbenes
Alkylcarbenes can undergo rearrangements with migration of hydrogen or alkyl group. These rearrangements are so rapid that insertion reactions and addition to double bonds, which are very common to : CH2' are seldom occur with alkyl or dialkylcarbenes.
In this rearrangement migratory power of the groups in
decreasing order is : H >> aryl> alkyl.
Nitrenes
Chemical species having a neutral, monovalent nitrogen are known as nitrenes. Nitrenes are nitrogen analogues of carbenes. Substituted nitrenes are simply named as substituted derivatives of nitrenes. For example:
Structure of Nitrenes
Similar to carbenes, there are two possible spin states in which nitrenes can exist, i.e., the singlet state (the two non-bonding electrons have their spins paired) and the triplet state (the two non-bonding electrons have parallel spins). In both the cases the normal lone pair remains paired.
In general, nitrenes obey Hund's rule and are ground state
triplets with two degenerate p-orbitals
containing a single electron each, and the lone pair in the sp-orbital.
Nitrenes also exist in singlet state. Nitrogen atom in the singlet state is usually represented as sp2 hybridised.
Stability of Nitrenes
The energy difference between the singlet and triplet states
is usually much larger for nitrenes than for carbenes, i.e., 34.5
kcal/mole for nitrene (: NH) and 7.7-10 kcal/mole for carbene (CH2>. This
energy difference is due to the electronegativity difference in carbon and
nitrogen. Nitrogen is more electronegative than carbon, therefore holds
its electrons closer to the nucleus which decreases energy and hence
increases stability. Strong pi donor substituents such as amino groups
greatly stabilise the singlet state as well as cause the nitrene to
exhibit nucleophilic character. Such nitrenes are ground state singlet.
Nitrenes are characterized by spectroscopy mainly UV and IR.
Nitrene (: NH) is very transient species and it is extensively characterized by
UV spectroscopy. It shows absorption maximum at 336 nm.
Formation of Nitrenes
(i) From azides
The most common method for generating nitrene intermediates is the photolysis or thermolysis of azides.
In this reaction azides may be alkyl, aryl, acyl, or sulphonyl.
(ii) From Sulphinylamines
Sulphinylamines on thermolysis give nitrenes.
Sulphinylamines are readily prepared from aniline and thionyl chloride.
(iii) From nitro and nitroso compounds
(iv) From small ring compounds
TYPES OF ORGANIC REACTIONS
Any classification of organic reactions must emphasize the
changes that occur in the carbon bondings of the substrate at the site of the
reaction. Thus a large variety of organic reactions may be placed in the
following major categories :
(1) Substitution reactions (2) Addition reactions (3)
Elimination reactions
(4) Molecular rearrangements (5) Molecular reactions
Study of Catalysis
A very important question regarding any mechanism is whether
or not it is catalysed or inhibited in any way. What is the effect of, for
example, heat, light, acid strength or solvent? For example, does the reaction
require the presence of acids, metal or peroxides? The answers to these
questions provide valuable information about the mechanism of a reaction.
Examples of reactions that do require catalysts include free-radical substitution and hydrogenation of alkenes. In the former, heat or light is necessary, and a metal is necessary in the latter:
A catalyst speeds-up a reaction by providing an alternate
reaction pathway that involves but does not consume the catalyst. This
alternate route has a lower 11G* and is therefore a more rapid reaction. Of
course, just as a mechanism must be compatible with the products, so must it be
compatible with its catalysts.
Comments
Post a Comment