General Organic Chemistry

1. Electronic Displacements in Organic Molecules
(a) Inductive effect: Inductive effect may be defined as the permanent displacement of electrons forming a
covalent bond towards the more electronegative element or group.
The inductive effect is represented by the symbol, the arrow pointing towards the more electronegative
element or group of elements. Thus in case of n-butyl chloride inductive effect may be represented as below.
Any atom or group, if attracts electrons more strongly than hydrogen, it is said to have a -I effect (electronattracting
or electron-withdrawing), viz NO2, Cl, Br, I, F, COOH OCH3, etc. while if atom or group attracts
electrons less stronlgy than hydrogen it is said to have +I effect (electron repelling or electron-releasing) viz,
CH3, C2H5, Me2CH and Me3C groups. The important atoms or groups which cause negative or positive
inductive effect are arranged below in the order of decreasing effect.
-I (Electron-attracting) groups:
+I (Electron-attracting) groups:
(b) Resonance and Resonance (Mesomeric) effect.
Resonance: The phenomenon in which two or more structures can be written for the true structure of a
molecules, but none of them can be said to represent if uniquely, is referred to as resonacne or mesomerism.
The true structure of the molecule is said to be a resonance hybrid of the various possible alternative
structures which themselves are known as resonating structures or canonical structures. Every two adjacent
resonating structures are represented by inserting a double headed arrow between them. Thus the actual
structure of benzene may be represented in the following two ways.
Necessary conditions for resonance

1. All resonating structures must have the same arrangement of atomic nuclei.
2. The resonating structurs must have the same number of paird and unpaired electrons. However, they differ
in the way of distribution of electrons.
3. The energies of the various limiting structures must be sane or nearly the same.
4. Resonating structures must be planar.
All the resonating structures do not contribute equal to the real molecule and hence only the major
contributing forms are used while represeenting a resonance hybrid.
The mesomeric effect may be defined as the permanent effect in which p electrons are transfered from a
multiple bond to an atom, or from a multiple bond to a single covalent bond or lone pair(s) of p-electrons
from an atom to the adjacent single covalent bond.
Like inductive effect, the mesomeric effect (denoted by M) may be +M and -M. It is +M when the
transference of electron pair is away from the atom and -M when transference of electron pair is toward the
atom. In general.
Some common atoms or groups which cause +M and -M effect are given below
+M groups. -Cl, -Br, -I, -NH2, -NR2, -OH, -OCH3
- M groups. -NO2, -CN, >C=O
(c) Electromeric effect: This type of temporary displacement of electrons take place in compounds
containing multiple covalent bonds (e.g. C=C, C=O, C°N, etc.) or an atom with a lone pair of electrons
adjacent to a covalent bond. The effect involves complete transference of a pair of electrons from a multiple
bond to an atom, or from a multiple bond to another bond, or from an atom with a free pair of electrons to a
bond. it is the p-electrons of a multiple bond, or the p-electrons of an atom, which are transfered. Since the
effect involes complete transference of electrons, it leads to the development of full + and - charges within
the molecule. It is important to note that the electromeric effect is purely a temporary effect and is brought
into play only the requirement of attacking reagent, it vanishes out as soon as the attacking reagent is
removed from reaction mixture.


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Aldol Condensation

In some cases, the adducts obtained from the Aldol Addition can easily be converted (in situ) to α,β-unsaturated carbonyl compounds, either thermally or under acidic or basic catalysis. The formation of the conjugated system is the driving force for this spontaneous dehydration. Under a variety of protocols, the condensation product can be obtained directly without isolation of the aldol.

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Mechanism of the Aldol Condensation

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resonence

RESONANCE OR MESOMERISM IN ORGANIC CHEMISTRY

 Sometimes, it is not possible to represent the molecule or ion with only one structure. More than one structure have to be proposed. But none of them explains all the observed properties of the molecule. The solution is to write a weighted average of all the valid structures, which explains all the properties. This condition is usually referred to as resonance or mesomerism or delocalization.
The representation of structure of a molecule as a weighted average of two or more hypothetical structures, which only differ by the arrangement of electrons but with same positions for atoms is referred to as resonance.
Salient features of resonance: 
* The hypothetical structures with different arrangement of electrons but with identical positions for atoms are called resonance structures or canonical forms or contributing structures. 
* The resonance structures are only imaginary and the actual structure of the molecule is considered as the hybrid of all the valid resonance structures.
Resonance hybrid: The weighted average of contributing structures is known as resonance hybrid. It is considered as the actual structure.
* The energy of resonance hybrid is always less than the energy of any of the contributing resonance structure. 
* The resonance structures are formed (only on paper!?) due to delocalization of electrons and not by changing the positions of atoms.
* The delocalization of electrons is shown using curved arrows.
One should keep in mind that the individual resonance structures do not exist and the molecule do not resonate (switch back and forth) between these structures. The actual molecule is simply the hybrid of all these imaginary resonance structures. Hence the delocalization of electrons is also imaginary process which helps in understanding the resonance.
Illustration: 
From the following structure (I) - representing urea, 

it is expected that: 

1) It should be a diacidic base, 

2) The bond length of C-N bond is equal to the normal C-N bond length and

3) No dipole moment since it is symmetrical. 

However it is observed that: 

1) Urea is a monoacidic base,

2) It has shorter than expected bond length for C-N bond and

3) It shows dipole moment. 

Hence, to account for above observations,  the actual structure of urea is represented as a resonance hybrid of following resonance structures. 

Note: The contributing structures are always shown to be linked by using double headed arrows ().
 

RULES THAT HELP IN WRITING VALID RESONANCE STRUCTURES

The valid resonance structures must satisfy the following rules: 
* They must be valid Lewis structures obeying octet rule. 
E.g. Carbon or Nitrogen with five bonds is not allowed.
In the structure (II), the nitrogen atom violated the octet. It has 10 electrons around it.

 

* They should possess same number of electrons and equal net charge.
* The number of unpaired electrons in them must be same. 
E.g. Following structure for butadiene is not valid. 

* The positions of atoms should be same in all the resonance structures.
E.g. The following are not the resonance structures, since the position of one hydrogen atom is not same. Indeed, they are different molecules, which are in dynamic equilibrium with each other. These are called tautomers. Also note that these molecules are linked by two half headed arrows and not by a single double headed arrow.

 

* The bond order of two connecting atoms may vary between two different resonance structures. 
* The resonance structures may or may not be equivalent. 
* The atoms that are part of the delocalized system must be arranged in one plane or nearly so. The reason is to get maximum overlap between the orbitals.
 

STABILITY OF RESONANCE STRUCTURES 

* The actual structure i.e., resonance hybrid of a molecule has lower energy than any of the contributing form and hence the resonance is a stabilizing phenomenon.
* Greater the number of contributing structures, greater is the stability of the resonance hybrid. 
* All the structures do not contribute equally to the hybrid. 
* Greater the stability of a resonance structure, larger is its contribution to the resonance hybrid.
 
Rules to decide the major contributor to the hybrid in the decreasing order of preference are given below: 
* The contributing structures that have atoms with full octets are more stable than the ones with open octets. 
* The contributing structure with more covalent bonds is more stable. 
E.g. Among the following, the structure II is more stable since all the atoms have octet configuration and there are more covalent bonds.

* Resonance structures with fewer charges are more stable than those with more charges.
E.g. The second structure with two negative charges is not only less stable.

* The structure with less charge separation is more stable. 
E.g. Among the following resonance forms of phenol, the structure I is more stable since it has no charge. Whereas the structures II and IV have less charge separation and are more stable than the structure III.

* The structure with charge dispersal or delocalization over more number of atoms is more stable. 
* The structures in which the atoms bearing the conventional charges are more stable i.e., the more electronegative atom should bear negative charge while the relatively less electronegative atom should bear positive charge. 
E.g.

 
Resonance stabilization energy: The difference between the energy of resonance hybrid and that of most stable resonance structure of a molecule is known as the resonance stabilization energy of that molecule.
 

EXAMPLES OF RESONANCE STRUCTURES

1) The following resonance structures can be written for benzene which are hypothetically possible due to delocalization of π electrons. The Kekule structures have more weightage than Dewar structures. 

 

The actual structure of benzene is thus shown to be the hybrid of these contributing structures. The bond order of every C-C bond is 1.5 and hence the every C-C bond length is reported to be same and equals to 1.39 A^o, which is in between the bond length values of C-C single bond (1.54 A^o) and C=C double bond (1.20 A^o).
Due to resonance, benzene gets extra stability and does not undergo electrophilic addition reactions. However it shows electrophilic substitution reactions. This phenomenon is known as aromaticity.

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 INDUCTIVE EFFECT :-

The C-Cl bond in the butyl chloride, CH₃-CH₂-CH₂-CH₂-Cl is polarized due to electronegativity difference. The electrons are withdrawn by the chlorine atom. Thus the first carbon atom gets partial positive charge. In turn, this carbon atom drags electron density partially from the next carbon, which also gets partial positive charge. Thus the inductive effect is transmitted through the carbon chain. 

But the inductive effect weakens away along the chain and is not significant beyond 3rd carbon atom.

TYPES OF INDUCTIVE EFFECT

The inductive effect is divided into two types depending on their strength of electron withdrawing or electron releasing nature with respect to hydrogen. 
1) Negative inductive effect (-I): The electron withdrawing nature of groups or atoms is called as negative inductive effect. It is indicated by -I. Following are the examples of groups in the decreasing order of their -I effect: 
NH₃⁺ > NO₂ > CN > SO₃H > CHO > CO > COOH > COCl > CONH₂ > F > Cl > Br > I > OH > OR > NH₂ > C₆H₅ > H 
 
2) Positive inductive effect (+I): It refers to the electron releasing nature of the groups or atoms and is denoted by +I. Following are the examples of groups in the decreasing order of their +I effect. 
C(CH₃)₃ > CH(CH₃)₂ > CH₂CH₃ > CH₃ > H
Why alkyl groups are showing positive inductive effect?
Though the C-H bond is practically considered as non-polar, there is partial positive charge on hydrogen atom and partial negative charge on carbon atom. Therefore each hydrogen atom acts as electron donating group. This in turn makes an alkyl group, an electron donating group.

 

APPLICATIONS OF INDUCTIVE EFFECT

Stability of carbonium ions: 
The stability of carbonium ions increases with increase in number of alkyl groups due to their +I effect. The alkyl groups release electrons to carbon, bearing positive charge and thus stabilizes the ion. 
The order of stability of carbonium ions is : 

Stability of free radicals: 
In the same way the stability of free radicals increases with increase in the number of alkyl groups. 
Thus the stability of different free radicals is:

Stability of carbanions: 
However the stability of carbanions decreases with increase in the number of alkyl groups since the electron donating alkyl groups destabilize the carbanions by increasing the electron density. 
Thus the order of stability of carbanions is:
 
Acidic strength of carboxylic acids and phenols: 
The electron withdrawing groups (-I) decrease the negative charge on the carboxylate ion and thus by stabilizing it. Hence the acidic strength increases when -I groups are present. 
However the +I groups decrease the acidic strength. 
E.g. 
i) The acidic strength increases with increase in the number of electron withdrawing Fluorine atoms as shown below. 
CH₃COOH < CH₂FCOOH < CHF₂COOH < CF₃COOH 
ii) Formic acid is stronger acid than acetic acid since the –CH₃ group destabilizes the carboxylate ion. 
On the same lines, the acidic strength of phenols increases when -I groups are present on the ring. 
E.g. The p-nitrophenol is stronger acid than phenol since the -NO₂ group is a -I group and withdraws electron density. Whereas the para-cresol is weaker acid than phenol since the -CH₃ group shows positive (+I) inductive effect. 
Therefore the decreasing order of acidic strength is:

Basic strength of amines: 
The electron donating groups like alkyl groups increase the basic strength of amines whereas the electron withdrawing groups like aryl groups decrease the basic nature. Therefore alkyl amines are stronger Lewi bases than ammonia, whereas aryl amines are weaker than ammonia. 
Thus the order of basic strength of alkyl and aryl amines with respect to ammonia is :CH₃NH₂ > NH₃ > C₆H₅NH₂ 

Reactivity of carbonyl compounds:
The +I groups increase the electron density at carbonyl carbon. Hence their reactivity towards nucleophiles decreases. Thus formaldehyde is more reactive than acetaldehyde and acetone towards nucleophilic addition reactions. 
Thus the order of reactivity follows:

 

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confermational analysis

Conformational analysis of ethane

A conformational analysis is a study of the energetics of different spatial arrangements of atoms relative to rotations about bonds. Conformational analyses are assisted greatly by a representation of molecules in a manner different to skeletal structures. A representation of a molecule in which the atoms and bonds are viewed along the axix about which rotation occurs is called a Newman projection (Figure 1).

Figure 1: Newman projection of ethane
In a Newman projection, the molecule is viewed along an axis containing two atoms bonded to each other and the bond between them, about which the molecule can rotate. In a Newman projection, the "substituents" of each atom composing the bond, be they hydrogens or functional groups, can then be viewed both in front of and behind the carbon-carbon bond. Specifically, one can observe the angle between a substituent on the front atom and a substituent on the back atom in the Newman projection, which is called the dihedral angle or torsion angle.
In ethane specifically, we can imagine two possible "extreme" conformations. In one case, the dihedral angle is 0° and the hydrogens on the first carbon line up with or eclipse the hydrogens on the second carbon. When the dihedral angle is 0° and the hydrogens line up perfectly, ethane has adopted the eclipsed conformation (Figure 2). The other extreme occurs when the hydrogens on the first carbon are as far away as possible from those on the second carbon; this occurs at a dihedral angle of 60° and is called the staggered conformation (Figure 2).

Figure 2: Conformations of ethane - staggered and eclipsed
The staggered conformation of ethane is a more stable, lower energy conformation than the eclipsed conformation because the eclipsed conformation involves unfavorable interactions between hydrogen atoms. Specifically, the negatively charged electrons in the bonds repel each other most when the bonds line up. Thus, ethane spends most of its time in the more stable staggered conformation.

Conformational analysis of butane

In butane, two of the substituents, one on each carbon atom being viewed, is a methyl group. Methyl groups are much larger than hydrogen atoms. Thus, when eclipsed conformations occur in butane, the interactions are especially unfavorable. There are four possible "extreme" conformations of butane: 1) Fully eclipsed, when the methyl groups eclipse each other; 2) Gauche, when the methyl groups are staggered but next to each other; 3) Eclipsed, when the methyl groups eclipse hydrogen atoms; and 4) Anti, when the methyl groups are staggered and as far away from each other as possible (Figure 3).

Figure 3: Conformations of butane: fully eclipsed, gauche, eclipsed, and anti
The fully eclipsed conformation is clearly the highest in energy and least favorable since the largest groups are interacting directly with each other. As the molecule rotates, it adopts the relatively stable gauche conformation. As it continues to rotate, it encounters less favorable eclipsed conformation in which a methyl group eclipses a hydrogen. As rotation continues, the molecule comes to the anti conformation, which is the most stable since the substituents are staggered and the methyl groups are as far away from each other as possible. An energy diagram is a graph which represents the energy of a molecule as a function of some changing parameter. An energy diagram can be made as a function of dihedral angle for butane (Figure 4).

Figure 4: Energy diagram for conformations of butane as a function of dihedral angle
Clearly, the anti and gauche conformations are significantly more stable than the eclipsed and fully eclipsed conformations. Butane spends most of its time in the anti and gauche conformations.
There is one final, very important point. At room temperature, approximately 84 kJ/mol of thermal energy is available to molecules. Thus, if the barrier to a rotation is less than 84 kJ/mol, the molecule will rotate. In ethane and butane, the barriers to rotation are significantly less than 84 kJ/mol. Therefore, even though the eclipsed conformations are unfavorable, the molecules are able to adopt them. In reality, since these conformations are not as stable, the molecules will quickly pass through them at room temperature and return a staggered conformation. Molecules are constantly converting between different staggered conformations all the time, quickly passing through eclipsed conformations in between. Thus, in alkanes, no single "true" conformation exists all the time; the molecule instead constantly converts between conformations, spending more time in those that are more stable. This constant conversion lies in stark contract to alkenes, which adopt the cis- or trans- (E- or Z-) conformations and retain them at room temperature; they do not interconvert because the barrier to rotation is too high.

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Isomers