Monday, 29 April 2013

Nucleophiles and Electrophiles

A reagent that brings an electron pair is called a nucleophile (Nu:) i.e., nucleus seeking and the reaction is then called nucleophilic. A reagent that takes away an electron pair is called electrophile (E+) i.e., electron seeking and the reaction is called electrophilic.
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. Some examples of nucleophiles are the negatively charged ions with lone pair of electrons such as hydroxide (HO- ), cyanide (NC-) ions and carbanions (R3C:-). Neutral molecules such as etc., can also act as nucleophiles due to the presence of lone pair of electrons. Examples of electrophiles include carbocations (C+H3) and neutral molecules having functional groups like carbonyl group (>C=O) or alkyl halides (R3C-X, where X is a halogen atom). The carbon atom in carbocations has sextet configuration; hence, it is electron deficient and can receive a pair of electrons from the nucleophiles. In neutral molecules such as alkyl halides, due to the polarity of the C-X bond a partial positive charge is generated on the carbon atom and hence the carbon atom becomes an electrophilic centre at which a nucleophile can attack.
Problem 12.11
Using curved-arrow notation, show the formation of reactive intermediates when the following covalent bonds undergo heterolytic cleavage.
(a) CH3-SCH3, (b) CH3-CN,(c) CH3-Cu

Problem 12.12
Giving justification, categorise the following molecules/ions as nucleophile or electrophile:
Nucleophiles: HS-,C2H5O-,(CH3)3N:,H2N:-
These species have unshared pair of electrons, which can be donated and shared with an electrophile.
Electrophiles: BF3,Cl+,CH3-C=O,N+O2.
Reactive sites have only six valence electrons; can accept electron pair from a nucleophile.
Problem 12.13
Identify electrophilic centre in the following: CH3CH=O, CH3CN, CH3I.
Among CH3HC*=O, H3CC*≡N, and H3C*-I, the starred carbon atoms are electrophilic centers as they will have partial positive charge due to polarity of the bond.
12.7.3 Electron Movement in Organic Reactions
The movement of electrons in organic reactions can be shown by curved-arrow notation. It shows how changes in bonding occur due to electronic redistribution during the reaction. To show the change in position of a pair of electrons, curved arrow starts from the point from where an electron pair is shifted and it ends at a location to which the pair of electron may move.
Presentation of shifting of electron pair is given below :

Movement of single electron is indicated by a single barbed ‘fish hooks’ (i.e. half headed curved arrow). For example, in transfer of hydroxide ion giving ethanol and in the dissociation of chloromethane, the movement of electron using curved arrows can be depicted as follows:

12.7.4 Electron Displacement Effects in Covalent Bonds
The electron displacement in an organic molecule may take place either in the ground state under the influence of an atom or a substituent group or in the presence of an appropriate attacking reagent. The electron displacements due to the influence of an atom or a substituent group present in the molecule cause permanent polarlisation of the bond. Inductive effect and resonance effects are examples of this type of electron displacements. Temporary electron displacement effects are seen in a moleculewhen a reagent approaches to attack it. This type of electron displacement is called electromeric effect or polarisability effect. In the following sections we will learn about these types of electronic displacements.
12.7.5 Inductive Effect
When a covalent bond is formed between atoms of different electronegativity, the electron density is more towards the more
electronegative atom of the bond. Such a shift of electron density results in a polar covalent bond. Bond polarity leads to various electronic effects in organic compounds.
Let us consider cholorethane (CH3CH2Cl) in which the C-Cl bond is a polar covalent bond. It is polarised in such a way that the carbon-1 gains some positive charge (δ+) and the chlorine some negative charge (δ-). The fractional electronic charges on the two atoms in a polar covalent bond are denoted by symbol δ (delta) and the shift of electron density is shown by an arrow that points from δ+ to δ- end of the polar bond.

In turn carbon-1, which has developed partial positive charge (δ+) draws some electron density towards it from the adjacent
C-C bond. Consequently, some positive charge (δδ+) develops on carbon-2 also, where δδ+ symbolises relatively smaller positive charge as compared to that on carbon – 1. In other words, the polar C – Cl bond induces polarity in the adjacent bonds. Such polarisation of σ-bond caused by the polarisation of adjacent σ-bond is referred to as the inductive effect. This effect is passed on to the subsequent bonds also but the effect decreases rapidly as the number of intervening bonds increases and becomes vanishingly small after three bonds. The inductive effect is related to the ability of substituent(s) to either withdraw or donate electron density to the attached carbon atom. Based on this ability, the substitutents can be classified as electron-withdrawing or electron donating groups relative to hydrogen. Halogens and many other groups such as nitro (- NO2), cyano (- CN), carboxy (- COOH), ester (-COOR), aryloxy (-OAr, e.g. – OC6H5), etc. are electron-withdrawing groups. On the other hand, the alkyl groups like methyl (-CH3) and ethyl (-CH2-CH3) are usually considered as electron donating groups.
Problem 12.14
Which bond is more polar in the following pairs of molecules: (a) H3C-H, H3C-Br (b) H3C-NH2, H3C-OH (c) H3C-OH, H3C-SH
(a)C-Br, since Br is more polar electronegetive then H, (b) C-O, (c) C-O
Problem 12.15
In which C-C bond of CH3CH2CH2Br, the inductive effect is expected to be the least?
Magnitude of inductive effect diminishes as the number of intervening bonds increases. Hence, the effect is least in the
bond between carbon-3 and hydrogen.

12.7.6 Resonance Structure

There are many organic molecules whose behaviour cannot be explained by a single Lewis structure. An example is that of
benzene. Its cyclic structure containing alternating C-C single and C=C double bonds shown is inadequate for explaining its
characteristic properties.

As per the above representation, benzene should exhibit two different bond lengths, due to C-C single and C=C double bonds. However, as determined experimentally benzene has a uniform C-C bond distances of 139 pm, a value intermediate between the C=C single(154 pm) and C=C double (134 pm) bonds. Thus, the structure of benzene cannot be represented adequately by the above structure. Further, benzene can be represented equally well by the energetically identical structures I and II.

Therefore, according to the resonance theory (Unit 4) the actual structure of benzene cannot be adequately represented by any of these structures, rather it is a hybrid of the two structures (I and II) called resonance structures. The resonance structures (canonical structures or contributing structures) are hypothetical and individually do not represent any real molecule. They contribute to the actual structure in proportion to their stability.
Another example of resonance is provided by nitromethane (CH3NO2) which can be represented by two Lewis structures, (I and II). There are two types of N-O bonds in these structures.

However, it is known that the two N-O bonds of nitromethane are of the same length (intermediate between a N-O single bond and a N=O double bond). The actual structure of nitromethane is therefore a resonance hybrid of the two canonical forms I and II.
The energy of actual structure of the molecule (the resonance hybrid) is lower than that of any of the canonical structures. The difference in energy between the actual structure and the lowest energy resonance structure is called the resonance stabilisation energy or simply the resonance energy. The more the number of important contributing structures, the more is the resonance energy. Resonance is particularly important when the contributing structures are equivalent in energy.
The following rules are applied while writing resonance structures:
The resonance structures have (i) the same positions of nuclei and (ii) the same number of unpaired electrons. Among the resonance structures, the one which has more number of covalent bonds, all the atoms with octet of electrons (except hydrogen which has a duplet), less separation of opposite charges, (a negative charge if any on more electronegative atom, a positive charge if any on more electropositive atom) and more dispersal of charge, is more stable than others.

Problem 12.16

Write resonance structures of CH3COO- and show the movement of electrons by curved arrows.
First, write the structure and put unshared pairs of valence electrons on appropriate atoms. Then draw the arrows one at a time moving the electrons to get the other structures.

Problem 12.17
Write resonance structures of CH2=CH-CHO. Indicate relative stability of the contributing structures.

[I: Most stable, more number of covalent bonds, each carbon and oxygen atom has an octet and no separation of opposite
charge II: negative charge on more electronegative atom and positive charge on more electropositive atom; III: does not contribute as oxygen has positive charge and carbon has negative charge, hence least stable].
Problem 12.18
Explain why the following two structures, I and II cannot be the major contributors to the real structure of CH3COOCH3.

The two structures are less important contributors as they involve charge separation. Additionally, structure I contains a carbon atom with an incomplete octe

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