Benzene

BENZENE Introduction Benzene is an organic compound with molecular formula C6H6. It has ring structure. Each bend of structure represents a location of carbon, although convention is not to show carbon of the ring. Each carbon is partially double bonded to other carbon. The circle in the middle of hexagon represents that carbon.

Double bonds are continuously changing between its all carbons. The benzene is resonance stabilized. Benzene is an aromatic compound. Resonance The resonance is a phenomenon due to which all the properties of the compound cannot be represented by one single structure.

Aromaticity (Huckel rule) The aromatic compounds have their characteristic sweet aroma. The compounds have alternate single and double bonds and follow Huckel rule. The rule says that the number of ‘Β’ electron in an aromatic compound are equal to 4n+2 where ‘n’ is a positive integer and can take the values 1,2,3,4,……etc.. [n = 1] 4n+2 =

4+2

=

6 = number of Β electrons in benzene.

Location of substituent Benzene is a highly stable compound and incoming reagent can substitute hydrogen from any of the six positions. Each position is well defined and shown in the following structure.

In the structure, ‘R’ represents an alkyl group. The three locations are defined as ortho position, meta position and para position. Nomenclature of Benzene Derivatives Benzene derivatives are formed by substituting at least one of the hydrogens of the benzene. Those compounds which have benzene ring present are called benzene derivatives. The most popular and useful derivatives carry common name. Quite a few of these common names have been accepted by IUPAC and are listed as IUPAC name. Some of these are shown below.

Effect of substituents: All hydrogen atoms of benzene ring are equivalent and hence only one product is formed, i.e. only one substituent or monosubstituted benzene. The mono-substituted has general molecular formula C6H5 – S where ‘S’ stands for any substituted atom or group. All the compounds shown above are mono-substituted.

A second substituent “E” can occupy any one of the remaining five positions.

The products are named as per the position occupied such as ortho, meta or para. The first substituent marked ‘S’ can be classified as electron donating or electron withdrawing group. Electron Donating Group (EDG) These are species which can donate electron to aromatic ring. EDG can be recognized by lone pairs on the atom adjacent to the p system, eg: -OMe except -R, -Ar or -vinyl (hyperconjugation, pi electrons) Electron Withdrawing Group (EWG)

These are species which withdraw electron from aromatic ring and can be recognized either by the atom adjacent to the p system having several bonds to more electronegative atoms, or, having a formal +ve or d +ve charge, eg: -CO2R, -NO2

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EDG is activating group and ortho / para director add electron density to the benzene system except halogens (-X). Though halogens are deactivating, the ortho / para director make it more nucleophilic. EWG is deactivating group and meta director removes electron density from the benzene system making it less nucleophilic.

In deciding the EWG or EDG tendency, hydrogen is taken as reference.

Table showing few electron donating and withdrawing groups

Substitution reaction Substitution reactions are the chemical reactions in which an atom, ion, or group of atoms or ions in a molecule is replaced by another atom, ion, or group. An example is the reaction in which the chlorine atom in the chloromethane molecule is displaced by the hydroxide ion, forming methanol.

Nucleophile A nucleophile is a species (an ion or a molecule) which are electron rich that will react with electron deficient species and is strongly attracted to a region of positive charge. The nucleophiles are represented as Nu — Nucleophiles are either fully negative ions, or else have a strongly δ - charge somewhere on a molecule. Common nucleophiles are hydroxide ions, cyanide ions, water and ammonia.

Each species contains at least one lone pair of electrons. The lone pair is either on an atom carrying a full negative charge, or a very electronegative atom carrying a substantial δ - charge. Nucleophilic substitution reaction Nucleophilic substitution reactions occur when an electron rich species, the nucleophile, substitutes another function group known as leaving group. The reaction occurs at an electrophilic carbon atom attached to an electronegative group (leaving group abbreviated as LG) that can be displaced as shown by the general scheme:

There are two type of nucleophilic substitution reaction SN1 and SN2. The SN1 is unimolecular and SN2 is bimolecular SN1 Substitution Reaction Mechanism SN1 represent substitution, nucleophilic, unimolecular reaction. The reaction proceeds in two steps. The first step is a rate determining slow step. The second step is a fast step. The mechanism is explained on the basis of trimethyl bromine. First step The first step is the formation of carbocation. In this step nucleophile is not involved. Hence the rate is independent of concentration of Nu- and it is directly proportional to concentration of electrophile being attacked. In SN1 reaction the leaving group simply breaks away on its own leaving behind a carbocation, of course leaving group leaves because of presence of Nu-.

Second step

Once the carbocation is formed, it would react immediately with a nucleophile like Nu-. The lone pair on the nucleophile is strongly attracted towards the positive carbon, and moves towards it to create a new bond.

As the initial slow step only involves one species, the mechanism is described as SN1 – substitution. Nucleophilic is the only one species taking part in the initial step. Elimination reactions may often accompany SN1 reaction because the nucleophile may act as a base to pull a proton from carbocation forming a carbon-carbon double bond. In some cases carbon chain rearrangement may occur if carbocation can arrange to a more stable form. SN2 Substitution Reaction Mechanism It is called SN2 mechanism as ‘S’ stands for substitution, ‘N’ for nucleophilic, and the ‘2’ is used since the initial stage of the reaction involves two species. It is a single step mechanism and rate of reaction depends on the concentration of nucleophile as well the electrophile which is attacked. Bromomethane is taken as an example to explain the mechanism and a general nucleophilic ion is represented as Nu-. The Nu- will have at least one lone pair of electrons. Nu- could be OH- or CN.

The lone pair on the Nu- ion will be strongly attracted to the + carbon, and will move towards it. In the process. the electrons in the C-Br bond will be pushed even closer towards the bromine, making it increasingly negative. The Nu- ion approaches the + carbon from the side away from the bromine atom. The large bromine atom hinders attack from its side and, being -, would repel the incoming Nu- anyway. This attack from the back is important. The detailed mechanism is as shown below

The order of reactivity is Methyl >10 > 20 > 30 In fact tertiary carbon does not even react by SN2 mechanism Steric Hindrance It is defined as repulsion between the electron clouds on bulky groups of a molecule or between molecules. It helps to describe how molecular groups interfere with other groups. In SN2 mechanism, incoming Nu- attacks carbon atom from behind due to steric hindrance. The bromine atom is highly electronegative which repels incoming Nu- as electron- electron repulsion. The bromine is also bulky atom; bigger in size hence leaves a very little space for entry from its side. If Nu- is a strong base and substrate is hindered, elimination reaction type E2 may occur. Nucleophile will pull a proton from carbocation and in the same step halogen leaves the carbon forming a carbon-carbon double bond. Effect of solvent 1. Polar protic solvents are polar and have the ability to form hydrogen bond. Polar aprotic solvents are polar but not able to form hydrogen bond. 2. Polar protic solvent stabilizes nucleophile and forms any carbocation. 3. Polar protic solvent increases the rate of SN1 while decreases rate of SN2 reaction. 4. Polar aprotic solvent increases the rate of SN2 reaction while inhibiting SN1. 5. A stable nucleophile slows SN2 reaction while stable carbocation increases SN1 rate.

Leaving group ( LG ) ¾ It is an atom (or a group of atoms) that is removed from a molecule as stable species taking with it the bonding electrons. ¾ The best leaving group are the most stable when they leave. ¾ Examples of leaving group are an anion (e.g. Cl-) or a neutral molecule (e.g. H2O). ¾ Electron withdrawing effect and polarizability makes a good leaving group. ¾ The better the leaving group, the more likely it is to depart. Nucleophilicity 1. The measure of affinity of a nucleophile for a carbon atom in SN2 reaction can be defined as nucleophilicity. 2. A negative charge and polarizability increases while electronegativity reduces nucleophilicity. 3. The basicity and nucleophilicity are not the same. A base is stronger nucleophile than its conjugate acid. 4. If a nucleophile behaves as a base, elimination reaction may occur. 5. In general nucleophilicity increases down the group and decreases from left to right in a periodic table

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