The substitution reactions
The preparation of this project on the topic-” substitution reactions.”: a profile’ would not have been possible without the valuable contribution of my TEACHERS.
I would like to give most specially thanks to my CHE sir Dr. Ashish kumar who is my chemistry teacher to giving me the important guidelines during making this project.
So, I hope this project will provide large and sufficient information about the different coordination numbers present in the coordination chemistry.
In substitution reaction, afunctional groupin a particularchemical compoundis replaced by another group. Inorganic chemistry, theelectrophilicandnucleophilicsubstitution reactions are of prime importance. Organic substitution reactions classified in several mainorganic reactiontypes depending on whether thereagentthat brings about the substitution is considered anelectrophileor anucleophile, whether areactive intermediateinvolved in the reaction is acarbocation, acarbanionor afree radicalor whether thesubstrateisaliphaticor aromatic. It also is helpful for optimizing a reaction with regard to variables such as temperature and choice of solvent Substitution reaction : chlorination of methane
These kind of substitution reactions happen when the reagent is a nucleophile, which means, an atom or molecule with free electrons.
- Anucleophilereacts with analiphaticsubstrate in anucleophilic aliphatic substitutionreaction.
- When the substrate is anaromaticcompound the reaction type isnucleophilic aromatic substitution.
- Carboxylic acidderivatives react with nucleophiles innucleophilic acyl substitution. This kind of reaction can be useful in preparing compounds
The Nucleophilic substitutions can be produced by two different mechanisms:
- Monomolecular nucleophilic substitution (SN1): In this case the reaction proceeds in stages, the compounds first dissociate in their ions and then this ions react between them. It’s produced by carbocations.
- Bimolecular nucleophilic substitution (SN2): In this case the reaction proceeds in only one stage. The attack of the reagent and the expulsion of the leaving group happen simultaneously.
- Electrophilesare involved inelectrophilic substitutionreactions and particularly inelectrophilic aromatic substitutions:
Electrophilic reactions to other unsaturated compounds thanarenesgenerally lead toelectrophilic additionrather than substitution.
Aradical substitutionreaction involvesradicals
- The term nucleophile comes from the Greek meaning ‘nucleus loving’, in other words nucleophiles seek positive charged centres. Nucleophiles have lone pairs of electrons and may carry a negative charge. There are many examples of nucleophiles, such asNH3,H2O,CN-,HC?C-, andOH-.
- Alkyl halides contain a halogen (X =F,Cl,BrorI) covalently bonded to a carbon atom. Due to the electronegativity differences between carbon and the halide, theC-Xbond is polar with a partial positive charge (?+) on the carbon atom and a partial negative charge (?-) on the halogen. Halogens are good leaving groups and can be replaced by an incoming nucleophile.
Nucleophilic substitution is the reaction of an electron pair donor (the nucleophile, Nu) with an electron pair acceptor (the electrophile). An sp3-hybridized electrophile must have a leaving group (X) in order for the reaction to take place.
Mechanism of Nucleophilic Substitution
The term SN2 means that two molecules are involved in the actual transition state:
The departure of the leaving group occurs simultaneously with the backside attack by the nucleophile. The SN2 reaction thus leads to a predictable configuration of the stereocenter – it proceeds with inversion (reversal of the configuration).
In the SN1 reaction, a planar carbenium ion is formed first, which then reacts further with the nucleophile. Since the nucleophile is free to attack from either side, this reaction is associated with racemization.
In both reactions, the nucleophile competes with the leaving group. Because of this, one must realize what properties a leaving group should have, and what constitutes a good nucleophile. For this reason, it is worthwhile to know which factors will determine whether a reaction follows an SN1 or SN2 pathway.
Common examples include
- Organic reductionswithhydrides, for example
- hydrolysisreactions such as
- Williamson ether synthesis
R-Br + OH-?R-OH+Br-(SN2) or
R-Br + H2O ? R-OH +HBr (SN1)
R-Br +OR’-?R-OR’+ Br- (SN2)
Electrophilic aromatic substitutionorEASis anorganic reactionin which an atom, usuallyhydrogen, appended to anaromatic systemis replaced by anelectrophile. The most important reactions of this type that take place arearomatic nitration,aromatic halogenation,aromatic sulfonation, and acylation and alkylatingFriedel-Crafts reactions.
Aromatic nitrationsto formnitro compoundstake place by generating a nitronium ion fromnitric acidandsulfuric acid.
Aromatic sulfonationofbenzenewith fumingsulfuric acidgives benzenesulfonic acid.
Aromatic halogenationof benzene withbromine,chlorineoriodinegives the corresponding aryl halogen compounds catalyzed by the corresponding iron trihalide.
TheFriedel-Crafts reactionexists as anacylationand analkylationwith acyl halides oralkyl halidesas reactants.
The catalyst is most typicallyaluminium trichloride, but almost any strongLewis acidcan be used. In Fridel-Crafts acylation, a full measure of aluminium trichloride must be used, as opposed to a catalytic amount.
Basic reaction mechanism
In the first step of thereaction mechanismfor this reaction, the electron-rich aromatic ring which in the simplest case isbenzeneattacks the electrophileA. This leads to the formation of a positively-charged cyclohexadienylcation, also known as anarenium ion. Thiscarbocationis unstable, owing both to the positive charge on the molecule and to the temporary loss ofaromaticity. However, the cyclohexadienyl cation is partially stabilized byresonance, which allows the positive charge to be distributed over three carbon atoms.
In the second stage of the reaction, aLewis baseBdonates electrons to the hydrogen atom at the point of electrophilic attack, and the electrons shared by the hydrogen return to thepisystem, restoring aromaticity.
An electrophilic substitution reaction on benzene does not always result in monosubstitution. While electrophilic substituents usually withdraw electrons from the aromatic ring and thus deactivate it against further reaction, a sufficiently strong electrophile can perform a second or even a third substitution. This is especially the case with the use ofcatalysts.
A radical is a species that contains unpaired electrons.
Typically formed by a homolytic bond cleavage as represented by the fishhook curved arrows:
RADICAL CHAIN MECHANISM
FOR REACTION OF METHANE WITH Br2
Step 1 (Initiation)
Heat or uv light cause the weak halogen bond to undergo homolytic cleavage to generate two bromine radicals and starting the chain process.
Step 2 (Propagation)
- A bromine radical abstracts a hydrogen to form HBr and a methyl radical, then
- The methyl radical abstracts a bromine atom from another molecule of Br2to form the methyl bromide product andanotherbromine radical, which can then itself undergo reaction 2(a) creating a cycle that can repeat.
Step 3 (Termination)
Various reactions between the possible pairs of radicals allow for the formation of ethane, Br2or the product, methyl bromide. These reactions remove radicals and do not perpetuate the cycle.
There are two components to understanding the selectivity of radical halogenations of alkanes:
- reactivity of R-H system
- reactivity of X.
The strength of the R-H varies slightly depending on whether the H is 1o, 2oor 3o. The following table shows the bond dissociation energy, that is the energy required to break the bond in a homolytic fashion, generating R.and H.
Halogen radical, X.
- Bromine radicals are less reactive than chlorine radicals
- Br.tends to be more selective in its reactions, and prefers to react with the weaker R-H bonds.
- The more reactive chlorine radical is less discriminating in what it reacts with.
The selectivity of the radical reactions can be predicted mathematically based on a combination of an experimentally determined reactivity factor, Ri, and a statistical factor, nHi. In order to use the equation shown below we need to look at our original alkane and look at each H in turn to see what product it would give if it were to be susbtituted. This is an exercise in recognizing different types of hydrogen, something that will be important later.
- Chang Raymond