Substitution reaction

Introduction

substitution reaction are also called displacement reactions .These are the reactions in which an atom or a group of atoms attached to a carbon atom in a molecule is replaced by some other atom or group of atoms without any change in the structure of the remaining part of the molecule. The product formed is known as the substitution product. The new atom or group which enters the molecule is called the substituent.

Some example of substitution reactions are:

CH₄+cl₂ → CH₃cl+Hcl

Methane Methyl chloride

Here H-atom of CH₄ is replaced by chloride atom.

CH₃-CH₂Br+KOH (aq) → CH₃CH₂OH+KBr

Ethyl bromide Ethyl alcohol

Here Br atom of ethyl bromide is replaced by OH group.

CH₃ -CH₂ = CH₂ → ClCH₂ -CH=CH₂ + Hcl

Propene Allyl chloride

Here H-atom of CH₃ group is replaced by cl-atom

Types of substitution reaction

Depending upon the nature of attacking reagent, substitution reactions can be classified as below:

a) Electrophilic Substitution Reactions : –

These are the reactions in which an atom or group in a molecule is replaced by an electrophile. Such reactions are shown by aromatic compound.

CH₅+HONO₂→ CH₅NO₂+H ₂0

CH₅+ NO₂→CH₅ NO₂+H ₂0 (H ₂s0 ₄)

Here Nitronium ion (NO₂ ions )acts as an electrophile and the process is called nitration.

Mechanism. Consider the chlorination of benzene in presence of halogen carrier(Fecl₃)

Fecl₃+cl-cl → Fecl₄⁻+cl(chloronium ion)

Electrophiles are involved in electrophilic substitution reactions and particularly in electrophilic aromatic substitutions:

Electrophilic reactions to other unsaturated compounds than arenes generally lead to electrophilic addition rather than substitution.

B) Necleophilic Substitution Reactions:-

These are the reaction sin which an atom or a group in a molecule is replaced by a nuclophile. Such reactions are shown by alkyl halides. Consider the action of aqueous KOH on methyl chloride.

HO⁻+H -CH₃ Cl→ Ho-cH₃OH+cl⁻

A nucleophile (literally “nucleus lover”) is a molecule or ion capable of acting as a Lewis base (i.e., an electron pair donor). Nucleophiles can be described as “electron-rich” while their targets or substrates can be described as “electron-deficient” (they are Lewis acids, i.e., electron pair acceptors). In a nucleophilic substitution the nucleophile takes the place of–or substitutes for–some atom or group on the substrate (called the “leaving group”):

Nu: + R:L → R:Nu+ + :L-

nucleophile substrate product leaving group

If the nucleophile is neutral (as shown above) the product will be charged since the leaving group takes both bonding electrons away with it. If the nucleophile is an anion then the product will be neutral:

Nu: – + R:L → R:Nu + :L-

Stronger bases make better nucleophiles (e.g., OH- is a better nucleophile than H2O). “Good” substrates include cations, central atoms with incomplete octets or double bonds (like sp2 carbons) or carbons with partial positive charges. Halogens are generally more electronegative than carbon and so organohalogen compounds are usually subject to nucleophilic attack at the carbon attached to the halogen (which would be the positive end of a dipole). For example, 2-chloro-2-methylpropane (commonly known as t-butyl chloride) will undergo nucleophilic substitution with hydroxide ion:

(CH3)3CCl + OH – → (CH3)3COH + Cl –

This is a typical synthetic route for producing an alcohol from an alkyl halide.

Nucleophilic substitution reactions have been studied for many years. It was noticed fairly early that while the overall reaction was similar in the vast majority of cases the kinetics of the process was not always the same. In some substitutions the concentration of the nucleophile had no effect on the rate. In others, the rate was directly proportional to the concentration of the nucleophile. This suggested that two different mechanisms must be at work. The factor which determines the mechanism employed is typically the nature of the substrate itself and NOT the particular nucleophile.

Necleophilic substitution reactions are further classified as:-

1)Necleophilic substitution Biomolecular :-

Such reaction sare shown by primary alkyl halides and involve a single step. The breaking of C-X bond and the making of C-OH bond takes place simultaneously. The neucleophile approaches the C-atom from the side opposite to that carrying the halogen. In such reactions, an inverted product is formed. The reaction involves the formation of a transition state.

The reaction follows the rate law, Rate=k[Alkyl halide][OH]. As the slow rate determining step involves two molecule(Alkyl halide and alkali); therefore, the reaction is known as bimolecular substitution reaction.

Most necleophilic substitutions, which involve the expulsion of an originally neutral substituent ,notably of halogen, from the aromatic ring,at temperature which are not particularly high,use the biomolecular mechanism SN2. This is established by their second order kinetics, which are documented by many records,

(NO₂)₂CH₃•Cl+OEt⁻→(NO₂)₂CH₃•OEt+Cl⁻

In substitution of this type,the rate of attack by different reagent on the same aromatic molecules follow the general order of nucleophilic strength towards carbon. This is the conclusion to which Bunnatt and Zahler come after having assembled data from many sources. The first mechanism is known as SN1 (substitution, nucleophilic, unimolecular) because only one molecule is involved in the first step–the rate determining step. Reactions occurring by this mechanism should exhibit first-order kinetics, i.e., the rate law should have the form “rate = k[substrate]1”. Because the nucleophile is not involved until after the slow step its concentration will have no effect on the rate.

The alternate mechanism is called SN2 (substitution, nucleophilic, bimolecular) because two molecules are involved in the rate determining (and only) step. Such reactions exhibit overall second-order kinetics. The rate is proportional to both the concentration of the substrate and the concentration of the nucleophile. Reactions like this will have a rate law in the form “rate = k[substrate] [nucleophile]

2) Nucleophilic Substitution Unimolecular:-

This mechanism is generally followed by tertiary alkyl halide. In the first step, tertiary alkyl halie breaks hydrolytically to form intermediate carbonium ion.

The formation of carbonium ion is the slow rate determing step.

In the second step, the nucleophile attacks tha carbonium ion to form an alcohol. Consider the action of aqueous KOH or tertiary Butyl bromide.

CH₃ CH₃

CH₃ ____ C—-Br ⇄ CH₃ ——C +Br⁻ (Slow Step)

CH₃ CH₃

CH₃ CH₃

CH₃ ____ C + OH⁻ ⇄ CH₃ ——C +OH (Fast Step)

CH₃ CH₃

(carbonium ion) Tert. butyl alcohol

Or The best established eample of nucleophilic aromatic substitution by the unimolecular mechanism ,SN1,is the uncatalysed decomposition of diazoium ions,in hydroxylic solvent,to give phenols or phhenolic ethers,accomplished often byaryl halides or others such substitution products, if the necessary necleophilic anions are present in the solution:

ArN₂+ → Ar+N₂ (Slow)

Ar+H₂o →Ar• OH+H (fast)

Ar+ROH→Ar•OR+H (Fast)

Ar +cl ⁻→ Ar• Cl (Fast)

These are SN1 mechanism .

The alternate mechanism is called SN2 (substitution, nucleophilic, bimolecular) because two molecules are involved in the rate determining (and only) step. Such reactions exhibit overall second-order kinetics. The rate is proportional to both the concentration of the substrate and the concentration of the nucleophile. Reactions like this will have a rate law in the form “rate = k[substrate] [nucleophile]

3) Free Radicals Subsitution Reactions[1.2.3]

These are the reactions in which an atom or group of atoms in a molecule is replaced by a free radical. The replacement of H- atom by a halogen atom is an example of free radicals substitution.In the free radical substitution reaction, the attacking reagent is a free radicals. These reactions are carried either at high temperature or in the presence of ultra-violet light.

In organic chemistry, a radical substitution reaction is a substitution reaction involving free radicals as a reactive intermediate

The reaction always involves at least two steps, and possibly a third.

In the first step called initiation (2,3) a free radical is created by photolysis. Homolysis can be brought about by heat or light but also by radical initiators such as organic peroxides or azo compounds. Light is used to create two free radicals from one diatomic species. The final step is called termination (6,7) in which the radical recombines with another radical species. If the reaction is not terminated, but instead the radical group(s) go on to react further, the steps where new radicals are formed and then react is collectively known as propagation (4,5) because a new radical is created available for secondary reactions.

Mechanism. The mechanism of free radicals substitution involves three steps:-

I) Initiation :- In this step, halogen molecule breaks homolyticallly to form free radicals. Consider the action of Br₂ on ethane in presence of sunlight.

Br-Br Br

II) Propagation step:— The Br formed in the first step reacts with alkane molecule to form new free radicals which in turn reacts with bromine molecule and the chain react ion starts and so on.

CH₃CH₃+Br →CH₃ CH₂+HBr

CH₃ CH₂+Br -Br →CH₃ CH₂Br+Br

3) Termination.:— In this step, the free radicals combine and the reaction stops.

Br + Br → Br₂

Similarly, consider action of Cl₂ on propene.

I) cl – cl→ 2Cl

II) CH₂=CH- CH₃+Cl→ CH₂=CH- CH₂+HCl

CH₂=CH- CH₂+Cl-Cl→ CH₂=CH-CH₂Cl+Cl

Substitution Reactions[1.2.3]

Substitution Reactions.

In an acid-base reaction such as CH3CO2H + NH3 → CH3CO2

– + NH4+ the N acts as a nucleophile (Greek for “loving the nucleus), the H acts

as an electrophile (“loves electrons”), and the O that accepts the pair of electrons acts as a leaving group. The acid-base reaction is the simplest model for a substitution reaction, which is a reaction in which a σ bond between atom 1 and atom 2 is replaced by a σ bond between atom 1 and atom 3. Substitution reactions are incredibly important in organic chemistry, and the most important of these involve substitutions at C. For example:

This substitution reaction, discovered in 1849, involves the nucleophilic O making a new bond to the electrophilic C, and the bond between the electrophilic C and the leaving group I breaking. Any Brønsted base can also act as a nucleophile, and any nucleophile can also act as a Brønsted base, but some compounds are particularly good bases and

particularly poor nucleophiles, whereas some are particularly poor bases and particularly good nucleophiles. Any Brønsted or Lewis acid can also act as an electrophile, but there are many electrophiles that are neither Brønsted nor Lewis acids (as in the example above). A haloalkane, e.g. CH3CH2Br, can in principle undergo either of two polar reactions when it encounters a lone pair nucleophile, e.g. MeO-. First, MeO- might replace Br- at the electrophilic C atom, forming a new C-O bond and giving an ether as the product. This is substitution, because the C-Br σ bond is replaced with a C-O σ bond. Second, MeO- might attack a H atom that is adjacent to the electrophilic C atom, giving MeOH, Br-, and an alkene as products. The electrons in the C-H bond move to form the π bond, and the electrons in the C-X bond leave with X-. This is elimination, because a new π bond is formed, and because the elements of the organic starting material are now divided between more than one product. Elimination requires that the substrate have a C-X bond and adjacent C-H bonds, while substitution requires only that the substrate have a C-X bond.

Nucleophilic aromatic substitution reaction

The name in the title in given to those substitution in whichnucleophilic reagent, such as Br⁻, combine with aromatic carbon and aprecltyuviously present substituent such as •Cl,•NO₂, becomes expelled along with its bonding electrons With considerable difficultly even •H may be expelled with its bonding electrons i.e at H⁻. Biomolecular substitution reaction electron attracting substituent especially one conjugated with aromatic system such as nitro,carbonyl,syano aids the attack of the reagent and a 2-or 4-situated hetero atom ,as n pyridine ,acts in a similar way. Neuclophilic aromatic substtion can proceed by several,mechanism. The Unimolecular and Biomolecular mechanism can definitely be recognized and other mechanism some of which are understood,can be seen to exist. The unimolecular mechanism is limited to the replacement of those substituents which are sufficiently loosely bound to undergo spontaneous heterlysis in solution. The biomolecular mechanism is muc more general, doubtless because it make much less severe demands on thequality of the explled group,so that a hydrogen shift involved.