Welcome back, Chemistry Enthusiasts! In our previous guide, we laid the critical groundwork by mastering the core concepts of General Organic Chemistry (GOC). We explored electronic effects, the stability of intermediates, and the fundamental rules that govern organic molecules. But what happens when these molecules actually collide and interact? Welcome to the fascinating world of Reaction Mechanisms.
If GOC is the alphabet of organic chemistry, Reaction Mechanisms are the sentences. Instead of blindly memorizing hundreds of complex chemical equations, understanding the mechanism allows you to logically predict the products of any reaction. In this post, we will decode the step-by-step pathways that reactants take to transform into products—making those tricky JEE, NEET, and Board exam questions feel like simple logical puzzles. Let’s grab a pen, master those curly arrows, and dive in!
If you haven’t mastered your basic electronic effects and intermediates yet, be sure to read our complete guide on [Master General Organic Chemistry: Unveiling the Secrets of Reaction Mechanism] before proceeding.
Reaction Mechanisms – Types of Reactions
Reactions are of mainly four types, namely:
- Addition Reactions
- Substitution Reactions
- Elimination Reactions
- Isomerisation Reactions
Addition Reaction
Reaction in which two or more molecules combine to form a single, larger product (often called an adduct) with no other byproducts left behind are called addition reactions. It is also of three types, namely:
- Electrophilic Addition Reaction
- Free Radical Addition Reaction
- Nucleophilic Addition Reaction
Electrophilic Addition Reaction
Because of presence of C=C bond in molecules, alkenes generally take part in addition reactions.
From the mechanism, the addition in alkenes is generally electrophilic in nature, which means that attacking reagent which carries the initial attack is an electrophile (E+). This is quitw expected also as there is high electron density in the double bond. The mechanism proceeds in two steps:
Step 1: The pi-electron cloud of the double bond causes the polarisation of the attacking molecule (E-Nu) which breaks to release the electrophile (E+) for the attack. The double bond simultaneously undergoes electromeric effect and the attack by the electrophile is accomplished in slow step (also called rate determining step) to form a carbocation intermediate.
Step 2: The nucleophile (Nu–) released in the slow step combines with the carbocation to give the desired addition product in the fast step.
Reactivity of Electrophilic addition reaction is directly proportional to the stability of carbocation formed in the rate determining step.
Addition of Halogen
It is an electrophilic addition reaction.
(a) The addition of Br2 on alkenes provides a useful test for unsaturation in molecules. The brown colour of bromine being rapidly discharged. This, decolourization of 5% Br2 in CCl4 by a compound suggest unsaturation in it. The product formed is colourless dibromo compound.
(b) I2 reacts with alkenes to form vicinal diiodide which are unstable and I2 gets eliminated to give original alkene.
Mechanism
It is interesting to note that the product which is mainly formed as a result of addition is trans in nature whereas the cis isomer is obtained in relatively smaller proportions. Since the carbocation intermediate is planer (sp2 hybridised), both cis and trans addition products must be formed almost in equal proportions. The trans product can be justified in case a cyclic halonium ion is formed by the initial electrophilic attack.
Ex: No carbocation rearrangement and anti-addition product is formed.
Addition of Halogen Acid
- Order of reactivity of hydrogen halide is HI > HBr > HCl
- Addition of alkene proceeds via the formation of more stable carbonium ion.
- Addition of HX on unsymmetrical alkenes (R-CH=CH2) takes place according to Markovnikov’s rule. Carbocation rearrangement is observed in the reaction.
Rearrangement in carbocatiion occurs as:
(i)
(ii)
(iii)
(iv)
(v)
Markovnikov’s Rule
(a) When molecule of HX add up on an unsymmetrical unsaturated hydrocarbon, the electrophile (H+) goes to the unsaturated carbon atom bearing more number of hydrogen atoms.
Mechanism
It is an electrophilic addition and is illustrated by the action of HCl to propene.
Primary carbocation (CH3-CH2-CH2+) is also formed but only in very small proportion, since it is less stable than the secondary carbocation. Markovnikov’s rule is based on the stability of carbocation.
Note: The electrophilic addition of HX to an unsymmetrical alkenes always occurs through the formation of a more stable carbocation intermediate.
(b) In the addition of HX to vinyl halide and analogous compound, the halogen attaches itself to the carbon atom, on which the halogen atom is already present.
Mechanism
All polar reagents of the general structure Y+Z– (such as H+X–, H+OH–, H+SO3H–, X+OH–) add on unsymmetrical unsaturated compound in accordance with Markovnikov’s rules. Such additions are called normal Markovnikov’s rule, where as additions in the opposite manner are referred to as abnormal or Anti-Markovnikov’s additions.
Addition of Hypohalous Acid
It is an electrophilic addition reaction and follows Markovnikov’s rule and anti addition occurs. The reagents used to carry out the reaction is X2/H2O or HOX.
Addition of Water
It is also called as Hydration of alkenes. Propane and higher alkanes react with water in the presence of acid to form alcohol. This reaction is known as acidic hydration reaction. Intermediate in this reaction is carbocation, so carbocation rearrangement may take place.
(i)
(ii)
Mechanism
Addition of NOCl
Hydroboration Oxidation Reaction
Borane readily reacts with alkenes giving trialkyl boranes The reaction is called hydroboration.
BH3 does not exist freely as monomer so a solvent THF (tetrahydrofuran) is used to stabilise it.
Ex:
BHR2 can also be taken.
Ex:
Note: The overall process appears to be addition of water according to Anti-Markovnikov’s rule and involves syn addition.
Oxymercuration-Demercuration Reaction
Mercuric acetate in water is treated with an alkene. The addition product on reduction with sodium borohydride in aqueous NaOH solution gives alcohol. It follows the Markovnikov’s rule.
Reagents used are: (i) (CH3COO)2Hg/H2O or (AcO)2Hg/H2O (Mercuric Acetate) (ii) NaBH4
Mechanism
Note: Intermediate is cyclic cation, so no carbocation rearrangement occur.
Summary
Free Radical Addition Reaction
Addition of HBr on alkene or alkyne in presence of peroxide.
Anti-Markovnikov’s Rule
- In the presence of peroxides, the addition of HBr on unsaturated unsymmetrical compound takes place contrary to Markovnikov’s rule. This is called as peroxide effect and is due to the difference in the mechanism of the addition.
- In the normal Markovnikov’s addition, the mechanism is ionic.
- In the presence of peroxide, the addition of HBr takes place via free radicals.
- Peroxide effect is not observed in case of HF, HCl and HI. Reactions follows electrophilic addition mechanism.
Mechanism
The mechanism consists of following three steps:
Step 1: Chain initiation:
Step 2: Chain propogation:
Step 3: Chain termination:
Nucleophilic Addition Reaction
NAR in Alkynes
In these reactions, some heavy metals cation like Hg2+, Pb2+, Ba2+ are used. These cation attracts the pi-electron of alkynes and decrease the electron density and hence a nucleophile can attack on alkynes.
Addition of Alcohol
In presence of BF3 and HgO alkynes react with alcohols and form acetal and ketal.
Addition of Arsenic Trichloride
In presence of AlCl3 and HgCl2 acetylene combines with AsCl3 to yield Lewisite gas. It is four times poisonous than mustard gas.
NAR in Aldehydes and Ketones
Due to strong electronegativity of oxygen, the mobile pi-electrons pulled strongly towards oxygen, leaving the carbon atom deficient of electrons. Carbon is thus readily attacked by nucleophile. The negatively charged oxygen is attacked by electron deficient electrophile.
- Reactivity of carbonyl group ∝ Magnitude of δ+ charge
- Reactivity of carbonyl group ∝ -I effect
- Reactivity of carbonyl group ∝ 1/+I effect
Addition of HCN
Addition of NaHSO3
Reaction with Alcohol
Reaction with Sodium Alkynide
Reaction with Grignard Reagent
Reaction with Glycol
Reaction with Water
It is a reversible reaction.
Note: Chloral (CCl3-CH=O) forms stable hydrate, i.e., Chloral hydrate (CCl3-CH(OH)2).
Reaction with Ammonia Derivatives
These are condensation or addition elimination reaction. These reacts proceeds well in weakly acidic medium.
Ammonia derivative are Z-NH2, where Z may vary.
Substitution Reaction
Reactions in which one atom or a group of substrate is replaced by other atom or group are called as substitution reactions. It is also of three types, namely:
- Free Radical Substitution Reaction
- Electrophilic Substitution Reaction
- Nucleophilic Substitution Reaction
Free Radical Substitution Reaction
Substitution reaction in alkanes show free radical mechanism.
Halogenation
Replacement of H-atom by halogen atom.
Halogenation is made on exposure of halogen and alkane mixture to UV light or at elevated temperature.
Reactivity order for halogens show the order: F2 > Cl2 > Br2 > I2
Reactivity order of hydrogen atom in alkane: Tertiary C-H > Secondary C-H > Primary C-H
- Fluorination: Reacts explosively even in dark. Fluorination can be achieved without violence when alkane is treated with F2 diluted with an inert gas like N2.
- Chlorination: The monochloro derivative of alkane is obtained as major product by taking alkane in large excess. When chlorine is in excess then perchloro derivative is obtained as major product.
At 12 noon, the reaction goes explosively as:
Mechanism
The mechanism consists of following three steps:
Step 1: Chain initiation:
Step 2: Chain propagation:
Step 3: Chain termination:
- Bromination: Br2 reacts with alkanes in a similar manner but less vigorously.
- Iodination: Iodine reacts with alkanes reversibly. HI formed as the by-product is a powerful reducing agent and is capable of reducing the CH3I to CH4. Iodination may be carried out in the presence of an oxidising agent such as HIO3, HNO3, HgO, etc. which decompose HI.
Iodination is very slow because energy of activation of the reaction is very large.
Nitration
It is also known as Vapour phase nitration. This involves the substitution of hydrogen atom of alkane with -NO2 group.
At ordinary temperature, alkanes do not react woth HNO3. But react with vapours of conc. HNO3 at 450 oC and in pressure.
Since the reaction is carried out at high temperature and pressure, so the C-C bonds of alkanes also break during the reaction and a mixture of nitroalkanes is formed.
Ex:
Sulphonation
Replacement of H atom of alkane by -SO3H is known as sulphonation. Alkane reacts with fuming H2SO4 or H2S2O7 (oleum).
Reactivity order of sulphonation: Tertiary H > Secondary H > Primary H
Note: The reaction is observed in higher alkanes and the alkanes having tertiary H.
Chlorosulphonation
Reaction with a mixture of SO2 and Cl2 at ordinary temperature in the presence of UV light is called chlorosulphonation. It is also known as Reed reaction.
Allylic or Benzylic Substitution
In this reaction, the substitution of H at allylic or benzylic position is done by Br. The reagent used in this reaction is Br2(low conc)/hν or NBS/hν.
Mechanism
The mechanism consists of following three steps:
Step 1: Chain initiation:
Step 2: Chain propagation:
Step 3: Chain termination:
Electrophilic Substitution Reaction
It is the characteristic reaction of arenes.
Mechanism
Note:
(i) Ortho/Para directing group or activating group: Group which direct electrophile on ortho and para position is called as ortho/para directing group. These groups increase electron density or increases reactivity of benzene ring are called as activating group. These groups are -NH2, -NHR, NR2, -OH, -OR, -NHCOCH3, -SH, -OCOR, -CH3, -CH2CH3, -CH(CH3)2.
Due to +M/+H effect of these groups, electron density at ortho and para position is increased so electrophile easily attack on ortho/para position.
(ii) Meta directing or deactivating group: Due to -M/-H effect of groups electron density at ortho and para position is less but more at meta position so electrophile attack on meta position. So, groups which direct electrophile on meta position are called meta directing groups. These groups decrease electron density in benzene ring and decrease reactivity of benzene ring, so are called as deactivating groups. These groups are: -CHO, -COOH, -COR, -COOR, -NO2, -CN, -SO3H, CX3.
(iii) Halogens are ortho/para directing group due to +M effect but are deactivating due to more -I effect than +M effect.
(iv) M and H effect does not depend upon distance which I effect depends on distance.
(v) M effect at meta position is considered zero.
Halogenation
Here, CH3 attached with benzene is ortho/para directing and activating group.
Nitration
Sulphonation
Friedel Craft Reaction
Alkylation or acylation of arenes in presence of lewis acid (FeCl3, ZnCl2, AlCl3) is called Friedel Craft Alkylation or Friedel Craft Acylation respectively.
Alkylation
In alkylation reaction, carbocation intermediate is formed, so rearrangement is possible.
Acylation
Gatterman’s Koch Reaction
Gatterman’s Aldehyde Synthesis
Summary
| Electrophile | Source | Name of ESR |
|---|---|---|
| Cl+ | Cl2 + AlCl3 or Cl2 + FeCl3 | Chlorination |
| Br+ | Br2 + AlBr3 or Br2 + FeBr3 | Bromination |
| NO2+ | conc HNO3 + conc H2SO4 | Nitration |
| SO3 | conc H2SO4 | Sulphonation |
| R+ (Alkyl carbocation) | RX + AlX3 or ROH + H+ | Friedel Craft Alkylation |
| R-C+=O (Acylinium ion) | RCOCl + AlCl3 | Friedel Craft Acylation |
Nucleophilic Substitution Reaction
Due to the electronegativity difference, the C-X bond is polarised. Carbon is having partial positive charge while halogen (X) is having partial negative charge. Thus, the C-atom of C-X bond becomes center to attack by a nucleophile (Nu–).
X– ion from R-X molecule is substituted by a Nu–.
There are two mechanism followed in nucleophilic substitution reaction:
(i) SN1 Reaction
(ii) SN2 Reaction
Mechanism
Unimolecular Substitution (SN1) Reaction
- SN1 stands for unimolecular nucleophilic substitution.
- The mechanism involves two steps, as follows:
Step 1: The alkyl halide ionises to give a planar carbocation. The carbocation ion is planar because the central positively charged carbon is sp2 hybridised.
Step 2: The nucleophile can attack the planar carbocation from either side to give the product.
- Ionisation is the rate determining step because it is the slowest step. In other words, the rate at which alcohol is formed depends upon the concentration of tertiary alkyl halide alone.
- Rate = k [R3C-Br]
- It is clear that the reaction follows first order kinetics, therefore this reaction is called as SN1 reaction.
- Reactivity order for SN1 reaction ∝ Stability of carbocation formed by halides
- Reactivity order of halides for SN1 varies as follows: Benzyl halide > Allyl 3o halide > Allyl 2o halide > Allyl 1o halide > 3o halide > 2o halide > 1o halide > Methyl halide.
- In case alkyl halide is optically active, SN1 reaction leads to racemisation.
Bimolecular Substitution (SN2) Reaction
- SN2 stands for bimolecular nucleophilic substitution. In this type of nucleophilic substitution reaction, bond breaking and bond formation takes place simultaneously. SN2 reaction follows concerted mechanism.
- Reactivity of alkyl halides in SN2 substitution is governed by steric factors. The bulkier the group, that less reactive it will be.
- Reactivity order of halide for SN2 varies as follows: Allyl halide > Methyl halide > 1o halide > 2o halide > 3o halide.
- Order of reactivity among 1o alkyl halide is: CH3X > C2H5X > C3H7X, etc.
- In case alkyl halide is optically active, SN2 reaction leads to Walden inversion.
For both SN1 and SN2,for a given alkyl group, the order of reactivity is: R-I > R-Br > R-Cl > R-F.
In addition to substitution reaction, alkyl halide also undergo elimination reactions to form alkene with the removal of a molecule of hydrogen halide (dehydrohalogenation). In dehydrohalogenation, hydrogen and halogen atoms are eliminated from two adjacent carbon atoms, the reaction also known as beta-elimination. It may proceed by E1 or E2 mechanism (analogous to SN1 and SN2 mechanism).
NSR of Alkyl Halide
NSR of Alcohol
Reaction with HX
Mechanism
In this reaction, intermediate carbocation is formed, so rearrangement takes place. Anhydrous ZnCl2 acts as dehydrating agent and absorbs H2O from the reaction, so good yield of halide is obtained. Also, it generates H+ from HCl.
Reactivity of alcohol ∝ Stability of intermediate carbocation.
So, the reactivity order of alcohol is 3o alcohol > 2o alcohol > 1o alcohol, and the reactivity order of HX is H-I > H-Br > H-Cl.
HI has maximum reactivity, so it reacts readily with 1o, 2o and 3o alcohols.
HCl and 1o alcohol are less reactive, so ZnCl2 or some amount of H2SO4 is needed to increase the reactivity. But at normal condition, no reaction occurs.
Note: conc HCl + anhydrous ZnCl2 is called as Lucas reagent, alcohol gives turbidity with lucas reagent.
We can distinguish between 1o, 2o and 3o alcohols by using Lucas Reagent. Turbidity is obtained as product which is due to formation of R-Cl.
- 3o alcohols give turbidity with lucas reagent immediately.
- 2o alcohols give turbidity with lucas reagent in 5 minutes.
- 1o alcohols give turbidity with lucas reagent in 30 minutes on heating.
Reaction with Phosphorus Halides
PBr3 and PI3 are less stable, thus for bromides and iodides (P+Br2) and (P+I2) mixture is used.
Reaction with Thionyl Chloride
This reaction is called as Darzen’s reaction. This process gives better yield of alkyl chloride as the side product obtained are gaseous in nature and can easily escape from the reaction mixture.
NSR of Ether
Reaction with HX
Reactivity order of the reaction of ether with HX is H-I > H-Br > H-Cl
Reaction with cold conc. HX
Ethers forms oxonium salts with cold conc. HCl as it is less reactive while with cold conc. HI and cold conc. HBr, the C-O bond breaks as they are more reactive.
Note: If oxonium ion give highly stable carbocation then SN1 reaction occurs. And if oxonium ion give less stable carbocation, then SN2 reaction occur in which X– attack on least hindered carbon atom.
Reaction with conc. HI or conc. HBr
Ether reacts with conc. HI or conc. HBr to form iodides or bromides with both the alkyl groups present in ether. In this case the reaction occurs as with cold conc. HI or cold conc. HBr forming an alcohol and an iodide or bromide, then the alcohol further reacts with HI or HBr to form alkyl iodide or bromide.
NSR in Halobenzene
Presence of deactivating group at ortho and para position makes the nucleophilic substitution easier. So the reactivity order towards nucleophilic substitution varies as:
NSR in Acid Derivatives
Acid derivatives are the derivatives of carboxylic acid, i.e., acid chloride, ester, amide and acid anhydride.
Where Z: -Cl, -OR, -NH2, -OCOR.
Z is a good leaving group.
Reactivity order of acid derivative towards nucleophilic substitution is: Acid Chloride > Acid Anhydride > Ester ≈ Carboxylic Acid > Amide.
Elimination Reaction
Reactions in which two atoms or groups of atoms are removed from a molecule, usually creating a carbon-carbon double or triple bond are called as elimination reaction. It is of two types:
- α-elimination or 1,1-elimination
- β-elimination or 1,2- elimination
α-elimination
In this reaction, removal of H and X is done from the same carbon atom. It produces an intermediate called as carbene.
Mechanism
β-elimination
In this reaction, removal of H and X is done from the adjacent carbon atoms. It produces an alkene. It is of two types:
- Unimolecular elimination reaction (E1)
- Bimolecular elimination reaction (E2)
Unimolecular Elimination (E1) Reaction
Mechanism
The acidic dehydration of alcohol proceeds through the formation of a carbocation intermediate as:
Step 1: Alcohol being a Lewis base accepts a proton (H+) from the acid in a reversible step.
Step 2: Due to the presence of positive charge on electronegative oxygen, its electron accepting tendency increases. As a result C-O bind becomes weak and cleaves.
This is a slow step and is regarded as rate determining step.
Step 3: Base removes alpha-H from carbocation and changes it into ethene in a fast step reaction.
Note: Since the intermediate formed in the reaction is carbocation, so the rearrangement is possible.
Ex:
Note: When two possible alkenes are formed by the elimination reaction then that alkene which contains maximum number of alkyl group on double bonded C-atoms (highly substituted alkene) is called as Saytzeff’s product and considered as the major product. While the alkene having less number of alkyl group on double bonded C-atoms (less substituted alkene) is called as Hoffmann’s product.
- Reactivity order of acidic dehydration of alcohols is 3o alcohol > 2o alcohol > 1o alcohol
- Rate = k[Substrate]
- Molecularity of reaction is 1
- Reactivity order of alkyl halides is 3o > 2o > 1o
Bimolecular Elimination (E2) Reaction
The bimolecular elimination reaction is of two types:
- Dehydrohalogenation of halides by using alcoholic KOH/NaNH2
- Pyrolysis of tetra alkyl ammonium ion
Dehydrohalogenation of halides by using alcoholic KOH/NaNH2
Mechanism
It is a one step mechanism in which bond breaking and bond formation takes place simultanoeusly. It is a concerted mechanism.
- Rate = k[Substrate][Base]
- Order of reaction is 2
- Reactivity order of alkyl halides is 3o > 2o > 1o
- No intermediate is formed in this reaction
Ex:
Pyrolysis of tetra alkyl ammonium ion
In this reaction, less substituted alkene is considered as the major product, i.e., Hoffmann’s product. The base attacks on β-hydrogen to form the alkene, so β-hydrogen should be present.
Ex:
Key Points
- SN2/E2 is favoured by high concentration of good nucleophile or strong base.
- SN1/E1 is favoured by low concentration of poor nucleophile or weak base.
- If an alkyl halide undergoes SN2 or SN1, then SN2 reaction will be favoured by high concentration of good nucleophile in presence of polar aprotic solvent whereas SN1 reaction is favoured by low concentration of poor nucleophile in presence polar protic solvent.
Polar protic solvent: H2O, CH3OH, HCOOH
Polar aprotic solvent: DMSO, CH3CN, C2H5OC2H5, DMF
Isomerisation Reaction
Reactions in which a molecule transforms into a new isomer with the exact same molecular formula but a different structural arrangement of its atoms are called as isomerisation reactions.
Reactions in a Flash
| Class of Compounds | Type of Reaction |
|---|---|
| Alkane | Free radical substitution |
| Alkene and Alkyne | Electrophilic addition |
| Alkyl halide | Nucleophilic substitution |
| Aldehyde and Ketone | Nucleophilic addition |
| Carboxylic Acid and its derivatives | Nucleophilic substitution |
| Aromatic compounds | Electrophilic substitution |
Conclusion
To wrap things up, mastering Reaction Mechanisms is the ultimate cheat code to scoring high in organic chemistry. Once you understand the ‘why’ and ‘how’ behind the movement of electrons, you will never have to rely on rote memorization again. Keep practicing the different pathways—from nucleophilic substitutions to electrophilic additions—and always pay attention to the stability of your intermediates.
Remember, organic chemistry is all about logical flow. Keep drawing those curly arrows and start predicting products like a pro! If you ever find yourself struggling to determine which carbocation is more stable or how the inductive effect changes a pathway, don’t worry. You can always jump back to our foundational guide on General Organic Chemistry to brush up on the basics. Keep practicing, and happy studying!