Why are Substitution Reactions Important Substitution reaction and classification

Why are Substitution Reactions Important Substitution reaction and classification

Substitution reaction refers to the reaction in which any atom or atomic group in a compound or an organic molecule is replaced by another atom or atomic group of the same type in a reagent. Reagent) → R-A (substitution product) + LB ( leaving group ) belongs to a class of chemical reactions.

Substitution reactions are very important in organic chemistry, while substitution reactions also exist in inorganic chemistry, and are not limited to organic chemistry.

Basic Information

  • The substitution reactions can be divided into three types : nucleophilic substitution , electrophilic substitution and homolytic substitution. If the substitution reaction occurs between the groups in the molecule , it is called intramolecular substitution. In some substitution reactions, molecular rearrangements occur simultaneously (see Rearrangement reactions ).
  • Note that substitution reactions can occur in inorganic chemistry , such as:

2 H 6 + BCl 3 ⇌ B 2 H 5 Cl + BHCl 2

2 H 6 + NH 3 → μ-NH 2 B 2 H 5 + H 2

  • In organic chemistry, electrophilic and nucleophilic substitution reactions are very important. Organic substitution reactions are classified into several organic reaction categories according to the following characteristics:
  • Whether the reaction- promoting reactant is an electrophile or a nucleophile .
  • Whether the intermediate in the reaction is a cation, an anion or a free radical or a two-step reaction occurs simultaneously.
  • Whether the reaction substrate is an aliphatic compound or an aromatic compound .
  • A detailed understanding of the reaction categories is not only helpful in predicting the reaction products, but also in optimizing the reaction from variables such as temperature and solvents.

Classification of substitution reactions

Nucleophilic substitution reaction

Abbreviated SN (S stands for “Substitution” in English, N stands for “Nucleophilic”, both take the first capital letter). Nucleophilic substitution reactions on saturated carbon are numerous. 

For example, haloalkanes can undergo nucleophilic substitution reactions with sodium hydroxide, sodium alkoxide or sodium phenate, thiourea, sodium thiolate, carboxylate, and ammonia or amine, respectively, to form alcohols, ethers, thiols, thioethers, and carboxylic acids. Esters and amines. Alcohols can react with halogen acids, phosphorus halides, or sulfoxides to form halogenated hydrocarbons.

 Haloalkanes are reduced to alkanes by lithium aluminum hydride , which is also the substitution of negative hydrogen ions for halogens in the reactants . When the nucleophilic atom of the reagent is carbon, a carbon-carbon bond is formed as a result of the substitution, thereby obtaining a carbon chain extension product, such as the reaction of a haloalkane with sodium cyanide, sodium alkynide or an enolate . 

In the nucleophilic substitution reaction, the C-X bond is first broken to generate a carbohydrate ion and then react with the reagent to form a C-Y bond. This reaction is called a single-molecule nucleophilic reaction and is referred to as SN1. 

The reaction that occurs when the C-X bond is broken and the C-Y bond is formed at the same time is called a bimolecular nucleophilic substitution reaction, denoted as SN2.

Due to the differences in reactant structure and reaction conditions, SN has two mechanisms, namely the single molecule nucleophilic substitution reaction SN1 and the bimolecular nucleophilic substitution reaction SN2. 

The process of SN1 is divided into two steps. In the first step, the reactants undergo bond cleavage (ionization) to generate the active intermediate carboions and leaving groups, in the second step, the carboions quickly combine with the reagents to become products. 

The total reaction rate is only proportional to the reactant concentration, and is not related to the reagent concentration. SN2 is a cooperative process in which old bond breaks and new bond formation occur simultaneously. 

The reaction rate is proportional to both the reactant concentration and the reagent concentration. Reactants that can generate relatively stable carbocations and leaving groups are prone to SN1, and reactants with small central carbon atom space barriers are prone to SN2. If the nucleophile is basic, the nucleophilic substitution reaction is often accompanied by an elimination reaction.

The ratio of the two depends on the structure of the reactant, the nature of the reagent, and the reaction conditions. Low temperature and weak alkaline are good for SN replacement

The nucleophilic substitution reactions of halogenated hydrocarbons are mainly of the following types:

hydrolysis. The reaction of adding a halocarbon to water to form an alcohol is called a hydrolysis reaction. The reaction is reversible:

RX + HOH → ROH + HX.

In fact, this reaction generally has no preparation value. Most halogenated hydrocarbons are prepared from the corresponding alcohols. However, because it is more difficult to introduce a hydroxyl group than a halogen atom in some complex molecules, the hydrolysis of halocarbons is sometimes used to synthesize the corresponding alcohol. 

In this case, RX + NaOH → ROH + NaX is often prepared by co-heating a halocarbon with an aqueous solution of potassium hydroxide or sodium hydroxide. The reaction can proceed completely because OH- is more nucleophilic than X-, the resulting hydrogen halide is neutralized by the base. Such as: Esterification of ethanol and acetic acid.

Reacts with sodium cyanide. Halogens react with potassium cyanide or sodium cyanide in an alcohol solution to form nitriles. In organic synthesis, it is often necessary to grow a carbon chain. The reaction of halogenated hydrocarbons with potassium (sodium) is one of the methods to grow a carbon atom. However, potassium cyanide (sodium) is highly toxic and requires special attention when used. Carboxylic acid (-COOH) and its derivatives can be synthesized through the conversion of nitrile group (-CN).

With sodium alkoxide, ammonia, silver nitrate, etc., the first two reactions are one of the methods for preparing ethers and amines. The latter reaction is often used for the identification of halogenated hydrocarbons (see below), but some inactive halogenated hydrocarbons do not react with silver nitrate, such as ArX, RCH = CHX, HCCl3, ArCOCHCl, and ROCHCHCl.

Reaction with organophosphorus. Halocarbons and trihydrocarbylphosphine effect obtained phosphonium salt, which loses a proton at a strong base to give a phosphorus ylide (ylide) or referred to by phosphorus YE Li (ylene). RCHX + RPRPCHRX where: X is I, Br, Cl, etc. R is alkyl or aryl. Phytolide is generally stable, but sometimes very lively. There are two kinds of limit expressions in terms of resonance expressions.

Electrophilic substitution reaction

The electrophilic substitution reaction mainly occurs on aromatic systems or electron-rich unsaturated carbons. In essence, the stronger electrophilic groups attack the negative electron system and replace the weaker electrophilic groups. Among them are sulfonation, nitration, halogenation, etc.

Homolytic substitution reaction

Referred to as SH (S stands for “Substitution” in English). It is a radical attack on an atom in the reactant molecule, which generates a reaction between the product and a new radical. This reaction is usually a chain transfer step of a free radical chain reaction. Some organic compounds will automatically oxidize in the air, and the process is also homolytic replacement. For example, benzaldehyde, cumene and tetralin can interact with oxygen to generate corresponding organic peroxides.

Aromatic Substitution Reaction

Aromatic electrophilic substitution reactions SEAr and aromatic nucleophilic substitution reactions SNAr (S stands for substitution, N stands for nucleophilic, Ar stands for aromatic), Ar stands for aryl. 

Aromatics can introduce nitro, halogen, sulfonic and alkyl or acyl groups on the aromatic ring through nitration, halogenation, sulfonation, and alkylation or acylation reactions,all of which are SEAr. For compounds that have substituents on the aromatic ring, the substituent has a positioning effect on the attack of the reagent. 

When the substituent on the benzene ring is an electron-donating group and a halogen atom, the electrophile enters its ortho and para positions more, when the substituent is an electron-withdrawing group, the meta product is mainly obtained. 

In addition, in addition to these normal reactions, sometimes the reagent can attack and replace the position of the original substituent, this situation is called in situ substitution.

SNAr requires certain conditions to proceed. Such as halogenated aromatic hydrocarbons are generally not prone to SNAr, but when halogen atoms are activated by ortho or para nitro groups, they are easily replaced. 

Halogenated aromatics can also be substituted under strong base conditions. In addition, the aromatic diazonium salt becomes a stable molecular nitrogen due to the cleavage of the leaving group, which is conducive to the generation of phenyl cations, and a reaction similar to SNl can also occur.

Difference between addition and substitution

Definition of substitution reaction: A reaction in which one atom or group of atoms in an organic molecule is replaced by another atom or group of atoms. Type comparison: Many reference books often compare it with displacement reactions but in fact it is more like metathesis reactions.

Example: Take the reaction between CH4 and Cl2 as an example. The principle is that one H is replaced by one Cl, that is, the C—H bond becomes a C—Cl bond. The remaining Cl produces HCI with the substituted H. The characteristic is that one H is replaced, one Cl2 is consumed, and one HCl is produced.

Addition reaction definition: The reaction in which unsaturated carbon atoms in organic molecules directly combine with other atoms or atomic groups to form new substances. Type comparison: From the point of view of the type of substance, it is similar to the chemical reaction. 

Example: Take the reaction between CH2 = CH2 and Br2 as an example. The principle is that one of the double bonds in C = C is broken, and two Cs each form a half bond, which are respectively combined with two Br. The characteristic is that double bonds become single bonds, and unsaturated becomes saturated.

Sanjay Bhandari


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