Substitution reaction refers to a reaction in which an atom or group of atoms in a compound or organic molecule is replaced by another atom or group of the same type in the reagent, expressed by the formula:
R-L (reaction matrix) + AB (offensive Reagents) → R-A (substituted product) + LB (leaving the group)
belong to the class of chemical reactions.
The substitution reaction is very important in organic chemistry, and the substitution reaction is also present in inorganic chemistry and is not limited to organic chemistry.
The substitution reaction can be classified into three types. Nucleophilic substitution, electrophilic substitution, and homo-cleavage substitution. If a substitution reaction occurs in the molecule group between the substituted referred molecules. In some substitution reactions, molecular rearrangement occurs simultaneously (see rearrangement reaction).
It should be noted that the substitution reaction can occur in inorganic chemistry, for example:
B 2 H 6 + BCl 3 ⇌ B 2 H 5 Cl + BHCl 2
B 2 H 6 + NH 3 → μ-NH 2 B 2 H 5 + H 2
1. In organic chemistry, electrophilic and nucleophilic substitution reactions are very important. Organic substitution reactions are classified into several organic reaction categories based on the following characteristics:
Whether the reactant that promotes the reaction is an electrophile or a nucleophile.
The intermediate in the reaction is a cation, an anion or a free radical or a two-step reaction occurs simultaneously.
The matrix of the reaction is an aliphatic compound or an aromatic compound.
A detailed understanding of the reaction categories is not only helpful in predicting reaction products, but also helps us optimize the reaction from variables such as temperature and solvent.
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Substitution reaction classification
Nucleophilic substitution reaction
Referred to as SN (S is English “Substitution” (replacement), N is “Nucleophilic” (nuclear), all take the first capital letter). There are many nucleophilic substitution reactions on saturated carbon.
For example, an alkyl halide can be subjected to a nucleophilic substitution reaction with sodium hydroxide, sodium alkoxide or sodium phenolate, thiourea, sodium thiolate, a carboxylate, and ammonia or an amine to form an alcohol, an ether, a thiol, a thioether, or a carboxylic acid, Esters and amines, etc.
The alcohol can react with a hydrohalic acid, a phosphorus halide or a thionyl chloride to form a halogenated hydrocarbon. The alkyl halide is reduced to an alkane by lithium aluminum hydride, which is also a substitution of a negative hydrogen ion for the halogen in the reactant.
When the nucleophilic atom of the reagent is carbon, the substitution results in a carbon-carbon bond, thereby obtaining a carbon chain growth product such as a reaction of a halogenated alkane with sodium cyanide, sodium acetylide or an enolate.
In the nucleophilic substitution reaction, the C-X bond is first cleaved to form a positive carbon ion and then react with a reagent to form a C-Y bond. This reaction is called a single molecule nucleophilic reaction and is recorded as SN1.
The simultaneous reaction of C-X bond cleavage and C-Y bond formation is called bimolecular nucleophilic substitution reaction and is referred to as SN2.
Due to the difference in reactant structure and reaction conditions, SN has two mechanisms, namely, a single molecule nucleophilic substitution reaction SN1 and a bimolecular nucleophilic substitution reaction SN2.
The process of SN1 is divided into two steps: in the first step, the reactant undergoes bond cleavage (ionization) to form a positive intermediate carbon ion and a leaving group: in the second step, the positive carbon ion rapidly combines with the reagent to form a product.
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The overall reaction rate is only proportional to the concentration of the reactants, regardless of the concentration of the reagent. SN2 is a synergistic 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. A reactant capable of generating a relatively stable carbocation and a leaving group is liable to undergo SN1, and a central carbon atom sterically hinders a small reactant from easily occurring SN2.
If the nucleophile is basic, the nucleophilic substitution reaction is often accompanied by an elimination reaction, the ratio of which depends on the structure of the reactants, the nature of the reagents, and the reaction conditions. Low temperature and weak alkali are beneficial to SN substitution
Substitution Reaction Mechanism
The nucleophilic substitution reactions of halocarbons mainly include the following categories:
The reaction of a halocarbon with water to form alcohol is called a hydrolysis reaction. The reaction is reversible:
RX + HOH → ROH + HX.
In fact, the reaction is generally of no preparative value. Most halogenated hydrocarbons are prepared from the corresponding alcohols.
However, since the introduction of a hydroxyl group in some complex molecules is more difficult than the introduction of a halogen atom, the hydrolysis of a halocarbon is sometimes employed to synthesize the corresponding alcohol.
In this case, the halocarbon is often co-heated with an aqueous solution of potassium hydroxide or sodium hydroxide to obtain RX+NaOH→ROH+NaX.
The reaction can be carried out completely because OH- is more nucleophilic than X- The hydrogen halide produced is again neutralized by a base. Such as ethanol and acetic acid esterification reaction.
Reacts with sodium cyanide. The halocarbon reacts with potassium cyanide or sodium cyanide in an alcohol solution to form a nitrile. In organic synthesis, it is often necessary to grow a carbon chain.
The reaction of a halocarbon with potassium cyanide (sodium) is one of the methods of increasing one carbon atom.
However, potassium cyanide (sodium) is highly toxic and must be used with special care. The carboxylic acid (-COOH) and its derivatives can be synthesized by the conversion of a nitrile group (-CN).
The first two reactions are one of the methods for preparing ethers and amines by reaction with sodium alkoxide, ammonia, silver nitrate and the like.
The latter reaction is often used for the identification of halocarbons (see below), but some inactive halocarbons do not react with silver nitrates, such as ArX, RCH=CHX, HCCl3, ArCOCHCl, and ROCHCHCl.
Reaction with organic phosphorus. Halocarbons and the 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
wherein X is I, Br, Cl, etc. and R is an alkyl group or an aryl group. Phosphorus ylide is generally stable, but sometimes it is very lively. There are two limit types in the form of resonance.
Electrophilic substitution reaction
The electrophilic substitution reaction mainly occurs on the aromatic system or the electron-rich unsaturated carbon.
In essence, the stronger electrophilic group attacks the negative electron system and replaces the weaker electrophilic group. Among them are sulfonation reactions, nitration reactions, halogenation reactions, etc.
Homogeneous split substitution reaction
Referred to as SH (S is English “Substitution” (replaced)). The reaction of a free radical to an atom in a reactant molecule to produce a product and a new free radical.
This reaction is usually a chain transfer step of a free radical chain reaction. Some organic substances will undergo auto-oxidation in the air, and the process is also homo-cracked.
Aromatic substitution reaction
The aromatic-electrophilic substitution reaction SEAr and the aromatic nucleophilic substitution reaction SNAr (S represents substitution, N represents nucleophilic, Ar represents aromatic), and Ar represents an aryl group.
The aromatic hydrocarbon can be introduced into the aromatic ring by nitration, halogenation, sulfonation, and alkylation or acylation, respectively, to introduce a nitro group, a halogen atom, a sulfonic acid group, and an alkyl group or an acyl group, which are all SEAr.
A compound having a substituent on the aromatic ring, and the substituent agent has a localization 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 the ortho and para positions more frequently. when the substituent is an electron withdrawing group, the Meta product is dominant.
In addition, in addition to these normal reactions, sometimes the reagent can attack the position of the original substituent and replace it, which is called in situ substitution.
SNAr requires certain conditions to proceed.
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For example, a halogenated aromatic hydrocarbon is generally less susceptible to SNAr, but when the halogen atom is activated by an ortho or para-nitro group, it is easily substituted.
Halogenated aromatic hydrocarbons can also be substituted under strong base conditions.
In addition, the aromatic diazonium salt can form stable molecular nitrogen due to the cleavage of the leaving group, which is favorable for the formation of a phenyl cation, and a reaction similar to SNl can also occur.
Addition and substitution reaction difference
Substitution reaction definition: 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 to displacement reactions, but in reality, it is more like a metathesis reaction. Example: Taking CH 4 and Cl 2 as an example, the principle is: one H is replaced by one Cl, that is, the C—H bond becomes a C—Cl bond. The remaining Cl produces HCl with the substituted H. The characteristic is that one H is replaced, one Cl 2 is consumed, and one HCl is produced.
Addition reaction definition: a reaction in which an unsaturated carbon atom in an organic molecule directly combines with other atoms or atomic groups to form a new substance.
Type comparison: Similar to the chemical reaction, it is similar to the chemical reaction. Example: Taking CH 2 =CH 2 and Br 2 as an example, the principle is that the double bond in C=C is broken, and the two Cs each form a half bond, which is combined with two Br.
The characteristic is that the double key changes to a single key and the unsaturated becomes saturated.