In this article we are going to talk about the very basic concept and before that the terminologies we are using in the organic chemistry especially in reaction mechanism. We all know the chemical reactions but what is the mechanism behind the reactions, how two species are reacting and forming the chemical bonds is something which is very important to answer the questions. So, today we are starting with reagents, types of reagents,
What are reagents in chemistry?
A reagent is a compound or mixture added to a system to cause a chemical reaction or test if a reaction occurs. A reagent may be used to tell whether or not a specific chemical substance is present by causing a reaction to occur with it.
In this article we will be discussing the different types of reagents like electrophilic reagents and nucleophilic reagents.
1. Electrophilic Reagents: they are also called as electrophile.
- Electrophile has electron deficient atom or vacant orbital or incomplete octate at the valence shell.
- These species carry either positive charge or electron deficient molecules. So a reagent which can accept an electron pair in a reaction is called an electrophile. Generally these contain two electrons less than the octet. These attack regions of high electron density in the substrate molecule to complete the octet. These are represented by E+.
- Electrophiles are lewis acid (A Lewis acid is therefore any substance, such as the H+ ion, that can accept a pair of nonbonding electrons. In other words, a Lewis acid is an electron-pair acceptor.)
- An electrophile is a species that accepts a pair of electrons to form a new covalent bond
There are 2 types of electrophiles.
(a) Neutral electrophile: These species carry neither positive charge nor negative charge
Example: AlCl3, BF3, Carbene, Nitrene, free radicals SO3, , Acid chlorides etc.
(b) Positive electrophile: they are also called Positively charged Electrophile
Examples: H+, H3O+, Cl+, Br+, NO2+, NO+, Carbonium ion, nitrosonium ion, diazonium ion
Note: cation of IA group and IIA group like Na+, Mg++, Ca++ and also NH4+ are not Positive electrophile
2. Nucleophilic reagents: they are also called as nucleophile
- A nucleophile is a chemical species that donates an electron pair to an electrophile to form a chemical bond in relation to a reaction.
- All molecules or ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.
- They have electron rich atoms and lone pairs of electron so they are negatively charged species
- Nucleophiles are from “nucleus loving”, or “positive-charge loving
- A nucleophile is a reactant that provides a pair of electrons to form a new covalent bond.
There are 2 types nucleophiles.
(a) Neutral nucleophile:
NH3, RNH2, H–O–H , R–OH , R–O–R , R–S–R.
- LiAlH4: it can donate H ion with a lone pair of electron
- LiBH4 : it can donate H ion with a lone pair of electron
- RMgX: Grignard Reagent: here R acts as a nucleophile but Mg++ is not an electrophile
(b) Negative nucleophile:
Examples: Cl– (chloride ion), Br– ( Bromide ion), I– (Iodide ion), OH– , CN– Carbanions like CH3–, CH3CH2– etc.
1. Amphiphile: they behave as electrophile as well as nucleophile
Examples: H2O, HCHO, R-CN
Here we have given a very basic concept about the various species that come under the category of reagents and play a very important role in the organic chemistry especially in reaction mechanism.
Lots of time in the various engineering entrance examination, questions have been asked on the category only by asking that which one is the electrophile, neutral electrophile, nucleophile etc.
Let’s see few questions as an example which can be asked ion your JEE, UPSEE/UPTU, VITEE, NEET and WBJEE.
Q1. Which of the following cannot react as a nucleophile?
Q2. Which of the following is not a typical electrophile?
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