Chemistry
Substitution Reaction
A substitution reaction in chemistry involves the replacement of an atom or group of atoms in a molecule with another atom or group of atoms. This process typically occurs when a reactant molecule interacts with a reagent, resulting in the substitution of one or more atoms or groups in the reactant molecule. Substitution reactions are fundamental to organic and inorganic chemistry.
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6 Key excerpts on "Substitution Reaction"
eBook - PDF- John A. Olmsted, Gregory M. Williams, Robert C. Burk(Authors)
- 2020(Publication Date)
- Wiley(Publisher)
However, the number of distinctly different types of organic reactions is surprisingly small. In this chapter, we will study three important types of reactions; namely, substitution, elimination, and addition reactions. In a Substitution Reaction, as the name suggests, one functional group is substituted for another. An example is shown below, where a nitro group substitutes for a hydrogen atom on a benzene ring: HNO 3 NO 2 H 2 O H 2 SO 4 + + In an elimination reaction, atoms or groups of atoms that are bound to adjacent carbon atoms are eliminated, generally as a small molecule. This results in the formation of a double bond between the carbon atoms. For example, ethanol can undergo a reaction to form ethene with the elimination of water: H OH H H H H H H H H H 2 O + And finally, in an addition reaction, a molecule is added across a double (or triple) bond, resulting in a single (or double) bond. An example is the chlorination of ethene to make 1,2-dichloroethane: Cl Cl H H H H H H H H Cl 2 + Chemical Space—How Many Possible Drug Compounds Are There? New drug molecules have been synthesized and tested by the thou- sands over the years. However, recent estimates suggest that only a tiny fraction of the potential medicines that could be made have been synthesized so far. Some estimates suggest that there are as many as 10 60 potentially interesting small molecules that we have yet to synthesize or test. This staggering number is not too different from estimates of the number of atoms in the universe—how can we possibly decide which compounds to spend time and effort on? Combinatorial chemistry involves the automated synthesis of huge libraries of different but related compounds. Pharmaceu- tical companies in particular have used robotic approaches to syn- thesize hundreds of thousands of new and unique compounds per year.
eBook - PDF- Metin Balcı(Author)
- 2021(Publication Date)
- Wiley-VCH(Publisher)
50 2 Nucleophilic Substitution Reaction 2.1 Types of Chemical Reactions During chemical reactions, the original chemical bonds between the atoms are broken and new bonds are formed. When we examine the reactions in terms of breaking and forming new bonds, we encounter three groups. 2.1.1 Polar Reactions Bond breaking in which the covalent bond between two chemical species is broken unequally so that the bonding electrons are retained by one of the chemical species. This kind of bond breaking is called heterolytic bond cleavage. When a neutral molecule undergoes heterolytic bond cleavage, one of the products will have a positive charge, while the other will have a negative charge. The charge distribution depends on the electronegativity difference between the atoms. A B A A Heterolytic bond cleavage B B + + 2.1.2 Radical Reactions Bond breaking in which the bonding electron pair is split evenly between the products so that one electron is retained by each of the original fragments of the molecule. This kind of bond breaking is called homolytic bond cleavage. When a neutral molecule undergoes homolytic bond cleavage, two free radicals are formed. Because radicals are very unstable, they react in a variety of ways (see Section 9.1). A B A + B Homolytic bond cleavage 2.1.3 Pericyclic Reactions Although most organic reactions take place by way of ionic or radical intermediates, many useful reactions occur in one-step processes that do not form reactive intermediates. They are not affected by solvent changes. Bond breaking and bond for- mation take place simultaneously in a concerted manner. For example, the Diels–Alder reactions fall into this group. + Diels–Alder reaction Polar reactions constitute a significant part of organic chemistry. Polar reactions should also be classified. In nucleophilic Substitution Reactions, an electronegative atom or an electron-attracting group is separated from the molecule and replaced by another atom or group.
eBook - PDF- T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
- 2017(Publication Date)
- Wiley(Publisher)
240 6 Not all substitutions are a good thing; for instance, we would not want to accidentally use salt in place of the needed amount of sugar in a batch of chocolate chip cookies. But with some substitutions, we get something even better. In organic chemistry that is often the case, since nucleophilic Substitution Reactions (which we will learn about in this chapter) allow the conversion of functional groups within a given molecule into entirely different functional groups, leading to new compounds with distinct properties. Moreover, nature utilizes a number of specific Substitution Reactions that are required for life. IN THIS CHAPTER WE WILL CONSIDER: • what groups can be replaced (i.e., substituted) or eliminated • the various mechanisms by which such processes occur • the conditions that can promote such reactions ▸ WHY DO THESE TOPICS MATTER? At the end of the chapter, we will show an example where just a few Substitution Reactions can convert table sugar into a sweetener that has no calories—a sugar substitute that is not salty, but is in fact 600 times sweeter than sugar itself! C H A P T E R Nucleophilic Reactions PROPERTIES AND Substitution ReactionS OF ALKYL HALIDES photo credit: (sugar bowl) Sylvie Shirazi Photography/Getty Images (salt pouring) Tom Grill/Getty Images (sugar pouring) Tom Grill/Getty Images 6.1 ALKYL HALIDES 241 6.1 ALKYL HALIDES • An alkyl halide has a halogen atom bonded to an sp 3 -hybridized (tetrahedral) carbon atom. • The carbon–halogen bond in an alkyl halide is polarized because the halogen is more electronegative than carbon. Therefore, the carbon atom has a partial positive charge (δ+) and the halogen has a partial negative charge (δ−). X C δ+ δ– • Alkyl halides are classified as primary (1°), secondary (2°), or tertiary (3°) accord- ing to the number of carbon groups (R) directly bonded to the carbon bearing the halogen atom (Section 2.5).
eBook - PDF- M Mortimer, P G Taylor, Lesley E Smart, Giles Clark(Authors)
- 2007(Publication Date)
- Royal Society of Chemistry(Publisher)
Not only have we introduced Substitution Reactions, but we have also introduced mechanistic organic chemistry. You will meet curly arrows, nucleophiles, electrophiles and leaving groups again and again and again in organic chemistry! An understanding of reaction mechanisms is invaluable because it enables organic chemists to make predictions about reactions that have not yet been carried out. When we discuss synthesis later, you will see how important it is to be able to predict reaction mechanisms accurately ! Complete the following definitions by choosing one of the alternative endings. (i) Heterolysis of a covalent bond produces ... (a) radicals; (b) ions. The s N 2 mechanism involves ... (a) one step; (b) two steps. (iii) The reactivity and concentration of a nucleophile affect the rate of ... (a) a reaction proceeding by the sN2 mechanism; (b) a reaction proceeding by the SNI mechanism. (ii) 179 Now that you have completed Part 2 The Mechanism of Substitution, you should be able to do the following: 1 Recognize valid definitions of and use in a correct context the terms, concepts and principles in the following table (All questions). List of scientific terms, concepts and principles used in Part 2 of this book. 2 Given the reactants and products of an organic reaction, (a) classify the reaction as a substitution, elimination, or addition reaction; (b) decide whether an ionic or a radical mechanism is operating. (Questions 1.1, 2.1 and CD-ROM Questions) Given the reactants and products of a nucleophilic Substitution Reaction, (a) specify the nucleophile, leaving group, and electrophilic carbon centre; (b) if possible, decide whether an SNl or SN2 mechanism is operating. (Questions 3.1, 5.1 and CD-ROM Questions) Use curly arrows to illustrate reaction mechanisms. (CD-ROM Questions) Write balanced equations to illustrate SN reactions. (Questions 3.2, 4.1 and CD-ROM Questions) 3 4 5 180
eBook - PDFMicrowave-Assisted Organic Synthesis
A Green Chemical Approach
- Suresh C. Ameta, Pinki B. Punjabi, Rakshit Ameta, Chetna Ameta, Suresh C. Ameta, Pinki B. Punjabi, Rakshit Ameta, Chetna Ameta(Authors)
- 2014(Publication Date)
- Apple Academic Press(Publisher)
85 5.16 Miscellaneous .................................................................................... 86 Keywords .................................................................................................... 94 References ................................................................................................... 94 70 Microwave-Assisted Organic Synthesis: A Green Chemical Approach 5.1 INTRODUCTION In a Substitution Reaction, a functional group in a particular chemical compound is replaced by another group. Substitution Reaction is also known as displacement reaction or replacement reaction. Substituted compounds are those chemical com-pounds, where one or more hydrogen atoms of a core structure have been replaced with a functional group like alkyl, hydroxy, halogen, etc. Organic substitution reac-tions are classified in several main organic reaction types depending on: (i) whether the reagent that brings about the substitution is an electrophile or a nucleophile. (ii) whether a reactive intermediate involved in the reaction is a carbocation, a carbanion or a free radical. (iii) whether the substrate is aliphatic or aromatic. Detailed understanding of a reaction type helps to predict the product outcome in a reaction. It is also helpful for optimizing a reaction with regard to variables such as tempera-ture and choice of solvent. Ju et al. (2006) carried out microwave synthesis of various azides, thiocyanates, and sulfones in an aqueous medium by reacting alkyl halides or tosylates with alkali azides, thiocyanates, or sulfinates at 110 o C for 20 min in the absence of any phase transfer catalyst, and a variety of reactive functional groups are tolerated. This is a practical, rapid, and efficient microwave synthesis method. 1.3 eq. of alkali azides, thiocyanates, or sulfinates are required to react with alkyl monohalide or alkyl monotosylates to give azides, thiocyanates, and sulfones, respectively.
eBook - PDF- T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
(c) Br (b) CH 3 Br (a) 6.2 NUCLEOPHILIC Substitution ReactionS Nucleophilic Substitution Reactions are among the most fundamental types of organic reactions. In a nucleophilic Substitution Reaction a nucleophile (Nu ⋅ ⋅ ) displaces a leaving group (LG) in the molecule that undergoes the substitution (the substrate). • The nucleophile is always a Lewis base (electron pair donor), and it may be negatively charged or neutral. • The leaving group is always a species that takes a pair of electrons with it when it departs. Often the substrate is an alkyl halide ( R − X ⋅⋅ ⋅⋅ ⋅ ⋅ ) and the leaving group is a halide anion ( ⋅ ⋅ X ⋅⋅ ⋅⋅ ⋅ ⋅ − ). The following equations include a generic nucleophilic Substitution Reaction and some specific examples. PRACTICE PROBLEM 6.2 Classify each of the following organic halides as primary, secondary, tertiary, alkenyl, or aryl. (e) I (d) F (c) Br (b) Cl (a) Br 6.2 NUCLEOPHILIC Substitution ReactionS 243 + + + OCH 3 CH 3 CH 2 CH 3 OH Nu R The nucleophile uses its electron pair to form a new covalent bond with the substrate carbon. + + + - CH 3 O - HO - Nu The nucleophile is a Lewis base that donates an electron pair to the substrate. Br - I - LG - The leaving group gains the pair of electrons that originally bonded it in the substrate. CH 3 CH 2 Br CH 3 I LG R The bond between the carbon and the leaving group breaks, giving both electrons from the bond to the leaving group. + I + - Cl Cl I - + + R— N—CH 3 R R + R—N R R I - H 3 C—I In nucleophilic Substitution Reactions the bond between the substrate carbon and the leaving group undergoes heterolytic bond cleavage. The unshared electron pair of the nucleophile forms the new bond to the carbon atom.
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