Reactions of Haloalkanes

11 Key excerpts on "Reactions of Haloalkanes"

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  eBook - ePub

  Principles of Organic Chemistry

  • Robert J. Ouellette, J. David Rawn(Authors)
  • 2015(Publication Date)
  • Elsevier

    (Publisher)

  7

  Nucleophilic Substitution and Elimination Reactions

  7.1 Reaction Mechanisms and Haloalkanes

  We introduced the concept of functional groups and their role in the organization of the structures of organic molecules in Section 1.9 . We described the importance of reaction mechanisms as an organizational device to classify chemical reactions in Section 2.9 . The details of the electrophilic addition reactions of alkenes (Section 4.9 ) and electrophilic substitution reactions of aromatic compounds (Section 5.5 ) are examples of two important reaction mechanisms. In this chapter we examine two more types of reactions mechanisms—nucleophilic substitution and elimination reactions. These mechanisms often occur in competition with one another and describe the reactions of several classes of compounds, such as haloalkanes (also called alkyl halides) and alcohols. In this chapter we focus on the substitution and elimination Reactions of Haloalkanes. These reactions illustrate the role of structure in determining the degree to which a given reaction mechanisms occurs.

  Reactivity of Haloalkanes

  Haloalkanes have a halogen atom bonded to an sp3 -hybridized carbon atom. As a result of the greater electronegativity of the halogens, the carbon atom of the carbon-halogen bond bears a partial positive charge and the halogen atom has a partial negative charge.

  where X = F, Cl, Br, I

  Since a carbon-halogen bond is polar, a haloalkane has two sites of reactivity. One is at the carbon atom bonded to the halogen atom. This carbon atom is electropositive and reacts with nucleophiles. The second site of reactivity in a haloalkane is the hydrogen atom bonded to the carbon atom adjacent to the carbon atom bonded to the halogen atom. This hydrogen atom is more acidic than the hydrogen atoms in alkanes because the halogen atom on the adjacent carbon atom withdraws electron density by an inductive effect.

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  Organic Chemistry

  • John M. McIntosh(Author)
  • 2018(Publication Date)
  • De Gruyter

    (Publisher)

  This will greatly simplify review. It is also important to realize that the reactions must be learned frontwards and backwards. That is – we will see a reac-tion where A gives B under certain conditions. You should remember this in terms of how A reacts and also how to prepare B. https://doi.org/10.1515/9783110565140-005 62 | 5 Reactions of Alkanes, Alkenes, and Alkynes 5.2.2 Halogenation The replacement of hydrogen atoms by halogen (usually chlorine) is another common reaction of alkanes. The products are called alkyl halides . (Alkyl is the term used to describe a general structure of the type C n H 2 n + 1 ). The reaction is used frequently in industrial processes CH 3 CH 3 + Cl 2 󳨀→ CH 3 CH 2 Cl + HCl The products, particularly if they are polyhalogenated (i.e., they contain several halo-gen atoms), are useful as flame retardants, insecticides, herbicides, and solvents. When alkanes of more complex structures are used, it is found that tertiary hydro-gens are replaced at a faster rate than secondary which, in turn, are replaced faster than primary hydrogens. It is frequently difficult to get clean replacement of one type to the complete exclusion of others and as a result, mixtures of products are com-monly obtained. If these mixtures can be used directly, this poses no problem, but frequently very undesirable properties are associated with the impurities. An exam-ple of this can be found in the chlorination of an organic molecule called phenol. The desired product – 2,4,6-trichlorophenol is contaminated with another product called dioxin, which has the reputation, perhaps undeserved, of being one of the most toxic compounds known. 5.3 Electrophilic Addition to Alkenes: Our First Mechanism E + E X X E Fig. 5.1 Alkenes (olefins) are electron-rich molecules; that is they contain more electrons than are required to hold the atoms together in the molecule. Therefore, they can be con-sidered to be nucleophilic compounds.

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  Introduction to Organic Chemistry

  • William H. Brown, Thomas Poon(Authors)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  190 YOU MAY HAVE HEARD of the term chlorofluorocarbons and their well‐documented harm to the environment. Chlorofluorocarbons belong to a larger class of compounds named haloalkanes or, in the common system of nomenclature, alkyl halides, compounds containing at least one halogen atom covalently bonded to an sp 3 hybridized carbon atom. The general symbol for an alkyl halide is R X, where X may be F, Cl, Br, or I: R X A haloalkane (An alkyl halide) In this chapter, we study two characteristic Reactions of Haloalkanes: nucleophilic sub- stitution and β‐elimination. We will see that haloalkanes can be quite useful molecules because they can be converted to alcohols, ethers, thiols, amines, and alkenes and are Haloalkane (alkyl halide) A compound containing a halogen atom covalently bonded to an sp 3 hybridized carbon atom; given the symbol RX. K E Y Q U E S T I O N S 7.1 How Are Haloalkanes Named? 7.2 What Are the Characteristic Reactions of Haloalkanes? 7.3 What Are the Products of Nucleophilic Aliphatic Substitution Reactions? 7.4 What Are the S N 2 and S N 1 Mechanisms for Nucleophilic Substitution? 7.5 What Determines Whether S N 1 or S N 2 Predominates? 7.6 How Can S N 1 and S N 2 Be Predicted Based on Experimental Conditions? 7.7 What Are the Products of β‐Elimination? 7.8 What Are the E1 and E2 Mechanisms for β‐Elimination? 7.9 When Do Nucleophilic Substitution and β‐Elimination Compete? H O W TO 7.1 How to Name Cyclic Haloalkanes 7.2 How to Recognize Substitution and β‐Elimination Reactions 7.3 How to Complete a Substitution Reaction 7.4 How to Predict the Type of Substitution Reaction a Haloalkane Will Undergo 7.5 How to Complete an Elimination Reaction 7.6 How to Draw Mechanisms 7.7 How to Predict the Type of β‐Elimination Reaction a Haloalkane Will Undergo C H E M I C A L C O N N E C T I O N S 7A The Environmental Impact of Chlorofluorocarbons 7B The Effect of Chlorofluorocarbon Legislation on Asthma Sufferers Carolyn A.

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  eBook - ePub

  Understanding Advanced Organic and Analytical Chemistry

  The Learner's ApproachRevised Edition

  • Kim Seng Chan, Jeanne Tan;;;(Authors)
  • 2016(Publication Date)
  • WS EDUCATION

    (Publisher)

  CHAPTER 7

  Halogen Derivatives

  7.1 Introduction

  Halogenoalkanes, also known as alkyl halides, are saturated organic compounds that contain the −C−X functional group (X = F, Cl, Br or I). They are important derivatives of alkanes and have the general formula Cn H2n+1 X. An example is bromoethane:

  Halogenoalkanes do not occur naturally. In fact, they are the by-products of the reaction of alkanes or alkenes with halogen, as these hydrocarbons are commonly found in petroleum. Halogenoalkanes are generally known as the “workhorse” in organic chemistry as they are very useful intermediates to be converted to other more important specialty chemicals of greater economic value. Some halogenoalkanes, such as chlorofluorocarbon, can also be harmful to the environment.

  Halogenoarenes (or aryl halides) are aromatic compounds with a halogen atom directly attached to the benzene ring. Similar to halogenoalkanes, halogenoarenes do not occur naturally and are in fact synthesized by reacting aromatic compounds isolated from petroleum with halogens.

  7.2 Nomenclature

  A halogenolkane is obtained when one or more hydrogen atoms of an alkane molecule have been replaced by halogen atoms via the free radical substitution reaction. Other than this, halogenoalkanes can also be obtained when hydrogen halide (HX) or the diatomic halogen molecules add across an alkene double bond through the electrophilic addition mechanism. Thus, one can simply perceive halogenoalkanes as substituted alkanes. Therefore, halogenoalkanes are named in a similar manner to alkanes — the suffix ends in — ane

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  eBook - ePub

  Organic Chemistry Study Guide

  Key Concepts, Problems, and Solutions

  • Robert J. Ouellette, J. David Rawn(Authors)
  • 2014(Publication Date)
  • Elsevier

    (Publisher)

  9

  Haloalkanes and Alcohols Introduction to Nucleophilic Substitution and Elimination Reactions

  Keys to the Chapter

  9.1 Functionalized Hydrocarbons

  In this chapter, we discuss the chemistry of haloalkanes and alcohols in which the halogen or hydroxy! group is bonded to an sp3 -hybridized carbon atom. Molecules with sp2 -hybridized carbon atoms bonded to a halogen or a hydroxyl group have different chemistry. We will discuss these compounds in later chapters. Haloalkanes and alcohols are classified as 1°, 2°, and 3° by the same method used to classify carbon atoms in alkanes.

  9.2 Nomenclature of Haloalkanes

  The rules for naming haloalkanes are very similar to the rules for naming alkanes (Section 4.3 ) and alkenes (Section 5.6 ). Halogen atoms and branching alkyl groups have equal priorities in terms of their positions in the parent chain. If no other functional groups are present, the chain is numbered from the end closest to the first substituent, whether it is a halogen or an alkyl group. However, the double bond of an alkene or the triple bond of an alkyne takes precedence in numbering a carbon chain, regardless of where the halogen is located or how many halogens there may be. The concept of the priority of one functional group over another is expanded with each new functional group we will study in later chapters. It is explicitly part of R ,S configurational nomenclature.

  9.3 Nomenclature of Alcohols

  The common names of simple alcohols are based on the name of the alkyl group bonded to the hydroxyl group, as in methyl alcohol. The IUPAC method of naming alcohols is based on the longest continuous carbon chain that contains a hydroxyl group. The chain is numbered to give the carbon atom bonded to the hydroxyl group the lowest possible number. The suffix -ol

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  eBook - PDF

  Chemistry, 5th Edition

  • Allan Blackman, Steven E. Bottle, Siegbert Schmid, Mauro Mocerino, Uta Wille(Authors)
  • 2022(Publication Date)
  • Wiley

    (Publisher)

  An E2 reaction occurs in one step: reaction with a base to remove H + , formation of the alkene and departure of the leaving group, all occurring simultaneously. 18.4 Explain how the reaction conditions and reagents infuence the reaction processes of haloalkanes. When a nucleophile is also a strong base, nucleophilic sub- stitution and  -elimination often compete with each other. Reactions of 2° and 3° haloalkanes in polar protic solvents give mixtures of substitution and elimination products. After the formation of the carbocation intermediate, either (a) H + is lost to give an alkene (E1) or (b) solvent adds to give a substitution product (S N 1). In polar protic solvents, the products formed depend only on the structure of the particular carbocation. For Reactions of Haloalkanes with reagents that act as both nucleophiles and bases, steric hindrance significantly retards S N 2 reactions, while branching at the -carbon or  -carbon(s) increases the rate of E2 reactions to give the alkene product. The greater the nucleophilicity of the attacking reagent, the greater is the S N 2 : E2 ratio. Conversely, the greater the basicity of the attacking reagent, the smaller is the S N 2 : E2 ratio. Pdf_Folio:941 CHAPTER 18 Haloalkanes 941 KEY CONCEPTS AND EQUATIONS Concept Section Description/equation Halogenation: a radical substitution reaction 18.1 Substitution of hydrogen atoms on an alkane can be achieved using high temperatures or intense light to induce a radical chain reaction. Only bromine and chlorine are practical for this halogenation reaction. CH 3 CHCHCH 3 CH 3 Cl CH 3 CHCH 2 CH 2 Cl CH 3 28% 16.5% CH 3 CCH 2 CH 3 CH 3 Cl 33.5% 22% Cl CH 2 CHCH 2 CH 3 CH 3 CH 3 CHCH 2 CH 3 CH 3 Cl 2 300 °C Nucleophilic substitution: S N 2 18.2 S N 2 reactions occur in one step, and both the nucleophile and the leaving group are involved in the transition state of the rate-determining step. The nucleophile may be negatively charged or neutral.

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  eBook - ePub

  Organic Chemistry

  An Acid-Base Approach, Third Edition

  • Michael B. Smith(Author)
  • 2022(Publication Date)
  • CRC Press

    (Publisher)

  Nature Chemistry 2019, 11, 213–221.

  20 Feng, K.; Quevedo, R.E.; Kohrt, J.T.; Oderinde, M.S.; Reilly, U.; White, M.C. Nature 2020, 580, 621–627.

  11.9 Organization of Reaction Types

  Substitution reactions can be organized as follows.

  What reactions are possible for alkyl halides?

  1. Alkyl halides undergo substitution reactions with various nucleophiles. Other halides, ethers, nitriles, and alkynes can be prepared.

  What reactions are possible for alcohols?

  1. Alcohols react with HBr or HCl to form alkyl bromides or alkyl chlorides.
  2. Alcohols react with sulfur or phosphorus halides to form alkyl halides.
  3. The stereochemistry of chiral alcohols can be inverted using the Mitsunobu reaction.

  What reactions are possible for ethers?

  1. Ethers are generally unreactive, but they react with HI or HBr to form alcohols and alkyl halides.
  2. Epoxides are reactive ethers that react with acids to form halo-alcohols.
  3. Epoxides react with water and acid to yield diols.
  4. Ethers react with nucleophiles at the less substituted carbon in nonaqueous solvents.

  What reactions are possible for alkanes?

  1. Alkanes are generally unreactive, but they react with bromine or chloride under radical conditions to yield alkyl halides.

  What reactions are possible for alkynes?

  1. Terminal alkynes can be coupled to give diynes using a copper catalyst.
  2. Haloalkynes are coupled with terminal alkynes using a copper catalyst.

  11.10 Biological Relevance

  Substitution reactions occur in many biological processes. Bis(2-chloromethyl)sulfide [ClCH2 CH2 SCH2 CH2 Cl], otherwise known as mustard gas , is a sulfide (a thioether) with primary alkyl chloride units and it is highly reactive. It was used as a poison gas in World War I as a vesicant, since exposure causes large blisters on exposed skin. It is also cytotoxic and mutagenic. These latter effects arise by a reaction with heterocyclic bases in DNA. As this compound was studied, chemical modification led to an amine derivative, 2-chloro-N-(2-chloroethyl)-N-methylethanamine, a so-called nitrogen mustard . It is one of the first clinically useful anti-cancer drugs. The anti-cancer activity arises from reaction as a DNA intercalating agent. An intercalating agent inserts itself into the DNA structure of a cell and binds to the DNA, causing damage. The nitrogen mustard first reacts via the N9 -nitrogen of a guanine that is part of a DNA strand (28), and a SN 2-like reaction at the aziridinium salt leads to 29, as shown in Figure 11.13 .21 Aziridines are discussed in Section 23.7. The three-membered ring is susceptible to attack by nucleophiles, similar to the reactivity of epoxides. Hence, 2-chloro-N-(2-chloroethyl)-N-methylethanamine is classified as an alkylating agent. The cross-linking (intercalating) ability arises when the tertiary amine unit in 29 reacts with the other primary alkyl chloride to give aziridinium salt 30. A second molecule of DNA (28) reacts to form 31. As suggested by 31, reaction with double stranded DNA leads to intercalation of the nitrogen mustard (Figure 11.14

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  eBook - ePub

  Electrochemical Reactions and Mechanisms in Organic Chemistry

  • J. Grimshaw(Author)
  • 2000(Publication Date)
  • Elsevier Science

    (Publisher)

  CHAPTER 2

  OXIDATION OF ALKANES, HALOALKANES AND ALKENES

  Radical-Cations

  Oxidation of alkanes involves the removal of an electron from either a carbon-hydrogen or a carbon-carbon σ-bond. These are dissociative processes where the radical-cation cannot be detected as an intermediate in either fluorosulphuric acid or acetonitrile.

  Oxidation of iodoalkanes involves removal of an electron from the halogen non-bonding orbital. The radical-cations of primary and secondary alkyl iodides can be identified in aqueous solution by their absorption spectra and have half-lives of microseconds [1 ]. They are formed during pulse radiolysis of the iodoalkane in aqueous solution in the presence of nitrous oxide. This system generates hydroxyl radicals, which remove an electron from the iodine atom lone pair. Iodoalkane radical-anions complex with the lone-pair on other heteroatoms to form a 2σ/1σ* three-electron bond. In aqueous solution, the radical-cation of iodomethane is involved in an equlibrium indicated by Equation 2.1 .

  Eq. 2.1

  Related three-electron bond radical-cations 1 are formed from dialkyl sulphides by oxidation with hydroxyl radicals generated using pulse radiolysis [2 ]. An isoelectronic three-electron bond between two nitrogen atoms in 2 is formed by reduction

  of the hydrazine dication with sodium in liquid ammonia. This species is sufficiently stable to be sublimed in vacuum as the tetrafluoroborate salt [3 ].

  Electrochemical oxidation of alkenes results in the removal on one electron from the alkene function to give a π-radical-cation where the electron deficiency is delocalised over the conjugated system. The majority of alkene radical-cations cannot be characterised because they readily lose an allylic proton in aprotic solventsor react with any nucleophile present in solution. Important exceptions are the radical-cations of the rigid adamantane derivatives 3 and 4

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  Chemistry

  • John A. Olmsted, Gregory M. Williams, Robert C. Burk(Authors)
  • 2020(Publication Date)
  • Wiley

    (Publisher)

  The generalized reaction is as follows: C C C C + HOH Acid catalyst H OH An example is the hydration of 2-methylpropene to prepare 2-methyl-2-propanol: OH H + HOH Acid catalyst Because the reactions follow Markovnikov’s rule, acid-catalyzed hydrations of alkenes do not yield primary alcohols, except in the special case of the hydration of ethene: CH 2 H OH H H H H H 2 C + HOH Acid catalyst 540 CH A P TER 11 Organic Chemistry—Reactions Electrophilic Addition of Bromine and Chlorine to Alkenes Alkenes react rapidly with chlorine and bromine to form vicinal dihalides (abbreviated vic-dihalide, meaning that the two halogens are on adjacent carbon atoms). Some specific examples of this reaction are given below: Cl Cl Cl Cl and Cl Cl + Cl 2 + Cl 2 When bromine is used for this reaction, it can serve as a test for the presence of carbon– carbon multiple bonds. If we add bromine to an alkene or alkyne, the red-brown colour of the bromine disappears almost instantly, as long as the alkene (or alkyne) is present in excess: C C C C Br Br Rapid decolourization of the red-brown Br 2 solution is a positive test for unsaturation. Coloured Colourless + Br 2 This behaviour contrasts markedly with that of alkanes, which do not react appreciably with bromine or chlorine at room temperature under the same conditions. Example 11.15 illustrates the addition of halogens to an alkene. EX AMPLE 11.15 Addition of Halogens to a Double Bond Predict all products of the addition reaction below. + ICl Strategy: We need to keep in mind that stereoisomers are possible, and remember Markovnikov’s rule. Solution: In ICl, the Cl atom is more electronegative, so the I atom will bond to the carbon atom that has more hydrogen atoms bonded to it, according to Markovnikov’s rule. Enantiomers are possible: I I Cl and Cl + ICl 11.4 Addition Reactions 541 Does the Result Make Sense? A vicinal dihalide is formed, and two enantiomers result, which makes sense.

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  Organic Chemistry

  • David R. Klein(Author)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  Many steps 3 2 1 TDBMS = O CH 3 OTBDMS O CH 3 OTBDMS O CH 3 O O H 3 COCO H Si t-Bu CH 3 CH 3 Try Problems 8.46, 8.70 PRACTICE the skill APPLY the skill need more PRACTICE? 390 CHAPTER 8 Addition Reactions of Alkenes REVIEW OF REACTIONS 1. Hydrohalogenation (Markovnikov) 2. Hydrohalogenation (anti -Markovnikov) 3. Acid-catalyzed hydration and oxymercuration-demercuration 4. Hydroboration-oxidation 5. Hydrogenation 6. Bromination 7. Halohydrin formation 8. Anti dihydroxylation 9. Syn dihydroxylation 10. Ozonolysis O O H 1) OsO 4 2) NaHSO 3 H 2 O NaOH, cold KMnO 4 1) RCO 3 H 1) O 3 2) DMS Br 2 , H 2 O X HX Br 2 ROOR HBr, H 3 O + H 2 Pt Br 1) Hg(OAc) 2 , H 2 O 2) NaBH 4 OH 1) BH 3 ∙ THF 2) H 2 O 2 , NaOH 2) H 3 O + Br Br En + OH En + OH Br En + OH OH En + OH OH En + 1 2 3 4 5 6 7 8 9 10 REVIEW OF CONCEPTS AND VOCABULARY SECTION 8.1 • Addition reactions are characterized by the addition of two groups across a double bond. SECTION 8.2 • Alkenes are abundant in nature. • Ethylene and propylene, both formed from cracking petroleum, are used as starting materials for a wide variety of compounds. SECTION 8.3 • Addition reactions are thermodynamically favorable at low temperature and disfavored at high temperature. SECTION 8.4 • Hydrohalogenation reactions are characterized by the addi- tion of H and X across a π bond, where X is a halogen. • For unsymmetrical alkenes, the placement of the halogen represents an issue of regiochemistry. Hydrohalogenation reactions are regioselective, because the halogen is gen- erally installed at the more substituted position, called Markovnikov addition. • In the presence of peroxides, addition of HBr proceeds via an anti-Markovnikov addition. • The regioselectivity of an ionic addition reaction is deter- mined by the preference for the reaction to proceed through the more stable carbocation intermediate. • When one new chiral center is formed, a racemic mixture of enantiomers is obtained.

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  Introductory Organic Chemistry and Hydrocarbons

  A Physical Chemistry Approach

  • Caio Lima Firme(Author)
  • 2019(Publication Date)
  • CRC Press

    (Publisher)

  Chapter Seventeen

  Alkenes (reactions)

  INTRODUCTION

  Unlike alkanes, alkenes undergo polar reactions of basically three types: pericyclic reactions, polymerization reactions, and electrophilic addition reactions. Most polar reactions in alkenes are stepwise, but alkenes also undergo concerted reactions. Similarly to alkanes, alkenes also undergo radical reactions, following radical addition instead of radical substitution as in alkanes.

  An important point in representing the mechanism of these reactions is the use of the most appropriate, correct view of the alkene. As shown before (Fig. 16.1 in chapter sixteen ), there are two views for representing alkenes: upper view where the π-bond electrons/orbital is shown above the plane containing vinylic carbon and their substituents (i.e., π-bond electrons are outside the plane of the paper); and side view where π-bond electrons/orbital is within the plane of the paper and the vinylic substituents are behind and at the front of the plane of the paper. In all alkene reactions, the π-bond electrons play the main role and then any appropriate mechanistic representation should take into account the correct approach of π-bond electrons to the reactant within the plane of the paper (since we are limited to a bi-dimensional representation in the paper) yielding the effective collision. As it was discussed in chapter ten, the effective collision requires reactants to approach each other at the correct orientation and with the minimum energy.

  The only alkene’s view that can correctly represent the approach between alkene’s π-bond electrons and its reactant within the plane of the paper is the side view. On the other hand, from the upper view, the approach between π-bond electrons and its reactant might result in a non-effective collision and does not lead to the expected transition state. In other words, the alkene’s side view is the correct orientation for alkene’s reaction yielding an effective collision, while the alkene’s upper view is the incorrect orientation for a bi-dimensional representation of a reaction since it yields a non-effective collision

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