Chapter 7_c27390996aac70f449da7bc79c1d84bb.pptx

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Alkyl Halides CHAPTER 7 1 In This Chapter •Nomenclature of Alkyl halides •Synthesis •Reactions of Alkyl halides •Nucleophilic Substitution Reactions •Elimination Reactions 2 Halides – Compounds of Halogen (X) Alkyl halides: Halogen, X, is directly bonded to sp3 carbon. Vinyl halides: X is bond...

Alkyl Halides CHAPTER 7 1 In This Chapter •Nomenclature of Alkyl halides •Synthesis •Reactions of Alkyl halides •Nucleophilic Substitution Reactions •Elimination Reactions 2 Halides – Compounds of Halogen (X) Alkyl halides: Halogen, X, is directly bonded to sp3 carbon. Vinyl halides: X is bonded to sp2 carbon of alkene. Aryl halides: X is bonded to sp2 carbon on benzene ring. H H H C C Br H H alkyl halide H C C H H Cl vinyl halide I aryl halide 3 Polarity and Reactivity Halogens are more electronegative than C. Carbon—halogen bond is polar, so carbon has partial positive charge. Carbon can be attacked by a nucleophile. Halogen can leave with the electron pair. 4 7.1 Naming Alkyl Halides Step # 1: Find the longest carbon chain, and name it as parent. If multiple bond is present, parent chain must contain it. Step # 2: Number the parent chain, from the end where substituent (Halo or Alkyl) comes first. Name of Halogen will be change to Halo – Chlorine to Chloro etc If substituents are at the equal distance, start numbering from the end where substituents with the alphabetical priority comes first. Step # 3 5 7.1 Naming Alkyl Halides Name Following Compounds CH3 CH3 H3C H2C H2C HC HC CH CH3 Br H2C H2C H2C H2C H2C CH2 CH2 Cl Br CH3 CH3 H2C HC CH CH3 Cl I CH3 6 7.1 Naming Alkyl Halides Draw the structure of following compounds a) 3-Bromo-3-ethylpentane methylhexane b) 2-Iodo-5-chloro-3- c) 4-(2-Fluoroethyl)heptane fluorocyclohexane d) cis-1-Bromo-3- 7 7.2 Preparing Alkyl Halides Alkenes undergo addition reaction with halogen acids (HCl, H-Br, H-I) to produce Markovnikov Alkyl halides (Chapter 4) H Cl H H C H + C H H Cl H C C H H H Chloroethane Alkenes reacts with halides to produce anti 1,2-Alkyl dihalide (chapter 4) 8 7.2 Preparing Alkyl Halides Alkane + Cl2 or Br2, heat or light replaces C-H with C-X but gives mixtures ◦Hard to control ◦Via free radical mechanism It is usually not a good idea to plan a synthesis that uses this method 9 7.2 Preparing Alkyl Halides Converting Alcohols to Alkyl Halides: Alcohols (R-OH) reacts with hydrogen halides (HX) to produces Alkyl halides (R-X). R OH + CH3 H3C C OH CH3 + R X H X H Cl Ether 0 oC + H OH CH3 H3C C Cl + H OH CH3 10 7.2 Preparing Alkyl Halides Converting Alcohols to Alkyl Halides: Reaction of tertiary C-OH with HX is fast and effective Add HCl or HBr gas into ether solution of tertiary alcohol 11 7.2 Preparing Alkyl Halides Converting Alcohols to Alkyl Halides: Primary and secondary alcohols react very slowly and often rearrange, so alternative methods are used. Thionyl chloride (SOCl2) or Phosphorus tribromide (PBr3) CH3 H2C OH + PBr3 H3C HC OH + SOCl2 Ether 35 oC CH3 H2C Br H3C HC Cl + H OH + H OH 12 7.2 Preparing Alkyl Halides Complete the following: OH CH3 Br CH3CH2CHCH2CHCH3 + ? CH3 CH3CH2CHCH2CHCH3 + H2O OH CH3 + HCl ? 13 7.3 Reactions of Alkyl Halides: Grignard Reagent Alkyl halides react with Mg metal to produce organo magnesium halides Grignard reagents. Grignard reagents are organometallic compounds containing carbon-metal Ether bond. R X + Mg R Mg X R is 1°, 2°, 3° alkyls, aryl, or alkenyl; X can be Cl, Br or I Carbon bonded with Mg is electron rich. The electron density on the carbon - + Mg make it nucleophile and basic. So Grignard reagents react with H2O, HCl or CH3 H2C Br CH3 H2C MgBr alcohol to produce hydrocarbonsEther by accepting H+ ion. - + CH3 H2C MgBr H2O CH3 H2C H 14 7.3 Reactions of Alkyl Halides: Grignard Reagent e.g.: Convert 4-methyl-1-pentanol into 2-methylpentane: 15 7.4 Nucleophilic Substitution Reactions Alkyl halides are polarized at the carbon-halide bond, making the carbon electrophilic. Nucleophiles will replace the halide in C-X bonds. 16 7.4 Nucleophilic Substitution Reactions In 1896, Walden showed that (-)-malic acid could be converted to (+)-malic acid by a series of chemical steps with achiral reagents This established that optical rotation was directly related to chirality and that it changes with chemical alteration ◦ Reaction of (-)-malic acid with PCl5 gives (+)chlorosuccinic acid ◦ Further reaction with wet silver oxide gives (+)-malic acid ◦ The reaction series starting with (-)-malic acid gives (+)-malic acid. The same reaction sequence yields (+)-malic acid to (-)-malic acid. 17 7.4 Nucleophilic Substitution Reactions Significance of Walden’s Cycle As (-)-malic acid converted to (+)-malic acid, which means that inversion in the configuration of stereocenter occurred. Now we called Walden’s cycle as Nucleophilic Substitution Reaction. Two types of Nucleophilic Substitution (SN) Reactions depending upon reaction pathways. 1.SN1 reaction 2.SN2 reaction 18 7.4 Nucleophilic Substitution Reactions In any SN reaction, a nucleophile (Nu: or Nu:-) attacks alkyl halides (R-X) and substitute leaving group (X:-) to produce R-Nu. 19 7.4 Nucleophilic Substitution Reactions What substitution product would be the following reactions? 1. CH2Br + 2. NaCN ? CH3 H 3C HC CH2Cl + HS ? 20 7.5 The SN2 Reaction • It is a bimolecular substitution reaction. • It is a single step reaction. No intermediate is formed. • Nucleophile attacks from 180away from the leaving group. • Bond breaking and new bond forming occurs at the same time. The transition state is planar. 21 7.5 The SN2 Reaction Rate of SN2 Reaction: Speed at which reaction took place is called rate of reaction. Rate is the change in concentration with time. The rate of SN2 depends upon concentration of both alkyl halide and nucleophile. If concentration of any reactant is doubled, rate of the reaction will also doubled. “2” in SN2 represents that reaction is bimolecular nucleophilic substitution reaction. 22 7.5 The SN2 Reaction Stereochemistry of SN2 Reaction: SN2 reaction results in the inversion of configuration. Incoming nucleophile attacks the substrate and begin pushing out the leaving group on the other side. This inverts the configuration of molecule. This inversion occur through a planar transition state. 23 24 7.5 The SN2 Reaction Steric effects in SN2 Reaction: SN2 reaction is Sensitive to steric effects. More bulky groups on alkyl halide, more difficult will be the substitution Methyl halides are most reactive, Primary are next most reactive, Secondary might react, Tertiary are unreactive by this path. No reaction at C=C (vinyl or aromatic halides) 25 7.5 The SN2 Reaction Steric effects in SN2 Reaction: The carbon atom in bromomethane is readily accessible resulting in a fast SN2 reaction. The carbon atoms in ethyl bromide is primary, therefore, attack is easy, isopropylbromide (secondary) is relatively hindered results in slower SN2 reaction, and t-butylbromide (tertiary) is completely hindered results in NO SN2 reactions. 26 7.5 The SN2 Reaction Which one will be the faster; A SN2 reaction of OH- ion with 1-bromopentane or with 2-bromopentane? 27 7.5 The SN2 Reaction The Leaving group in SN2 Reaction: A good leaving group also change the rate of SN2 reaction Stable anions that are weak bases are usually excellent leaving groups Q.s. Arrange the following compounds in order of increasing reactivity towards SN2 reaction? CH3I, CH3Br, CH3F 28 7.6 The SN1 Reaction The SN1 reaction is a unimolecular nucleophilic substitution. It is a two step reaction with a carbocation intermediate. First step is the loss of leaving group before arrival of nucleophile, which generates carbocation. Second step is the attack of nucleophile, giving a substituted product. 29 7.6 The SN1 Reaction Rate of SN1 Reaction: The rate of SN1 Reaction depends only on the concentration of substrate (R-X) In SN1- 1 means it is a unimolecular. SN1 can occur only when stable carbocations intermediate are formed. The more stable carbocations intermediate, the more faster will be the SN1 reaction. That’s why tertiary alkyl halides and tertiary alcohols do not undergo SN2 type of reactions. They follow SN1 route. 30 7.6 The SN1 Reaction Stereochemistry of SN1 Reaction: SN1 type of reactions yields racemic mixture. Formation of planar, achiral, carbocation allows attack of incoming nucleophile either from top face or bottom face equally. 31 7.6 The SN1 Reaction What will be the products reaction of (S)-3-methyl-3-octanol with HBr? Show stereochemistry of starting material and product? 32 7.6 The SN1 Reaction The Leaving Group in SN1 Reaction: SN1 type of reactions critically dependent on leaving group The larger halides ions are better leaving groups In acid, OH of an alcohol is protonated and leaving group is H2O, which is still less reactive than halide Worst Leaving Group Good Leaving Group 33 7.7 Eliminations: The E2 Reaction • Elimination is an alternative pathway to substitution • Opposite of addition • Generates an alkene 34 7.7 Eliminations: The E2 Reaction Zaitsev Rule: Elimination reaction of unsymmetrical alky halide give mixture of alkene. According to Zaitsev, If more than one elimination product is possible, the most-substituted alkene is the major product (most stable). 35 7.7 Eliminations: The E2 Reaction Elimination reaction can take place by different pathways: 1. E2 Reaction 2. E1 Reaction 3. E2 Reaction: It is a bimolecular reaction. When alkyl halide is treated with strong base (OH- of OR-), this type of reaction took place. This is a concerted reaction: the proton is abstracted, the double bond forms and the leaving group leaves, all in one step. 36 7.7 Eliminations: The E2 Reaction 37 7.7 Eliminations: The E2 Reaction Qs. Predict the product of reaction between 1-chloro-1methylcyclohexane and KOH? 38 7.8 Eliminations: The E1 Reaction Unimolecular elimination. Two groups lost: a hydrogen and the halide. Nucleophile acts as base. Competes with SN1 and E2 at 3° centers 39 7.8 Eliminations: The E1 Reaction Mechanism: Step 1: halide ion leaves, forming a carbocation. Rate limiting step. Unimolecular. Step 2: Base abstracts H+ from adjacent carbon forming the double bond. 40 Homework 7.22, 7.23, 7.26, 7.27, 7.28, 7.29, 7.33, 7.48, 7.51, 7.54 41

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