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CHM 2211C th 6 edition Notes Chapter 10 Alkyl Halides By Dr. Andrea Wallace Coastal Georgia Community College Edited by John T. Taylor Florida Community College at Jacksonville 1 Chapter 10: Alkyl Halides Alkyl halides occur widely in nature and have many uses in industrial processes. Uses of Alkyl Halides – see p. 316 – solvents, anesthetic, refrigerant, fumigant Epibatidine (p. 317) – is found on the skin of an Ecuadorian frog and is 200 x more potent than morphine in blocking pain. 10.1 Naming Alkyl Halides Rule 1) Find the parent chain – longest continuous chain (that contains a double or triple bond if one is present). Rule 2) Assign lowest numbers to the branches – alkyl and halo. a) Apply prefixes – di, tri, tetra b) Alphabetize Rule 3) If the parent chain can be properly numbered from either end by Rule 2, assign them alphabetically. Common names: Alkyl group + Halide Example: Iodomethane (IUPAC) or methyl iodide (common) Example: 2-Chlorpropane (IUPAC) or isopropyl chloride (common) Problem 10.1, p. 318 Give the IUPAC names of the following alkyl halides. a) d) f) Problem 10.2, p. 318 Draw the structures corresponding to the following IUPAC names: d) 1,1-Dibromo-4-isopropylcyclohexane 2 10.2 Structures of Alkyl Halides Table 10.1, p. 319 CH3F CH3Cl CH3Br CH3I ----C-X Bond Length ________________ Why? ______________________________ ---C-X Bond Strength _______________ Why? ______________________________ --Dipole Moment (Bond Polarity) ___________ Why? _________________________ Halogens are more electronegative than Carbon. All have substantial dipole moments. Figure: 10.3 Preparation of Alkyl Halides Electrophilic Addition 1) Alkene + X2 2) Alkene + HX X2 = Cl2 or Br2, HX = HCl, HBr, or HI Free Radical Substitution of Alkanes See Figure 10.1, p. 320 for Mechanism Alkane + X2 3 10.4 Radical Halogenation of Alkanes Why is this not necessarily the best choice? What are the possible products of CH4 + Cl2? There are even more products possible when more than one type of Hydrogen is present. Example: + Cl2 --h Butane Ratios: (only showing monochlorinated products – di, tri, tetra, etc, are possible) Another Example: + Cl2 --h 2-Methylpropane Ratios: Compare equivalent Hydrogen’s and Ratios: Butane 2-Methylpropane ____eq. Primary ____eq. Secondary ___ eq. Primary ___ eq. Tertiary Primary accounts for ___% of pdt. Secondary accounts for ___% of pdt. Primary accounts for ___% of pdt. Tertiary accounts for ___% of pdt. 4 Secondary is _____ times more likely. See Reactivity Figure on p. 321. Tertiary is _____ times more likely. Primary Tertiary < Secondary < This shows relative reactivity towards Chlorination. Why? Table 5.3, p. 154 Energy needed to break bond Primary Secondary Tertiary Radical Stability -------_____________________ Stability The more stable radical forms faster. See Figure 10.2, p. 322. Bromination is even more selective. + 2-methylpropane Br2 --h 2-bromo-2methylpropane > 99% + 1-bromo-2methylpropane < 1% Why? Hammond Postulate – the transition state most closely resembles the species (reactant or product ) to which it is closest in energy. H = -50 kJ for X = Cl H = +13 kJ for X = Br The bromine reaction is more ____________ and thus more product-like (more similar to the radical). The reaction shows selectivity that reflects the stability of the radical. 5 Problem 10.3, p. 323 Draw and name all monochloro products you would expect to obtain from radical chlorination of 2-methylpentane. Which, if any, are chiral? Problem 10.4, p. 323 Taking the reactivities of 1o, 2o, and 3o hydrogen atoms into account, what product(s) would you expect to obtain from monochlorination of 2-methylbutane? What would the approximate percentage of each product be? (Don’t forget to take into account the number of each type of hydrogen.) 10.5 Allylic Bromination of Alkenes Find the allylic positions on cyclohexene. Reaction: Allyic Bromination of an Alkene NBS h, CCl4 Cyclohexene 3-Bromocyclohexene The presence of h causes the Br2 to form Br radicals. Br2 --hv 2 Br . 6 Mechanism: What type of intermediate is formed? ________________________________ What step of the mechanism is shown above? __________________________ What step of the mechanism is occurring when the Br radical forms (shown above the “mechanism”)? ________________________________ What contributes to the stability of the allylic radical? _____________________ Why does bromination occur exclusively at the allylic position? Compare bond dissociation energies at the other positions – alkyl, allylic, and vinylic. The bond dissociation energy is less because the radical formed is more stable. --------------------------_________________ Radical Stability---------- 10.6 Stability of the Allyl Radical: Resonance Revisited Why are allylic radicals so stable? Draw the resonance structures for allyl radical. Delocalization of electrons via resonance gives increased stability. The greater the # of resonance structures, the greater the stability of the molecule. Example: Propyl radical Electron is localized on Carbon – it is _________ stable. 7 Bromination of unsymmetrical alkenes yields an unequal mixture of products. Reaction at less hindered end is more stable. 1-octene NBS h, CCl4 3-Bromo-1-octene (17%) 1-Bromo-2-octene (83%) (53/47- trans/cis) Mechanism: Useful Reaction: NBS h, CCl4 Cyclohexene KOH 3-Bromocyclohexene 1,3-Cyclohexadiene What type of reaction occurs when KOH is added? ____________________ Problem 10.5, p. 327 Draw three resonance forms for the cyclohexadienyl radical. Problem 10.6, p. 327 The major product of the reaction of methylenecyclohexane with N-bromosuccinimide is 1-(bromomethyl)cyclohexene. Explain. 8 Problem 10.7 b., p. 327 What products would you expect from reaction of the following alkenes with NBS? If more than one product is formed, show the structures of all. 9 10.7 Preparing Alkyl Halides from Alcohols Most general method of preparation of alkyl halides. Reaction: Alcohol Hydrohalic acid Alkyl Halide Water where X = Cl, Br, or I Works best for 3o Alcohols. 1o and 2o alcohols react very slowly and require very high temperatures – generally not practical. Reactivity of Alcohols with Hydrohalic acids Methyl Primary Secondary Tertiary ------_______________ reactivity ------- Example: HCl(aq), 25oC t-butyl alcohol + H2O t-butyl chloride Primary and Secondary alcohols are best converted to alkyl halides by reaction with Thionyl Chloride (SOCl2) or Phosphorus Tribromide (PBr3). Examples: PBr3 Ether, 35 oC 2-Butanol SOCl2 10 Pyridine 2-Butanol Why does this reaction work better for primary and secondary alcohols? These reagents are less acidic and less likely to cause acid catalyzed rearrangements. (Mechanisms are covered in Chapter 11.) Problem 10.8, p. 369 How would you prepare the following alkyl halides from the appropriate alcohols? a) 2-chloro-2-methylpropane b) 1-bromo-5-methylhexane 10.8 Grignard Reagents Grignard reagents are organometallic reagents, RMgX Preparation of Grignard Reagents: R-X Alkyl halide + Mg Ether or THF R-Mg-X Grignard Reagent where R = 1o, 2o, or 3o alkyl, aryl, or alkenyl (all work equally well – best to use THF with aryl and alkenyl) X = Cl, Br, or I (Cl is less reactive than Br or I, organofluorides rarely react with Mg) Examples: Bromobenzene Phenylmagnesium bromide 11 2-chlorobutane sec-butylmagnesium chloride Polarity of C-Mg bond: Think of the C as partial negative or even negative like a carbanion. These species do act like bases and react with acids (proton donors) such as H2O, ROH, RCOOH, and RNH2 to yield hydrocarbons. Example: 1-bromobutane butane Problem 10.10, p. 330 How might you replace a halogen substituent with a deuterium atom if you wanted to prepare a deuterated compound? 10.9 Organometallic Coupling Reactions Preparation of Organometallic Reagents 2 Li pentane 1-bromobutane Alkyllithium (Butyllithium) Lithium Bromide 12 Alkyllithiums are basic and act as nucleophiles. They are similar to RMgX (Grignard Reagents). One of the most valuable reactions of alkyllithiums is the preparation of diorganocopper compounds or Gilman reagents. Preparation of Gilman Reagents + Methyllithium ether Copper(I) Iodide Lithium Dimethylcopper (Gilman Reagent) Lithium Iodide Reaction of Gilman Reagent with Alkyl Halides (Cl, Br, or I) to Produce Alkanes Ether, 0oC Lithium Dimethylcopper (Gilman Reagent) Ethyl Iodide Propane This organometallic coupling reaction is very versatile. It also works with vinylic halides and aryl halides (not just alkyl halides). See p. 331. Example: Problem 10.11, p. 332 How would you prepare the following compounds using an organocopper coupling reaction? More than one step is required in each case. a) 3-methylcylohexene from cyclohexene c) Decane from 1-pentene 13 10.10 Oxidation and Reduction in Organic Chemistry Inorganic Definitions: Oxidation - ____________________ of electrons Reduction - ____________________ of electrons Still true in organic, but the definition is different. Oxidation is a gain of _______________________onto C and/or a loss of __________ onto C. Reduction is a gain of _______________________onto C and/or a loss of __________ onto C. Examples on p. 333 Methane + Cl2 Chloromethane _________________ Why? __________________ Chloromethane 1) Mg, ether/ 2) H3O+ Why?____________________________ Methane _______________________ More Examples on p. 333 See Figure 10.5 on p. 334. --------------------------_________________ Oxidation Level ----------------- 14 Problem 10.12, p. 334 Rank each of the following series of compounds in order of increasing oxidation level: (Strategy: Compounds that have the same number of carbon atoms can be compared by adding the number of C-O, C-N, and C-X bonds in each and then subtracting the number of C-H bonds. The larger the resultant value, the higher the oxidation level.) a) b) CH3CN, CH3CH2NH2, H2NCH2CH2NH2 Problem 10.13, p. 334 Tell whether each of the following reactions is an oxidation, a reduction, or neither. Explain your answers. 15