Nuggets of Knowledge for Chapter 9 – Reactions of Alkyl HalidesChem 2310I. Introduction to Alkyl HalidesTypes of Organic Halides• Organic halides can be organized into four major categories. These compounds undergo verydifferent kinds of reactions.o Aryl halides have a halogen connected directly to a benzene ring. o Vinyl halides have a halogen connected directly to a C=C.o Acid chlorides have a chloride connected directly to a C=O.o Alkyl halides have a halogen connected to an sp3 carbon. These are the compounds thatwill be discussed in this chapter.• Alkyl halides can be further classified as methyl, primary 1o, secondary 2o, or tertiary 3o.o methyl – a halogen attached to a methyl groupo primary (1o) – halogen attached to an sp3 carbon with 1 other carbon on ito secondary (2o) – halogen attached to an sp3 carbon with 2 other carbons on ito tertiary (3o) – halogen attached to an sp3 carbon with 3 other carbons on it• Alkyl halides can also be labeled as benzyl or allyl.o allyl halide – halogen attached to an sp3 carbon next to a C=C o benzyl halide – halogen attached to an sp3 carbon next to a benzene ring• If there are two halogens in the right positions, alkyl halides can be labeled as geminal or vicinal.o geminal dihalide – two halogens on the same carbono vicinal dihalide – two halogens on carbons that are next to each other• All alkyl halides undergo the same reactions, but they are affected by whether they are 3o, 2o, 1o,methyl, allyl, or benzyl.Where organic halides are found• Organic compounds containing halogens are rarely found in nature. o An important exception is thyroid hormone. Iodine is necessary in our diet in order tosynthesize thyroid hormone. It is usually obtained from iodized salt. People who do notget enough iodine in their diet suffer from goiter, an enlarged thyroid gland.• Alkyl halides are often used in medical, industrial, and commercial applications:o 1,1,1-trichloroethane (CH3CCl3) is used as a dry cleaning solution.o Chloroform (CHCl3) was one of the first anesthetics, but it isn’t used now on humansbecause of the small window between an effective dose and death.o Freon-12 (CF2Cl2) was a widely used refrigerant, but it has been replaced because ofdamage to the ozone layer.o Dichloromethane (CH2Cl2) is used as a solvent.o Halothane is an anesthetic currently in use.o Chlordane is a pesticide.Preparation of Alkyl Halides• One way to prepare alkyl halides is the halogenation reaction discussed in chapter 4.o In the presence of light or heat, alkanes react with Cl2 or Br2 to produce alkyl halides. o Multiple products are usually observed; these can be separated by fractional distillation,which is only practical on an industrial scale. o F2 is not used because it is so energetic the reaction becomes explosive; I2 is not usedbecause the reaction is too slow.• This reaction follows a radical mechanism, and is the example used in chapter 7.o Initiation step: homolytic cleavage of Cl2 or Br2 o Propagation steps: 1) attack of the halide radical on the alkane, taking a H and creating a carbon radical; 2) attack of the carbon radical on a chlorine or bromine moleculeo Termination steps: combination of halide radicals and carbon radicals.• Radicals are stabilized by hyperconjugation and resonance. o hyperconjugation makes radicals more stable: 3o > 2o > 1o > methylo radicals with resonance structures are more stable than those without• A reagent called N-bromosuccinimde (NBS) is often used in place of Br2. It produces smallamounts of Br2 because of an unfavorable equilibrium. The Br2 reacts, causing more to becreated. o NBS is much easier to work with than Br2 because it is solid and doesn't causedangerous fumes.• We will encounter other (better) ways to form alkyl halides in future chapters.Spectroscopy of Alkyl Halides• Alkyl halides have the following IR bands: o C-H 3000-2850 cm-1o CH2 bending 1460, 1380 cm-1o C-X 800-500 cm-1 (often hard to find)• Alkyl halides have the following characteristic NMR chemical shift values: o Chemical shift of H’s on carbons next to halogens: 3-4 ppmReactivity of Alkyl Halides• The carbon-halogen bond is a polar covalent bond. o The halogens higher on the Periodic Table are more electronegative, making the bondmore polar. All are more electronegative than carbon.o The halogens lower on the Periodic Table are larger, making the bond weaker. o When these two factors are combined, the result is that carbon-iodine bonds areweakest, carbon-bromine bonds are next, carbon-chlorine bonds are also reactive, butcarbon-fluorine bonds are too strong to react easily.• Because of the polarity and weakness of the carbon-chlorine, carbon-bromine, and carbon-iodinebonds, they can react in three ways:o They can dissociate to form a carbocation and a halide ion. The carbocation will reactfurther, either as an electrophile or acid.o They participate in displacement reactions with a nucleophile, forming a new bond tothe nucleophile and forming a halide ion.o They can participate in acid/base reactions with a base by giving up a hydrogen next tothe carbon with the halogen. A carbon-carbon double bond is formed, and a halide ion isformed.o In each of these reactions, the carbon-halogen bond is broken, and a halide ion isformed.Nomenclature of alkyl halides• Some functional groups have two ways to name them: IUPAC names, sometimes calledsubstitutive names, and common names. o IUPAC names follow the rules we discussed in the chapter on alkanes. o Common names are much simpler, but only work for a few of the compounds.• Common names for alkyl halides are formed by using the substituent name for the alkyl groupattached to the halide. For example, CH3Br is called methyl bromide.o The following are substituents we learned in Ch 4; review their structures if you don’tremember them: methyl, ethyl, propyl, isopropyl, cyclopropyl, butyl, isobutyl, sec-butyl,and tert-butyl and so on.o Substituent names for alkyl groups that contain a C=C are vinyl (directly attached to theC=C) and allyl (one carbon away from the C=C).o The name for alkyl groups that contain a benzene ring are phenyl (directly attached tothe benzene ring) and benzyl (one carbon away from the benzene ring). However, onlybenzyl is used in common names for alkyl halides. Aryl halides are named differently. o Common names are not used if there is not a simple name for the alkyl substituent.• IUPAC, or