22 Functional Group Chemistry Problems 1.1 (i)Identify the o-and r-bonds in the following:(a) CH.CH,OH:(b)CH,CHO:(c)CH,CH=CH,. (ii)Identify the t-bonds in the following and comment on their structure:(d)CH,=CHCHO:(e)benzene. 1.2 Identify the functional groups in the following structur and indicate the regions of electron excess(+)and electron deficiency CH CH-CH (b) CH,-CH2 CH3 (c) CH3、 H CH CN (d)CH3-C=C-H (e) CH: CH-B NO, 1.3 Name the following compounds: (a)CH3 CH2CHs (b)CH3 (c) HCH2CH-CH3 C=CHCH3 CHCH> CH CH-CH (d) 0 ()CH-CH:CH CH CCH-N CH3 CH g)CH h C=CH-CO,H CH NO
22 Functional Group Chemistry
General Principles 23 1.4 Identify the following species as nucleophiles or electrophiles (a)Br";(b)Br;(c)CN;(d)NO,*;(e)NO,(f)NH,(g)RC=C (h)MeCO" 1.5 Draw resonance structures for the following: (a)CH3-CH2-C-OH a--- 0 1.6 Draw the three-dimensional structures of the following,indi cating the in o(b)eractions that may exist:()buann it tane in its eclips form about the 2,3-bond ed f dibromoethane in its most stable form;(d)cyclopropane:(e)1.2. epoxyethane(ethylene oxide)showing the lone pairs of electrons on the oxygen:(f)cis-and trans-1,4-dimethylcyclohexane in the chair form;(g)trans-cyclohexane-1,2-dicarboxylic acid:(h)cyclo- hexene
General Principles 23
2 The Chemistry of the o-Bond Aims This chapter of the book describes the reactions of o-bonded fund tional the end of this chapter you should be able to understand: .The difference in reactivity between a methyl,methylene and a methine hydrogen towards radical reagents .The methods of preparing alkyl halides and the role of alkyl halides in the preparation of other functional groups The variation in lcohols. ethers.thiols and The factors which affect nucleophilic substitution of these func- tional groups The factors which affect the balance between substitution and elimination reactions The factors which govern the stereochemistry and regiochem- istry of these reactions The formation of orga ometallic compounds from alkyl halide and thei role as carbanion thus revers g the reactive char acter of the carbon atom of the alkyl halide 2.1 Alkanes 2.1.1 Preparation of Alkanes Methods for the preparation of alkanes may be divided into a number methods that e the hydrogenolysisof escondly there are methods that involye the redue there 24
The Chemistry of the 0-Bond 2.1 Alkanes 2A Preparation of Alkanes Methods for the preparation of alkanes may be divided into a number of functional groups; secondly, there are methods that involve the of groups. Firstly, there are methods that involve the 24
The Chemistry of the o-Bond 25 tionof unsaturated systems;and thirdly,there are methods that involve the formation of the C-C bonds of the alkane. Hydrogenolysis of the toluene-4-sulfonate of an alcohol may be carried out with a nucleophilic hydride such as lithium aluminium hydride.There are also a series of radical methods based on the reduction of alkyl halides with tri-butyltin hydride(Bu SnH).Finally,the sourc of the e hydroge the mplified d by the with water The reduction of carbonyl compounds to hydrocarbons may be achieved under acidic conditions (e.g.the Clemmensen reduction with zine and concentrated hydrochloric acid).basic conditions (e.g.the Wolff-Kishner reduction of a hydrazone with alkali)or neutral condi tions s(g.the catalytic reduct of thioketals with Ra ey nickel).The hgo8conond onyl group may represent the residue from an earlier step in the syn- The catalytic reduction of alkenes and alkynes are important methods for the synthesis of alkanes.The hydroboration and hydrosilylation of alkenes are alternatives to catalytic methods.Again,both the alkene and alkyne may have plaved an important role in the construction of the hydrocarbon chain The formatio of C-C bonds in hydr carbon synthesis by arious ons has a long history.While the Wurtz coupl ng (Scheme 2.la)is of historical interest,modem coupling methods using organometallic reagents are more specific.Electrolytic methods(Scheme 2.1b)may lead to the generation of the free radicals involved in the cou- pling process. (a2 Br +2Na ∠+2NaB C02 -20,C6H54+2C02 Scheme 2.1 2.1.2 Reactions of Hydrocarbons Although the older name for the alkanes,the paraffins,arises from the Latin parum affinis,meaning little reactivity'.nevertheless under the appropriate conditions the aliphatic C-H bond can be quite reactive. Hydrogen is more electropositive than carbon and hence there is a tendency for the C-H bond to react in the sense CH'.However,very few of the reactions of alkanes are of an ionic character.The normal or straight-chain hydrocarbons are unattacked by treatment with con-
The Chemistry of the o-Bond 25 of unsaturated systems; and thirdly, there are methods that involve the formation of the C-C bonds of the alkane. Hydrogenolysis of the toluene-4-sulfonate of an alcohol may be carried out with a nucleophilic hydride such as lithium aluminium hydride. There are also a series of radical methods based on the reduction of alkyl halides with tri-n-butyltin hydride (Bu,SnH). Finally, the source of the hydrogen may be the electrophilic proton, exemplified by the decomposition of organometallic reagents such as the Grignard reagent with water. The reduction of carbonyl compounds to hydrocarbons may be achieved under acidic conditions (e.g. the with zinc and concentrated hydrochloric acid), basic conditions (e.g. the of a hydrazone with alkali) or neutral conditions (e.g. the catalytic reduction of thioketals with Raney nickel). The carbonyl group may represent the residue from an earlier step in the synthesis of a compound. The catalytic reduction of alkenes and alkynes are important methods for the synthesis of alkanes. The hydroboration and hydrosilylation of alkenes are alternatives to catalytic methods. Again, both the alkene and alkyne may have played an important role in the construction of the hydrocarbon chain. The formation of C-C bonds in hydrocarbon synthesis by various coupling reactions has a long history. While the Wurtz coupling (Scheme 2. la) is of historical interest, modern coupling methods using organometallic reagents are more specific. Electrolytic methods (Scheme 2. lb) may lead to the generation of the free radicals involved in the coupling process. (a) 2 mBr + 2Na - + 2NaBr Scheme 2.1 2.1.2 Reactions of Hydrocarbons Although the older name for the alkanes, the paraffins, arises from the Latin parurn afjnis, meaning ‘little reactivity’, nevertheless under the appropriate conditions the aliphatic C-H bond can be quite reactive. Hydrogen is more electropositive than carbon and hence there is a tendency for the C-H bond to react in the sense C--H+. However, very few of the reactions of alkanes are of an ionic character. The normal or straight-chain hydrocarbons are unattacked by treatment with con-
26 Functional Group Chemistry centrated sulfuric acid or molten sodium hydroxide.Indeed.refluxing n-hexane with concentrated sulfuric acid is part of the method for it The chemistry of the alkanes is dominated by the abstraction of a hydrogen atom by various free radicals.The bond dissociation energies of C-H bonds decrease in the order primary secondary tertiary benzylic and allylic,and thus free radical reactions tend to occur at ter- tiary,benzylic or allylic centres. The sequence of a radical chain reaction is exemplified by the photo chemical chlorination of methane(Box 2.1). Box 2.1 The Photochemical Chlorination of Methane CL,→C+C initiation CH+C→CH,·+HCI abstraction CH·+CLCHC1+C gation CI+C CL propa atior Overal:CH,CH,C→CH,CL,→CHCl,→CC In the case of the fluorination and chlorination of methane.both the hydrogen abstraction and propagation steps are exothermic,leading to a vigorous reaction.Although the abstraction of a hydrogen atom from methane by a bromine atom is endothermic,the generation of the halo oetomeactopagation step is surfciently xothermic to vith brom ne.lnthecaseofiodinatio n the hydro gen atom abstraction step is sufficiently endothermic to inhibit reaction. The photochemical or radical-induced chlorination of other hydrocar. bons can be quite vigorous and mixtures of mono-and polychlorinated products are obtained. Branched chain hydrocarbons have a greater reactivity,with the rel ative reactivity order being tertiary C-H>secondary C-H>primary C-H.Thus,r -methylpropane The same reactivity order found with oxidation.A number of these reactions may have radical character.Oxidation with chromium(VI) oxide(Cro)may lead to a tertiary alcohol. When a free radical is formed and held tightly within a molecule,reac tions of quite high site specificity(regioselectivity)may be observed.The free radical may be generated from anther functional group in the mol- ecule.but the result is the substitution at a c tre that in a formal se has the cha eristics of a hydro sen Such tions are observe in the photolysis of nitrite esters(RONO)(the Barton reaction),the
26 Functional Group Chemistry centrated sulfuric acid or molten sodium hydroxide. Indeed, refluxing n-hexane with concentrated sulfuric acid is part of the method for its purification. The chemistry of the alkanes is dominated by the abstraction of a hydrogen atom by various . The bond dissociation energies of C-H bonds decrease in the order primary > secondary > tertiary > benzylic and allylic, and thus free radical reactions tend to occur at tertiary, benzylic or allylic centres. The sequence of a radical chain reaction is exemplified by the photochemical of methane (Box 2.1). In the case of the fluorination and chlorination of methane, both the hydrogen abstraction and propagation steps are exothermic, leading to a vigorous reaction. Although the abstraction of a hydrogen atom from methane by a bromine atom is endothermic, the generation of the halogen atom in the propagation step is sufficiently exothermic to allow a slower overall reaction with bromine. In the case of iodination the hydrogen atom abstraction step is sufficiently endothermic to inhibit reaction. The photochemical or radical-induced chlorination of other hydrocarbons can be quite vigorous and mixtures of mono- and polychlorinated products are obtained. Branched chain hydrocarbons have a greater reactivity, with the relative reactivity order being tertiary C-H > secondary C-H > primary C-H. Thus, of 2-methylpropane with nitric acid in a sealed tube can lead to 2-nitro-2-methylpropane. The same reactivity order is found with . A number of these reactions may have radical character. Oxidation with chromium(V1) oxide (CrO,) may lead to a tertiary alcohol. When a free radical is formed and held tightly within a molecule, reactions of quite high site specificity (regioselectivity) may be observed. The free radical may be generated from another functional group in the molecule, but the result is the substitution at a centre that in a formal sense has the characteristics of a hydrocarbon. Such reactions are observed in the photolysis of nitrite esters (RONO) (the Barton reaction), the