Haloforms iodoform

This IS called the haloform reaction because the trihalomethane produced is chloroform (CHCI3) bromoform (CHBrj) or iodoform (CHI3) depending on the halogen used... 

The haloform reaction using iodine was once used as an analytical test in which the formation of a yellow precipitate of iodoform was taken as evidence that a substance... 

In general, ketones don t undergo oxidation however, methyl ketones undergo a haloform reaction. In a haloform reaction, the oxidation converts the methyl group to a haloform molecule (usually iodoform (CHI3)), which leaves the Ccirbon backbone one carbon atom shorter. The oxidant in a haloform reaction is sodium hypohalite (NaOX), which forms by the reaction of sodium hydroxide (NaOH) with a halogen (X, where X = Cl, Br, or 1). 

This sequence is called the haloform reaction because it results in the production of chloroform, bromoform, or iodoform, depending upon the halogen used. The haloform reaction is a useful method for identification of methyl ketones, particularly when iodine is used, because iodoform is a highly insoluble, bright-yellow solid. The reaction also is very effective for the synthesis of carboxylic... 

When a base is used with a methy] ketone, the alpha carbon will become completely halogenated. This trihalo product reacts further with the base to produce a carboxylic acid and a haloform (chloroform, CHCI j bromoform, CHBr, or iodoform, CHIj). This is called the Haloform Reaction. 

The haloform reaction (converts methyl ketones to acids and iodoform Chapter 22)... 

When the halogen is iodine, the haloform product (iodoform) is a solid that separates out as a yellow precipitate. This iodoform test identifies methyl ketones, which halo-genate three times, then lose CI3 to give iodoform. 

The conversion of a methyl ketone to a carboxylate ion and a haloform (CHX3) by treatment with a halogen and base. The iodoform reaction uses iodine to give a precipitate of solid iodoform, (p. 1056)... 

If excess base and hal< en ore osed, a m yl ketone ts triply hsi-g cnated and then cleaved by base in the haloform reaction. Ihe inodi are a caibougrlic acid plus a so-called Aalcfarm nucleophilic acyl subotitution of CXa by OH. That U, a earbanion ac as a leavii group. 

Consequently, for the haloform case (Table 4.10), since repulsion for chlorine is less than that for fluorine, chlorine as a substituent facilitates carbanion formation much more than fluorine. The enhanced acidities of bromoform and iodoform have been attributed to the release of steric strain on deprotonation, while the increased availability of [Pg.110]

A trihalo compound may be formed from the halogenation of a methyl ketone. The electron-withdrawing effect of the trihalomethyl group makes the carbonyl group very sensitive to nucleophilic addition. Consequently, in the presence of a mild base the trihalomethyl compound easily decomposes with the formation of chloroform, bromoform or iodoform, depending on the halogen. The other product of the haloform reaction is a carboxylic acid (Scheme 3.86). 

In the haloform reaction, the three H atoms of the CH3 group are successively replaced by X to form an intermediate that is oxidatively cleaved with base. Mechanism 23.5 is written with I2 as halogen, forming CHI3 (iodoform) as product. 

Iodoform test (Section 23.7B) A test for the presence of methyl ketones, indicated by the formation of the yellow precipitate, CHI3, via the haloform reaction. 

Aldehydes may be converted to ( )-alkenyl halides by the reaction of CrCh with a haloform in THF. The highest overall yields for the conversion were with iodoform, but somewhat higher (E) (Z) ratios were observed with bromoform or chloroform. Other low-valent metals, such as tin, zinc, manganese and vanadium, were ineffective. As the examples in Table 19 indicate, the reaction is selective for the ( )-isomer, except in the case of an a,3-unsaturated aldehyde. In addition, the reaction with ketones is sufficiently slow for chemoselectivity to be observed for mixed substrates. 

Study carefully the pKgS for the haloform series, CHX3— they may not do what you think they should Chloroform is much more acidic than fluoroform even though fluorine is more electronegative (likewise with bromoform and chloroform). The anion CFamust be slightly destabilized because of some backdonation of electrons. The anion from chloroform and bromoform may also be stabilized by some interaction with the d orbitals (there aren t any on fluorine). The conjugate base anion of bromoform is relatively stable— you will meet this again in the bromoform/iodoform reaction (Chapter 21). 

If excess base and halogen are used, a methyl ketone is triply hali genated and then cleaved by base in the haloform reaction. The produc are a carboxylic acid plus a so-called haloform (chloroform, CHClg bromo-form, CHBrg or iodoform, CHL<). Note that the. second step of the reaction is a nucleophilic acyl substitution of CX3 by OH. That is, a carbanion acts as a leaving group. 

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