Halogens inductive effect

Haloalkane,. see Alkyl halide Haloform reaction, 854-855 Halogen, inductive effect of, 562 resonance effect of, 563 Halogenation, aldehydes and, 846-848... 

According to Memorization Task 19.5, halogens are net EAS deactivators. Based on this and info in Model 12, which is stronger in the case of halogens, inductive effects or resonance effects (Circle one and explain your reasoning.)... 

Each of these intermediates can be hthiated in the 2-position in good yield. The reactivity toward hthiation is due to the inductive effect of the nitrogen atom and coordination by oxygen from the N-substituent. A wide variety of electrophiles can then carry out substitution at the 2-position. Lithiation at other positions on the ring can be achieved by halogen—metal exchange 3-hthio and 5-hthioindoles have also been used as reactive intermediates. 

One further point inductive effects and resonance effects don t necessarily act in the same direction. Halogen, hydroxyl, alkoxyl, and amino substituents, for instance, have electron -withdrawing inductive effects because of the electronegativity of the -X, -O, or —N atom bonded to the aromatic ring but have resonance effects because of the lone-pair electrons on those same —X, -O, or —N atoms. When the two effects act in opposite directions, the stronger of the two dominates. 

Inductive effects (Section 16.4) are also important in determining alcohol acidities. Electron-withdrawing halogen substituents, for example, stabilize an alkoxide ion by spreading out the charge over a larger volume, thus making the alcohol more acidic. Compare, for example, the acidities of ethanol (p/tcrt-butyl alcohol (p/

Because inductive effects operate through cr bonds and are dependent on distance, the effect of halogen substitution decreases as the substituent moves farther from the carboxyl. Thus, 2-chlorobutanoic acid has p/chlorobutanoic acid has p/chlorobutanoic acid has p/C., = 4.52, similar to that of butanoic acid itself. 

The applicability of these methods is restricted to polyhalogenated compounds, since the inductive effect of the halogen atoms is necessary for facilitating formation of the carbanion. 

Since the (n-Bu)3Sn radical (R ) is nucleophilic (17), a partial negative charge must be produced at the methine carbon whose chlorine is being abstracted. The rate of this abstraction should clearly be enhanced by electron-withdrawing groups on R due to their stabilization of this charge by inductive effects. As observed, the removal of Cl from EW (or W diad) is expected to be more facile than from EVE (or VE diad) as a result of a -halogen effect in the former structure. 

If the competition data are compared with electronegativity values for the halogens 85>, then tetrafluorobenzyne is clearly in an anomalous position. The only reasonable explanation available at present is that tetrafluorobenzyne is so destabilized by the inductive effect of the fluorine atoms that it has lost a considerable amount of the selectivity which arynes normally show. Estimates for the heats of formation of the isomeric dichlorobenzynes and for tetrachlorobenzyne have recently been made from mass spectrometric studies and these do indicate a low stability for tetrachlorobenzyne 86>. Evidently more data are required for the tetrahalogenobenzynes. 

The effect of introducing electron-withdrawing substituents into simple aliphatic acids is more marked. Thus halogen, with an inductive effect acting in the opposite direction to alkyl, might be expected to increase the strength of an acid so substituted, and this is indeed observed as pKa values show ... 

The presence of groups such as OH, OMe, or halogen having an electron-withdrawing inductive effect, but an electron-donating meso-meric effect when in the o- and p-positions, may, however, cause the p-substituted acids to be weaker than the m- and, on occasion, weaker even than the unsubstituted acid itself, e.g. p-hydroxybenzoic acid ... 

A very similar situation is encountered in the electrophilic addition of unsymmetrical adducts (e.g. HBr) to vinyl halides (e.g. CH2=CHBr), where the inductive effect of halogen controls the rate, but relative mesomeric stabilisation of the carbocationic intermediate controls the orientation, of addition (p. 185). 

With ketones such as (94), that have alternative groups of a-H atoms to attack, two questions arise (a) which group, the CH2 or the CH3, is attacked preferentially, and (b) when one H has been substituted by halogen, will a second halogen become attached to the same or to the other a-carbon atom. So far as (a) is concerned, it is found that bromination of, for example, MeCH2COCH3, yields 1- and 3-bromobutanones in virtually equal amount (both these bromoketones then undergo very rapid further reaction, cf. p. 296). The inductive effect exerted by a simple alkyl group, R, thus appears to have relatively little effect on the acidity of H, or on the... 

CX3 is a good leaving group because of the electron-withdrawing inductive effect of the three halogen atoms this activates the carbonyl carbon atom in (100) to nucleophilic attack, and also stabilises the... 

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