Alkyl groups reactivity

Pyrylium salts alkyl groups reactivity, 3, 662 aromaticity, 3, 640 arylammes from, 3, 657 benzenoid compounds from, 3, 656, 658 benzisoxazol-3-yl-synthesis, 6, 124 bicyclic... 

Kinetic data are available for the nitration of a series of p-alkylphenyl trimethylammonium ions over a range of acidities in sulphuric acid. - The following table shows how p-methyl and p-tert-h xty augment the reactivity of the position ortho to them. Comparison with table 9.1 shows how very much more powerfully both the methyl and the tert-butyl group assist substitution into these strongly deactivated cations than they do at the o-positions in toluene and ferf-butylbenzene. Analysis of these results, and comparison with those for chlorination and bromination, shows that even in these highly deactivated cations, as in the nitration of alkylbenzenes ( 9.1.1), the alkyl groups still release electrons in the inductive order. In view of the comparisons just... 

Introduction of Non-functional Alkyl and Reactive Allyl Groups... 

The effect of alkyl groups in the 5-position on the reactivity of the thiazole nitrogen is analogous to that found for 3-alkylpyridines, in other words, a simple inductive effect. In passing from the unsubstituted heterocycle to the methyl derivative, the rate constant doubles a further increase in substitution produces a much less pronounced variation. 

It is well known that in nitrogen-containing heterocyclic compounds the reactivity of alkyl groups is enhanced. In the thiazole series, alkyl groups in the 2-position are reactive towards carbonyl compounds and condensations may be realized. 

In general alkyl substituents increase the reactivity of a double bond toward elec trophilic addition Alkyl groups are electron releasing and the more electron rich a dou ble bond the better it can share its tt electrons with an electrophile Along with the observed regioselectivity of addition this supports the idea that carbocation formation rather than carbocation capture is rate determining... 

The order of alkyl halide reactivity in nucleophilic substitutions is the same as their order m eliminations Iodine has the weakest bond to carbon and iodide is the best leaving group Alkyl iodides are several times more reactive than alkyl bromides and from 50 to 100 times more reactive than alkyl chlorides Fluorine has the strongest bond to car bon and fluonde is the poorest leaving group Alkyl fluorides are rarely used as sub states m nucleophilic substitution because they are several thousand times less reactive than alkyl chlorides... 

We saw m Section 8 2 that the rate of nucleophilic substitution depends strongly on the leaving group—alkyl iodides are the most reactive alkyl fluorides the least In the next section we 11 see that the structure of the alkyl group can have an even greater effect... 

The most frequently used organocuprates are those m which the alkyl group is pri mary Steric hindrance makes secondary and tertiary dialkylcuprates less reactive and they tend to decompose before they react with the alkyl halide The reaction of cuprate reagents with alkyl halides follows the usual 8 2 order CH3 > primary > secondary > tertiary and I > Br > Cl > F p Toluenesulfonates are somewhat more reactive than halides Because the alkyl halide and dialkylcuprate reagent should both be primary m order to produce satisfactory yields of coupled products the reaction is limited to the formation of RCH2—CH2R and RCH2—CH3 bonds m alkanes... 

The carbonyl carbon of a ketone bears two electron releasing alkyl groups an aldehyde carbonyl group has only one Just as a disubstituted double bond m an alkene is more stable than a monosubstituted double bond a ketone carbonyl is more stable than an aldehyde carbonyl We 11 see later m this chapter that structural effects on the relative stability of carbonyl groups m aldehydes and ketones are an important factor m then rel ative reactivity... 

Primary alkyl groups are more reactive than secondary and tertiary. PivaUc acid accelerates the rate of protonolysis of trialkylboranes with water and alcohols (207,208). The reaction can be controlled to give excellent yields of dialkylbotinic acids and esters. 

The reactions of trialkylboranes with bromine and iodine are gready accelerated by bases. The use of sodium methoxide in methanol gives good yields of the corresponding alkyl bromides or iodides. AH three primary alkyl groups are utilized in the bromination reaction and only two in the iodination reaction. Secondary groups are less reactive and the yields are lower. Both Br and I reactions proceed with predominant inversion of configuration thus, for example, tri( X(9-2-norbomyl)borane yields >75% endo product (237,238). In contrast, the dark reaction of bromine with tri( X(9-2-norbomyl)borane yields cleanly X(9-2-norbomyl bromide (239). Consequentiy, the dark bromination complements the base-induced bromination. 

Addition to Carbonyl Compounds. Unlike Grignard and alkykitliium compounds, trialkylboranes are inert to carbonyl compounds. The air-catalyzed addition to formaldehyde is exceptional (373). Alkylborates are more reactive and can transfer alkyl groups to acyl halides. The reaction provides a highly chemoselective method for the synthesis of ketones (374). 

Acylation. Acylation is the most rehable means of introducing a 3-substituent on the indole ring. Because 3-acyl substituents can be easily reduced to 3-aLkyl groups, a two-step acylation—reduction sequence is often an attractive alternative to direct 3-aLkylation. Several kinds of conditions have been employed for acylation. Very reactive acyl haUdes, such as oxalyl chloride, can effect substitution directiy without any catalyst. Normal acid chlorides are usually allowed to react with the magnesium (15) or 2inc (16) salts. The Vilsmeier-Haack conditions involving an amide and phosphoms oxychloride, in which a chloroiminium ion is the active electrophile, frequentiy give excellent yields of 3-acylindoles. 

AH higher a-olefins, in the presence of Ziegler-Natta catalysts, can easily copolymerise both with other a-olefins and with ethylene (51,59). In these reactions, higher a-olefins are all less reactive than ethylene and propylene (41). Their reactivities in the copolymerisation reactions depend on the sise and the branching degree of their alkyl groups (51) (see Olefin polya rs, linear low density polyethylene). 

Substituted heat-reactive resins are most widely used in contact-adhesive appHcations and, to a lesser extent, in coatings (77,78) -butylphenol, cresol, and nonylphenol are most frequendy used. The alkyl group increases compatibiHty with oleoresinous varnishes and alkyds. In combination with these resins, phenoHcs reduce water sensitivity. Common appHcations include baked-on and electrical insulation varnishes, and as modifiers for baking alkyds, rosin, and ester gum systems. Substituted heat-reactive resins are not used for air-dry coatings because of theh soft, tacky nature in the uncured state substituted nonheat-reactive phenoHcs are the modifying resin of choice in this case. 

Neoprene—phenohc contact adhesives, known for thein high green strength and peel values, contain a resole-type resin prepared from 4-/-butylphenol. The alkyl group increases compatibiHty and reduces cross-linking. This resin reacts or complexes with the metal oxide, eg, MgO, contained in the formulation, and increases the cohesive strength of the adhesive. In fact, the reactivity with MgO is frequently measured to determine the effectiveness of heat-reactive phenoHcs in the formulation. 

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