Rate of addition

In the foregoing reactions, the attacking species has been an electrophile, with a positively charged intermediate. Bearing these two points in mind, suggest what would be the effect of an electron donating substituent on the double bond. 

The electron donating groups will have the effect of increasing the electron density in the n orbital, and so increase the attraction towards the incoming electrophile, and also result in greater stabilisation of the resultant cation. 

Suggest an order of reactivity for ethene, propene and phenylethene. phenylethene propene ethene 

Suggest why the phenyl group enhances the rate so much. 

The phenyl group is capable of delocalising the positive charge much more effectively by mesomeric interactions than the alkyl groups may do by inductive interactions alone. It will be readily apparent that the more inductive groups there are increasing the electron density on the double bond, the more likely there is to be an attack by an electrophile and the less likely there is to be an attack by a nucleophile. 

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Other properties of association colloids that have been studied include calorimetric measurements of the heat of micelle formation (about 6 kcal/mol for a nonionic species, see Ref. 188) and the effect of high pressure (which decreases the aggregation number [189], but may raise the CMC [190]). Fast relaxation methods (rapid flow mixing, pressure-jump, temperature-jump) tend to reveal two relaxation times t and f2, the interpretation of which has been subject to much disagreement—see Ref. 191. A fast process of fi - 1 msec may represent the rate of addition to or dissociation from a micelle of individual monomer units, and a slow process of ti < 100 msec may represent the rate of total dissociation of a micelle (192 see also Refs. 193-195). 

Equip a 1-litre three-necked flask with a powerful mechanical stirrer, a separatory funnel with stem extending to the bottom of the flask, and a thermometer. Cool the flask in a mixture of ice and salt. Place a solution of 95 g. of A.R. sodium nitrite in 375 ml. of water in the flask and stir. When the temperature has fallen to 0° (or slightly below) introduce slowly from the separatory funnel a mixture of 25 ml. of water, 62 5 g. (34 ml.) of concentrated sulphuric acid and 110 g. (135 ml.) of n-amyl alcohol, which has previously been cooled to 0°. The rate of addition must be controlled so that the temperature is maintained at 1° the addition takes 45-60 minutes. AUow the mixture to stand for 1 5 hours and then filter from the precipitated sodium sulphate (1). Separate the upper yellow n-amyl nitrite layer, wash it with a solution containing 1 g. of sodium bicarbonate and 12 5 g. of sodium chloride in 50 ml. of water, and dry it with 5-7 g. of anhydrous magnesium sulphate. The resulting crude n-amyl nitrite (107 g.) is satisfactory for many purposes (2). Upon distillation, it passes over largely at 104° with negligible decomposition. The b.p. under reduced pressure is 29°/40 mm. 

Place 125 ml. of concentrated ammonia solution (sp. gr. 0-88) in a 600 ml. beaker and surround the latter with crushed ice. Stir the ammonia solution mechanically, and introduce the n-caproyl chloride slowly by means of a suitably supported separatory funnel with bent stem. The rate of addition must be adjusted so that no white fumes are lost. The amide separates immediately. Allow to stand in the ice water for 15 minutes after all the acid chloride has been introduced. Filter oflF the amide at the pump use the flltrate to assist the transfer of any amide remaining in the beaker to the Alter (2). Spread the amide on sheets of Alter or drying paper to dry in the air. The crude n-capro-amide (30 g.) has m.p. 98-99° and is sufficiently pure for conversion into the nitrile (Section 111,112) (3). Recrystallise a small quantity of the amide by dissolving it in the minimum volume of hot water and allowing the solution to cool dry on filter paper in the air. Pure n-caproamide has m.p. 100°. 

In a 500 ml. three-necked flask, equipped with a mechanical stirrer, thermometer and dropping funnel, place 300 ml. of 88-90 per cent, formic acid and add 70 ml. of 30 per cent, hydrogen peroxide. Then introduce slowly 41 g. (51 ml.) of freshly distilled cyclohexene (Section 111,12) over a period of 20-30 minutes maintain the temperature of the reaction mixture between 40° and 45° by cooling with an ice bath and controlling the rate of addition. Keep the reaction mixture at 40° for 1 hour after all the cyclohexene has been added and then allow to stand overnight at room temperature. Remove most of the formic acid and water by distillation from a water bath under reduced pressure. Add an ice-cold solution of 40 g. of sodium hydroxide in 75 ml. of water in small portions to the residual mixture of the diol and its formate take care that the tempera... 

Too fast a rate of addition may cause aggregation of the lithium. If this occurs, the addition should be interrupted and the rate of stirring diminished until the aggregate has disintegrated. 

Table 6 3 shows that the effect of substituents on the rate of addition of bromine to alkenes is substantial and consistent with a rate determining step m which electrons flow from the alkene to the halogen Alkyl groups on the carbon-carbon double bond release electrons stabilize the transition state for bromonium ion formation and increase the reaction rate... 

The reciprocal of a radical reactivity ratio is sometimes used to quantitatively express the reactivity of monomer M2 by comparing its rate of addition to radical Mi - relative to the rate of Mi adding to Mi-. 

Because polymerizations are accompanied by the Hberation of considerable heat, the chances of a violent or mnaway reaction must be avoided. This is most easily done by gradual addition of the reactants to the kettle. Usually the monomers are added from weighing or measuring tanks situated close to the kettle. The rate of addition of monomer is adjusted to permit removal of heat. A supply of inhibitor is kept on hand to stop the polymerization if the cooling becomes inadequate. 

The degree of polymerization is controlled by the rate of addition of the initiator. Reaction in the presence of an initiator proceeds in two steps. First, the rate-determining decomposition of initiator to free radicals. Secondly, the addition of a monomer unit to form a chain radical, the propagation step (Fig. 2) (9). Such regeneration of the radical is characteristic of chain reactions. Some of the mote common initiators and their half-life values are Hsted in Table 3 (10). 

Such a reaction is controlled by the rate of addition of the acid. The two-phase system is stirred throughout the reaction the heavy product layer is separated and washed thoroughly with water and alkaU before distillation (Fig. 3). The alkaU treatment is particularly important and serves not just to remove residual acidity but, more importantiy, to remove chemically any addition compounds that may have formed. The washwater must be maintained alkaline during this procedure. With the introduction of more than one bromine atom, this alkaU wash becomes more critical as there is a greater tendency for addition by-products to form in such reactions. Distillation of material containing residual addition compounds is ha2ardous, because traces of acid become self-catalytic, causing decomposition of the stiU contents and much acid gas evolution. Bromination of alkylthiophenes follows a similar pattern. 

The rate of addition depends on the concentration of both the butylene and the reagent HZ. The addition requires an acidic reagent and the orientation of the addition is regioselective (Markovnikov). The relative reactivities of the isomers are related to the relative stabiUty of the intermediate carbocation and are isobutylene 1 — butene > 2 — butenes. Addition to the 1-butene is less hindered than to the 2-butenes. For hydrogen bromide addition, the preferred orientation of the addition can be altered from Markovnikov to anti-Markovnikov by the presence of peroxides involving a free-radical mechanism. 

Butylene isomers also can be expected to show significant differences in reaction rates for metaHation reactions such as hydroboration and hydroformylation (addition of HCo(CO). For example, the rate of addition of di(j -isoamyl)borane to cis-2-huX.en.e is about six times that for addition to trans-2-huX.en.e (15). For hydroformylation of typical 1-olefins, 2-olefins, and 2-methyl-l-olefins, specific rate constants are in the ratio 100 31 1, respectively. 

In a study of the relative rates of addition of 20 dienophiles to cyclopentadiene, TCNE was at the head of the Hst, eg, it added 7700 times as rapidly as maleic anhydride (15). Reaction with most 1,3-dienes takes place rapidly and in high yield at room temperature. TCNE has often been used to characterize 1,3-dienes, including those that are unstable and difficult to isolate (16). 

Control rate of addition of solids, so as not to exceed inerting capacity... 

One kilo (10.4 moles) of fuifural (Note i) is placed in a 4-I. copper can (Note 2) provided with a mechanical stirrer and surrounded by an ice bath. The stirrer is started and the furfural is cooled to 5-8°. When the temperature has fallen to this range, 825 g. of 33.3 per cent technical sodium hydroxide solution (Note 3) is added from a separatory funnel at such a rate that the temperature of the reaction mixture does not exceed 20°. This requires twenty to twenty-five minutes. The rate of addition will depend on the efficiency of the cooling. The stirring is continued for one hour after the addition of the sodium hydroxide solution. 

Too rapid a rate of addition may cause the solution to boil. 

After about two-thirds of the mixture has been added the rate of addition may be increased somewhat. The time required for the addition depends on the efficiency of the... 

When preparing larger amounts, it would probably be better to control the temperature by external cooling as well as by the rate of addition of the acid. 

The rate of addition of the bromine should be so regulated that a large excess of unreacted bromine does not accumulate in the reaction mixture. The amount of bromine present may be roughly estimated by the color of the solution and by the amount of bromine vapor carried into the condenser. 

To maintain this temperature at the rate of addition of the nitrite indicated will probably require the occasional addition of cracked ice to the reaction mixture. Only occasional checking of the temperature is necessary. If the temperature is kept below 0°, slightly better yields are obtained. The addition of Dry Ice directly to the ether layer accomplishes this easily. Temperatures above 10° cause a loss in yield, but apparently create no hazard. 

The rate of addition is regulated by the rate of evolution of hydrogen bromide. The yield of product is essentially the same whether the reaction mixture is held at 80-85° or at room temperature. 

A mixture of 300 ml. of water, 150 ml. of concentrated nitric acid, and 0.2 g. of sodium nitrite (Note 2) is placed in a 2-1. threenecked flask equipped with a stirrer and a thermometer. The stirred mixture is warmed to 45°, and 2 g. of l,2,4-triazole-3(5)-thiol is added. When oxidation starts, as indicated by the evolution of brown fumes of nitrogen dioxide and a rise in temperature, a bath of cold water is placed under the reaction flask to provide cooling and an additional 99 g. (total, 101 g. 1 mole) of 1,2,4-triazole-3(5)-thiol is added in small portions over the course of 30-60 minutes. The rate of addition and the extent of cooling by the water bath are so regulated as to keep the temperature close to 45 7° all during the addition. The water bath is kept cold by the occasional addition of ice. 

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