Effect of Alcohol Structure on Reaction Rate

For the general case of R = any alkyl group, how many bonded pairs of electrons are involved in stabilizing R3C+ by hyperconjugation How many in R2CH In RCH2  

To summarize, the most important factor to consider in assessing carbocation stability is the degree of substitution at the positively charged carbon. 

We will see numerous reactions that involve carbocation intermediates as we proceed through the text, so it is important to understand how their structure determines their stability. 

For a proposed reaction mechanism to be valid, the sum of its elementary steps must equal the equation for the overall reaction and the mechanism must be consistent with all experimental observations. The S l process set forth in Mechanism 4.1 satisfies the first criterion. What about the second  

One important experimental fact is that the rate of reaction of alcohols with hydrogen halides increases in the order primary secondary tertiary. This reactivity order parallels the carbocation stability order and is readily accommodated by the mechanism we have outlined. 

The rate-determining step in the S l mechaiusm is dissociation of the alkyloxonium ion to the carbocation. 

The rate of any chemical reaction increases with increasing temperature. Thus the value of k for a reaction is not constant, but increases as the temperature increases. 

The rate of this step is proportional to the concentration of the alkyloxonium ion  

We saw in Section 4.8 that the reactivity of alcohols with hydrogen halides increases in the order primary secondary tertiary. To be vahd, the mechanism proposed in Fig-nre 4.7 and represented by the energy diagram in Fignre 4.13 mnst account for this order of relative reactivity. When considering rate effects, we focns on the slow step of a reaction mechanism and analyze how that step is inflnenced by changes in reactants or reaction conditions. 

As mentioned, the slow step in the SnI mechanism is the dissociation of the alkyl-oxoninm ion to the carbocation. The rate of this step is proportional to the concentration of the alkyloxoninm ion  

Consider what happens when the alkyloxoninm ion dissociates to a carbocation and water. The positive charge resides mainly on oxygen in the alkyloxonium ion but is shared between oxygen and carbon at the transition state. 

Inferring the structure of the transition state on the basis of what is known about the species that lead to it or may be formed by way of it is a practice with a long history in organic chemistry. A justification of this practice was advanced in 1955 by George S. Hammond, who reasoned that if two states, such as a transition state and an intermediate derived from it, are similar in energy, then they are similar in structure. This rationale is known as Hammond s postulate. In the formation of a carbocation from an alkyloxonium ion, the transition state is closer in energy to the carbocation than it is to the alkyloxonium ion, and so its structure more closely resembles the carbocation and it responds in a similar way to the stabilizing effects of alkyl substituents. 

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As in organic chemistry where m values for bromides are rather lower than for chlorides, for example m — 0.92 for t-butyl bromide compared with m = 1.00 for t-butyl chloride, m values for cobalt(ra)-amine-bromide complexes are, at around 0.2, rather lower than for the analogous chlorides. Whereas in this work the effect of solvent structure on reaction rates has been used to gain further insight into reaction mechanisms, the opposite approach has also been used, in a study of aquation of tra/u-[Co(en)2Cl2]+ in alcohol-water mixtures, in which variation of rate with solvent composition has been used as a probe of solvent structure variation. Rates of aquation of both cis- and trans-[Co ea)2C have been determined in aqueous acetonitrile (0 < mole fraction MeCN < 0,104). For both complexes aquation rates decrease only slightly as the proportion of acetonitrile increases, with the cir-complex slightly more sensitive to solvent variation. The kinetic effects observed here are smaller than those observed in t-butyl alcohol-water solvent mixtures. ... 

Optimization of the reaction protocol and the study of the effect of promoter structure on selectivityin the catalytic enantioselective cyclopropanation with bis(halomethyl)zinc reagents have been reported recently. Prior formation of a zinc alkoxide and the use of added zinc iodide are critical for efficient catalytic enantioselective cyclopropanation of allylic alcohols using chiral bis(sulfonamides), the simple A7,iV -dimesylcyclohexane-l,2-diamine being the most effective in terms of rate and enantioselectivity. 

Toluene alkylation with isopropyl alcohol was chosen as the test reaction as we can follow in a detail the effect of zeolite structural parameters on the toluene conversion, selectivity to cymenes, selectivity to para-cymene, and isopropyl/n-propyl ratio. It should be stressed that toluene/isopropyl alcohol molar ratio used in the feed was 9.6, which indicates the theoretical toluene conversion around 10.4 %. As you can see from Fig. 2 conversion of toluene over SSZ-33 after 15 min of T-O-S is 21 %, which is almost two times higher than the theoretical toluene conversion for alkylation reaction. The value of toluene conversion over SSZ-33 is influenced by a high rate of toluene disproportionation. About 50 % of toluene converted is transformed into benzene and xylenes. Toluene conversion over zeolites Beta and SSZ-35 is around 12 %, which is due to a much smaller contribution of toluene disproportionation to the overall toluene conversion. A slight increase in toluene conversion over ZSM-5 zeolite is connected with the fact that desorption and transport of products in toluene alkylation with isopropyl alcohol is the rate controlling step of this reaction [9]... 

The zero slope found for transesterification (series 45) can be explained in accordance with the general view on acid-catalyzed reactions of organic acids and esters. The first step is the protonation of the acid or ester, which is followed by interaction with the alcohol (or water in ester hydrolysis). The absence of any observable influence of the alcohol structure on rate indicates that the rate-determining step must be the protonation of the ester. This is in contrast to the homogeneous reaction, in which this step is usually very rapid. The parallel dehydration of the alcohols exhibited a large structure effect on rate (Case 7 from Table II), confirming the independence of the two reaction routes. 

Our compatriot N. A. Menshutkin made a great contribution to the development of the kinetics. In 1877 he studied in detail the reaction of formation and Iqrdrolysis of esters from various acids and alcohols and was the first to formulate the problem of the dependence of the reactivity of reactants on flieir chemical structure. Five years later when he studied the hydrolysis of tert-zmy acetate, he discovered and described the autocatalysis phenomenon (acetic acid formed in ester hydrolysis accelerates the hydrolysis). In 1887-, studying the formation of quaternary ammonium salts from amines and alkyl halides, he found a strong influence of the solvoit on the rate of this reaction (Menschutkin reaction) and stated the problem of studying the medium effect on the reaction rate in a solution. In 1888 N. A. Menschutkin introduced the term chmical kinetics in his monograph Outlines of Development of Chemical Views. ... 

Effect of Structure. The rate at which different alcohols and acids are esterified as weU as the extent of the equiHbrium reaction are dependent on the stmcture of the molecule and types of functional substituents of the alcohols and acids. Specific data on rates of reaction, mechanisms, and extent of reaction are discussed in the foUowing. More details concerning stmctural effects are given in References 6, 13—15. 

As has been mentioned previously, one is most likely to find analogies to catalytic reactions on solids with acidic and/or basic sites in noncatalytic homogeneous reactions, and therefore the application of established LFERs is safest in this field. Also the interpretation of slopes is without great difficulty and more fruitful than with other types of catalysts. The structure effects on rate have been measured most frequently on elimination reactions, that is, on dehydration of alcohols, dehydrohalogenation of alkyl halides, deamination of amines, cracking of the C—C bond, etc. Less attention has been paid to substitution, addition, and other reactions. 

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