Elimination 2-propanol dehydration

When some straight and branched-chain aliphatic alcohols, such as n-propanol, n-butanol and isopropanol, are subjected to high temperatures, dehydrogenation products predominate over dehydration (51). Presumably the eliminations take place via a six-membered transition state and are catalyzed by hydrogen halides in the homogeneous phase (52) to produce olefins. On the other hand, gas phase dehydration over solid catalysts is a valuable process for the preparation of olefins and ethers. 

Dehydrohalogenation of l-phenyI-2-chloropropane, or dehydration of l-phenyl-2-propanol, could yield two products l-phenylpropene or 3-phenyl-propene. Actually, only the first of these products is obtained. We saw earlier (Secs. 5.14 and 5.23) that where isomeric alkenes can be formed by elimination, the... 

Dimethyl-1-propanol cannot undergo dehydration because there is no H available for elimination on the carbon adjacent to the carbon holding the OH. 

Because of the instability of 1° carbocations and the conclusion that 1° alcohols undergo acid-catalyzed oxygen exchange by an Sn2 process, there may be some question whether 1° carbocations are true intermediates in elimination reactions. An alternative mechanistic possibility for dehydration of 1° alcohols is an acid-catalyzed E2 mechanism. Narayan and Antal proposed such a mechanism for the specific acid-catalyzed dehydration of 1-propanol with sulfuric acid in supercritical water (Figure 10.44). ... 

Synthesis from sucrose ester, aluminium diiso-propoxide monoacetylacetonate and diisocyanate. In pre-liminary experiments, sucrose "penta"soyate or sucrose "penta" dehydrated castor ester reacted in toluene at IIQOC with an equimolar proportion of aluminium diiso-propoxide monoacetylacetonate. (The latter was prepared separately by reaction of equimolar proportions of aluminium isopropoxide and acetylacetone in refluxing toluene with elimination of 1 mole 2-propanol). The modified sucrose ester reacted with 1.1 mole toluene diisocyanate (the maximum without gelation) and dibutyl tin dilaurate as catalyst. The performance of air-dried or stoved coatings was similar to material derived by the mono-/diisocyanate route in terms of alkali resistance, but they were more brittle. 

The fact that primary carbenium ions are unstable suggests that the exchange with solvent is an Sn2 process with primary alcohols. If true, then are primary carbenium ions ever intermediates in dehydration reactions Studies have shown that it depends upon the case. Neopentyl alcohol does form a primary carbenium ion, whereas 1-propanol does not. Acid-catalyzed elimination of 1-propanol to form propene occurs by a concerted E2 reaction (Eq. 10.90). Similarly, whether a secondary alcohol eliminates in acid via an El or E2 pathway depends on the case. 

This addition process is the reverse of the acid-induced elimination of water from alcohols (dehydration, Section 11-7). Its mechanism is the same in reverse, as illustrated in the hydration of 2-methylpropene, a reaction of industrial importance leading to 2-methyl-2-propanol (tert-butyl alcohol). 

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