Example An elimination reaction

The influence of reaction temperature, T, and the concentration of base, [Bore], on the yield of an elimination reaction was studied. The ranges of variation of the experimental variables were 20 C T 60 °C, and 1.0 5 [Base] 2.0. 

It was assumed that a response surface model with linear terms augmented with a 

The corresponding design expressed in scaled variables, Xj (temperature), X2 (base concentration) is given in Table 3.2. Such tables will be called design matrices. 

The model can also be written in matrix notation as below. It is very convenient to use matrices to describe the calculations involved in the modelling process. A short summary of matrix calculus is given in an appendix at the end of this book. 

If we put the values of the scaled variables in the model for the five experimental runs we obtain 

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Figure 3-3. Representative, simple examples of a substitution, an addition, and an elimination reaction showing the number, n, of reaction partners, and the change in n, An, during the reaction.

The entropy change, AS, is a measure of the change in the amount of molecular randomness, or freedom of motion, that accompanies a reaction. For example, in an elimination reaction of the type... 

An elimination reaction is, in a sense, the reverse of an addition reaction. It involves the elimination of two groups from adjacent carbon atoms, converting a saturated molecule into one that is unsaturated. An example is the dehydration of ethanol, which occurs when it is heated with sulfuric acid ... 

This step is an example of an elimination reaction, a reaction in which two groups or atoms on neighboring carbon atoms are removed from a molecule, leaving a multiple bond (Fig. 18.8). 

The states of reactants and products are often not given for organic reactions, because the reaction may take place at the surface of a catalyst or it may take place in a nonaqueous solvent, as here. The reaction is another example of an elimination reaction and is carried out in hot ethanol, with sodium ethoxide, NaCH CH,0, as the reagent. Some Cl I3CH2CH=CH2 is also formed in this reaction. 

In the MPVO reaction, several side-reactions can occur (Scheme 20.23). For example, an aldol reaction can occur between two molecules of acetone, which then leads to the formation of diacetone alcohol. The latter acts as a good ligand for the metal of the MPVO catalyst, rendering it inactive. Moreover, the aldol product may subsequently eliminate water, which hydrolyzes the catalyst. The aldol reaction can be suppressed by adding zeolite NaA [84, 92]. 

Some interesting modifications with respect to the base-induced isomerization have recently been developed. For example, conversion of 4-hydroxy-l-thiophenyl-2-alkynes 299 into the corresponding 4-hydroxy-substituted thiophenylallenes 300 was achieved by treatment with potassium hexamethyldisilazide at low temperature (Scheme 8.79) [165], If the hydroxyl group is protected as the THP ether an elimination reaction occurred, resulting in the formation of an enyne instead of allene 300. 

For a number of other pharmacologically active unsaturated compounds, it is assumed that a reactive allene is formed in situ by an alkyne isomerization [160] or an elimination reaction [161]. The prime example of the formation of such a highly reactive allene through chemical activation of an unsaturated precursor is the ene-diyne antibiotic neocarzinostatin (Scheme 18.57) [162],... 

In the first example of an elimination reaction, the strong acid, H2SO4, does not count as a reactant. It is not directly involved in the reaction. It is a catalyst a compound that speeds up a reaction but is not consumed by it. 

Glassify each reaction in two different ways for example, as oxidation and as an elimination reaction. 

The synthetic results have been summarized In Table I. In most cases the displacement of trlflate anion occurred rapidly ( 30 min) at or below reflux temperature however. In two examples (see Table I, compotinds 4 and 13) an elimination reaction was found to predominate. 

Eliminations belong to one of the most diverse reaction types [76] and numerous solvent-free pyrolyses (sometimes quantitative melt reactions) provided useful syntheses [58,81-87]. However, quantitative solid-state eliminations are rare (examples are found in the halogenations of 110,112, and 114 (Scheme 12)). If an elimination reaction cannot be performed purely thermally or photo-chemically, usually a catalyst or other auxiliary has to be added and it is then no longer waste-free. 

Mg+" reacts with alkyl halides in the gas phase via a range of substrate-dependent pathways Not all halides are reactive—examples of unreactive substrates include methyl chloride, vinyl chloride, trichloro and tetrachloro ethylene. Reaction with ethyl chloride proceeds via an elimination reaction (equation 18) followed by a displacement reaction (equation 19). For larger alkyl halides, such as isopropyl chloride, chloride abstraction also occurs (equation 20). For multiply halogenated substrates such as carbon tetrachloride, oxidative reactions occur (equations 21 and 22), although organometallic... 

During an elimination reaction, atoms are removed, or eliminated, from adjacent carbons on a carbon chain, producing a small molecule, which is often water. A double bond forms between the adjacent carbons, producing an alkene. For example, ethyl alcohol (ethanol) can undergo an elimination reaction to form ethylene by the loss of H and OH, which form H2O (water). 

The El reaction involves the formation of a planar carbocation intermediate. Therefore, both syn and anti elimination can occur. If an elimination reaction removes two substituents from the same side of the C—C bond, the reaction is called a syn elimination. When the substituents are removed from opposite sides of the C—C bond, the reaction is called an anti elimination. Thus, depending on the substrates El reaction forms a mixture of cis (Z) and trans (E) products. For example, tert-hutyl bromide (3° alkyl halide) reacts with water to form 2-methylpropene, following an El mechanism. The reaction requires a good ionizing solvent and a weak base. When the carbocation is formed, SnI and El processes compete with each other, and often mixtures of elimination and substitution products occur. The reaction of t-butyl bromide and ethanol gives major product via El and minor product via SnI-... 

The second special mechanism that we shall not consider is actually not a separate mechanism but is an electrophilic addition to one of the aromatic double bonds followed by an elimination reaction. An example is shown below ... 

Then the differences in rate caused by the electronic effect of the substituent are correlated by the Hammett equation log(kz/kH) = poz, where kz is the rate constant obtained for a compound with a particular meta or para substituent, ku is the rate constant for the unsubstituted phenyl group, and crz is the substituent constant for each substituent used. The proportionality constant p relates the substituent constant (electron donating or wididrawing) and the substituent s effect on rate. It gives information about the type and extent of charge development in the activated complex. It is determined by plotting log(kz/kQ) versus ov for a series of substituents. The slope of the linear plot is p and is termed the reaction constant. For example, the reaction shown above is an elimination reaction in which a proton and the nosy late group are eliminated and a C-N n bond is formed in their place. The reaction is second order overall, first order in substrate, and first order in base. The rate constants were measured for several substituted compounds ... 

The reaction that produces the alkene involves the loss of an HBr molecule to form a C=C double bond. It is therefore an example of an elimination reaction. 

The ester group is lost by an elimination reaction. The adjacent carbonyl group makes the hydrogen more acidic, so it is more readily removed by the base. This is an example of an EI cb mechanism. (See the Focus On box on page 333.)... 

An example of an elimination reaction is the reaction of an alxyl halide with base to yield an acid and an atkene ... 

The aromatization of triazolines can take place either by rearrangement reactions, by direct oxidation or by an elimination reaction. A recent review discusses this topic (78BSF(2)485>. From the numerous reactions some illustrative examples are described below. 

In an elimination reaction, two groups X and Y are removed from a starting material. Two a bonds are broken, and a it bond is formed between adjacent atoms. The most common examples of elimination occur when X = H and Y is a heteroatom more electronegative than carbon. 

Figure 14 shows another example of an elimination reaction one whose results can be seen easily. When sucrose reacts with concentrated sulfuric acid, water is eliminated, which leaves behind mostly carbon. Carbon is the black substance you can see forming in the photos and rising out of the beaker on the far right. 

Alkyl halides undergo an elimination reaction to yield alkenes on treatment with strong base. For example, chlorocyclohexane gives cyclohexene on reaction with NaNHg ... 

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