Group 4 - Alkyl Protons

The mechanism of the alkylation reaction is similar to curing. The methylo1 group becomes protonated and dissociates to form a carbonium ion intermediate which may react with alcohol to produce an alkoxymethyl group or with water to revert to the starting material. The amount of water in the reaction mixture should be kept to a minimum since the relative amounts of alcohol and water determine the final equiHbrium. 

The initiating step in these reactions is the attachment of a group to the sulfoxide oxygen to produce an activated intermediate (5). Suitable groups are proton, acyl, alkyl, or almost any of the groups that also initiate the oxidations of alcohols with DMSO (40,48). In a reaction, eg, the one between DMSO and acetic anhydride, the second step is removal of a proton from an a-carbon to give an yUde (6). Release of an acetate ion generates the sulfur-stabilized carbonium ion (7), and the addition of acetate ion to the carbonium ion (7) results in the product (eq. 15) ... 

Determining the degree of substitution using standard proton nmr refles on the integral ratio between the ceUulosic ring protons ( i 5.0-2.96) and the ester alkyl protons ( i 1.26 for butyryl and propionyl and i 2.06 for acetyl methyl groups). This simple procedure is used extensively to determine the extent of esterification and is currently the fastest, easiest way for determining the DS of mixed cellulose esters. 

Alkyl groups attached to pyridopyrimidines adjacent to a nitrogen are activated , i.e. they are readily deprotonated and react with electrophilic reagents as their anions, or resonance stabilized equivalents, e.g. (64). This ready deprotonation, of course, leads to facile exchange of the alkyl protons for deuterium (Sections 2.15.2.2.1, 2.15.4.2), but, in... 

From a general point of view, the tautomeric studies can be divided into 12 areas (Figure 20) depending on the migrating entity (proton or other groups, alkyl, acyl, metals. ..), the physical state of the study (solid, solution or gas phase) and the thermodynamic (equilibrium constants) or the kinetic (isomerization rates) approach. 

Finally, a brief word about aldehydes. They are included at the end of this group for convenience only and should be spotted easily. Aldehydes bound to aromatic rings give sharp singlets at 10.2-9.9 ppm, whilst in alkyl systems, they give sharp signals at 10.0-9.7 ppm, which couple to adjacent alkyl protons with a relatively small coupling constants (2-4 Hz). 

The source of the alkyls is usually some main group alkylating agent, and the groups that are replaced on M to form a dialkyl complex usually are halides. When the alkyl group contains one or more (3 protons, then the predominant re-... 

The structure of the products is determined by the site of protonation of the radical-anion intermediate formed after the first electron-transfer step. In general, electronreleasing substituents favor protonation at the ortho position, whereas electron-attracting groups favor protonation at the para position.143 Addition of a second electron gives a pentadienyl anion, which is protonated at the center carbon. As a result, 2,5-dihydro products are formed with alkyl or alkoxy substituents, and 1,4-products are formed from aromatics with electron-attracting substituents. The preference for protonation of the central carbon of the pentadienyl anion is believed to be the result of the greater 1,2 and... 

In another related study (using hexadecyltrimethyl ammonium bromide micelles), isopropyl benzene was solubilized, and the chemical shifts of aromatic and alkyl protons were observed. The results suggest that the isopropyl benzene molecules are oriented such that the isopropyl groups are buried more deeply in the core of the micelle, while the benzene ring is in the more hydrated palisade layer. This plus the conclusion of Item 3 is consistent with the description presented in Section 8.3, which located the benzene in a relatively polar portion of the micelle. 

Because of the greater acidity of a vinylic than an alkyl proton, vinyl halides, RHC=CRX, are more likely than alkyl halides to undergo EjcB elimination. However, when the proton is not rendered even more acidic by a vicinal electron-withdrawing group, and when the basic catalyst is not too strong, E2 reaction obtains. Then anti elimination is much the preferred pathway. 

Dimethylbenzene has two types of protons those attached directly to the benzene ring and those of the methyl groups. Aryl protons are significandy less shielded than alkyl protons. As shown in text Table 13.1 they are expected to give signals in the chemical shift range 8 6.5-8.5 ppm. Thus, the... 

An alkyl halide can undergo an elimination reaction if it has a susceptible proton situated on a (1-carbon, i.e. the carbon next to the C-X group. This proton is lost during the elimination reaction along with the halide ion. In some respects, there is similarity here between alkyl halides and carbonyl compounds (Following fig.). Alkyl halides can have susceptible protons at the (1-position whilst carbonyl compounds can have acidic protons at their a-position. By comparing both structures, it can be seen that the acidic/ susceptible proton is attached to a carbon neighbouring an electrophilic carbon. 

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