Alpha Position carbon

The carbon—carbon double bond is the distinguishing feature of the butylenes and as such, controls their chemistry. This bond is formed by sp orbitals (a sigma bond and a weaker pi bond). The two carbon atoms plus the four atoms ia the alpha positions therefore He ia a plane. The pi bond which ties over the plane of the atoms acts as a source of electrons ia addition reactions at the double bond. The carbon—carbon bond, acting as a substitute, affects the reactivity of the carbon atoms at the alpha positions through the formation of the aHyUc resonance stmcture. This stmcture can stabilize both positive and... 

Substitution Reactions. The chemistry at alpha positions hinges on the fact that an aHyUc hydrogen is easy to abstract because of the resonance stmctures that can be estabUshed with the neighboring double bond. The aHyUc proton is easier to abstract than one on a tertiary carbon these reactions are important in the formation of alkoxybutenes (ethers). 

An alpha hydroxy acid is an organic carboxylic acid in which an additional hydroxyl functional group (-OH) is present at the alpha position, i.e., on the carbon adjacent to the carboxyl functionality, -COOH. Figure 13.10.1 presents the struc-... 

Alpha olefins are straight-chain hydrocarbons having a double bond in the number one carbon-carbon position. That s called the alpha position, and hence the name alpha olefin, (There are beta, gamma, etc., compounds around, too.) The chains can have as few as four carbons (butene-1) or more... 

Alpha olefins. Straight-chain olefins with the double bond in the alpha position, i.e., in the first carbon-carbon bond of the chain. Alpha olefins start with the butenes and go up to the C30 s and higher. 

Rule B. A secondary olefin will tend to rearrange to a tertiary olefin when a tertiary carbon atom is in alpha position to a double-bonded carbon atom. The rearrangement of 3-methyl-l-butene to 2-methyl-2-butene is well known (Norris and Reuter, 66). 

Rule D. When a quarternary carbon atom is the alpha position to a secondary or tertiary double-bonded carbon atom, a pinacol type of rearrangement involving migration of a methyl group may occur. Examples of this type of rearrangement include isomerization of... 

Further examination of the results indicated that by invocation of Pearson s Hard-Soft Acid-Base (HSAB) theory (57), the results are consistent with experimental observation. According to Pearson s theory, which has been generalized to include nucleophiles (bases) and electrophiles (acids), interactions between hard reactants are proposed to be dependent on coulombic attraction. The combination of soft reactants, however, is thought to be due to overlap of the lowest unoccupied molecular orbital (LUMO) of the electrophile and the highest occupied molecular orbital (HOMO) of the nucleophile, the so-called frontier molecular orbitals. It was found that, compared to all other positions in the quinone methide, the alpha carbon had the greatest LUMO electron density. It appears, therefore, that the frontier molecular orbital interactions are overriding the unfavorable coulombic conditions. This interpretation also supports the preferential reaction of the sulfhydryl ion over the hydroxide ion in kraft pulping. In comparison to the hydroxide ion, the sulfhydryl is relatively soft, and in Pearson s theory, soft reactants will bond preferentially to soft reactants, while hard acids will favorably combine with hard bases. Since the alpha position is the softest in the entire molecule, as evidenced by the LUMO density, the softer sulfhydryl ion would be more likely to attack this position than the hydroxide. 

An additional complication and opportunity is provided by the fact that the placement of the methyl group on the alpha-position introduces a chiral carbon. The R- and S-isomers have been compared (see in the Qualitative Comments section) and the S-isomer is clearly three or four times more potent that the R-isomer. These were assayed completely blind, with the code having been broken only after the completion of the study. The dramatic differences in potency let the assignment of the more active isomer be made without hesitation. This S-isomer is the d-or dextrorotary one, and has the absolute configuration of the active member of the isomer... 

Exercise 17-21 a. A useful modification of aldol addition to methanal, known as the Mannich reaction, uses a secondary amine (usually as its hydrochloride salt) to selectively introduce one carbon atom at the alpha position of an aldehyde or ketone. The actual product is the salt of an amino ketone. For example,... 

It is quite obvious, that with one carbon atom lying on that alpha-position, you are precisely half-way between no carbons and two carbons. And there was one letter of the alphabet that lies precisely half-way between an H and a J. So, an natural naming pattern developed. The I compounds were already pretty well known by... 

Halogens add to ketones at the alpha carbon in the presence of a base or an arid-When a base is used, it is dffieult to prevent haJogdnation at more than one of the alpha positions. The base is also consumed by the reaction with water as a by-product, whereas the acid acts as a true catalyst and is not consumed. 

As previously described, ethers can be alkylated photochemically at the carbon alpha to the ether oxygen. In compounds having both an ether and a carbonyl function a "competitive alkylation process may take place. Tetrahydro-y-pyrone can serve as a good model for this "competitive alkylation between a ketone and an ether. This molecule has a ketone function having two methylene groups at its alpha positions, and an ether function having also two adjacent methylenes. When tetrahydro-y-pyrone was left in sunlight with 1-octene the 1 1 adduct was obtained in about 70 % yield. Alkylation occurred at the carbon alpha to the carbonyl function (24) ... 

However, if two trifluoromethyl groups are attached to the beta carbon, the cumulative effect of the two trifluoromethyl groups polarizes the double bond so that lower electron density is in alpha position, and the nucleophile becomes attached to the alpha carbon. The reaction product is L, ethyl 2-(P-hydroxyethylamino)-3-trifluoromethyl-4,4,4-tri-fluorobutanoate [117]. 

Supporting evidence for the mechanism comes from the observation that the bromination and iodination proceed at the same rates. The deuterium exchange is also comparable in absolute rate. Very extensive work with the optically active sec-butyl phenyl ketone, C2H5—CH(CH3)COC6H6, has shown that the acid-catalyzed iodination, bromination, and inversion have identical rates. The base-catalyzed, OD, rates of deuteration and inversion have also been shown to be equal. If the enol and enolate ion can be considered to be planar about the a carbon atom, then these results provide very strong support for the slow enolization step. In fact it is difficult to find any other reasonable interpretation of the data. The enol mechanism is also compatible with the well-known susceptibility of H atoms, in the alpha position to one or more C==0 groups, to substitution reactions. 

The correlation equation obtained using the data summarized in Table 10 was 0.938. The modest correlation coefficient may be due to the use of commercial linear primary alcohol ethoxylates from two different manufacturers. Variatiofts in hydrophobe linearity, hydrophobe carbon number distribution about the average value, and EO chain length distribution about the average value were not considered in equation 8. This interpretation is supported by the observation that inclusion of data for three secondary (methyl branched at the alpha position of the hydrophobe) alcohol ethoxylates in the analysis resulted in a decrease of the correlation coefficient to <0.90. 

Optically active esters in which the activity is due to asymmetry of the a-carbon atom ate racemized by the Bouveault-Blanc and catalytic hydrogenation procedures. The optically active alcohols may be prepared by the addition of small pieces of sodium to a stirred mixture of an ethereal solution of the ester and aqueous sodium acetate at 0°. A slight acidity is maintained by periodic additions of acetic acid (Prin s method).An asymmetric center in the alpha position to a carboxyl group is not racemized by lithium aluminum hydride. ... 

By the method of halogenation just described the halogen always enters the alpha position. In the case of isomeric branched chain compounds, in which the alpha carbon has no remaining hydrogen atom united to it, direct substitution does not take place. To form halogen acids from acids of this character other methods of preparation must be employed. 

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