alpha

Examples of alpha in the following topics:

• Reactions at the α-Carbon

  • Many aldehydes and ketones undergo substitution reactions at an alpha carbon, as shown in the following diagram (alpha-carbon atoms are colored blue).
  • If the alpha-carbon is a chiral center, as in the second example, the products of halogenation and isotopic exchange are racemic.
  • First, these substitutions are limited to carbon atoms alpha to the carbonyl group.
  • Cyclohexanone (the first ketone) has two alpha-carbons and four potential substitutions (the alpha-hydrogens).
  • This is demonstrated convincingly by the third ketone, which is structurally similar to the second but has no alpha-hydrogen.

• Modes of Radioactive Decay

  • Alpha particles have greater mass than beta particles.
  • By passing alpha particles through a very thin glass window and trapping them in a discharge tube, researchers found that alpha particles are equivalent to helium (He) nuclei.
  • An alpha particle (α\alpha) is made up of two protons and two neutrons bound together.
  • Alpha decay occurs because the nucleus of a radioisotope has too many protons.
  • Alpha particles can be completely stopped by a sheet of paper.

• Enolate Intermediates

  • Many of the most useful alpha-substitution reactions of ketones proceeded by way of enolate anion conjugate bases.
  • Esters and nitriles are even weaker alpha-carbon acids than ketones (by over ten thousand times), nevertheless their enolate anions may be prepared and used in a similar fashion.
  • The presence of additional activating carbonyl functions increases the acidity of the alpha-hydrogens substantially, so that less stringent conditions may be used for enolate anion formation.
  • The influence of various carbonyl and related functional groups on the equilibrium acidity of alpha-hydrogen atoms (colored red) is summarized in the following table.
  • Note that each of these compounds has two acidic alpha-hydrogen atoms (colored red).

• Reactions at the α-Carbon

  • Many aldehydes and ketones were found to undergo electrophilic substitution at an alpha carbon.
  • Acid-catalyzed alpha-chlorination and bromination reactions proceed more slowly with carboxylic acids, esters and nitriles than with ketones.
  • The chiral alpha-carbon in equation #2 is racemized in the course of this exchange, and a small amount of nitrile is hydrolyzed to the corresponding carboxylic acid.
  • This difference may be used to facilitate the alpha-halogenation of carboxylic acids.
  • The final product is the alpha-halogenated acid, accompanied by a trace of the acyl halide.

• The Alkyl Moiety

  • Alkyl halides in which the alpha-carbon is a chiral center provide additional information about these nucleophilic substitution reactions.
  • The nucleophile must approach the electrophilic alpha-carbon atom from the side opposite the halogen.
  • In the SN2 transition state the alpha-carbon is hybridized sp2 with the partial bonds to the nucleophile and the halogen having largely p-character.
  • The consequence of rear-side bonding by the nucleophile is an inversion of configuration about the alpha-carbon.
  • Second, the rear-side approach of the nucleophile to the alpha-carbon will be subject to hindrance by neighboring alkyl substituents, both on the alpha and the beta-carbons.

• Claisen Condensation

  • The last equation shows a mixed condensation between two esters, one of which has no alpha-hydrogens.
  • Note that this stabilization is only possible if the donor has two reactive alpha-hydrogens.
  • In the case of mixed condensations, complex product mixtures are commonly avoided by using an acceptor ester that has no alpha-hydrogens.
  • There are three ester functions, each of which has at least one alpha-hydrogen.
  • Only one of these, that on the left, has two alpha-hydrogens and will yield an enolizable beta-ketoester by functioning as the donor in a Dieckmann cyclization.

• Balancing Nuclear Equations

  • Common light particles are often abbreviated in this shorthand, typically p for proton, n for neutron, d for deuteron, α representing an alpha particle or helium-4, β for beta particle or electron, γ for gamma photon, etc.
  • This fits the description of an alpha particle.
  • This could also be written out as polonium-214, plus two alpha particles, plus two electrons, give what?
  • In order to solve this equation, we simply add the mass numbers, 214 for polonium, plus 8 (two times four) for helium (two alpha particles), plus zero for the electrons, to give a mass number of 222.
  • Describes how to write the nuclear equations for alpha and beta decay.

• Peptides & Proteins

  • The conformational flexibility of peptide chains is limited chiefly to rotations about the bonds leading to the alpha-carbon atoms.
  • The color shaded rectangles in the lower structure define these regions, and identify the relatively facile rotations that may take place where the corners meet (i.e. at the alpha-carbon).

• The Aldol Reaction

  • The products of aldol reactions often undergo a subsequent elimination of water, made up of an alpha-hydrogen and the beta-hydroxyl group.
  • The first reaction demonstrates that ketones having two sets of alpha-hydrogens may react at both sites if sufficient acceptor co-reactant is supplied.
  • The base-catalyzed reaction proceeds via an enolate anion donor species, and the kinetically favored proton removal is from the less substituted alpha-carbon.
  • Finally, reaction #4 has two reactive alpha-carbons and a reversible aldol reaction may occur at both.
  • Second, aldehydes lacking alpha-hydrogens can only function as acceptor reactants, and this reduces the number of possible products by half.

• Disaccharides

  • Notice that the glycoside bond may be alpha, as in maltose and trehalose, or beta as in cellobiose and gentiobiose.
  • Enzyme-catalyzed hydrolysis is selective for a specific glycoside bond, so an alpha-glycosidase cleaves maltose and trehalose to glucose, but does not cleave cellobiose or gentiobiose.
  • This leaves the anomeric carbon in ring B free, so cellobiose and maltose both may assume alpha and beta anomers at that site (the beta form is shown in the diagram).
  • Its alpha-anomer is drawn in the diagram.
  • It is a non-reducing disaccharide composed of glucose and fructose joined at the anomeric carbon of each by glycoside bonds (one alpha and one beta).