Know More
Weblinks
http://www.organic-chemistry.org/synthesis/C1H/reductionsunsaturatedcompounds.shtm https://en.wikipedia.org/wiki/Reductions_with_metal_alkoxyaluminium_hydrides https://en.wikipedia.org/wiki/Sodium_borohydride https://en.wikipedia.org/wiki/Diisobutylaluminium_hydride https://en.wikipedia.org/wiki/Rosenmund_reduction https://en.wikipedia.org/wiki/Lindlar_catalyst https://en.wikipedia.org/wiki/Adams%27s_catalyst https://en.wikipedia.org/wiki/Raney_nickel https://en.wikipedia.org/wiki/Birch_reduction
CHEMISTRY Paper 9: ORGANIC CHEMISTRY-III (Reaction Mechanism-2) Module 16: Reduction by Metal hydrides – Part-I
Suggested readings
Organic Chemistry by Clayden, Greeves, Warren and Wothers
March’s Advanced Organic Chemistry, Reaction, Mechanism and structure by Michael B. Smith and Jerry March
A Guidebook to Mechanism in Organic Chemistry, Sixth edition by Peter Sykes
CHEMISTRY Paper 9: ORGANIC CHEMISTRY-III (Reaction Mechanism-2) Module 16: Reduction by Metal hydrides – Part-I
Glossary
A
Adams's catalyst: Adams's catalyst, also known as platinum dioxide, is usually represented as platinum(IV) oxide hydrate, PtO2•H2O. It is a catalyst for hydrogenation and hydrogenolysis in organic synthesis.
B
Birch reduction: The Birch reduction is an organic reaction where aromatic rings undergo a 1,4-reduction to provide unconjugated cyclohexadienes. The reduction is conducted by sodium or lithium metal in liquid ammonia and in the presence of an alcohol
C
Conjugate addition: Conjugate addition is the vinylogous counterpart of direct nucleophilic addition. A nucleophile reacts with a α,β-unsaturated carbonyl compound in the β position. The negative charge carried by the nucleophile is now delocalized in the alkoxide anion and the α carbon carbanion by resonance.
E
Electron donating group: In organic chemistry, an electron donating group (EDG) or electron releasing group (ERG) is an atom or functional group that donates some of its electron density into a conjugated π system via resonance or inductive electron withdrawal, thus making the π system more nucleophilic. CHEMISTRY Paper 9: ORGANIC CHEMISTRY-III (Reaction Mechanism-2) Module 16: Reduction by Metal hydrides – Part-I
Electrophile: In chemistry, an electrophile (literally electron lover) is a reagent attracted to electrons. Electrophiles are positively charged or neutral species having vacant orbitals that are attracted to an electron rich centre.
H
Heteroatom: In organic chemistry, a heteroatom is any atom that is not carbon or hydrogen in a ring structure. Usually, the term is used to indicate that non-carbon atoms have replaced carbon in the backbone of the molecular structure.
L
Lactone: An organic compound containing an ester group —OCO— as part of a ring.
Lindlar catalyst: A Lindlar catalyst is a heterogeneous catalyst that consists of palladium deposited on calcium carbonate which is then poisoned with various forms of lead or sulphur. It is used for the hydrogenation of alkynes to alkenea.
N
Nucleophile: A nucleophile is a chemical species that donates an electron pair to an electrophile to form a chemical bond in relation to a reaction. All molecules or ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.
Nucleophilic addition reaction: In organic chemistry, a nucleophilic addition reaction is an addition reaction where a chemical compound with an electron-deficient or electrophilic double or triple bond, a π bond, reacts with electron-rich reactant, termed a nucleophile, with disappearance of the double bond and creation of two new single, or σ, bonds.
O
Oxidation: Oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion.
Oxidation-reduction reaction: An oxidation-reduction (redox) reaction is a type of chemical reaction that involves a transfer of electrons between two species. An oxidation- reduction reaction is any chemical reaction in which the oxidation number of a molecule, atom, or ion changes by gaining or losing an electron.
CHEMISTRY Paper 9: ORGANIC CHEMISTRY-III (Reaction Mechanism-2) Module 16: Reduction by Metal hydrides – Part-I
Oxidation state: An oxidation state is a number that is assigned to an element in a chemical combination. This number represents the number of electrons that an atom can gain, lose, or share when chemically bonding with an atom of another element.
Oxidizing agent: An oxidizing agent is a chemical species that removes an electron from another species.
R
Raney nickel: Raney nickel is a fine-grained solid composed mostly of nickel derived from a nickel-aluminium alloy.
Reducing agent: A reducing agent (also called a reductant or reducer) is an element or compound that loses (or "donates") an electron to another chemical species in a redox chemical reaction. Since the reducing agent is losing electrons, it is said to have been oxidized.
Reduction: Reduction is any chemical reaction that involves the gaining of electrons. It refers to the side that accepts electrons. When iron reacts with oxygen it forms a chemical called rust. In that example, the iron is oxidized and the oxygen is reduced. Reduction is the opposite of oxidation.
Rosenmund reduction: The Rosenmund reduction is a hydrogenation process in which an acyl chloride is selectively reduced to an aldehyde.
S
Stereocenter: A stereocenter or stereogenic center is an atom bearing groups such that an interchanging of any two groups leads to a stereoisomer. The most common stereocenters are chiral centers (such as asymmetric carbon atoms) and the double-bonded carbon atoms in cis-trans alkenes.
Value addition
Raney nickel
Safety
A square orange sticker with a flame picture. Raney nickel is flammable. A square orange sticker with a black cross on it. Nickel metal is classified as "Harmful".
CHEMISTRY Paper 9: ORGANIC CHEMISTRY-III (Reaction Mechanism-2) Module 16: Reduction by Metal hydrides – Part-I
Due to its large surface area and high volume of contained hydrogen gas, dry, activated Raney nickel is a pyrophoric material that should be handled under an inert atmosphere. Raney nickel is typically supplied as a 50% slurry in water. Care should be taken never to expose Raney nickel to air. Even after reaction, Raney nickel contains significant amounts of hydrogen gas, and may spontaneously ignite when exposed to air.
Raney nickel will produce hazardous fumes when burning, so the use of a gas mask is recommended when extinguishing fires caused by it. Additionally, acute exposure to Raney nickel may cause irritation of the respiratory tract and nasal cavities, and causes pulmonary fibrosis if inhaled. Ingestion may lead to convulsions and intestinal disorders. It can also cause eye and skin irritation. Chronic exposure may lead to pneumonitis and other signs of sensitization to nickel, such as skin rashes ("nickel itch").
Nickel is also rated as being a possible human carcinogen by the IARC and teratogen, while the inhalation of fine aluminium oxide particles is associated with Shaver's disease. Care should be taken when handling these raw materials during laboratory preparation of Raney nickel.
Development
Murray Raney graduated as a mechanical engineer from the University of Kentucky in 1909. In 1915 he joined the Lookout Oil and Refining Company in Tennessee and was responsible for the installation of electrolytic cells for the production of hydrogen which was used in the hydrogenation of vegetable oils. During that time the industry used a nickel catalyst prepared from nickel(II) oxide. Believing that better catalysts could be produced, around 1921 he started to perform independent research while still working for Lookout Oil. In 1924 a 1:1 ratio Ni/Si alloy was produced, which after treatment with sodium hydroxide, was found to be five times more active than the best catalyst used in the hydrogenation of cottonseed oil. A patent for this discovery was issued in December 1925.
Subsequently, Raney produced a 1:1 Ni/Al alloy following a procedure similar to the one used for the nickel-silicon catalyst. He found that the resulting catalyst was even more active and filed a patent application in 1926. This is now the preferred alloy composition for Raney nickel catalysts.
Following the development of Raney nickel, other alloy systems with aluminium were considered, of which the most notable include copper, ruthenium and cobalt. Further research showed that adding a small amount of a third metal to the binary alloy would promote the activity of the catalyst. Some widely used promoters are zinc, molybdenum and chromium. An alternative way of preparing enantioselective Raney nickel has been devised by surface adsorption of tartaric acid.
CHEMISTRY Paper 9: ORGANIC CHEMISTRY-III (Reaction Mechanism-2) Module 16: Reduction by Metal hydrides – Part-I
Adams's catalyst
Development
Prior to the development of Adams's catalyst, organic reductions were carried out using colloidal platinum or platinum black. The colloidal catalysts were more active but posed difficulties in isolating reaction products. This led to more widespread use of platinum black. In Adams's own words:
"...Several of the problems I assigned my students involved catalytic reduction. For this purpose we were using as a catalyst platinum black made by the generally accepted best method known at the time. The students had much trouble with the catalyst they obtained in that frequently it proved to be inactive even though prepared by the same detailed procedure which resulted occasionally in an active product. I therefore initiated a research to find conditions for preparing this catalyst with uniform activity."
Safety
Little precaution is necessary with the oxide but, after exposure to H2, the resulting platinum black can be pyrophoric. Therefore, it should not be allowed to dry and all exposure to oxygen should be minimized.
CHEMISTRY Paper 9: ORGANIC CHEMISTRY-III (Reaction Mechanism-2) Module 16: Reduction by Metal hydrides – Part-I