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The thiourea hydrogen bonds to the nitro group and stabilizes the incoming negative charge, while the amine acts a specific base to activate the nucleophile. This is an example of bifunctional catalysis. Hydrogen-bond catalysis is a type of organocatalysis that relies on use of hydrogen bonding interactions to accelerate and control organic ...
Hydrogen bond: A hydrogen bond is a specific type of dipole-dipole interaction between a partially positive hydrogen atom and a partially negative electron donor that contain a pair of electrons such as oxygen, fluorine and nitrogen. The strength of hydrogen bond depends on the chemical nature and geometric arrangement of each group.
Consequently, hydrogen bonds between or within solute molecules dissolved in water are almost always unfavorable relative to hydrogen bonds between water and the donors and acceptors for hydrogen bonds on those solutes. [44] Hydrogen bonds between water molecules have an average lifetime of 10 −11 seconds, or 10 picoseconds. [45]
The Guerbet reaction, reported in 1899, [5] is an early example of a hydrogen auto-transfer process. The Guerbet reaction converts primary alcohols to β-alkylated dimers via alcohol dehydrogenation followed by aldol condensation and reduction of the resulting enones.
For example, an iridium(I)\chiral phosphine\I 2 system is effective in the asymmetric hydrogenation of activated (alkylated) 2-pyridiniums [67] or certain cyclohexanone-fused pyridines. [68] Similarly, chiral Brønsted acid catalysis with a Hantzsh ester as a hydride source is effective for some 2-alkyl pyridines with additional activating ...
Use of oxygen or sulfur as the nucleophilic atom causes minor differences in catalysis. Compared to oxygen, sulfur's extra d orbital makes it larger (by 0.4 Å) [29] and softer, allows it to form longer bonds (d C-X and d X-H by 1.3-fold), and gives it a lower pK a (by 5 units). [30]
Note that no chemical bonds are formed between adsorbate and adsorbent, and their electronic states remain relatively unperturbed. Typical energies for physisorption are from 3 to 10 kcal/mol. [ 2 ] In heterogeneous catalysis, when a reactant molecule physisorbs to a catalyst, it is commonly said to be in a precursor state, an intermediate ...
One example is titanium dihydride, which forms when titanium sponge is heated to 400-700 °C under an atmosphere of hydrogen. These reactions typically require high surface area metals. The direct reaction of metals with H 2 is a step in catalytic hydrogenation. For solutions, classic example involves Vaska's complex: [7]