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1. Reversible Addition Reactions

 Hydration and Hemiacetal Formation

It has been demonstrated (above) that water adds rapidly to the carbonyl function of aldehydes and ketones. In most cases the resulting hydrate (a geminal-diol) is unstable relative to the reactants and cannot be isolated. Exceptions to this rule exist, one being formaldehyde (a gas in its pure monomeric state). Here the weaker pi-component of the carbonyl double bond, relative to other aldehydes or ketones, and the small size of the hydrogen substituents favor addition. Thus, a solution of formaldehyde in water (formalin) is almost exclusively the hydrate, or polymers of the hydrate. Similar reversible additions of alcohols to aldehydes and ketones take place. The equally unstable addition products are called hemiacetals.

R₂C=O   +   R'OH     R'O–(R₂)C–O–H   (a hemiacetal)

Enamine Formation

The previous reactions have all involved reagents of the type: Y–NH₂, i.e. reactions with a 1º-amino group. Most aldehydes and ketones also react with 2º-amines to give products known as enamines. Two examples of these reactions are presented in the following diagram. It should be noted that, like acetal formation, these are acid-catalyzed reversible reactions in which water is lost. Consequently, enamines are easily converted back to their carbonyl precursors by acid-catalyzed hydrolysis. A mechanism for enamine formation may be seen by pressing the "Show Mechanism" button.

Reduction by Complex Metal Hydrides

Addition of a hydride anion to an aldehyde or ketone would produce an alkoxide anion, which on protonation should yield the corresponding alcohol. Aldehydes would give 1º-alcohols (as shown) and ketones would give 2º-alcohols.

RCH=O   +   H:^(–)     RCH₂O^(–)   +   H₃O^(–)     RCH₂OH

Two practical sources of hydride-like reactivity are the complex metal hydrides lithium aluminum hydride (LiAlH₄) and sodium borohydride (NaBH₄). These are both white (or near white) solids, which are prepared from lithium or sodium hydrides by reaction with aluminum or boron halides and esters. Lithium aluminum hydride is by far the most reactive of the two compounds, reacting violently with water, alcohols and other acidic groups with the evolution of hydrogen gas. The following table summarizes some important characteristics of these useful reagents.

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Sagar Singh

B.Tech, IIT Delhi

sagarsingh24.iitd@gmail.com

Read the book "Reactions in Organic Chemistry" by Mukul C Ray.