Inorganic Esters – Esters of Inorganic Acids

Esters of Inorganic Acids

– In addition to forming esters with carboxylic acids, alcohols form inorganic esters with inorganic acids such as nitric acid, sulfuric acid, and phosphoric acid.

– In each type of ester, the alkoxy (-OR) group of the alcohol replaces a hydroxyl group of the acid, with loss of water.

– We have already studied tosylate esters, composed of paratoluenesulfonic acid and alcohols (but made using tosyl chloride).

– Tosylate esters are analogous to sulfate esters , which are composed of sulfuric acid and alcohols.

Inorganic Esters - Esters of Inorganic Acids

Sulfate Esters

– A sulfate ester is like a sulfonate ester, except there is no alkyl group directly bonded to the sulfur atom.

– In an alkyl sulfate ester, alkoxy groups are bonded to sulfur through oxygen atoms. Using methanol as the alcohol,

Inorganic Esters - Esters of Inorganic Acids

– Sulfate ions are excellent leaving groups. Like sulfonate esters, sulfate esters are good electrophiles.

– Nucleophiles react with sulfate esters to give alkylated products.

– For example, the reaction of dimethyl sulfate with ammonia gives a sulfate salt of methylamine, CH3NH3+ CH3OSO3

Nitrate Esters

– Nitrate esters are formed from alcohols and nitric acid.

– The best-known nitrate ester is “nitroglycerine,” whose systematic name is glyceryl trinitrate.

– Glyceryl trinitrate results from the reaction of glycerol (1,2,3-propanetriol) with three molecules of nitric acid.

Inorganic Esters - Esters of Inorganic Acids

– First made in 1847, nitroglycerine was found to be a much more powerful explosive than black powder, which is a physical mixture of potassium nitrate, sulfur, and charcoal.

– In black powder, potassium nitrate is the oxidizer, and sulfur and charcoal provide the fuel to be oxidized.

– The rate of a black powder explosion is limited by how fast oxygen from the grains of heated potassium nitrate can diffuse to the grains of sulfur and charcoal.

– A black powder explosion does its work by the rapid increase in pressure resulting from the reaction.

– The explosion must be confined, as in a cannon or a firecracker, to be effective.

– In nitroglycerine, the nitro groups are the oxidizer and the CH and CH2 groups are the fuel to be oxidized.

– This intimate association of fuel and oxidizer allows the explosion to proceed at a much faster rate, forming a shock wave that propagates through the explosive and initiates the reaction.

– The explosive shock wave can shatter rock or other substances without the need for confinement. Because of its unprecedented explosive power, nitroglycerine was called a high explosive.

– Many other high explosives have been developed, including picric acid, TNT (trinitrotoluene), PETN (pentaerythritol tetranitrate), and RDX (research department explosive).

– Nitroglycerine and PETN are nitrate esters. Picric acid and TNT are nitrobenzene derivatives, not esters.

– Pure nitroglycerine is hazardous to make, use, and transport.

– Alfred Nobel’s family were experts at making and using nitroglycerine, yet his brother and several workers were killed by an explosion.

– In 1866, Nobel found that nitroglycerine soaks into diatomaceous earth to give a pasty mixture that can be molded into sticks that do not detonate so easily.

– He called the sticks dynamite and founded the firm Dynamit Nobel, which is still one of the world’s leading ammunition and explosives manufacturers.

– The Nobel prizes are funded from an endowment that originated with Nobel’s profits from the dynamite business.

Phosphate Esters

– Alkyl phosphates are composed of 1 mole of phosphoric acid combined with 1, 2, or 3 moles of an alcohol.

– For example, methanol forms three phosphate esters.

Inorganic Esters - Esters of Inorganic Acids

– Phosphate esters play a central role in biochemistry.

– The following Figure shows how phosphate ester linkages compose the backbone of the nucleic acids RNA (ribonucleic acid) and DNA (deoxyribonucleic acid).

Phosphate ester groups bond the individual nucleotides together in DNA. The “base” on each of the nucleotides corresponds to one of the four heterocyclic bases of DNA

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