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The Esters. Fischer Esterification. This is not necessarily the best method of preparing esters, but it does serve to illustrate that esters are essentially the products of a reaction between a carboxylic acid and an alcohol. Lactones.

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The esters l.jpg
The Esters

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Fischer Esterification

This is not necessarily the best method of preparing esters, but it does serve to illustrate that esters are essentially the products of a reaction between a carboxylic acid and an alcohol.

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Lactones

Lactones are cyclic esters formed by the intramolecularinteraction of an alcohol functional group with a carboxylic acid functional group:

a lactone

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The Triacylglycerols

  • A very important class of esters, from a biological point of view, is in the triacylglycerols -- the common simple fats.

  • “Triacylglycerol” is a relatively modern term; you may also hear of these substances as “simple lipids” or as “triglycerides.”

  • The triacylglycerols are esters of long-chain carboxylic acids combined with glycerol.

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The Triacylglycerols

The nature of the R groups can vary.

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Glycerol

Since glycerol has three -OH groups, it can react with three molecules of carboxylic acid to form a triester.

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The Fatty Acids

  • The carboxylic acid components of triacylglycerols are known as fatty acids.

  • Fatty acids are simple, long-chain carboxylic acids -- their name comes from the simple fact that they occur in fats!

  • Naturally-occurring fatty acids always have:

    • an unbranched carbon chain

    • an even number of carbon atoms

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The Fatty Acids

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Saturated versus Unsaturated Fatty Acids

  • When the carbon chain (excluding the C=O bond) contains single bonds only, the fatty acid is known as a saturated fatty acid, and the triacylglycerol thus becomes a saturated fat.

  • When the carbon chain contains one or more C=C double bonds, the fatty acid becomes an unsaturated fatty acid, and the triacylglycerol thus becomes an unsaturated fat.

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Unsaturated Fatty Acids

  • Naturally-occurring unsaturated fatty acids have the following characteristics:

    • The first double bond appears at carbon #9

    • If there are two or more double bonds, they are separated by a -CH2- group -- the double bonds are never conjugated.

    • Double bonds always have the cis geometry. The so-called trans fatty acids are not naturally-occurring.

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Hydrogenation of Fats

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Hydrogenation of Fats

  • The hydrogenation of an unsaturated fat converts it into a saturated fat.

  • This is an important process in the food industry, where unsaturated (liquid) fats are converted into saturated (solid) fats.

  • Often, in an effort to retain some of the nutritional benefits of an unsaturated fat, food manufacturers will only partially hydrogenate a liquid fat -- just sufficiently for the material to turn semi-solid. This would be a partially hydrogenated fat.

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Olestra (“Olean”)

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Fischer Esterification

This is the most direct way of preparing esters.

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Fischer Esterification

  • This is a reversible reaction; the equilibrium constant is only slightly greater than unity (between 1 and 10).

  • To be successful, this reaction requires that one be able to shift the equilibrium to the right

  • This is accomplished by:

    • removing water by azeotropic distillation

    • using an excess of whichever reagent is inexpensive.

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Fischer Esterification

84% yield

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Preparation of Esters(Best Method)

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Also Good:

You would use this alternative when the acid anhydride is cheaper than the corresponding acid chloride. There are relatively few commercially-available acid anhydrides, however.

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Methyl Ester Formation

  • Carboxylic acids can be converted to methyl esters by the action of diazomethane.CH2N2

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Methyl Ester Formation

Notice that the CH2 of diazomethane appears to insert itself between the O and the H of the O-H bond!

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Diazomethane

  • Diazomethane is a highly toxic and dangerously explosive gas.

  • In spite of its hazardous nature, it can be handled relatively safely in dilute ether solutions (and in small quantities)

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Diazomethane(continued)

  • In dilute solution it is a very useful reagent for the preparation of methyl esters, particularly in cases where only small quantities of carboxylic acid are available.

  • This reaction proceeds in nearly quantitative (100%) yields!

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Reactivity of Diazomethane

  • The high reactivity of diazomethane arises from the fact that it possesses an exceedingly reactive leaving group, the nitrogen molecule (which is exceptionally stable).

  • As the following mechanism shows, a nucleophilic substitution reaction on the protonated diazomethane molecule transfers a methyl group to the oxygen atom of the carboxylic acid, while liberating nitrogen gas (a very stable product).

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Mechanism of Methyl Ester Formation

This process is very favorable energetically, owing to the great stability of N2.

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Reactions of Esters

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By far the most important reaction of the esters is hydrolysis.

Hydrolysis can take place in either acidic or basic solution.

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Hydrolysis of Esters

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Example

Can anyone suggest why an ester might not make a suitable perfume?

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Another Example

Notice that, in a basic medium, the product of the hydrolysis is a carboxylate ion. The basic hydrolysis of an ester is often known as a saponification.

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Saponification – The World’s Second-Oldest Known Organic Reaction

  • Recall that the triacylglycerols (simple lipids or fats) are esters.

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If we hydrolyze a fat in aqueous base: Reaction

The products are three equivalents of carboxylate ion.

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If our starting fat were something like Reactiontristearin, which would be representative of an animal fat:

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Then basic hydrolysis would give us three equivalents of the corresponding sodium carboxylate.

This is the structural formula for ordinarysoap! (“Grandma’s lye soap”)

“Saponification” (from Latin: sapo = soap). Saponification is a soap-making reaction.

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Transesterification corresponding sodium carboxylate.

  • Sometimes, it is more convenient to convert the alkyl group of an ester to another alkyl group.

  • This process is known as transesterification.

  • Transesterification can take place under either acidic or basic conditions, although most commonly it is done under acidic conditions.

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Transesterification corresponding sodium carboxylate.

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Transesterification (in base) corresponding sodium carboxylate.

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Example corresponding sodium carboxylate.

Thus, we’ve converted a methyl ester to a benzyl ester.

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A Problem: corresponding sodium carboxylate.

Suppose we have a sample of a fat.

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  • What are R corresponding sodium carboxylate.1, R2, and R3?

  • With what frequency do they appear in a sample of the fat?

  • What is their relative distribution in the sample?

  • We can’t analyze the fat sample directly, because it isn’t very soluble and it isn’t volatile.

  • We need to convert it from a glycerol ester to some more convenient form of ester.

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Transesterification corresponding sodium carboxylate.

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Reaction of Esters with Organometallic Reagents corresponding sodium carboxylate.

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We can outline (crudely) the mechanism as follows: corresponding sodium carboxylate.

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BUT: The ketone can also react with the organometallic reagent! So, addition of organometallic reagent happens a second time.

So, the product is a tertiary alcohol.

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Example reagent! So, addition of organometallic reagent happens a second time.

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Reduction of Esters reagent! So, addition of organometallic reagent happens a second time.

Lithium aluminum hydride (LiAlH4) reduces esters through two 2-electron reduction steps, all the way to the primary alcohol.

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Crude Outline of the Mechanism reagent! So, addition of organometallic reagent happens a second time.

BUT: The aldehyde can also be reduced by LiAlH4!

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And so…. reagent! So, addition of organometallic reagent happens a second time.

Sodium borohydride (NaBH4) will not attack esters.

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Example reagent! So, addition of organometallic reagent happens a second time.

99 % yield

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Reduction of Esters to Aldehydes reagent! So, addition of organometallic reagent happens a second time.

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Example reagent! So, addition of organometallic reagent happens a second time.

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