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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
The Esters

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fischer esterification
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

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
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 triacylglycerols5
The Triacylglycerols

The nature of the R groups can vary.

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glycerol
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 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 acids8
The Fatty Acids

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saturated versus unsaturated fatty acids
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
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 fats12
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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fischer esterification17
Fischer Esterification

This is the most direct way of preparing esters.

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fischer esterification18
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 esterification19
Fischer Esterification

84% yield

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also good
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
Methyl Ester Formation
  • Carboxylic acids can be converted to methyl esters by the action of diazomethane.CH2N2

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methyl ester formation23
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
  • 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
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
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
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
Reactions of Esters

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slide30

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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example
Example

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

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another example
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
Saponification – The World’s Second-Oldest Known Organic Reaction
  • Recall that the triacylglycerols (simple lipids or fats) are esters.

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slide37

If we hydrolyze a fat in aqueous base:

The products are three equivalents of carboxylate ion.

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slide38

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

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slide39

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
Transesterification
  • 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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transesterification41
Transesterification

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example43
Example

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

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a problem
A Problem:

Suppose we have a sample of a fat.

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slide45
What are R1, 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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transesterification46
Transesterification

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slide47
The methyl esters are much more volatile.
  • The transesterified fat sample can now be analyzed by gas chromatography.

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slide50

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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example51
Example

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reduction of esters
Reduction of Esters

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
Crude Outline of the Mechanism

BUT: The aldehyde can also be reduced by LiAlH4!

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and so
And so….

Sodium borohydride (NaBH4) will not attack esters.

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example55
Example

99 % yield

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example57
Example

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