Unlike amides, esters are structurally flexible functional groups because rotation about the C-O-C bonds has a lower energy barrier. Their flexibility and low polarity affects their physical properties on a macroscopic scale; they tend to be less rigid, leading to a lower melting point, and more volatile, leading to a lower boiling point, than the corresponding amides.
The pK a of the alpha-hydrogens, or the hydrogens attached to the carbon adjacent to the carbonyl, on esters is around 25, making them essentially non-acidic except in the presence of very strong bases. Esters are more polar than ethers, but less so than alcohols. They participate in hydrogen bonds as hydrogen bond acceptors, but cannot act as hydrogen bond donors, unlike their parent alcohols and carboxylic acids. This ability to participate in hydrogen bonding confers some water-solubility, depending on the length of the alkyl chains attached.
Since they have no hydrogens bonded to oxygens, as alcohols and carboxylic acids do, esters do not self-associate.
Consequently, esters are more volatile than carboxylic acids of similar molecular weight. Esters are usually identified by gas chromatography, taking advantage of their volatility.
This peak changes depending on the functional groups attached to the carbonyl. Esters react with nucleophiles at the carbonyl carbon. The carbonyl is weakly electrophilic, but is attacked by strong nucleophiles such as amines, alkoxides, hydride sources, and organolithium compounds. The C-H bonds adjacent to the carbonyl are weakly acidic, but undergo deprotonation with strong bases. After protonation, water adds to the carbonyl carbon causing the formation of a tetrahedral alkoxide intermediate.
Then a proton transfers to the —OR group, increasing its ability to act as a leaving group. Reforming the carbonyl double bond causes the elimination of an alcohol HOR as a leaving group, creating a protonated carboxylic acid. In the last step of the mechanism, water acts as a base, removing a hydrogen, to form a carboxylic acid and regenerating the acid catalyst.
Lactones Cyclic esters undergo typical reactions of esters including hydrolysis. Hydrolysis of the lactone under acidic conditions creates a hydroxyacid. Esters can also be cleaved into a carboxylate and an alcohol through reaction with water and a base. The reaction is commonly called a saponification from the Latin sapo which means soap. This name comes from the fact that soap used to me made by the ester hydrolysis of fats. Saponification reaction utilize a better nucleophile hydroxide and are typically faster than an acid catalyzed hydrolysis.
The carboxylation ions produced by saponification are negatively charged and very unreactive toward further nucleophilic substitution which makes the reaction irreversible. The base-promoted hydrolysis of an ester follows the typical nucleophilic acyl substitution mechanism. A full equivalent of hydroxide anion is used, so the reaction is called base-promoted and not base catalyzed.
The mechanism of ester saponification begins with the nucleophilic addition of a hydroxide ion at the carbonyl carbon to give a tetrahedral alkoxide intermediate. The carbonyl bond is reformed along with the elimination of an alkoxide -OR leaving group yielding a carboxylic acid. The alkoxide base deprotonates the carboxylic acid to for a carboxylate salt and an alcohol as products.
The last deprotonation step essentially removes the carboxylic acid from the equilibrium which drives the saponification towards completion. Because the carboxylic acid is no longer part of the equilibrium the reaction is effectively irreversible. This mechanism is supported by experiments performed using isotopically labeled esters. When the ether-type oxygen of the ester was labeled with 18 O, the labeled oxygen showed up in the alcohol product after hydrolysis.
An alternative mechanism would be if the hydroxide participated in an S N 2 reaction to create the carboxylate product. If this were to happen the alcohol reaction product would not contain the labeled oxygen. Ester saponification in biological systems, called hydrolytic acyl substitution reactions, are common. In particular, acetylcholinesterase, an enzyme present in the synapse, catalyzes hydrolysis of the ester group in acetylcholine which is a neurotransmitter that triggers muscle contraction.
Like many other hydrolytic enzymes, the acetylcholinesterase reaction proceeds in two phases: first, a covalent enzyme-substrate intermediate is formed when the acyl group of acetylcholine is transferred to an active-site serine on the enzyme a transesterification reaction. A water nucleophile then attacks this ester, driving off acetate and completing the hydrolysis.
Transesterification is a reaction where an ester is converted to a different ester through reaction with an alcohol. Because there is typically very little difference in stability between both esters, the equilibrium constant of this reaction is usually near one.
Transesterifications also shows that great care should be taken when an ester containing compound is used in a reaction involving an alcohol. The reaction follows the basic mechanism of a nucleophilic acyl substitution. The alkoxide leaving group of the ester is replace by an incoming alkoxide nucleophile creating a different ester.
Protonation allows the alcohol reactant to add to the ester carbonyl. Proton transfer to the ester's alkoxy group increases it ability to act as a leaving group. It is possible to convert esters to amides through direct reaction with ammonia or amines.
However, these reactions are not commonly used because the formation of an amide using an acid chloride is a much simpler reaction. Esters can undergo hydride reduction with LiAlH 4 to form two alcohols. We are talking about molecules of very similar sizes and so the potential for temporary dipoles should be much the same in all of them.
What matters, though, is how close together the molecules can get. The presence of carbon-carbon double bonds in the chains gets in the way of tidy packing. The hydrocarbon chains are, of course, in constant motion in the liquid, but it is possible for them to lie tidily when the substance solidifies. If the chains in one molecule can lie tidily, that means that neighbouring molecules can get close. That increases the attractions between one molecule and its neighbours and so increases the melting point.
Unsaturated fats and oils have at least one carbon-carbon double bond in at least one chain. There isn't any rotation about a carbon-carbon double bond and so that locks a permanent kink into the chain. That makes packing molecules close together more difficult.
If they don't pack so well, the van der Waals forces won't work as well. This effect is much worse for molecules where the hydrocarbon chains either end of the double bond are arranged cis to each other - in other words, both of them on the same side of the double bond:. If they are on opposite sides of the double bond the trans form the effect isn't as marked. It is, however, rather more than the diagram below suggests because of the changes in bond angles around the double bond compared with the rest of the chain.
Trans fats and oils have higher melting points than cis ones because the packing isn't affected quite as much. Naturally occurring unsaturated fats and oils tend to be the cis form.
Note: Follow this link if you aren't sure about cis and trans forms around a carbon-carbon double bond. If this is the first set of questions you have done, please read the introductory page before you start. What are esters? A common ester - ethyl ethanoate The most commonly discussed ester is ethyl ethanoate. The formula for ethyl ethanoate is: Notice that the ester is named the opposite way around from the way the formula is written.
A few more esters In each case, be sure that you can see how the names and formulae relate to each other. Use the BACK button on your browser to return to this page. Fats and oils Differences between fats and oils Animal and vegetable fats and oils are just big complicated esters.
A simple introduction to their structures Fats and oils as big esters Esters can be made from carboxylic acids and alcohols. The diagram shows the relationship between the ethanoic acid, the ethanol and the ester. This isn't intended to be a full equation. Water, of course, is also produced. If you make an ester of this with ethanoic acid, you could attach three ethanoate groups. Now, make the acid chains much longer, and you finally have a fat.
Saturated and unsaturated fats and oils If the fat or oil is saturated , it means that the acid that it was derived from has no carbon-carbon double bonds in its chain. Those same terms will then apply to the esters that are formed. All of these are saturated acids, and so will form saturated fats and oils: Oleic acid is a typical mono-unsaturated acid:.
Linolenic acid is an omega 3 acid for the same reason. Physical properties Simple esters I am thinking here about things like ethyl ethanoate.
Boiling points The small esters have boiling points which are similar to those of aldehydes and ketones with the same number of carbon atoms.
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