Sciencemadness Discussion Board - Why are some esters formed easily and others not?

Why are some esters formed easily and others not?

woelen - 23-3-2018 at 01:58

I noticed that some esters are formed very easily, while others need more demanding conditions for their formation.

The nitrite ester is formed easily by adding a solution of sodium nitrite to a solution of an alcohol in dilute strong acid. E.g. if you add NaNO2 to a cold solution of methanol in 10% H2SO4, then the ester bubbles out as gas. With higher soluble alcohols (e.g. iso-propanol or n-propanol) you get an oily yellow layer of the ester which separates from the aqueous layer.
A similar thing is true for the hypochlorite esters. If you add a 2 : 1 by volume mix of acetic acid and methanol to 10% bleach, then bubbles of CH3OCl escape, if you do the same with higher alcohols you again get an oily layer which separates from the aqueous layer. This reaction is nearly immediate at room temperature.
Boric acid (and also boron oxide) also very easily and nearly quantitatively makes borate esters with alcohols, also in aqueous solution. E.g. H3BO3, which is only sparingly soluble in water, dissolves very well in solutions of organics with lots of OH-groups in it. E.g. glycerol, sugar.

All of these are very easy reactions, in aqueous solution. Just mixing the chemicals and you have instant ester formation.

H3BO3, HOCl and HNO2 are weak acids, but not all weak acids make esters easily, e.g. acetic acid, formic acid do not make esters easily. Weakness of the acid may be a necessary condition, but it is not a sufficient condition. For some weak acids (e.g. acetic acid) you need strong dehydrating agents to get a decent production of ester, and even then, the reaction runs slowly and requires some heating. With the weak acids H2CO3 and H2SO3 in aqueous solution you also don't get esters. For making esters of these you need the extreme reagents COCl2 and SOCl2, combined with the alcohol.

Is there an underlying general rule for very easy ester formation with certain acids? I now found 3 types of esters with this property, but there may be more and if some rule can be formulated it may be possible to predict more of these. This is quite an interesting branch of chemistry and it is fun to experiment with this kind of esters. They have quite interesting properties.

LearnedAmateur - 23-3-2018 at 02:24

From what I’ve seen, nitrites and hypochlorites form easily because the precursor acids are unstable. Take the former for instance, HNO2 rapidly disproportionates above 5C to form NO and NO2 as well as water - the lack of equilibrium here, as well as the negligible solubility shunts the reaction to the product side.

On the other hand, carboxylic acids do exist in equilibrium and are obviously quite stable acids; an equilibrium is set up between the acid and alcohol, and the ester and water. Combine this with a somewhat soluble product, and hydrolysis of the ester occurs to a higher extent.

This is just what I’ve learned during the course of experimentation - acid stability and solubility play key roles. Wasn’t aware that you could get hypochlorite esters so that’ll be something I need to have a play around with in the near future! Will the 5% household stuff do (containing NaOH and an/ionic surfactants) or do you need purer NaOCl?

[Edited on 23-3-2018 by LearnedAmateur]

subskune - 23-3-2018 at 12:52

And then there is terephthalic acid. Esterification is very tough. While you can get acceptable yields from benzoic and acetic with sulfuric acid and refluxing or azeotropic alcohol destillation, terephthalic never really worked for me. I extra dried the acid, destilled the sulfuric freshly to 98 and added far more than needed as dessicant and refluxed with etoh for days, result not a single bit, very frustrating

woelen - 23-3-2018 at 14:50

Quote: Originally posted by LearnedAmateur

Wasn’t aware that you could get hypochlorite esters so that’ll be something I need to have a play around with in the near future! Will the 5% household stuff do (containing NaOH and an/ionic surfactants) or do you need purer NaOCl?

I expect it to work with household bleach as well. Use the cheapest stuff, without surfactants and perfumes.
Be careful with hypochlorite esters. The methyl ester explodes on ignition (it is a gas), the ethyl ester and propyl esters are volatile liquids, which burn like nitrocotton when ignited.
Do not use more than half a ml of the alcohol, mix this with 1 ml of acetic acid and slowly add this mix to bleach. Do this in a petri dish, not in a long glass test tube. Ignite the gas mix shortly after the bubbling starts. You will get an explosion, but not dangerous. If you do this in a test tube, then I cannot guarantee that the test tube does not shatter on ignition of the gas.
Methyl hypochlorite also is said to be lighht sensitive and it is said it can explode in bright light. I myself never had this, but if I were you, I would not do the experiment in bright daylight. I only did it under normal TL-light.

[Edited on 23-3-18 by woelen]

DraconicAcid - 23-3-2018 at 15:05

I've used bleach to turn a large secondary alcohol into a ketone, and we didn't seem to have any ester formation (other than possibly as an intermediate).

AvBaeyer - 23-3-2018 at 18:26

Consider that esters of nitrous acid, hypochlorous acid and boric acid do not form by the same mechanism as a carboxylate ester. For example, in the case of HNO2 and an alcohol, the reaction is likely an addition of the alcohol to NO followed by a proton loss. Similarly, for hypochlorites the alcohol is probably reactiing with Cl+ followed by loss of a proton. (Hypochlorites are the intermediates in the oxidation of secondary alcohols to ketones.) Boric acid [(HO)3B] is a bit different. The first step is probably a complex between the alcohol and the boric acid utilizing boric acid's unfilled orbital. Loss of water gives a mono ester which in a similar manner ultimately gives the triester.

By the way, higher alcohols form relatively stable nitrites (isoamyl and t-butyl) which are used in non-aqueous diazotizations. t-Butyl hypochlorite is also relatively stable and is used in non-aqueous oxidation and chlorination reactions. However, great care must be taken in the preparation and use of these compounds. See Organic Syntheses for more information.

Short comment on terphthalic acid. Did you see the mono ester from your reaction? Typically, a benzoic acid with a strongly electron withdrawing substituent in the para position esterifies with difficulty. This would be the case if you have the mono ester of terphthalic acid. The second ester will be difficult to form.

AvB

Melgar - 23-3-2018 at 19:17

Phosphate esters also seem to form easily. Sulfate esters not so much. However, sulfuric acid certainly can contribute to the formation of other esters. I wonder if it has to do with their affinity for water, compared to the alcohol?

clearly_not_atara - 23-3-2018 at 23:19

I thought nitrite and hypochlorite esters form because of the relative stability of the cations Cl+ and NO+, and borates by formation of tetracoordinate boron species. In other words it seems like if there's a sort of stable Lewis acid intermediate the ester will form quickly. Can't think of any others that would be available, though... maybe VO2+?

subskune - 26-3-2018 at 11:24

Quote:

Short comment on terphthalic acid. Did you see the mono ester from your reaction? Typically, a benzoic acid with a strongly electron withdrawing substituent in the para position esterifies with difficulty. This would be the case if you have the mono ester of terphthalic acid. The second ester will be difficult to form.

I don't know the properties of the mono ester. Therefore I can't be sure. After reflux I did hot filtration to remove the terepthalic acid powder and mixed the filtrate with loads of water, nothing fell out of solution. The terepthlic acid powder I filtered of didn't melt, so there was really nothing in it. It seems as nothing happened at all.

JBSC - 23-5-2018 at 20:12

WOW I want to put my 2 cents in but you guys are so much smarter than me on the subject. So far i have only isolated one ester. Made alot in test tubes but that is more testing than synthesis. Anyways thanks for all the good information.

fusso - 23-5-2018 at 22:23

NO2- + ROH + H+ <--> RONO +H2O

2NO2- + 2H+ <--> H2O + NO2 + NO

Consider 2nd rxn equilibrium lies towards the right, shouldn't nitrite esters be quite unstable?

Swinfi2 - 24-5-2018 at 00:04

I'd go with LearnedAmateur's answer, its to do with equilibriums.

If you have a low boiling ester you could distil it off, (maybe not the explosive ones) or try to use dry reagents and add a desiccant. Any time water is formed it can go backwards under reaction conditions.

This is kinda making me want to make some esters too haha, but alas, contamination and hastiness ruined all my sulphuric acid while i was making Et2O .