Saponification is common in school with the esterification connected. However, that is not entirely correct. But from the beginning: What is a saponification anyway?
The saponification describes the irreversible Hydrolysis of an ester in an aqueous solution of a basic substance (e.g. by a base).
arise in the process alcohol and the corresponding Salt the carboxylic acid, i.e soap.
Saponification – reaction equation
During saponification, a ester cleaved. These are the products that are formed in this reaction salt of carboxylic acid and Alcohol.
Figure 1: Saponification reaction equation
The saponification takes place in aqueous solutions that hydroxide ions included, instead. That means she walks in one alkalinerespectively basicenvironment.
So, concretely, you can say that you can make soap by… ethyl acetate with a hydroxide ion react. This then results from that salt of carboxylic acid and Alcohol.
The mechanism of saponification of carboxylic acid esters
The saponification follows Addition-Elimination Mechanism. The mechanism of saponification consists of the following three steps:
1st step: Nucleophilic attack
As you can see, the hydroxide ion is (OH-) negative loaded. Thus this is nucleophilic. The carbon atom, however, is partially positively chargedwhat you at the δ+ can see.
The nucleophilic hydroxide ion then attacks the carbon atom. This means that the upper bond of the hydroxide ion is folded up – it forms one with the C atom binding out. This immigrates electron pair the double bond C=O to the oxygen. This is also due to the fact that the carbon atom can form a maximum of four bonds. Thus, the oxygen of the former C=O double bond is negatively charged.
The hydroxide ion is now attached to the carbon atom bound. It has no charge more. You can now use the hydroxide ion to hydroxyl group assign. This first step of saponification can still be reversed – it is reversible.
2nd step: Elimination of the alcoholate ion
Figure 3: Second step of saponification
The oxygen, which was previously bound to the carbon atom via a double bond, «wants» its own negative not keep charge. He are the pair of electrons just picked up again away. As mentioned in the first step, oxygen can only form four bonds—that Alcoholate ion (⁻OR) splits himself consequently away. The ion is called the alcoholate ion because it is a Alcohol without a proton is. That is also the reason for that negative Charge.
So you get one in the second step of the saponification carboxylic acid and a alcoholate ion. This step is still there reversible.
In organic chemistry, the letter «R» (= radical) is a placeholder for a side chain on a main chain.
3rd step: proton transition
Figure 4: Third step of saponification
Now for the last and irreversible Step of saponification: Because the alcoholate ion has a very strong base is, also wants his give off a negative charge and pick up a proton instead. So «steals» the alkoxide ion becomes the proton of the carboxylic acid. To do this, as you can see on the left side of the equation, it must attack the hydroxyl group.
On the right you can now see that the proton has split off and is now bound to the alcoholate ion. This will give you one Alcohol.
You call that the resulting molecule without the proton salt of carboxylic acid. Here you can also see why this step is irreversible: the salt of the carboxylic acid is missing now the proton for a reverse reaction. The alcoholate ion «does not want» to give up the hydrogen atom either. Thus, mostly also forms no carboxylic acid more out.
saponification of fats
With lye how caustic soda or caustic potash fats can be saponified. Among the fats you can animal fat (e.g. from meat) or vegetable oils (e.g. olive oil).
In the saponification of fats ester groups split – the trihydric alcohol glycerol. In addition, the respective alkali salts of the fatty acids found in fats. You know these alkaline salts under the name «soap«.
Figure 5: Saponification with caustic soda to curd soap
Saponification vs. Esterification
Note: Ester saponification is not the reverse of esterification!
the acid hydrolysis is the reverse reaction of esterification. Thus, esterification and acidic hydrolysis reversible – so reversible. That is why esterification is also one equilibrium reaction.
However, this is not the case with saponification. One saponification is mostly irreversible. This is due to the carboxylic acid that a proton is missing.
Saponification – The most important thing
- Saponification describes the irreversible hydrolysis of an ester in an aqueous solution of a basic substance (e.g. with a lye).
- Saponification belongs to the addition-elimination mechanism.
- The salts of the carboxylic acid are called soap.
- The mechanism of saponification is divided into three steps:
- 1. Nucleophilic attack of the hydroxide ion on the ester, formation of the orthocarboxylic acid monoester anion
- 2. Elimination of the alcoholate ion and formation of the carboxylic acid.
- 3. Proton transfer from the carboxylic acid molecule to the alcoholate ion (irreversible step).
- The saponification is usually irreversible, since a proton is missing on the carboxylic acid for a reverse reaction.
can you caustic soda saponify, you get curd soap, one Sodium Soap – chemically speaking sodium salts. The saponification with caustic potash in turn delivers you soft soap, one potassium soap.
Here you can see the reaction equation of the saponification caustic soda – it arises glycerin and curd soap.
Figure 6: Saponification with caustic potash to soft soap
In this example you can also see the reaction equation for the production of a soft soapso a saponification of caustic potash: