Instead, the following happens:. As you can see, the carboxyl group donates an extra proton hydrogen nucleus to the water molecule. This makes the ethanoic ion negatively charged. As an acid, albeit a weak one, it donates a proton or a positively charged hydrogen ion when it reacts to a base. The weak acidic properties of this chemical means that it has a number of wide-ranging applications, including:.
The pH of ethanoic acid varies depending on its concentration. For instance, at a concentration of 1. This is an indication that only 0. The pH level of any dissolved substance demonstrates its tendency to either donate protons, making it acidic, or accept protons, making it alkaline.
The protons here refer to the nucleus of the hydrogen atom in a solution at a particular concentration. While acids have pH levels less than seven, bases or alkaline solutions have pH levels higher than seven. The true measure of acidity or alkalinity strength is the dissociation constant. This is calculated based on the formulas below:. Basically, the dissociation constant is the proportion between the number of intact molecules of a substance in a solution and the ions that are dissociated.
The logarithmic version is used to simplify the number by removing the scientific notation. Ethanoic acid turns blue litmus paper red. Carboxylic acids The carboxylic acids form a homologous series. Like all homologous series, the carboxylic acids: have the same general formula differ by CH 2 in molecular formulae from neighbouring compounds show a gradual variation in physical properties , such as their boiling points have similar chemical properties Functional group The functional group in the carboxylic acids is the carboxyl group, -COOH.
Structures The table shows four carboxylic acids, their molecular formulae and their structures. Acid properties The carboxylic acids have the typical properties of acids. For example, they: dissolve in water to form acidic solutions with pH values less than 7 react with metals to form a salt and hydrogen react with bases to form a salt and water react with carbonates to form a salt, water and carbon dioxide These properties are due to the —COOH functional group.
Making esters Carboxylic acids can react with alcohols to make esters. That means that the ethanoate ion won't take up a hydrogen ion as easily as it would if there wasn't any delocalisation.
Because some of it stays ionised, the formation of the hydrogen ions means that it is acidic. In the phenoxide ion , the single oxygen atom is still the most electronegative thing present, and the delocalised system will be heavily distorted towards it. That still leaves the oxygen atom with most of its negative charge. What delocalisation there is makes the phenoxide ion more stable than it would otherwise be, and so phenol is acidic to an extent. However, the delocalisation hasn't shared the charge around very effectively.
There is still lots of negative charge around the oxygen to which hydrogen ions will be attracted - and so the phenol will readily re-form. Phenol is therefore only very weakly acidic. If the hydrogen-oxygen bond breaks to release a hydrogen ion, an ethoxide ion is formed:. This has nothing at all going for it. There is no way of delocalising the negative charge, which remains firmly on the oxygen atom.
That intense negative charge will be highly attractive towards hydrogen ions, and so the ethanol will instantly re-form. You might think that all carboxylic acids would have the same strength because each depends on the delocalisation of the negative charge around the -COO - group to make the anion more stable, and so more reluctant to re-combine with a hydrogen ion. In fact, the carboxylic acids have widely different acidities. Why is ethanoic acid weaker than methanoic acid?
It again depends on the stability of the anions formed - on how much it is possible to delocalise the negative charge. The less the charge is delocalised, the less stable the ion, and the weaker the acid.
The only difference between this and the ethanoate ion is the presence of the CH 3 group in the ethanoate. But that's important! Alkyl groups have a tendency to "push" electrons away from themselves. That means that there will be a small amount of extra negative charge built up on the -COO - group.
Any build-up of charge will make the ion less stable, and more attractive to hydrogen ions. Ethanoic acid is therefore weaker than methanoic acid, because it will re-form more easily from its ions. The other alkyl groups have "electron-pushing" effects very similar to the methyl group, and so the strengths of propanoic acid and butanoic acid are very similar to ethanoic acid. Note: If you want more information about the inductive effect of alkyl groups, you could read about carbocations carbonium ions in the mechanism section of this site.
Use the BACK button on your browser to return to this page if you choose to follow this link. The acids can be strengthened by pulling charge away from the -COO - end. You can do this by attaching electronegative atoms like chlorine to the chain. We begin by considering the conjugate bases. In both species, the negative charge on the conjugate base is held by an oxygen, so periodic trends cannot be invoked. For acetic acid, however, there is a key difference: a resonance contributor can be drawn in which the negative charge is localized on the second oxygen of the group.
The two resonance forms for the conjugate base are equal in energy. What this means is that the negative charge on the acetate ion is not located on one oxygen or the other: rather it is shared between the two. The delocalization of charge by resonance has a very powerful effect on the reactivity of organic molecules, enough to account for the difference of over 12 pK a units between ethanol and acetic acid. The acetate ion is that much more stable than the ethoxide ion, all due to the effects of resonance delocalization.
Why should the presence of a carbonyl group adjacent to a hydroxyl group have such a profound effect on the acidity of the hydroxyl proton? To answer this question we must return to the nature of acid-base equilibria and the definition of pK a , illustrated by the general equations given below. We know that an equilibrium favors the thermodynamically more stable side, and that the magnitude of the equilibrium constant reflects the energy difference between the components of each side.
In an acid base equilibrium the equilibrium always favors the weaker acid and base these are the more stable components.
Water is the standard base used for pK a measurements; consequently, anything that stabilizes the conjugate base A: — of an acid will necessarily make that acid H—A stronger and shift the equilibrium to the right. Both the carboxyl group and the carboxylate anion are stabilized by resonance, but the stabilization of the anion is much greater than that of the neutral function, as shown in the following diagram.
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