Certain functional groups have been identified in Organic Chemistry for ease of identification and nomenclature. One such group is an alcohol group which has one or more hydroxyl groups (OH) connected to the carbon through non-ionic bonding, the general formula being R-OH.
So, what is the ph of alcohol? The pH value of alcohol is 7.33. Alcohol cannot be defined as acidic or alkaline, but rather defined as amphiprotic because the nature of the bond between carbon and hydroxyl group is non-ionic but slightly polar in nature. Aliphatic alcohol acts as a weak acid in the presence of any strong base. Aromatic alcohols behave weakly acidic at standard conditions because the resonance allows the hydrogen to kick off.
This pH property allows two cleavages to take place in alcohol; that is R-O bond can also be broken, and the O-H bond as well (as it behaves weakly acidic). This gives a wide range of reactions to be performed using alcohol as a substrate.
But in this conclusion, a variety of indirect questions need to be understood, and here is the logic behind these conclusions.
What exactly is the pH?
When we define the pH of any compound, we are essentially talking about the concentration of the hydrogen ions present in the solution.
This concentration is the molarity of the hydrogen ions, that is, the moles of hydrogen ion present in one liter of solution.
In most exact terms, pH is defined as the negative logarithm (to the base 10) of hydrogen ion concentration.
pH = – Log10[H+]
Because of this negative sign, the pH is inversely proportional to the concentration of hydrogen ions. A typical pH scale starts from 1 and ends at 14, with 7 being the neutral point.
Acidic pH ranges in the pH less than 7 and bases in the pH greater than 7.
The smaller the value of pH, the greater the acidic strength that is a strong acid.
For bases, the larger the pH value stronger is the base.
For example, the pH of Hydrochloric Acid (HCl) is 3.01, and the pH of Acetic acid (CH3COOH) is 4.76.
Therefore, acetic acid is a weak acid, and hydrochloric acid is a strong acid. In the case of alkalis, the pH of Sodium Hydroxide (NaOH) is 14, and the pH of sodium bicarbonate (NaHCO3) is 8.
This confirms that Sodium Hydroxide is a stronger alkali than Sodium Bicarbonate.
For pH to be determined, the compound should exist in the ionic form such that the concentration can be measured.
Why can alcohol not be regarded as an acid?
In alcohols, there are no ionic bonds present leading to the generation of ions to a large extent upon dissociation. The bond is covalent but slightly polar in nature.
This slightly polar nature is not sufficient for the hydrogen to behave as the acidic hydrogen and therefore exists as an ion and the other part is called alkoxide.
Alkoxides are a very important reaction intermediate because of strong nucleophilic nature.
But in order to make this hydrogen acidic, we need to make the C-O bond more polar.
This is done by changing the electro-positivity of the carbon. Increasing the electro-positivity will increase the polar nature of the bond, in turn resulting in acidic hydrogen. This weak acidic nature is why alcohols are also termed brownsted acids.
Introducing electron-withdrawing groups (-I and –R effect group) at the carbon center will lead to the pulling of electrons away from carbon and thus making it more electron-deficient, which is electropositive in nature.
This will help the C-O bond to be more polar; thus, the hydrogen will be more acidic, giving out a weakly acidic pH range. (5-7 pH range)
For example. –
1 – Butanol is a very simple aliphatic alcohol that is also present in the human body in tissues and blood as well. The pH of 1-Butanol is very close to 7, because of which it is to be regarded as a very weak acid.
On the same carbon, if we attach a chloro group (electron-withdrawing group through negative inductive effect), we will get a compound as 1-Chloro butanol.
This compound will have a pH lower than 1-butanol, which means it will be more acidic.
Similarly, if we look at butanoic acid, the C=O bond makes, the same carbon more electropositive, and that is why the carboxylic acid compounds are stronger acids as compared to the corresponding alcohol.
The pH of butanoic acid is close to 4; thus, it is much stronger than the corresponding alcohol because of the electron-withdrawing group present on the same carbon.
Why can alcohol not be regarded as a base?
This is one of the trickiest questions in the pursuit of the end answer. This is because the nature of the compound looks very similar to the strong metal bases or alkalis such as Sodium Hydroxide (NaOH), Potassium Hydroxide (KOH).
This enables the creation of confusion as in alcohols to be regarded as an alkali.
Sometimes, it can be regarded as a weak base (Because Ph is greater than 7), but this is complete because of the influence of the surroundings.
Therefore, depending upon the surrounding conditions, pH is determined for alcohols, and they exist as acids as well as bases. Hence, they are also called amphiprotic compounds.
However, there are some underlying differences in the nature of the bond. In the case of Bases or alkalis, the OH group is connected to the metal center.
For example, the Hydroxide ion is bonded to the sodium metal ion through the ionic bond in Sodium Hydroxide.
If this base is added to an aqueous medium, it will dissociate into negatively charged hydroxide anions and positively charged sodium metal cations.
Being a polar molecule, alcohol is easily soluble in water. here is the article related to the polarity of alcohol.
On the other hand, the alcohol has the hydroxyl group, which is connected to the carbon using a covalent bond and not an ionic bond.
Because of this, in the solution state, the hydroxyl group will not exist as a hydroxide ion, which is a primitive of the compound to be called an alkali.
Hence, the idea of alcohol being regarded as a base based on similarities of the chemical formula is to be disregarded.
These two classes of compounds will not react to the condition in a similar fashion because of the underlying difference in bond nature.
For example – the pH of Potassium Hydroxide (KOH) is in the range of 13-14, making it a very strong base, and the pH of ethanol is 7.33 (very close to water, hence almost neutral).
The pH of Ethanol
Ethanol is a very important chemical as it is the largest part of the alcohol that is consumed. In the chemical domain, it is also largely used as a solvent and a reagent.
Ethanol is one of the simplest aliphatic alcohols present with the structure as follows –
Ethanol has a pH of 7.33 in comparison to water at 7.
Hence, the ethanol is regarded as neutral since the pH is very close to 7.
Depending on the surroundings, the pH will change. As in the presence of potassium Hydroxide (KOH), the pH will drop below 7, making it a weak acid.
Which is more acidic Phenol and Ethanol?
In the case of aromatic alcohols, such as phenol, the peroxide that is formed after the O-H bond cleavage, is stabilized because of the resonance effect of the benzene ring since the electronegative character of the oxygen will be reduced.
This will allow the aromatic alcohols to be more acidic in nature than ethanol (aliphatic compound) since the compound formed after the reaction is stable.
This is not the case with aliphatic alcohols, and that is why they exist as weak acids only in the presence of a strong base for the alkoxide to form.
Apart from this, both types of alcohol display similar kinds of reactions.
Properties of Alcohol
The boiling point of alcohol compounds is more than hydrocarbons of equal molecular mass because the attached hydroxyl bond has hydrogen bonding.
hydroxyl (O-H) bond is hydrophilic, making hydrogen bonding with water increases its solubility in water, and the alkyl group is hydrophobic.
As a result, the solubility of alcohol in water decreases as the chain of carbon increases.
The primary alcohol compound is more acidic as compared to secondary and tertiary alcohols.
Unlike water, alcohol does not conduct electricity due to a lack of free electrons. Check out the interesting article related to alcohol conducting electricity.
Alcohol has a lower boiling point than water; it evaporates at a faster rate as compared to water. Here is an article related to the evaporation of alcohol.
Conclusion
Alcohols are to be regarded as amphiprotic since they can exist as acidic and alkaline, depending upon the condition of the surroundings. Therefore, it cannot be regarded as a base or an acid. While basic chemistry cannot be changed for the same, chemists have meticulously used these limitations to develop more chemistry and go to par by using these limitations themselves.
