What is the pKa of phenol?
Phenol (C6H5OH, also called carbolic acid) has a pKa of 9.89.
The pKa value for phenol is 10.0, and two series of substituted phenols are given: 2-fluorophenol, 3-fluorophenol, and 4-fluorophenol have pKa values of 8.7, 9.3, and 9.9, respectively; 2-nitrophenol, 3-nitrophenol, and 4-nitrophenol have pKa values of 7.2, 8.4, and 7.2, respectively.
Even though phenol is 106 more acidic than water, its pKa of 10 tells you that the acidity constant is 10-10 or that only one part in 1010 is ionized, the rest exists as phenol with H attached to oxygen.
Phenol is more acidic than ethanol.
Phenols generally have lower pKa's than aliphatic alcohols because of resonance stabilization with the aromatic ring.
pKa = -log[Ka]
These values reveal the deprotonation state of a molecule in a particular solvent.
Sulfuric acid is the strongest acid on our list with a pKa value of –10, so HSO4- is the weakest conjugate base.
Hence, p−methyl phenol will be the least acidic and will have the highest pKa value.
Properties of phenol as an acid
The pH of a typical dilute solution of phenol in water is likely to be around 5 - 6 (depending on its concentration). That means that a very dilute solution isn't really acidic enough to turn litmus paper fully red.
Phenol is more acidic than alcohols due to stabilisation of phenoxide ion through resonance. Presence of electron withdrawing group increases the acidity of phenol by , stabilising phenoxide ion while presence of electron releasing group decreases the acidity of phenol by destabilising phenoxide ion.
What pKa is a strong acid?
Definition of Strong Acids
More precisely, the acid must be stronger in aqueous solution than a hydronium ion (H+ ), so strong acids have a pKa < -1.74.
A strong acid will have a pKa of less than zero. More precisely – pKa is the negative log base ten of the Ka value (acid dissociation constant). It measures the strength of an acid — how tightly a proton is held by a Bronsted acid.
pKa is the negative log of the acid dissociation constant or Ka value. A lower pKa value indicates a stronger acid. That is, the lower value indicates the acid more fully dissociates in water.
| Names | |
|---|---|
| log P | 1.48 |
| Vapor pressure | 0.4 mmHg (20 °C) |
| Acidity (pKa) | 9.95 (in water), 18.0 (in DMSO), 29.1 (in acetonitrile) |
| Conjugate base | Phenoxide |
Because the charge is less concentrated on oxygen, the anion will have a smaller tendency to combine with a proton; therefore, the phenoxide ion is a weaker base than an alkoxide ion (and, conversely, phenol is a stronger acid than an alcohol).
Salt or cocrystal formation depends on the "rule of pKa", which states that components with a pKa value of less than three will produce cocrystals; salt requires a pKa value higher than three.
Remember that when the pH is equal to the pKa value, the proportion of the conjugate base and conjugate acid are equal to each other. As the pH increases, the proportion of conjugate base increases and predominates.
The pKa measures how tightly a proton is held by a Bronsted acid. A pKa may be a small, negative number, such as -3 or -5. It may be a larger, positive number, such as 30 or 50. The lower the pKa of a Bronsted acid, the more easily it gives up its proton.
Yes you are limited to weak acids that have a pKa of +/-1 of your desired buffers pH value. This is a rule of thumb used in many labs.
In addition, the smaller the pKa value, the stronger the acid. For example, the pKa value of lactic acid is about 3.8, so that means lactic acid is a stronger acid than acetic acid.
What is pKa value examples?
| Compound | pKa Value |
|---|---|
| Formic acid | 3.75 |
| Acetic acid | 4.75 |
| Phenol | 10 |
| Water | 14 |
- First, determine the pH value.
- Next, determine the concentration of conjugate base.
- Next, determine the concentration of weak acid.
- Next, gather the formula from above = pKa = pH – Log(CB/WA).
- Finally, calculate the pKa.
A low value of pKa (e.g. -10) indicates a strong acid. A high value of pKa (e.g. 50) indicates a weak acid.
If the pH of solution is greater than the pKa, the group is in the conjugate base form (deprotonated). If the pH of solution is less than the pKa, the group is in the conjugate acid form (protonated). (Note: glycine can serve as a buffer in 2 different buffer ranges).
Phenol is more acidic than ethanol because after losing a proton (H+), phenol forms phenoxide ion which is stabilised by resonance whereas ethoxide ion is not.
Hence p-nitrophenol is more acidic than phenol.
The phenol derivative picric acid has a pKa of 0.25, lower than that of trifluoroacetic acid.
Conversely, the pKa of phenol is 10. At pH 8, the environment is considered acidic for phenol and it remains primarily protonated. It is also important to remember that organic chemistry does NOT have to occur in water so pKa values can be as high as 50.
phenol is a weak acid.
Phenols are organic compounds containing a benzene ring bonded to a hydroxyl group. They are also known as carbolic acids. Phenols react with active metals like sodium, and potassium to form phenoxide. This reaction of phenol with metals indicates its acidic nature.
Which has higher PH alcohol or phenol?
Alcohols and Phenols
Phenol is more acidic than cyclohexanol and acyclic alcohols because the phenoxide ion is more stable than the alkoxide ion. In an alkoxide ion, such as the one derived from cyclohexanol, the negative charge is localized at the oxygen atom.
Phenols are more acidic than alcohol.
Hence, phenol is more acidic than water.
Strong acids completely dissociate in aq solution (Ka > 1, pKa < 1).
The smaller pKa, the stronger the acid. The smaller pKb, the stronger the base.
pKa and pH are two concepts in physical chemistry that refer to a system's acidity. The fundamental distinction between pKa and pH is that pKa denotes an acid's dissociation, whereas pH denotes a system's acidity or alkalinity.
Weak acids have large pKa's (2 — 50); they are stable because they have high affinity for their protons and want to stay bound to them.
A larger pKa for the conjugate acid indicates a stronger base. For indicating base strength directly we use pKb, in which case a smaller value is more basic.
Compound C has the lowest pKa (most acidic): the oxygen acts as an electron withdrawing group by induction.
phenol is a weak acid.
Why is phenol strong acid than water?
Since phenol has a propensity to lose H+ ion and form phenoxide ions, it is more acidic than water.
To measure the pH of the phenol directly, Ambion recommends mixing 2 ml of water saturated phenol with 5 ml of methanol and 13 ml water to form a uniform solution, which can be measured with a pH meter. For phenol/chloroform solutions, they recommend mixing 2 ml of phenol/chloroform with 8 ml methanol and 10 ml water.
Both alcohols and phenol are weak acid, the alcohols are less acidic than phenol because it is very tough to remove ′H′ ion from alcohol. Phenol can lose ion easily because phenoxide ion formed is stabilized to some extent. This is as the negative charge on the oxygen atom is delocalized around the ring.
Thus phenol prefers to lose hydrogen as cation to form phenoxide ion and hence phenols are acidic in nature.
Among the following, least acidic is O-Cresol as it has the electron donating group CH3 at ortho position which destabilises the phenoxide ion and hence less acidic.
Hence, p−methyl phenol will be the least acidic and will have the highest pKa value.
Phenols are organic compounds containing a benzene ring bonded to a hydroxyl group. They are also known as carbolic acids. Phenols react with active metals like sodium, and potassium to form phenoxide. This reaction of phenol with metals indicates its acidic nature.
Properties of phenol as an acid
The pH of a typical dilute solution of phenol in water is likely to be around 5 - 6 (depending on its concentration). That means that a very dilute solution isn't really acidic enough to turn litmus paper fully red.
pKa values that we have seen range from -5 to 50.
Therefore the release of H+ from the acid becomes easy, that means its acidic strength increases. Now higher the acidity, lighter is the value of Ka and lower is the value of pKa. Hence, benzoic acid has a higher pKa value among the four.
Is phenol strong acid or weak base?
phenol is a weak acid.
Phenol (Carbolic Acid)- C6H6O
Phenol is an aromatic compound. The chemical formula of this organic compound is C6H6O. Phenol is also known as Carbolic acid.
Hence, phenol is more acidic than water.
Phenol is more acidic than ethanol because after losing a proton (H+), phenol forms phenoxide ion which is stabilised by resonance whereas ethoxide ion is not.
So, the conjugate base is more stable than phenol. So, p-nitrophenol is more acidic than phenol. Therefore, it is clear that phenol is less acidic than p-nitrophenol.