In this lesson, we consider about acidity of organic compounds and what compounds show acidic characteristics. Acidity of organic compounds depends on many different factors. Some organic compounds are very acidic and some are less. Also there are basic organic compounds.
At the end of this tutorial, you should have the ability to decide which organic compound is more acidic from given compounds and what are the reasons for it. We study all acidic organic compounds in this lesson.
As an example,
Most of the acidic organic compounds are weak acids. Dissociation constant of acids (Ka) value tells us about the acidity or strength of the acid. When Ka value is high, acidic strength is high.
When we study about acidity of compounds, we have to look their reactions with following compounds and products and then observe reaction rates.
|Organic compound type||Acidity|
|Alkanes||No acidic characteristics|
|Alkenes||No acidic characteristics|
|Alkynes with acidic hydrogen||has weak acidic characteristics, reacts with sodium|
|Alkynes (no acidic hydrogen)||No acidic characteristics|
|Benzene||No acidic characteristics|
|Alkyl halides||No clear acidic characteristics|
|Alcohols||has weak acidic characteristics, acidic than alkynes|
|Phenol||has weak acidic characteristics, acidic than alcohols|
|Aldehyde||Acidic, reacts with strong alkali such as NaOH|
|Ketone||Aldehydes with less molecular mass are soluble|
|Carboxylic acid chlorides||Reacts with water and form soluble strong acid (HCl) and a weak acid (carboxylic acid).|
|Amines||Less basic than amines.|
Alkane, alkene and alkyne are hydrocarbons and there are different characteristics of acidity of these compounds.
Alkyne with acidic hydrogen react with strong metals such as sodium or potassium and H2 gas is emitted. But those alkyne do not react with NaOH(aq).
When we look their structure, it is clear that 2-butyne does not have acidic hydrogen. But propyne has a acidic hydrogen atom. Therefore propyne is more acidic than 2-butyne.
Alkane and alkene do not react with NaOH(aq). However alkyne with acidic hydrogen has some acidic properties than alkane and alkene because alkyne with acidic hydrogen react with sodium.
Benzene does not react with sodium. That means acidity of benzene is below than alkyne.
Alkyl haide compounds react with aqueous NaOH or KOH to give alcohols. But this reactions does not belong to acid base reaction.
Alcohols react with sodium and emit hydrogen gas. It says alcohols are acidic organic commpounds.
But alcohols do not react with aqueous NaOH to give the salt and water. Also alcohols do not react with aqueous Na2CO3 or NaHCO3.
pKa of ethanol = 16
pKa of methanol = 15.5
Lower the pKa value, acidic strength increases.
Phenol is more acidic than alcohols.
Acidity depends on polarity of O-H bond. When polarity of O-H bond increases, acidity also
Electron density of -O-CH3 is high. Therefore it gives electrons to the benzene ring which decreases polarity of O-H bond.
Electron density of -NO2 is less. Therefore it attracts electrons from benzene ring. Then O-H bond gives electrons to the ring which increases polarity of O-H bond. When more NO2 groups connect to the ring, polarity of O-H bond more increases.
Aqueous carboxylic acids are much acidic than other acidic organic compounds.
Carbonic acid is a weak organic acid and a dibasic acid. Carbonic acid can release two hydrogen ions in aqueous state. It's pK value is 6.4 .
pKa of methanoic acid (formic acid) is 3.75. Therefore methanoic acid is much acidic than carbonic acid.
When acid chloride is added into the water, acid chloride (ex: CH3COCl )
hydrolyses into strong acid ( HCl ) and carboxylic acid ( CH3COOH ).
Due to exist of strong acid in the aqueous solution it is more acidic than aqueous carboxylic acid solution.
When a nitro group is attached to the benzene ring, it attracks the electrons of benzene ring ( act as an deactivator). Also this attraction effects to the -OH group and increases the polarity of hydrogen atom. Therefore acidity of 4-nitrophenol is higher than phenol.
Most of the acidic organic compounds are weak acids. But acidic strength depends on the types of organic compound. Carboxylic acids are much acidic than other acidic organic compounds. But carboxylic acids are weak acids too.