Acidic and Acidity of Organic Compounds and Organic Chemistry

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,

Carboxylic acid compounds are more acidic than alcohol compounds.



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.

  • With sodium or potassium - whether hydrogen gas is eliminated or not.
  • Aqueous NaOH or KOH - Give a salt and water as products.
  • Na2CO3 or NaHCO3 - They are weak bases. To react with these weak bases, acidity strength of organic compound should be high.


Acidic strength of organic compounds

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
  • Acidic than alcohols and phenols.
  • Reacts with sodium, aqueous NaOH, Na2CO3, NaHCO3
Carboxylic acid chlorides Reacts with water and form soluble strong acid (HCl) and a weak acid (carboxylic acid).
Amides Basic.
Amines Less basic than amines.




Acidity alkane, alkene and alkyne - Hydrocarbons

Alkane, alkene and alkyne are hydrocarbons and there are different characteristics of acidity of these compounds.


Alkyne

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).


Acidity of propyne and 2-butyne

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

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.

acidic H alkyne


Benzene

Benzene does not react with sodium. That means acidity of benzene is below than alkyne.


Alkyl halide compounds

Alkyl haide compounds react with aqueous NaOH or KOH to give alcohols. But this reactions does not belong to acid base reaction.


Alcohols

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.


Ethanol and methanol acidity

pKa of ethanol = 16

pKa of methanol = 15.5

Lower the pKa value, acidic strength increases.


Phenol


Phenol is more acidic than alcohols.



Acidity of phenol derivatives

Acidity depends on polarity of O-H bond. When polarity of O-H bond increases, acidity also increases.
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.



Carboxylic Acids

Aqueous carboxylic acids are much acidic than other acidic organic compounds.

  • Carboxylic acids react with sodium and emit hydrogen gas.
  • Also carboxylic acids react with aqueous NaOH and produce sodium carboxylate and water.
  • Carboxylic acids react with aqueous Na2CO3 or NaHCO3 and emit CO2 gas.

Carbonic acid | H2CO3

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 .


Methanoic acid and carbonic acid

pKa of methanoic acid (formic acid) is 3.75. Therefore methanoic acid is much acidic than carbonic acid.


Acidity of alcohols, phenol, carbonic acid and carboxylic acid

  • All of those acids react with Na and H2 gas is emitted.
  • Phenol, carbonic acid and carboxylic acids react with NaOH. Only alcohol compounds do not react with NaOH. Therefore alcohols are less acidic than other compound types.
  • Solid Na2CO3 and NaHCO3 react with carboxylic acids and emit CO2.
acidity of ethanol phenol carbonic carboxyic acids

Acidity of acid chlorides

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.

acid chloride and water


Questions



Acidity of 4-nitrophenol than phenol

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.



Are there strong acidic organic compound such as HCl or H2SO4

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.



References


pKa Values of Organic and Inorganic Acids


Last edit: 20 Sep 2019

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