We know that an acidic compound can donate hydrogen ion while a basic compound can give hydroxyl ion. The number of H+ and OH- ion from a chemical compound can represent with the help of their acidity or basicity. Let’s discuss what is acidity and basicity? The number of hydrogen ion from the compound is called as basicity of that acid while the number of hydroxyl ion from a base is called as acidity of respective compound.
The acidity or basicity of a certain element depends upon the position of that element in the periodic table. For simple organic compounds like alkane (ethane), amines (methylamine) and alcohol (methanol) the concept of acidity and basicity is entirely dependent on the molecular structure of the molecule. For example the acidity of given organic compounds increases from alkane to alcohols. On the contrary the basic nature of their conjugated bases increases in the opposite direction as given below;
As we move from carbon to oxygen via nitrogen, their nature can be observed with the help of their conjugate base as for the more stable the conjugate base, the strength of the acid also increases. There is negative charge on these elements in their conjugated base like negative charge on C in ethyl anion and negative charge in N in methylamine anion and on O in methoxide anion. We know that the electro negativity of elements increases from C to O and a more electronegative atom can better bear a negative charge compare to less electronegative element.
Hence the methoxide anion would be the most stable and least basic) and ethyl anion is the least stable and most basic. Similar concept can be applicable on water and ammonia. Out of these two compounds; ammonia is more basic compare to water as O-atom is more electronegative and tightly hold the lone pair of electrons compare to N-atom in ammonia. Hence N-atom more tends to form bond through its lone pair and becomes more basic. From top to bottom in the periodic table the concept of acidity can be observed which we can apply on the basic nature of halides.
From fluoride to iodide the basic nature decrease which increases the acidic nature of hydrohalic acid from HF to HI. The concept of acidic and basic nature of organic compounds is very useful in the study of biological organic chemistry such as this trend proves that thiols are more acidic compare to alcohols. The periodic trend of acidity in the periodic table through periods and groups can be written as given below;
The resonance effect is also helpful in the determination of acidity and basicity of groups which have exchangeable proton (H+) bound to different elements. Let’s take an example of this effect.
The resonance in ethanol and acetic acid can be used to determine the acidic and basic nature of these organic compounds. After losing H+ ion; RCOOH converts into RCOO- ion which is stabilised due to equal distribution of negative charge over both oxygen atoms. That is the reason; RCOOH can easily convert into RCOO- ion as RCOOH is less stable compare to RCOO-. On the contrary; alcohols (ROH) can be stabilised after losing H+ ion as they form RO- (alkoxide ion) which is less stable due to positive inductive effect of alkyl group.