The pKa values of acetylene, ehtylene and ethane are 25, 44 and 50 respectively. This means that acetylene is the strongest acid of the three.Explain the order of acidity for the above hydrocarbons
The triple bond of acetylene makes the C atom slightly negative and H slightly positive and the polar bond acts as an acid, although a weak one. Ethylene works the same way but the C atom is less negative, the bond is less polar, and the acidity of ethylene is less than that of acetylene. The same reasoning holds for ethane and the C-C single bond.
The order of acidity for the hydrocarbons acetylene, ethylene, and ethane is explained by their respective pKa values.
A lower pKa value indicates a stronger acid. In this case, the pKa values for acetylene, ethylene, and ethane are 25, 44, and 50, respectively.
Therefore, acetylene has the lowest pKa value (25) among the three hydrocarbons, making it the strongest acid. Ethylene has a higher pKa value of 44, indicating it is less acidic than acetylene. Lastly, ethane has the highest pKa value of 50, suggesting it is the least acidic of the three hydrocarbons. So, the order of acidity is acetylene > ethylene > ethane.
To understand the order of acidity for the given hydrocarbons (acetylene, ethylene, and ethane), we need to consider their ability to donate a proton (H+) in an acidic solution. Acidity is determined by the stability of the resulting conjugate base (the species that remains after the acid donates a proton). The stronger the conjugate base, the stronger the acid.
In this case, we are given the pKa values of the hydrocarbons which can provide insights into their relative acid strengths. The pKa is a measure of the acidity of an acid. It is the negative logarithm (base 10) of the acid dissociation constant (Ka) for an acid, where a higher value of pKa indicates a weaker acid.
Let's analyze each hydrocarbon:
1. Acetylene: pKa = 25
Acetylene (C2H2) has a triple bond between the two carbon atoms. The presence of this triple bond makes the hydrogen attached to the carbon more easily dissociated, resulting in a relatively stable conjugate base. Thus, acetylene is a stronger acid compared to the other two hydrocarbons.
2. Ethylene: pKa = 44
Ethylene (C2H4) has a double bond between the two carbon atoms. While it is not as acidic as acetylene, it is still more acidic than ethane. The presence of the double bond allows for some delocalization of electron density across the pi bond, which stabilizes the resulting conjugate base.
3. Ethane: pKa = 50
Ethane (C2H6) lacks any multiple bonds between the carbon atoms, making it the least acidic among the three hydrocarbons. Without any pi bonds, ethane cannot delocalize electron density and stabilize the conjugate base as effectively as acetylene or ethylene.
Therefore, based on the given pKa values, the order of acidity for the hydrocarbons is:
Acetylene > Ethylene > Ethane