Question

Consider each pair of bases, and explain which one is more basic. Draw their conjugate acids, and show which one is a stronger acid.

(a)

(b)

(c)

(d)

Short answer

(a)

(b)

(c)

(d)

Step-by-step solution

Conjugate acid-base pair

The residual part of acid after losing a proton (H⁺) will have a tendency to accept a proton (H⁺) . Therefore, it will behave as a base. These pairs of substances which differ from one another by a proton (H⁺) are known as conjugate acid-base pairs. Consider a general example of an acid:

Hybridization

The mixing of atomic orbitals to give equal number of symmetrically hybridized orbitals is known as hybridization. The different types of hybridization are sp , sp² , sp³ , sp³d, sp³d² or d²sp³, sp³d³. sp hybridization is linear with a bond angle of 180⁰, sp² hybridization is trigonal planar with a bond angle of 120⁰ and sp³ hybridization is tetrahedral with a bond angle of 109.5⁰.

Explanation

(a)

Add a proton (H⁺) to the base to get the conjugate acid.

The deprotonated carbon differs in their hybridization. The more percentage of the s-orbital character of the electron pair on the carbon is, the more stabilized the charge is which implies negative charge is closer to the positive nucleus and hence it is less basic. There is 50% of the s-orbital character in the alkyne carbon pair ( sp hybridization), and 33% in the alkene carbon lone pair (sp² hybridization).

(b)

Add a proton (H⁺) to the base to get the conjugate acid.

The alcohol oxygen lone pair have less s-character (sp³ hybridized), hence it is more basic.

(c)

Add a proton (H⁺) to the base to get the conjugate acid.

The amine-nitrogen lone pair have less s-character (sp³ hybridized), hence it is more basic.

(d)

Add a proton (H⁺) to the base to get the conjugate acid.

The given bases are not even comparably strong. The negative charge on the hybridized carbon makes it really a strong base. But lone pair on nitrile nitrogen is not readily available for sharing. This molecule can be considered as neutral, since lone pair is present at the more electronegative nitrogen atom and it is hybridized.