Solveeit Logo
Login

Question

Question: Select the correct order of acidic strength. (This question has multiple correct options.) A) \(...

Select the correct order of acidic strength.
(This question has multiple correct options.)
A) BF3>Me3B{ BF }_{ 3 }{ >Me }_{ 3 }{ B }
B) CH3≡C−H{ CH }_{ 3 }{ \equiv C-H} << CH2=CH−H>CH3−CH2−H{CH }_{ 2 }{ =CH-H>CH }_{ 3 }{ -CH }_{ 2 }{ -H }
C) BF3{ BF }_{ 3 } << Me3B{Me }_{ 3 }{ B }
D) CH3≡C−H>CH2=CH−H>CH3−CH2−H{ CH }_{ 3 }{ \equiv C-H > CH }_{ 2 }{ =CH-H>CH }_{ 3 }{ -CH }_{ 2 }{ -H }

Explanation

Solution

Hint: Acidic strength refers to its ability to lose a proton H+{ H }^{ + }.
Back bonding is an additional pi bond formed between two atoms of which one has an empty orbital and another must-have lone pair.

Complete step-by-step answer:
The order of acidic strength is alkane < alkene < alkyne. and this is because of their hybridization and hence alkanes are the least electronegative whereas, an alkyne is the most electronegative.
{ H-C\equiv HC\rightarrow }{ H-C\equiv C }^{ \\_ }
{ R-CH=CH }_{ 2 }{ \rightarrow R-CH=C^{ \\_ }H^{ + } }
H−CH2−CH3→H−CH2C−H2{ H-CH }_{ 2 }{ -CH }_{ 3 }{ \rightarrow H-CH }_{ 2 }{ C }^{ - }{ H }_{ 2 }
The C−{ C }^{ - } is sp hybridized, so it can hold negative.
The carbon with a triple bond is more electronegative and hence, can hold a negative charge and is most stable. Hence, an alkyne is more stable than alkene and alkene is more than alkane.
So, the correct order of acidic strength will be
CH3≡C−H>CH2=CH−H>CH3−CH2−H{ CH }_{ 3 }{ \equiv C-H>CH }_{ 2 }{ =CH-H>CH }_{ 3 }{ -CH }_{ 2 }{ -H }

Now, among boron trihalides, maximum pΠ−pΠ{ p\Pi -p\Pi }, back bonding exists in BF3{ BF }_{ 3 }. This is because as the size of the halogen atom increases down the group, the overlapping of the vacant 2p{ 2p } orbital of boron cannot take place easily and efficiently with the p-orbitals of high energy levels. While there is an absence of π-back bonding in Me3B{ Me }_{ 3 }{ B }.
As we know that back bonding is inversely proportional to acidic strength, so Me3B{ Me }_{ 3 }{ B } will be more acidic than BF3{ BF }_{ 3 }.
Hence, the correct options are C and D.

Note: The possibility to make a mistake is that you may choose option A. But BF3{BF}_{3 } << Me3B{Me }_{ 3 }{ B } is more acidic as there is a pi back bonding present in BF3{BF}_{3} molecule which will increase the stability.