Question

Which reaction involves a decrease in the bond angle at a carbon atom? A. bromoethane refluxed with ethanolic sodium hydroxide B. complete combustion of methane in air C. ethanol heated with conc. H2SO4 D. polymerisation of ethene

Which reaction involves a decrease in the bond angle at a carbon atom?
A. bromoethane refluxed with ethanolic sodium hydroxide
B. complete combustion of methane in air
C. ethanol heated with conc. H2SO4
D. polymerisation of ethene

Answer 1

D. Polymerization of ethene involves a decrease in the bond angle at a carbon atom.

Explanation:

A. Bromoethane refluxed with ethanolic sodium hydroxide

The equation is

`"CH"_3"-"underbrace("C")_color(red)(sp^3)"H"_2"Br" stackrelcolor(blue)("OH"^"-" color(white)(mm))(→)"CH"_3"-"underbrace("C")_color(red)(sp^3)"H"_2"OH"`

The hybridization of the carbon atom does not change, so the bond angle does not change.

B. Complete combustion of methane in air

The equation is

`underbrace("C")_color(red)(sp^3)"H"_4 stackrelcolor(blue)("O"_2color(white)(mm))(→) "O="underbrace("C")_color(red)(sp)"=O"`

The hybridization goes from `"sp"^3` to `"sp"`, so the bond angle increases from 109.5 ° to 180 °.

C. Ethanol heated with conc. `"H"_2"SO"_4`

The equation is

`"CH"_3underbrace("C")_color(red)(sp^3)"H"_2"OH" stackrelcolor(blue)("H"_2"SO"_4, Δ color(white)(mm))(→)"CH"_2"="underbrace("C")_color(red)(sp^2)"H"_2`

The hybridization goes from `"sp"^3` to `"sp"^2`, so the bond angle increases from 109.5 ° to 120 °.

D. Polymerization of ethene

The equation is

`n"CH"_2"="underbrace("C")_color(red)(sp^2)"H"_2 → ("-CH"_2"-"underbrace("C")_color(red)(sp^3)"H"_2"-")_n`

The hybridization goes from `"sp"^2` to `"sp"^3`, so the bond angle decreases from 120 ° to 109.5 °.