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

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 °.