An alkene on ozonolysis gives methanal as one of the products. Its structure is

Ozonolysis of compound $X$ followed by reduction yields two moles of the same aldehyde. What is $X$?

The major product of the following chemical reaction is: $CH_3-CH(CH_3)-CH=CH_2 + HBr \xrightarrow{(C_6H_5CO)_2O_2} ?$

Glycol on treatment with $PI_3$ mainly gives -

The order of increasing reactivity towards $HCl$ of the following compounds will be:
$(1)$ $CH_2 = CH_2$
$(2)$ $(CH_3)_2C = CH_2$
$(3)$ $CH_3CH = CHCH_3$

Find the product of the reaction: $CH_3 - CH = CH_2 + HBr \rightarrow$ ?