Consider the following molecule $(X)$. The reaction of $(X)$ with $HBr$ gives the major product:

Major product of the given reaction is $CH_2=CH-CH_2-I \xrightarrow[CCl_4]{HI(excess)}$

Compound $X$ on heating with $Zn$ dust gives compound $Y$,which on treatment with $O_3$ followed by reaction with $Zn$ dust and $H_2O$ gives propionaldehyde. The structure of $X$ is:

Arrange the following compounds in the decreasing order of their reactivity towards electrophilic addition of $HBr$:
$(a)$ $CH_3-C(CH_3)=CH_2$
$(b)$ $CH_3O-C(CH_3)=CH_2$
$(c)$ $CH_3OCH_2-C(CH_3)=CH_2$

The carbon-carbon bond length in an ethylene molecule is:

An alkyl bromide produces a single alkene when it reacts with sodium ethoxide and ethanol. This alkene undergoes hydrogenation and produces $2-$methylbutane. What is the identity of the alkyl bromide?