What is the major product obtained from the debromination of meso-$2,3$-dibromobutane?

Rank the following alkenes in order of increasing $\lambda_{max}$:
$(1)$ $A$ conjugated diene in a bicyclic system.
$(2)$ An isolated diene in a bicyclic system.
$(3)$ $A$ conjugated diene in a bicyclic system (different substitution pattern).

$A$ hydrocarbon $X$ adds one mole of hydrogen to give another hydrocarbon and decolourises bromine water. $X$ reacts with $KMnO_4$ in the presence of acid to give two moles of the same carboxylic acid. The structure of $X$ is:

An alkene $'A'$ on ozonolysis gives a mixture of ethanal and pentan$-3-$one. Write the structure and $IUPAC$ name of $'A'$.

What is the major product of the following reaction?
$1\text{-methylcyclohexene} + Br_2 \xrightarrow{H_2O} ?$

Unsaturation in organic compounds is detected by ...........