Identify which functional group is $NOT$ present in the given compound.

What is obtained when $4$-methylbenzenesulfonic acid reacts with sodium acetate?

The degree of unsaturation (also known as double bond equivalent or index of hydrogen deficiency) for a compound with the molecular formula $C_{20}H_{22}O_2$ is:

Two unknown compounds $X$ and $Y$,both having molecular formula $C_4H_8O$,give the following results with four chemical tests.

Compound	Bromine	$Na$ metal	Chromic acid	Lucas reagent
$X$	decolourises	bubbles	Orange to Green	No reaction
$Y$	No reaction	No reaction	No reaction	No reaction

Compound $X$ and $Y$ respectively are:

Fill in the blanks:
$(1)$ Two isomers of butane are $n$-butane and isobutane. The relation of the number of $\sigma$ and $\pi$-bonds in them is .......... .
$(2)$ .......... is present in the structure of a heterocyclic compound.
$(3)$ Cyclohexene is a .......... type and furan is a .......... type of compound.
$(4)$ .......... type of $3-D$ model indicates the atomic volume but does not indicate the bond.

Which of the following has the maximum enol content?