$C_2H_5OH$ reacts with which of the following to produce a sweet,fruity smell?

Formic acid on heating with concentrated $H_2SO_4$ at $373 \ K$ gives $X$,a colourless substance and $Y$,a good reducing agent. The number of $\sigma$ and $\pi$ bonds in $X$ and $Y$ are respectively:

The reaction is fastest when $X$ is
$R-C(=O)-X + Nu^- \rightarrow R-C(=O)-Nu + X^-$

Which of the following substances will not undergo the Hell-Volhard-Zelinsky reaction?

Some pairs of acids are given below. Select the pair in which the second acid is stronger than the first.

The correct order of $pK_{a}$ of the following compounds is:
$I. CCl_3COOH$
$II. CF_3COOH$
$III. NO_2CH_2COOH$
$IV. NCCH_2COOH$