(a) Why is `DMF` (dimethyl formamide) a useful solvent ?
(b) Why is formyl chloride unstable ?

Explanation

(a) Nearly te same `(C-Cl)` bond length in `(R-overset(O)overset(||)(C)-CI)` and `(R-Cl)` suggests that the extent of delocalisation by `Cl` in `(R-overset(O)overset(||)(C)-CI)` is very small or non-existent `[(-I)` and `(+R)` effects of `Cl, (+R)` effect or `pi-bond` overlap is not effective because of `2p` of `C` and `3p` of `Cl` overlap]. So, the contribution of resonance hybrid structure `(II)` is very small or non-existent.

(b) The relative importance of delocalisation decreases in the order `RCONH_2 gt RCOOR' gt RCOX`. as the `EN` (electronegativity) of `G` increases in the same period (i.e., `(-I)` effect increase, from `N gt O gt X`.
The `(C-Cl)` bond is longer than the `(C-F)` bond, although `EN` of `F` is greater than `Cl`. However, `(+R)` effect is not effective in `(C-Cl)` but effective in `(C-F)` because of the effective overlap of `2p` of `C` with `2p` of `F`.
( c) Resonance structure of anhydride.

Cross conjugation in the resonance structures of anhydride decreases the delocalisation of each `O` of `(C=O)` group and gives `(C-O) sigma-bond` less double-bond characte. Thus, the `(C-O)` bond length is longer in anhydrides than in esters.
(d) Resonance structure of esters :

Due to delocalisation of `overline e^, s` in esters, the positive charge on `C` of the `(C=O)` group is more spread out. This results in a smaller dipole moment and weater dipole-dipole attraction in esters than in ketones. So, esters have low boiling in ester points than ketones.
Acids have strong intermolecular `H-`bonding, while esters do not have. So the boiling points of esters are lower than those of acids.
( e) `1^@` and `2^@` amides exist as dimer in the solid and pure liquid state through intermolecular `H-`bonding. `1^@` amides have two `H` atoms. While `2^@` amides have one `H` atom. So intermolecular `H-`bonding is more in `1^@` amide than in `2^@` amides. `3^@` amdies lack `H` atoms and thus do not have `H-`bonding.
So, the boinling points of `1^@ amide (I) gt 2^@ amide (II) gt 3^@ amide (III)`.

(f) `DMF, (H-overset(O)overset(||)(C)-underset(Me)underset(|)(N)-Me)` , is an aprotic solvent, i.e., it has no `H` atom for `H-bonding` and has a high dipole moment. So `DMF` does not form `H-bonds` with anions although it dissolves many ionic compounds. The salts are dissolved through solvation of the cation by the attraction towards the end of the `(C-O)` dipole. The positive end of the dipole shielded within the molecule and solvates anion very weakly.
(g) `(H-overset(O)overset(||)(C)-CIoverset(Delta)rarr C-=O +HX)`
The decomposition reaction is favoured since the `Delta H_(f)^(@)` of `CO` is high (more negative) which indicates the stability of `(C -= O)` bond. Formyl chloride is unstable above `213 K(-60^@ C)`.