  Among all the interstellar complex organic molecules (iCOMs), acetaldehyde is
one of the most widely detected species. The question of its formation route(s)
is, therefore, of a major interest regarding astrochemical models. In this
paper, we provide an extensive review of the gas-phase formation paths that
were, or are, reported in the literature and the major astrochemical databases.
Four different gas-phase formation routes stand out : (1) CH$_3$OCH$_3$ + H$^+$
/ CH$_3$CHOH$^+$ + e$^-$, (2) C$_2$H$_5$ + O($^3$P), (3) CH$_3$OH + CH and (4)
CH$_3$CH$_2$OH + OH / CH$_3$CHOH + O($^3$P). Paths (2) and (3) were not studied
neither via laboratory or theoretical works in the low temperature and density
regime valid for the ISM. Thus, we carried out new accurate quantum chemistry
computations. A theoretical kinetics study at low temperatures (7-300 K),
adopting the RRKM scheme, was also performed. We confirm that reaction (2) is
efficient in forming acetaldehyde in the 7-300 temperature range (alpha = 1.21
x 10$^{-10}$ cm$^3$ s$^{-1}$ and beta = 0.16). On the contrary, our new
computations disprove the formation of acetaldehyde through reaction (3) (alpha
= 1.84, 0.67 x 10$^{-13}$ cm$^3$ s$^{-1}$ and beta = -0.07, -0.95). Path (1)
was showed to be inefficient too by recent computations, while path (4) was
formerly considered for glycolaldehyde formation, having acetaldehyde as a
by-product. In conclusions, of the four above paths only two, the (2) and (4),
are potentially efficient gas-phase reaction routes for the formation of
acetaldehyde and we encourage astrochemical modellers to only consider them.
Comparison with astronomical observations suggest that path (4) may actually
play the major role.
