Aldehyde

An aldehyde has a group $\mathrm{RC}(=\mathrm{O})\mathrm{H}$, written $\mathrm{CHO}$ at the top of the illustrated open-chain Fischer projections; $R$ denotes the attached remainder of the molecule.

Trichloroacetaldehyde; alkaline cleavage can yield chloroform and formate.

The two-carbon aldehyde also called ethanal. It can be an intermediate in a historical chloroform preparation from ethanol.

$\mathrm{PhCHO}$.

The conjugated aldehyde $\mathrm{C}{\mathrm{H}}_2=\mathrm{CHCHO}$. A carbon nucleophile can add at its carbonyl carbon or, in a conjugate addition, at the terminal beta carbon. Atom mapping distinguishes the two products.

The planar molecule ${\mathrm{H}}_2\mathrm{CO}$. With the C=O bond along $z$ and the molecule in the $xz$ plane, its $C_{2v}$ vibrational representation is $3A_1+2B_1+B_2$. Interchanging the $x$ and $y$ axes interchanges the $B_1$ and $B_2$ labels; physical activity is unchanged.