$4$ axit hữu cơ 2 chức hay gặp nhất:
$HOOC-COOH$ (axit oxalic)
$HOOC-CH_2-COOH$ (axit malonic)
$HOOC-[CH_2]_4-COOH$ (axit ađipic)
$p-HOOC-C_6H_4-COOH$ (axit terephtalic)
Các axit 2 chức khác như:
$HOOC-CH_2-CH_2-COOH$
$HOOC-CH=CH-COOH$
$HOOC-C\equiv C-COOH$