Undecane (also known as hendecane) is a liquid alkane hydrocarbon with the chemical formula CH3(CH2)9CH3.

It is used as a mild sex attractant for various types of moths and cockroaches, and an alert signal for a variety of ants.

[4] Undecane may also be used as an internal standard in gas chromatography when working with other hydrocarbons.

Since the boiling point of undecane (196 °C) is well known, it may be used as a comparison for retention times in a gas chromatograph for molecules whose structure has been freshly elucidated.

For example, if one is working with a 50 m crosslinked methyl silicone capillary column with an oven temperature increasing slowly, beginning around 60 °C, an 11-carbon molecule like undecane may be used as an internal standard to be compared with the retention times of other 10-, 11-, or 12- carbon molecules, depending on their structures.