# Naming unsaturated hydrocarbons

$\ce{CH2=CH-CH=CH-CH2-CH3}$

Is this hexa-1,3-diene or hex-1,3-diene?
And why?
When do you add an extra a and when do you not?

• Ignoring the numbers for a moment. This will probably help you in most exams: You generally never see 2 vowels together. When 2 vowels come together, remove the first one. e.g.: methane + al = methanal (notice that I removed 'e' from the combined word), butane + one = butanone, but, in case of dione because the suffix doesn't start with a vowel, one has to just join the two: butane + dione = butanedione. Thus, in your case, we just have to join the two: hexadiene, there is no need of removing a vowel. – Harshal Gajjar Mar 28 '16 at 18:12

According to the general methodology described in the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), a double bond is expressed by changing the ending ‘ane’ of the name of the corresponding saturated parent structure to ‘ene’.

P-31.1.1.1 The presence of one or more double or triple bonds in an otherwise saturated parent hydride (…) is denoted by changing the ending ‘ane’ of the name of a saturated parent hydride to ‘ene’ or ‘yne’. (…)

For example, the name ‘hexane’ is changed to ‘hex-1-ene’.

If the compound contains multiple double bonds, a multiplicative prefix is placed before the ending ‘ene’ and the letter ‘a’ is inserted between the root of the name of the parent structure and the multiplicative prefix.

P-31.1.1.2 The multiplying prefixes ‘di’, ‘tri’, etc., are placed before endings denoting unsaturation to indicate the number of multiple bonds of each kind, as required, for example, ‘diene’ and ‘triyne’. For euphonic reasons, when the endings ‘ene’ and ‘yne’ are preceded by a multiplying prefix and a locant the letter ‘a’ is inserted. (…)