Root naming

[Jennifer Ngo 1M]

Hope this question makes sense.. These are two examples taken from Problem Set 2

(E )-3-bromo-2,4-dimethylhepta-2,4-diene
(Z)-4-ethyl-6-methylhept-3-enal

Why is one hepta- and one hept-? What are the rules when it comes to adding/dropping the -a?

[Chem_Mod]

A polyene (multiple double bonds) will have the "a" in the name. The second example is an aldehyde with one double bond.

[Diego Cortez 2J]

Hope this question makes sense.. These are two examples taken from Problem Set 2

(E )-3-bromo-2,4-dimethylhepta-2,4-diene
(Z)-4-ethyl-6-methylhept-3-enal

Why is one hepta- and one hept-? What are the rules when it comes to adding/dropping the -a?

Can someone explain to me when to use trans and cis?