Chemistry Forum

Can you tell me which are the geometrical isomers of 3,4,7,8 tetramethyl 3,7 decadiene and geometrical isomers of 2,4 heptadiene? thank you in advance.

I find that the SMILES representation is inevaluable for this. (See also: Daylight Theory: SMILES)

CCC(C)=C(C)CCC(C)=C(C)CC

3,4,7,8-tetramethyl-3,7-decadiene will have cis/trans isomerism around the double bonds. The chain is otherwise symmetrical from end to end, so there will be three isomeric patterns.

CC/C(C)=C(C)/CC/C(C)=C(C)/CC : (3E,7E)-3,4,7,8-tetramethyl-3,7-decadiene

CC/C(C)=C(C)\CC/C(C)=C(C)/CC : (3Z,7E)-3,4,7,8-tetramethyl-3,7- decadiene or (3E,7Z)-3,4,7,8-tetramethyl-3,7-decadiene (Depending on which end you count from; otherwise identical.)

CC/C(C)=C(C)\CC/C(C)=C(C)\CC : (3Z,7Z)-3,4,7,8-tetramethyl-3,7-decadiene

CC=CC=CCC
2,4-heptadiene will have stereoisomers around the double bonds, but the chain is not end-to-end symmetrical so there will be four.
C/C=C/C=C/CC ; (2E,4E)-hepta-2,4-diene
C/C=C/C=C\CC ; (2E,4Z)-hepta-2,4-diene
C/C=C\C=C\CC ; (2Z,4E)-hepta-2,4-diene
C/C=C\C=C/CC ; (2Z,4Z)-hepta-2,4-diene