Chemistry help urgent Alevel

Alevelhelp.1

Help!!! I’m stuck!

I don’t get if these molecules are cis isomers or trans.

1)

(edited 11 months ago)

Reply 1

danhas20

cis as highest priority groups(heaviest functional group attached to one of the c's in c=c) are on the same side of the molecule

Reply 2

AspiringMed :)

They are both cis isomers as they both have 2 functional groups on the same side of the covalent bond

Reply 3

Alevelhelp.1

Original post by AspiringMed :)

They are both cis isomers as they both have 2 functional groups on the same side of the covalent bond

Thanks, do we compare the groups/ atoms on on the same carbon together or on different carbons. Im so lost 😭
Because the definition is that it arises from having 2 different groups on each carbon atom of the c-c double bond.

My finals are in 2 weeks 💀
For e.g
For this question do I compare the 2 things I circled in green or the two things I circled In pink?

(edited 11 months ago)

Reply 4

danhas20

Original post by Alevelhelp.1

This will explain everything, trust me https://youtu.be/UYdwwtTjBi0

Reply 5

Pigster

Original post by Alevelhelp.1

Help!!! I’m stuck!

I don’t get if these molecules are cis isomers or trans.

Neither are either.

In order to have cis trans, first of all you need to have E/Z isomerism.

To have E/Z you need a non-rotating double bond and then you look at each carbon atom (attached by said double bond) and check - are both the groups attached to each carbon different?

Here, in your example #1 the left hand C is attached to 2x CH3 groups and the right C has 2x H atoms. Neither end of the C=C has two different groups attached - we need both to be different. You can fix it by moving a CH3 from the left to the right and swapping it with an H from right to left - making but-2-ene.

Likewise, you #2 has 2x Br on the LHS and 2x H on the RHS.

If you had drawn but-2-ene we could then check whether it is E or Z by checking the relative atomic masses of the atoms bonded to each C atom and applying the point of first difference rule if there is/are a draw.

BUT... you asked about cis trans. In order to see if you have cis/trans, you first need it to have E/Z. If it does, you check to see whether the carbon on the LHS of the C=C has a group in common with the C on the RHS. Using but-2-ene as an example, The LHS has two groups in common, so it can have cis trans. Using 2-chlorobut-ene as a perhaps better example, the LHS C has 1x CH3 and 1x Cl - the RHS C has 1x CH3 and 1x H. Therefore, both C atoms have a CH3 attached, so it has cis trans.

To determine whether it is cis or trans, you only look at the group in common and see whether the two groups are diagonally opposite each other (trans) or next to each other (cis).