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Isomerism help needed pls :)

Hello, am trying to get through some chemistry homework on isomerism and have worked out a few answers of my own, however I would really appreciate some further input, suggestions and help on what I am stuck with ....

1)State the type of isomerism which occurs between the following pairs of compounds. in each case, briefly justify your answer.

For this question, please refer to the attached file, and structure diagrams labelled 1 - 4

So far, I think number 1 is deometric isomerism tho can't seem to explain it properly. I think number 2 is an example of functional group isomerism because each compound has a different functional group and I think number 4 is an example of chain isomerism because there is a different carbon skeleton. As for number three, I really can't work it out.

2) The second part of this homework refers to the diagram labelled number 5.

What type of isomerism is shown by 2-hydroxypropanoic acid (lactic acid)?
What structural feature of the molecule is responsible for the isomerism?
Draw out the structure of the isomers and discuss how they are related to each other.

I don't have any answers for this last question, for you to build on, but any answers with explanations to help me understand will be most greatfully received. Thankyou in advance, xx
Reply 1
Hi, these are my answers:

1. Trans isomerism as there are the same functional groups about the double bond.
2.simple stuctural/functional group isomerism as carbonyl group changes from aldehyde to ketone position.
3.again structual isomerism as OH group changes from an alcohol to ester whilst the overall molecular formula is maintained (applies to 2 also)
4. i think this is cis/trans isomerism again. not sure. that double bond annoys me!
5. Optical isomerism due to 4 different groups being attached to the second carbon atom...

Lactic acid exhibits optical isomerism as 4 different groups are attached to a carbon atom known as the asymmetric carbon or chiral carbon. this means one isomer of the acid will rotate the plane of plane-polarised light anticlockwise and the other isomer will rotate the plane of plane-polarised light clockwise.

Please find my attachment for lactic acid. One optical isomer is a mirror image on the other.

Vince
Reply 2
Thankyou ever so much. Your explanations do help my understanding, and I appreciate your help :smile: x
vinny221
Hi, these are my answers:

1. Trans isomerism as there are the same functional groups about the double bond.
2.simple stuctural/functional group isomerism as carbonyl group changes from aldehyde to ketone position.
3.again structual isomerism as OH group changes from an alcohol to ester whilst the overall molecular formula is maintained (applies to 2 also)
4. i think this is cis/trans isomerism again. not sure. that double bond annoys me!
5. Optical isomerism due to 4 different groups being attached to the second carbon atom...


I agree basically, but some additional points:

1. The different isomers exist because the C=C exhibits restricted rotation, that is it can not rotate freely, at room temperature atleast. In the picture, the one on the left is the trans- isomer, the other is the cis- isomer.
4.this is chain isomerism, or "position isomerism" since the functional group, C=C is just in a different place in the carbon chain
5. I could never picture optical isomerism, but yea its that. The different isomers can be called enantiomers btw, and are prefixed with (+) or (-), or D- or L-, respectively. D- for dextro- and L- for Levo- presumably, or something like that. (left and right)

-Joe