Isomerism!

i can't tell the difference between structural and stereoisomerism. i also do not understand e-z isomerism, nor cis-trans isomerism.

please help!

(edited 10 years ago)

Reply 1

Dynamo123

7

Structural isomerism is involved with changes in the structure. This may be due to position of functional group, double or triple bond on C chain, etc, or may be due to change in carbon length on either side of functional group(fg). It might also be due to branching.
Stereoisomerism is concerned with the way alkenes undergo change of fg on either side of unrotateable CdoublebondC.
For more on it:
http://w ww.chemguide.co.uk/basicorg/isomerism/geometric.html#top
http://ww w.chemguide.co.uk/basicorg/isomerism/ez.html#top

Reply 2

Qwertish

13

Original post by genuinelydense

i can't tell the difference between structural and stereoisomerism. i also do not understand e-z isomerism, nor cis-trans isomerism.

please help! :colondollar:

Structural isomers have the same molecular formula but a different structural formulae. For example, propanone and propanal are structural isomers: they both have the same molecular formula (C₃H₆O), but different structural formulae (CH₃COCH₃ for propanone and CH₃CH₂CHO is propanal).

Stereoisomers have the same structural forumla but have a different arrangement of bonds in space. E-Z isomerism (or geometrical isomerism) is a sub-set of stereoisomerism, where the two groups attached to each of two carbon atoms either side of a double bond (this is a specific case for A Level organic chemistry) and rotated one way or the other. Cis-trans is the same as E-Z (but remember that cis is the same as Z and trans is the same as E, not the other way around :P).

Original post by genuinelydense

i can't tell the difference between structural and stereoisomerism. i also do not understand e-z isomerism, nor cis-trans isomerism.

please help! :colondollar:

Try this general isomerism summary

and this explanation of the CIP rules for EZ assignment

Reply 4

genuinelydense

OP

11

Original post by Qwertish

Structural isomers have the same molecular formula but a different structural formulae. For example, propanone and propanal are structural isomers: they both have the same molecular formula (C₃H₆O), but different structural formulae (CH₃COCH₃ for propanone and CH₃CH₂CHO is propanal).

Stereoisomers have the same structural forumla but have a different arrangement of bonds in space. E-Z isomerism (or geometrical isomerism) is a sub-set of stereoisomerism, where the two groups attached to each of two carbon atoms either side of a double bond (this is a specific case for A Level organic chemistry) and rotated one way or the other. Cis-trans is the same as E-Z (but remember that cis is the same as Z and trans is the same as E, not the other way around :P).

so stereoisomers arise from the single bonds' ability to rotate?

e.g. chloromethane 1.png

and

are both stereoisomers of chloromethane?

(edited 11 years ago)

Reply 5

Qwertish

13

Original post by genuinelydense

so stereoisomers arise the single bond's ability to rotate?

eg
H Cl
| |
H-C-H and H-C-H are both stereoisomers of chloromethane?
| |
Cl H

You can't have geometric isomers with single bonds, because the molecule can rotate about the C-C bond (making the two examples you gave effectively equivalent). With double bonds, the extra bond (the pi bond) prevents rotation about C=C, so


Cl    Cl
 \   /
  C=C
 /   \
H     H

and


Cl   H
 \   /
  C=C
 /   \
H    Cl

are different, because the molecule can't rotate about the C=C without undergoing a chemical reaction.

It's easier to show this with a model tbh :tongue:

Stereoisomers are a superset of isomers, containing geometric, and optical under them. Optical isomers are non-superimposable mirror images of one another. An example of an optical isomers is the chemicals which gives lemons and oranges their flavour (limonin); lemons have one isomer and oranges have the other.

(edited 11 years ago)

Reply 6

Pride

21

Original post by Qwertish

You can't have stereoisomers with single bonds, because the molecule can rotate about the C-C bond (making the two examples you gave effectively equivalent). With double bonds, the extra bond (the pi bond) prevents rotation about C=C, so


Cl    Cl
 \   /
  C=C
 /   \
H     H

and


Cl   H
 \   /
  C=C
 /   \
H    Cl

are different, because the molecule can't rotate about the C=C without undergoing a chemical reaction.

It's easier to show this with a model tbh :tongue:

Well not necessarily, around chiral carbons you can. Optical isomerism is an example of stereoisomerism...

Reply 7

Pride

21

Original post by Qwertish

Structural isomers have the same molecular formula but a different structural formulae. For example, propanone and propanal are structural isomers: they both have the same molecular formula (C₃H₆O), but different structural formulae (CH₃COCH₃ for propanone and CH₃CH₂CHO is propanal).

Stereoisomers have the same structural forumla but have a different arrangement of bonds in space. E-Z isomerism (or geometrical isomerism) is a sub-set of stereoisomerism, where the two groups attached to each of two carbon atoms either side of a double bond (this is a specific case for A Level organic chemistry) and rotated one way or the other. Cis-trans is the same as E-Z (but remember that cis is the same as Z and trans is the same as E, not the other way around :P).