I can't figure out why isomers change their structure...

[katsui]

I'm new to chemistry, I'm looking at isomers, chiral topics, but I can't figure out why isomers change their structure, why chirals want to change in polarized light, why cis and trans do not remain constant but produce two different structures and which of these is the real structure, where do you think I'm going wrong?

 

[mjc123]

Not being clear, for a start. That's understandable if you're new to chemistry, but if we're to help you, we need you to be more clear and explicit about what you mean, and ideally provide some examples of the things you don't understand.

What do you mean by "isomers change their structure"? Can you give examples?
What do you mean by "chirals want to change in polarized light"?
What do you mean by "cis and trans do not remain constant but produce two different structures"? What do you mean by "which of these is the real structure?"? Examples?

 

[Borek]

My take is you misunderstand the idea - it is not like "isomers change their structure", it is like "isomers exist". Say, you have dimethyl ether and ethanol - they are two different compounds, that happen to share the same atomic composition (C₂H_^O). They are isomers, but they don't change automagically one into another.

There is plenty of fine prints here, but that should be your starting point.

 

[katsui]

Not being clear, for a start. That's understandable if you're new to chemistry, but if we're to help you, we need you to be more clear and explicit about what you mean, and ideally provide some examples of the things you don't understand.

What do you mean by "isomers change their structure"? Can you give examples?
What do you mean by "chirals want to change in polarized light"?
What do you mean by "cis and trans do not remain constant but produce two different structures"? What do you mean by "which of these is the real structure?"? Examples?

Chirals reflect under polarised light, creating an image that does not overlap with each other. I know that this situation causes our taste perception to change in the pharmaceutical industry. Why does it show reflectivity, is it useful (such as energy saving) ?
Then we have two images, which one is real ?

It is defined as the isomer shown by compounds with the same molecular formula that have different sequences in space.I know that the cis-trans isomers want to rotate, but they are prevented by the double bond, so the trans isomer comes out.I don't fully understand why this transformation is happening.At the same time, when we say that the sequence in space is different, which one is real?

My take is you misunderstand the idea - it is not like "isomers change their structure", it is like "isomers exist". Say, you have dimethyl ether and ethanol - they are two different compounds, that happen to share the same atomic composition (C₂H_^O). They are isomers, but they don't change automagically one into another.

There is plenty of fine prints here, but that should be your starting point.

Yes, I was thinking of a compound being converted. So dimethyl ether and ethanol are not related to each other except in formula ?

 

[DrJohn]

Chiral means two isomers exist. They don't change back and forth, two structures exist that are mirror images of each other, two different (but similar) compounds. A bit like hands - you have two hands, one is a right hand and one is a left hand, they don't suddenly become a left hand and a right hand. They are fixed.

Cis and trans isomers do not change either. In CHCl=CHCl there are two different structures and two different compounds that are fixed. One has the two Cl atoms on opposite sides (trans) and the same with the two H atoms, and the other compound has the two Cl atoms on the same side, Same with the two H atoms. They are two different chemicals, with different structures. CHCl2CHCl2 has no double bond, so the groups at each end spin around like tops relative to each other, but always the same compound, unlike the first molecule.

The transformation you are imagining does not occur from cis to trans.

Dimethyl ether and ethanol have the same number of each atoms present, but a totally different structure. Different functional groups are present.

Now there are molecules that are called fluctional molecules, which featured in my research a lot. Groups connected by a single bond say a CH3 group and a CH3CH2 group in an ether like CH3-O-CH2CH3 can rotate like a spinning top around their C-O bonds. So no fixed orientation of the groups can be found in the liquid and gaseous state (I am excluding solid state crystal structures to avoid unnecessary problems!).

But PF4NHPF2 is fluctional and some of its rotation and distortion due to bonds vibrating and bending can be slowed down at low temperatures and the structure sort of frozen. Oh, and it is a planar N, not your common or garden pyramidal N. ;)

The PF4 group has two axial Fs, two equatorial Fs and the NHPF2 group in an equatorial position. And at room temps the axial and equatorial Fs vibrate such that that seem to swap positions. So in an NMR spectrum you see four equivalent Fs in the PF4 group. But at low temperatures, -50C and below, with less energy around they don't swap positions, and you see two equivalent equatorial F atoms and two non-equivalent axial Fs because the H on the N can hydrogen bond to one of them and stops the groups rotating about the PF4-N bond. There is one fixed "frozen" structure in the solution at low temperatures.

Other compounds I made have groups spinning around a single bond and you can imagine them adopting one of two possible orientations of the groups at room temperature when a gas, due to hydrogen bonding holding them in slightly different orientations. But at low temperatures, the groups adopt the lowest energy structure. So in some of my compounds there were two distinct N-H vibrations in the IR spectra at room temperature in the gaseous state, but when frozen as a solid down to -100C or so, then warmed up for a few seconds, say to -50c and cooled again, they slowly transform to the most stable structure and after several warm then cool cycles of the solid only one N-H was present, the most stable structure / orientation of the groups. In the IR spectrum you can see the N-H vibrations start at about 60% one and 40% the other, then 70% to 25%, and so on until it is all just the one frozen structure and one N-H vibration. And only the one structure. Warm the sample up to room temp, and you see two sets of signals again.
I would spray the gas (in a vacuum) onto a plate at -100C to trap the molecules in their room temp orientations - literally freeze them to capture the shapes they were on, then remove the coolant from the plate, count to five or ten, and replace it. Sometimes I started at -196C, then removed that and added a different coolant at about -100C.

But the examples you started with do not do this sort of thing.