another question

[Sarahisme]

i can't see how these two molcules are the same....??

[budullewraagh]

oh they look the same. AHAHAAHAHAH. but organic is soooo weird like that.

 

these are enantiomers. while they appear to be the same, they are different. they rotate plane polarized light in opposite directions. it should be noted that they rotate light in equal amounts. in a given solution, if you have equal parts of an isomer that is optically active and its isomer, the solution is considered "racemic" and will not rotate the light. if you have unequal parts of chiral enantiomers of a compound, the solution is "scalemic."

[Sarahisme]

are you sure? because my answers booklet says that they are the same

[Sarahisme]

oh and while were on this topic, could you please please please (i have test tomorrow) explain how these bonds come to Z ?

 

Thanks

[Sarahisme]

also how in world is this meso?! aghhh stress...!!

[woelen]

are you sure? because my answers booklet says that they are the same

Indeed, they are the same, they are not enantiomers. The drawing of molecules like these in 2D is somewhat confusing. Imagine the right picture, and rotate the molecule, such that the H is pointing upwards. Then you have CH3 coming out of the plane, towards you, Br in the plane and NH2 going out of the plane, pointed away from you. This is the same situation as in the left picture.

[woelen]

also how in world is this meso?! aghhh stress...!!

I assume that this picture is 1,2-cyclohexane-diol (1,2-dihydroxy cyclohexane). In this molecule, the bonds between the C-atoms are single. This means that the two items, connected at the C-atoms are pointing downwards and pointing upwards. So, you can have that both OH-groups are pointing upwards (relative to the plane, in which the 6-membered-C-ring is) or both of them are pointing downwards, or they point in opposite directions. Because the C-atoms are in a ring, there is no free rotational motion, so, a OH-group pointing downwards will remain so forever (assuming that the molecule as a whole is kept in the place and not rotated around an axis in the plane). In this way, just from plain 3D-geometry, you can easily see that a molecule is meso or not. In the picture you give, both OH-groups are pointing upwards.

[mezarashi]

 

Another of my cruddy paints

[Sarahisme]

I assume that this picture is 1,2-cyclohexane-diol (1,2-dihydroxy cyclohexane). In this molecule, the bonds between the C-atoms are single. This means that the two items, connected at the C-atoms are pointing downwards and pointing upwards. So, you can have that both OH-groups are pointing upwards (relative to the plane, in which the 6-membered-C-ring is) or both of them are pointing downwards, or they point in opposite directions. Because the C-atoms are in a ring, there is no free rotational motion, so, a OH-group pointing downwards will remain so forever (assuming that the molecule as a whole is kept in the place and not rotated around an axis in the plane). In this way, just from plain 3D-geometry, you can easily see that a molecule is meso or not. In the picture you give, both OH-groups are pointing upwards.

 

 

so your saying that you could "move" the right hydrogen that is going into the page, so that it will point downwards, and so this would now look like it is 'mirroring' the left hand bit, hence making it meso

 

??

[Bluenoise]

It's hard to see because of how it was drawn, visualizing the 3d representation in your head shows how it's meso.

 

Here I've made a 3d render of it. Omiting unnecessary hydrogens.

 

As you can see it's meso. It contains two chiral centre's but the molecule is actually achiral because of how their arranged.

[Sarahisme]

so was what i said correct? or way off track? :S

[Sarahisme]

*in my precious post

[Bluenoise]

Yeah you could redraw it and move the hydrogens to positions that make it easier to visulize if that helps, just don't flip them to the other face of the ring.

 

But it's not necessary really. All you have to realize is that the hydrogens are on the same side of the plane of the molecule and the alcohol groups are on the other side of the plane.

[Crash]

Its Meso because even though it has chiral carbons it still has a plane of symmetry

[Sarahisme]

ok thanks everyone