I need help about chiral , achiral

[joonkimdds]

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I am reading Kaplan blue book page 337.

it says Stereocenter(carbon with 4 different substituent) is asymmetric because it lacks symmetry.

the first picture i drew below is stereocenter and they are non-superimposable thus chiral.

then it shows carbon with 3 different substituent(so it's not stereocenter)
but this time, it says we can rotate 180 degree along the y-axis and make them superimposable upon its mirror image.

why can't we rotate 180 degree along the y-axis and make the stereocenter superimposable too? because i can.

I just drew it and proved(LOL ) that i can rotate stereocenter 180 degree and made it superimposable. would u check the picture and see if i did something wrong? or am i better than kaplan blue book(sigh...)

and could you also take a look at the picture below?
it was from my textbook saying that molecules with a plane of symmetry is always achiral.
if you look at the picture, it has a plane of symmetry but it can't be superimposable so it should be chiral.
did this book make a mistake or am i misunderstanding something?
this picture is just like my left and and right hand connected pinky to pinky and there is a plane of symmetry between pinkies.

 

[StudentDentist]

Top Picture:

Top drawings:

When you "rotated it 180 deg", you actually didn't rotate it. You drew the mirror image. Upon rotation, the H comes out, and the Cl goes in. That's your mistake.

Bottom Picture:

When a molecule has a plane of symmetry, it's achiral. Bottom line. Somewhere in the geometry, you're thinking it's nonsuperimposable, when in actuality, it is.

"if you look at the picture, it has a plane of symmetry so it can be superimposable so it should be Achiral.

 

[joonkimdds]

Bottom Picture:

When a molecule has a plane of symmetry, it's achiral. Bottom line. Somewhere in the geometry, you're thinking it's nonsuperimposable, when in actuality, it is.

"if you look at the picture, it has a plane of symmetry so it can be superimposable so it should be Achiral.

but if you look at your left hand and right hand, and if you draw a plane of symmetry between your hands, isn't it exactly the same as what's drawn?
(you connect your pinky to pinky)
and we called our hands chiral so why is the picture achiral?
our hands has a plane of symmetry too if we connect pinky to pinky.

p.s. thanks for the first one 😀 but I am still confused with the second one.

 

[StudentDentist]

but if you look at your left hand and right hand, and if you draw a plane of symmetry between your hands, isn't it exactly the same as what's drawn?
(you connect your pinky to pinky)
and we called our hands chiral so why is the picture achiral?
our hands has a plane of symmetry too if we connect pinky to pinky.

p.s. thanks for the first one 😀 but I am still confused with the second one.

at the very least, you've combined two separate concepts, so it's no longer clear to you. to fix what you said...

if you look at your left hand and right hand, and draw a plane between them, that plane is called a mirror, not a plane of symmetry. We take the mirror image of the molecule. The plane of symmetry exists within the molecule.

Sort thru the terminology and see if the confusion still exists.

the hand, by analogy, is a molecule. Treat is as a molecule...if that makes sense.

 

[joonkimdds]

at the very least, you've combined two separate concepts, so it's no longer clear to you. to fix what you said...

if you look at your left hand and right hand, and draw a plane between them, that plane is called a mirror, not a plane of symmetry. We take the mirror image of the molecule. The plane of symmetry exists within the molecule.

Sort thru the terminology and see if the confusion still exists.

the hand, by analogy, is a molecule. Treat is as a molecule...if that makes sense.

Ok, if that's how i need to distinguish them...i understand.

but then, how are they superimposable to each other? do i just slide them toward each other?

 

[StudentDentist]

Ok, if that's how i need to distinguish them...i understand.

but then, how are they superimposable to each other? do i just slide them toward each other?

how're the hands superimposable on each other? they're not.

put your hands together, palms touching each other. ...the thumbs face one direction and the fingers align. yet, the palms are pointing the opposite directions.

 

[joonkimdds]

how're the hands superimposable on each other? they're not.

put your hands together, palms touching each other. ...the thumbs face one direction and the fingers align. yet, the palms are pointing the opposite directions.

no i mean how is the structure above is superimposable each other?

i slided the molecule just like how left and right hands were slided toward each other.
I understand that the hands are not superimposable, but isn't the first picture(right above the hand picture) also not superimposable?
I don't see the difference between two pictures that i just posted.

 

[StudentDentist]

Like I said before, this is an issue of terminology. you're mixing up concepts.

You do NOT cut a molecule in half and try to superimpose THAT. You take a mirror image of the molecule, THEN you try to superimpose the two mirror images.

your left hand's a molecule. take a mirror image, and you get the right hand. the two hands are two different molecules.

The ENTIRE carbon molecule (5 balls) is a molecule. You take a mirror image of ALL that, and you get another 5 balls.

What you did was ...to take one ball, and 3 half balls....and take the "mirror image" to get the other 3 half balls, and one ball. This is improper.

 

[joonkimdds]

Like I said before, this is an issue of terminology. you're mixing up concepts.

You do NOT cut a molecule in half and try to superimpose THAT. You take a mirror image of the molecule, THEN you try to superimpose the two mirror images.

your left hand's a molecule. take a mirror image, and you get the right hand. the two hands are two different molecules.

The ENTIRE carbon molecule (5 balls) is a molecule. You take a mirror image of ALL that, and you get another 5 balls.

What you did was ...to take one ball, and 3 half balls....and take the "mirror image" to get the other 3 half balls, and one ball. This is improper.

U r the Genius!

 

[erichen]

that makes so much sense! 😀