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/sci/ - Science & Math

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2586250 No.2586250 [Reply] [Original]

Okay, /sci/. You think you're so smart?
I know not all of you are chemfags, but let's see if anyone can tell me why:
A. [6]-annulene (benzene) is planar but [8]-annulene is nonplanar.
B. H2O is liquid at 293K and 100kPa, but H2S, H2Se, and H2Te are all gases under those conditions.
C. R-limonene smells like citrus fruit, but L-limonene smells like pine trees.
Any takers?

>> No.2586258

Horrible way of disguising homework thread

>> No.2586263
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2586263

>> No.2586271

if you dont know B. you are retarded (Protip: it has to do with hydrogen bonds)

>> No.2586273

>>2586258
Nope. If I had chem homework to do, do you think it'd cover chirality, aromaticity/antiaromaticity, and phase differences all in the same set?
I can give satisfactory answers to all three, and I will therefore pass judgement on any answers submitted.

>> No.2586292

Wow did you even fucking go to class you lazy nigger

1) torsional strain
2) polarity
3) enantiomers react differently to chemical receptors in animals

go away

>> No.2587543
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2587543

>>2586292
OP back again. Wrong, wrong, and wrong...or at least not exactly right.
A. Benzene has a total of six electrons in its molecular pi orbitals, and thus all of them are paired up if the molecule is planar. "Hopscotch board" diagram:
_1L_ _1L_
_1L_
[8]-annulene, however, has a total of eight electrons in the pi orbitals, and so two remain unpaired when the molecule is planar, causing the molecule to behave as a diradical.
_1_ _1_
_1L_ _1L_
_1L_
The high instability inherent in this state causes the molecule to remain nonplanar so that the pi orbitals do not conjugate across the molecule. Torsional strain has nothing to do with it; in fact, (nonplanar) cyclohexane has less torsional strain than (nonplanar) cyclooctane.

>> No.2587550
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2587550

>>2587543
>>2586292
Continued...
B. Oxygen is much more electronegative than the other chalcogens (since there are fewer electrons shielding the positive charge of the nucleus), and so there is a greater separation of charge (polarity) between the hydrogen atoms and the oxygen atom than in water's heavier counterparts. Therefore, water is capable of hydrogen bonding to itself, while hydrogen sulfide, selenide, and telluride display negligible hydrogen bonding character. This difference in intermolecular force strength causes the phase diagram for water to be vastly different than those for H2S, H2Se, and H2Te. (You get half credit for your answer.)
C. Different enantiomers of a chiral compound have identical properties except for their interactions with other chiral compounds. This is because the intermediate formed by the interaction between the two chiral molecules will necessarily have two chirality centers, and a difference at one center but not the other will result in two diastereomers of the intermediate. Diastereomers of a chiral compound do behave differently, and can generally be resolved from each other. Since life as we know it makes use of chiral molecules in functionally 100% enantiomeric excess, the (chiral) receptor proteins in our noses are capable of differentiating between the two enantiomers of various substances. (Again, half credit for what you wrote.)